In order for the Single Antenna system to become a useful and valuable solution capable of achieving excellent results at a particular location it is essential to understand how it is different from a traditional antenna. An exhaustive description of this would be too much to address but an overview may help a prospective builder or user have success rather than dismal failure.
A simplified block diagram of an entire broadband SA & SDR system follows. This includes not only the physical antenna conductors but simplifications of the active electronics involved.
If Antenna Factor is known this SA System can provide absolute electric field measurements as well as comparison with other receiving systems worldwide equally well qualified. Antenna factor is a measure of the transfer from an incident electric field, normally expressed in units of volts/meter to the voltage presented to an SDR or other measuring device of known impedance. If a suitable antenna measurement range which can produce known fields is available this factor can be directly measured but for an extremely broadband system such as this one and one that operates at very long wavelengths a range of this sort is not possible. For this reason the antenna factor for the SAS is estimated using both measurement and theoretical modeling.
The SAPreamp, CAT5 cable and ShackBoard can be assembled and the entire assembly can be measured with a VNA having suitable frequency range. A TinyVNA is able to operate over most of the SAS systems potentially useful 1 kHz-200 MHz range. But modern VNAs are normally 50 ohm systems. They give the most precise measurements when the DUT is also approximately a 50 ohm device. For the SAS this condition isn't met because the input impedance is very high compared to 50 ohms. When the SAS is measured with a 50 ohm VNA noisy measurements can result particularly at the low frequency end of its range.
To characterize the SAS a combination of methods are used. The antenna itself can be well approximated as a Short Dipole. For a 6m dipole this holds well to at least there it is one tenth of a wavelength long at 5 MHz. Additional models and methods can be used to characterize it at higher frequency where it becomes resonant and above. Theoretical models or microwave simulators may also be used to provide estimates of a longer dipole. Using a combination of methods, models and measurement the antenna factor for a given SAS system using a particular dipole size can be gained. Combined with the SAPreamplifier's input model, signal transfer from the radiation resistance to the SAPreamp input may be obtained. These two blocks may then be combined with VNA measurement (at 50 ohms) to produce an overall antenna factor which allows a reasonably accurate measurement of the absolute field strength anywhere within the entire frequency range of the system.
According to the short dipole model the radiation resistance, Ra, of a 6m dipole becomes extremely small at low frequency while the equivalent capacitance of the reactive portion of the impedance remains constant. For a 6m dipole this value is about 10.8 pF.
So although intercepted power, pattern and gain remain constan t, the available voltage becomes very small. This creates a 20 dB/decade slope. Even so, from ITU Quiet Rural region measurements the incoming noise has a 28.6 dB/decade negative slope. These effects cancel and make low noise system design most difficult at the high frequency end of the range. This upper limit is usually taken to be where the dipole is one tenth of a wavelength long or in the case of a 6m dipole, at 5 MHz.
A theoretical model for the antenna factor of a 6m dipole apart from active stages indicates a 7.55 voltage gain from the incident field to a high impedance voltmeter at its terminals.
For the SAS the entire active antenna system is considered in the model, from the dipole itself to the ShackBoard output and as delivered into a 50 ohm SDR or other measuring device. The dipole alone exhibits very high Q below 5 MHz where it behaves as a 2 ohm resistor in series with a 10.8 pf capacitor. When this is connected to the SAPreamp input, the antenna's reactance along with the SAPreamp input filtering, made up of components shown below
along with a bleeder resistor and buffer OpAmp input characteristics seen in the schematic create a frequency selective filter.
This filtering is provided as a means of preventing overdrive in typical use cases where AM, SW and FM broadcast stations may produce very large field strength at the antenna and otherwise drive the system into non-linear operation.
It should be understood that the series capacitance of the dipole varies along with dipole length and modifies the above response but this can be included in any antenna factor estimate. The circuits following the antenna and input filtering, including the SAPreamp, CAT5 connecting cable and ShackBoard taken together are seen to have approximately unity gain. Precise filtering shape changes with component values but generally looks as the red trace below:
Measurement from the monopole terminals to the 50 ohm SDR input using 100 feet of CAT5 cable interconnecting the SAPreamp to the ShackBoard is shown below. This was performed using a VNA having a 50 ohm reference impedance. The gain from Preamp input to ShackBoard output is approximately 0 dB from near DC to over 200 MHz.
With all of these factors put together, the dipole model, shaping model and active stage measurement can provide the antenna factor and a reasonably good approximation of the incident electric field.
Also, the power within the radiation resistance can be obtained and used for comparison and study. Knowledge of a geographical region and comparison of the system's measured noise floor with worldwide ITU estimates for expected noise floors in a region of that same type provides a metric to gauge a system's performance. It provides a method to know when the regional limiting performance has been reached and identifies areas where that is not the case. This knowledge is essential for improving achieving and maintaining optimum receive system performance.
[Placeholder for 6m SAS Antenna Factor Plot]
The SA broadband antenna system has much in common with the SDR shown below it in the block diagram. Both of these have major differences when compared to older, conventional analog receive system types. Unlike analog receivers, each of these is fundamentally very broadband. Rather than filtering in frequency to separate incoming signals prior and during amplification, these systems accept a very broad frequency spectrum and sample the unfiltered aggregate. Selectivity is created within the DSP processes that follow the hardware and which are inherent in any SDR.
Because there isn't selectivity within the range to be received, all of the components and circuits have a unique challenge - they must simultaneously tolerate the instantaneous aggregate of all input signals present while also providing the ability to process very weak signals. They must have very high dynamic range.
Unlike older analog receive system architectures, when these attributes aren't provided, when the instantaneous total of all signals impinging on the system exceeds some maximum, failure occurs. Also unlike previous analog receivers, this failure tends to occur suddenly rather than gradually. When an analog receiver experiences overload it tends to generate intermodulation distortion (IMD) which creates unwanted signals and noise. When a broadband, probe type receive system experiences too high input level from all signals within the entire spectrum it ceases to function almost instantly. That temporary failure may occur at stages within the active antenna system portion or, if the overload first occurs within the SDR, at that LNA preamplifier. It can also happen if the maximum tolerable ADC level is exceeded.
The characteristics of signal overload are so different and the consequences so much greater and more immediate that they need to be understood and recognized when they occur because the entire system becomes unusable for the duration of the overload. Because overloading occurs as the instantaneous vector sum of all signals over the entire broad bandwidth of the system it may not be persistent. As an example if there are many MW AM broadcast band signals simultaneously modulating, the instantaneous modulation peaks of all these different contents together may be very much greater than the carrier magnitude of any one of them alone. Even if there are only two equal size large signals having 100% modulation the instantaneous peak power may be 12 dB greater than the carrier power of either alone. If there are many such signals overloading may occur when the individual carriers are each lower than this. This forces a requirement of sufficient "headroom" on a these systems.
Also, for the broadband probe/SDR systems, out-of-band signals which are not even visible in a display may contribute to overloading. Even if a system is only being used to operate over, say, 0-30 MHz large FM broadcast signals in the 100 MHz region may contribute to overload.
In summary, because these broadband systems are so different from previous receivers which used tuned antennas and analog filtering for selectivity, to prevent overload that manifests differently it is essential to understand and recognize the failure modes and eliminate them in order for the system to operate properly.
Along with the necessity to eliminate overload by component selection within the SA preamplifier and Shack Board hardware, it is also important to select best antenna size and to optimize weak signal reception. This is an issue of optimizing an SA system to be able to detect and demodulate signals down to the propagated noise level at the antenna. It is why the measured ITU noise levels have been selected as goals for system design. Not every location has the same limitations or potential. Some very good locations may be able to achieve the ITU "Quiet Rural" levels. For an omnidirectional antenna system, this is approximately the best that can be done anywhere. If a particular locations ultimate ITU level is higher than this there is no benefit in outfitting it with an antenna system that can potentially do better. There may be practical benefit in NOT using such a capable system since dipole antenna size might be reduced with no degradation in performance while at the same time a smaller dipole might have more ability to tolerate very large signals. Up to about a half wavelength in size larger antennas generally have larger signal levels at their terminals, so deploying a less sensitive system may have overall benefits.
Because of this need for it is particularly important to make an initial assessment of any proposed site prior to deployment in order to best select antenna size and perhapscomponent values to give the greatest likelihood of providing the best strong-signal protection while simultaneously providing the highest system sensitivity. The topics below are intended to deploy a broadband receive-only antenna system kit and, once deployed, to optimize its performance for a specific location.
Begin by applying Silicone Rubber to the channel in the enclosure cover using a wide spatula or painter's knife to create a gasket in the channel. Silicone rubber is preferred because it shrinks very little as it cures. This can be a little messy but clean up should be possible using mineral spirits if required. Don't worry about a little extra rubber where it was not intended but rather let it set for a few hours and come back to do clean up when it is partially set. Let it sit for a few DAYS to fully cure. Note that this enclosure should probably be considered "water resistant" rather than "water proof". Better designs are welcomed. FreeCad source code for the current design is provided. Below is a picture of a portion of the enclosure cover filled with gasket material.
With both the Preamp and Shack Board PCBs complete, operation can be verified by simply connecting them with a standard, short CAT5 cable, connecting the RF output to a spectrum analyzer or broad band SDR or even narrow band receiver. With total current verified to be in the vicinity of 100 mA, simply touching each of the antenna pads on the preamp with a finger should result in significant and similar change in output observed for each of the inputs. If a problem is found, SMA connectors can be slipped onto the antenna pads, temporarily tacked to the PCB and a jumper wire solder across the adjacent grounding pads to connect the shield. This has been left unconnected to reduce capacitance during normal operation.
When this stage is successfully completed, .5mm diameter magnet wire perhaps AWG #24 - #26, may be prepared in two lengths each a little bit longer than 3 meters. One end should be tinned, passed through the small entry hole in the PCB enclosure and soldered to its antenna pad for each monopole. These conductors may be coiled pending attachment of the assembly to the fiberglass mast 3m below the top.
The mast can be extended to full length and plastic clips attached with brass 6-32 screws and nuts at the junction of each of the six upper sections. This placement further assures that a section won't collapse during use. Once the mast is complete with clips, the Preamp enclosure can be placed at or just below the third section, counting from the top, and the enclosure, cover and CAT5 cable placed in approximate position with the mast lying on the earth. Tywraps are then run through the cover and enclosure but left loose while the CAT5 cable is being connected to the RJ45 connector on the PCB. That cable will be formed 90 degrees and set in the exit groove of the enclosure while the cover is placed over the top. Prior to securing the cover some additional silicone rubber should be used to seal the antenna wire and the CAT5 passage areas. The cover is secured with six more 6-32 screws into the tapped holes in the enclosure. Don't over-tighten these screws since the threads are in plastic and only require enough torque needed to snugly close the cover gasket onto the enclosure.
The CAT5 cable and individual monopole conductors are routed through the intermediate mast clips in both directions along the mast with the conductors tied off at the end clips around the arms. It's OK to wind a fe turns around the end arms.
For reference, here are the standard CAT5 cable pairing connections used between the SAPreamp and the Shack Board RJ45 connectorss and the pin connections used.
When complete, the antenna assembly should look like the picture and be ready for paint, if desired. Spraying everything with flat green or brown camouflage paint is not required but makes the finished system much less noticeable. This may be important in some HOA environments.
In general, the Single Antenna receive system can't be better than its location. While capable of rejecting unwanted noise and signal ingress from some common mechanisms such as poor symmetry, balance and feedline coupling, it won't be better than the near-field and far-field noise environment. Placed too close to local sources that generate fields producing high field gradient such that the tips of the dipole see different potentials, the system will convert these to differential signals which may raise the system noise floor and degrade SNR of propagated signals. Small changes in position and polarization may provide a great deal of improvement in this respect. Keeping the mounting location well away from residences and sources of mains power, network signals is usually a good start. Mounting the dipole on or next to a building may be a bad choice. The SA has been designed to allow a 100' CAT5 cable to be used. Taking advantage of as much physical separation as a site allows usually helps final results.
The Field Probe, mentioned earlier, may be helpful in identifying "quieter spots" in a candidate back yard or placement region. Mounting close to buildings and other conductors is probably to be avoided. Eliminating sources of unwanted signals may be an option but is like playing "whack-a-mole" since new sources may always arise later. It's better by far to site the SA well in the first place, to the degree that is possible.
At this point initial powered testing may begin. With the mast vertical and bottom section either clamped to a short non-conductive post or else placed in the screw-in ground mount and freestanding, the CAT5 may be run to the Shack Board location and the entire system powered up for testing.
With mast, dipole, preamp, CAT5 cable and Shack Board installed and connected initial verification of the entire system can be performed. This step confirms that the common mode rejection capability of the system is being provided. This is most easily done through using the two switches on the Shack Board which are provided for that purpose.
Looking at a broad spectrum, either from a spectrum analyzer that covers at least 0-30 MHz or from an SDRs such as the KiwiSDR or WEB-888 which can provide the same display, simply observe that broadband display and select the "Disconnect Preamp12V" switch that turns OFF the bias to the SA Preamp's buffer stage. Confirm that ALL signals & noise being displayed drop in amplitude by 20 dB or more and that nothing near the previous level remains. This provides an assurance that there isn't a path for unwanted signal or noise ingress between the Preamp and the SDR detector.
Do not attempt to proceed until any signals and more importantly, their mechanism of ingress are identified and removed. Mechanisms that can produce these unwanted responses that would decrease system sensitivity by decreasing SNR of desired, propagated signals must be removed first. These important steps verifiy that "The antenna really is the antenna." Signals being produced are truly from the dipole acting as a differential source rather than from feedline, power supply, unbalance or some other form of unwanted ingress. This test can be periodically repeated to provide assurance that a system has not changed in this regard.
Once this first verification is complete, return the "Disconnect Preamp12V" switch to normal ON mode and thereby the bias to the input buffers Then, using the "Short the Dipole" switch on the Shack Board engage the mechanical relay at the dipole terminals. Again look for a very large drop in all responses. This test verifies that only differential signals from the antenna are significant and that common mode rejection of the system is being provided.
It's important to recognize that engaging the "Short the Dipole" switch DOES NOT remove signals from the Preamp input. It only assures that differential signals produced at the dipole center - across the mono-poles - are removed. Common mode signals appearing between the Dipole, which has become a center-fed conductor is still an antenna against the reference ground of the preamp. This 'ground' is the potential that exists at the end of the CAT5 cable. Because the entire system is now mono-pole with a horizontal element working against the CAT5 common potential, the Preamp and in particular the buffer input stages are likely still dealing with large signals. This situation is depicted in the last graphic presented at the end of Whip_Tipps DL4ZAO (Google Translation). Connecting the monopole connection only assures that the differential signals from the dipole which is the desired antenna are dominant. Strong common mode signals may still be causing overload or IMD.
Even after the assurances provided by the verification methods it is possible that some unwanted signals may remain since near-field interference, local sources of unwanted signals and noise, can be converted to differential signals if and when the change in their fields across the length of the dipoles is sufficient to create a differential signal. This is to say, if the dipole lies along the gradient of very strong near field sources the profile of the field strength will NOT follow an inverse square law as do propagated signals from very far away.
Since those signals are, in a sense, "real" the first line of defense against them will be in changing antenna siting slightly in either x, y or z directions or perhaps even altering polarization to minimize the response in the system. If the source of a particular offending source is only quenched without changing antenna system siting then a susceptibility may remain and may not be recognized when a another similar unwanted source arises. Ongoing vigilance is necessary to assure that a Single Antenna system is not responding to nearby unwanted interference that can restrict its overall performance.
Here is an example of a successful verification of a 6m SA receive system. The annotated trace is a web888 response when the "dipole shorted" switch is engaged. This demonstrates that signals coming in are truly differential from the dipole rather than other unwanted ingress. There is an additional 6 dB of attenuation added at the output of the Shack Board which prevents the system from overcoming the WEB888 noise at 60 MHz
It is important to understand that antenna size and component values within the SAPreamp and ShackBoards only provide a compromise between maximum sensitivity and avoidance of overload for typical amateur locations. This compromise will no doubt be less than perfect for any particular location. Arriving at the best choice of values is an ongoing effort. It may not be complete. The full-length 6m dipole is capable of achieving lower than ITU noise floor (noise temperature) values for a "Quiet Rural" location. This may be a considerably more difficult target than some common locations near residences, cities or such are capable of providing. This means that it may be possible to use a dipole that is shorter than 6m and not significantly degrade the resulting system. Analyzing this sort of change is a significant project but if it is found that there are extremely strong local signals anywhere in the 1 kHz - 200 MHz spectrum it could be that IMD within the Preamp or Shack Board will seriously degrade performance and a change will be required for best function.
It's for the above reasons that using a Field Probe prior to deployment can be very helpful. If it is determined that a signal much larger than about -10 dBm is present at the output of the Shack Board it may indicate that the preamp itself is being overloaded.
In this case antenna length adjustment and component value changes may be made most easily before final installation.
The following sections are an attempt to describe the degrees of freedom for these kinds of system optimizations.
There are four ITU noise regions being compared to the noise floor of a 6m SA antenna system:
The Output_noise_dBm plots model the system built with initial component values, those which should be capable of exceeding ITU "Quiet Rural" measurements. However, if at the particular deployment location, there are very large signals present that cause distortion then after verifying that the distortion is produced within the SA system rather than in a SDR that follows, antenna size or Preamp PCB component values must be changed to provide protection from this type of overload.
Isolating the source of distortion to either the SA system or the SDR is most easily done by inserting 10 dB or so of extra attenuation at the output of the Shack Board. If there is no change or the evidence persists then the SA Preamp needs to be adjusted.
Adjusting antenna size or the RC/R shaping in the input buffers allows a degree of level adjustment and of pre-filtering to reduce the potential for overload. Traditional LC filters cannot be easily created with such high impedance antenna connections and wide bandwidths.
Depending upon the magnitude and frequency of an offending local signal, different courses of action may be indicated.
The Shack Board has pads for 0 ohm jumper resistors or shorting wire which bypass 50 ohm resistors that set the output source impedance. With them absent there should be about -6 dB gain through the Shack Board, measuring from the differential CAT5 signal to an SDR termination having a 50 ohm input. If these resisters are jumpered the gain can be increased to 0 dB. This should rarely be necessary unless the SDR receiver or receive system is particularly insensitive and needs higher signal level.
Usually adjustments will be made after isolating the portion of the spectrum that needs to be adjusted. Very generally
If the degree of overload and its frequency can be identified, possible ways to reduce the level to take a system out of overload without compromising sensitivity at other frequencies may be understood in what follows.
To begin to demonstrate the effect of changes, in the following Cp has been increased 10X from 33pf to 330pf. This reduces gain from LF through MW while having little effect at HF. Adjustment of this value and/or Rp might be useful to reduce strong AM broadcast band interference without significantly altering HF performance:
In the next comparison Rc = 2k is compared with Rc=500 ohms which shows the most change in HF levels
Simply shortening the dipole from 6m to 3m can reduce levels across the entire spectrum but potentially with a consequential degradation of the overall noise floor. This might seem drastic but in regions such as "Residential" or "City" where the limiting broadband noise floor is significantly higher than "Quiet Rural", there may be a lot of unreachable headroom. Raising the SA noise floor and reducing the sensitivity may not have any significant adverse effect on the SNR of recovered signals.
All of these values are interactive so a model and simulator such as QUCS-S:qucsator or Spice is useful in selecting them once a particular target result is known.
As an example, the spectral plots below demonstrate two types of site noise. One type is a generally flat and unfeatured broadband noise floor with little variation. This is characteristic of the noise described by the ITU measurements. In this example, that level is between "Quiet Rural" and "Rural". Also evident are some more complex shapes that are not from noise of the type described by the ITU. Instead, these regions - 5MHz to 10 MHz for example - show structure that is attributable to local, probably near-field, noise ingress. While careful positioning of the SA might reduce this structured noise, it probably will have little or no effect on the wider, flatter noise will be a limit for that site and nearby regions as a whole.
Changes to shaping which do not increase the SA noise floor above the broadband, flatter noise will likely not affect recovered SNR since it is being set by the general location. If a large signal that drives the SA preamp into visible distortion is present then simply decreasing the dipole length may be sufficient. In this particular example, a 60 kHz transmission from the 20km distant 110kW transmitter of WWVB is producing an output signal above -10 dBm. This large signal aggregating with multiple large AM BCB signals occasionally pushes some part of the system, either the SA preamp or the SDR that follows, into distortion which causes noise floor lift across the entire spectrum. Here only a slight decrease in the value of Rp might be very beneficial.
If value changes are made, they must be performed equally on both sides of the preamplifier for each monopole.
Each situation will require analysis of signal levels during day, season and even over a solar cycle to determine optimum adjustments. A general solution for the adjustments is beyond the scope of this article. Don't forgot to look above the range visible on the SDR since the SA preamp generally has flat response beyond 200 MHz. A large local VHF or UHF signal may be causing or exacerbating overload yet be unseen.
Here is a spectrogram obtained using a KiwiSDR showing the results of occasional overloading peaks in the receive system.
Short term sporadic lines across the spectrum from these kinds of overload are most easily seen on wide frequency display such as WebSDR as is used on a KiwiSDR or Web-888 by selecting a broad span with spectral and waterfall averaging set to 1 - to no averaging at all. Because they are brief and sporadic, averaging tends to make them difficult to recognize. Notice that these generate components above the MUF which is an indication that they are locally created rather than over-the-air.
These events can occur when the instantaneous sum of all signals becomes large enough to either over-run the ADC within the SDR or create distortion within the SA preamp, or both. When they occur they may exhibit in a manner similar to broadband impulse noise such as lightning. This may be coincident with an observable change in amplitude of existing signals. Overloading may be due to or exacerbated by large out-of-band signals such as FM broadcast near 100 MHz. Study and testing is necessary to determine which is the cause in order to determine a best solution.
In this example the problem was more extreme when a 6m dipole was used but much less troublesome when antenna length was only reduced to 5m. This is the case because the signal levels are from local rather than ionospheric sources so are very stable in amplitude. If the overload had involved HF ionospheric signals this would probably not have been the case.
[A special comment to anyone having particular difficulty keeping the input high impedance buffers from overloading even with value changes and contemplating major design change:]
When overload occurs in this stage, the symptom is generally as shown above. The mechanism of overload and nonlinearity that will be observed is usually that of a tendency toward oscillation during the signal peaks. This happens when the slew rate capability of the ADA4817 OpAmp is insufficient for the high signal level and thereby creates an extra pole in the open loop response with the attendant extra phase shift. In this situation the circuit is no longer stable and oscillation occurs. It may be useful to recognize this if significant design change is contemplated.
The following 10 kHz to 200 MHz spectrogram was taken soon after local sunset in Fort Collins, Colorado using a TinySA-Ultra with RBW=300 kHz to capture the output from a 5m SA system having an older CAT driver with gain set to 14 dB - a previous setting which is a bit high of what may be a desirable for general purpose use. Approximately 30m of CAT5 cable is between the SA Preamp and the Shack Board. The Shack Board being used had 0 dB gain.
This plot also demonstrates the ability of the SA system to operate at VHF. While there are many signals below 30 MHz, except for nearby 110 kW WWVB at 60 kHz, the FM broadcast stations near 100 MHz are the strongest signals over the entire range. The large amplitude of these is partially because they are largely line-of-sight and running high power .They are probably contributing significantly to the peak instantaneous amplitude. Because they are higher frequency they also demand higher slew rate and large signal performance from the input buffer OpAmps. These may not be directly detrimental at the SDR that is used if it has a lowpass filter as most do but since all of these signals are present at the SA Preamp they do contribute to maximum instantaneous signal and may cause overload in either the high impedance buffer stage or the CAT cable driver there. The strongest FM BCB signal is measured to be -32 dBm on a nearby 75MHz - 2000 MHz Biconical indicating that even with CAT5 cable loss the overall SA system may still have 14dB - 9dB = 5 dB of gain as expected.
This system as shown is operating very near to input buffer overload. At lower HF frequencies reducing dipole length to,say, 3 meters could be expected to reduce the signals delivered by 20log(5/3) = 4.5 dB. However, because at that length it will still be a half wavelength long, voltage levels and output at the Shack Board will likely NOT be reduced that much. This may be a situation which would require extra effort to prevent overloading. Component values, particularly Rc, could have a similar effect but might need to be set much lower than their nominal value. In quiet environments this could have a negative effect on the system noise floor and in an extreme case might limit the LF-HF performance to cause it to be worse than the regional ITU noise.
On this topic, it may be that neither simple arbitrary reduction of dipole length or reduction of Rc reduces levels enough to avoid overload. There may be other approaches that can help. If the main source of strong signals is from FM BCB near 100 MHz a region where the previous approaches have less effect, careful selection of dipole length and/or the addition of 100 MHz parallel "traps" in the monopole's conductors might help. To understand this, consider the radiation resistance for a short dipole using the standard, non-SWTL, model. This is generally considered valid only for dipoles shorter than one-tenth wavelength with that point for a 6m dipole identified by the label:
In actuality, above one tenth wavelength the radiation resistance for a dipole does not follow this curve well . Measured impedance at large electrical size alternates between low and high values. Low values at odd half wavelengths, e.g. the "first resonance" of 72 ohms give way to high values at even half-wavelengths. The SWTL understanding of a dipole offers an idea of what is occurring. When the dipole elements, the monopoles, are considered as 377 ohm SWTLs "connected" to a region of space where radiation resistance is actually occurring, having apertures from each of these regions at the tip separating with increased length along with a central region becoming significant near even-wavelength size, then the signal available to the SA Preamp can be understood to become asymptotically twice that of a short dipole as the length goes from small to large. The apertures go from complete overlap to complete separation. Thus although the pattern varies wildly as the length is increased and apertures move apart the total effective aperture increases only to twice its initial value where the total gain approaches 2*3/2 =3 or about 5 dBi .
Using this understanding, large out-of-band (in terms of HF use) signals may possibly be suppressed by selecting dipole size near even wavelengths where the impedance is very high. Working against the impedance of the preamp input this may provide additional attenuation. I suspect that this technique works only when each dipole tip sees essentially the same field. Thus I'd expect dipoles that are high to provide the most reduction. Low dipoles where the lower tip is near the earth and more likely impacted by absorption from clutter likely will not show the same reduction in unwanted signal strength.
There is potentially another benefit to selecting special dipole length to reduce large signals coming from the broadside direction. This can be understood by recognizing that the broadside gain of a dipole falls to zero at certain lengths. Coincident with this are maxima away from broadside. The first null of this sort occurs at approximately 1.9 wavelengths. Although this technique has not yet been exhaustively tested, by selecting a 1.9*3m = 5.7m total length it is possible to reduce response of the system to 100 MHz signals coming from the horizon. This occurs when the waves propagating along each monopole cancel a central field at the feedpoint. It requires that both tips of the dipole are exposed to similar fields which may only be the case when the entire antenna is significantly about ground and local clutter. Initial examination of this have showed promise.
Caution! The following ideas have not yet been well tested or well modeled but might be worth investigating. Initial measurements do show significant effect when there is good LOS to the offending signal from both of the dipole tips.
Another possibility for reducing 100 MHz signal could be through the use of a small portion of each monopole's conductor to wind a coil "trap" This can either be thought of as a parallel resonant circuit- for 100 MHz or using the SWTL understanding of a dipole, as an extension of its electrical length. Whichever way it's viewed, placing a high impedance in series at a lower impedance location can attenuate the signal amplitude.
The following Smith Chart shows the modeled S-parameter for a 1m dipole made from very thin conductor over a frequency range from 90MHz to 999MHz. It is plotted as viewed from a reference impedance of 754 ohms - twice the impedance of free space as is appropriate for the differential measurement point at the dipole's center when the conductors are considered from the point of view of the SWTL model. Unfortunately the Markers are referenced to a 50 ohm chart due to the tool being used so the actual impedance they report needs to be scaled by about 754/50 ~ 15
The marker at ~600MHz is where the dipole is 2 wavelengths long when considering a 100 MHz interferer so near its second high impedance resonance and indicating an impedance of ~5100 ohms. The marker at ~750 MHz where the dipole is 2.5 wavelengths long shows the next low impedance resonance of ~82 ohms . Locating a trap at this position might create a minimum response at the SA preamp input which appears approximately as the value of Rc since at 100 MHz Rp is shunted by Cp.
A coil placed at a location nearest the Preamp that rotates the high impedance at the tip to a low impedance, at an odd number of quarter waves from the tip, that is, might have usefulness for arbitrary dipole lengths. . Additional "coiled length" might be added (modulo inter-turn capacitance) to rotate a low impedance presented to a value considerably higher. . The should produce the most reduction of the unwanted signal level at the target wavelength. Expanding on this idea for sites with multiple strong VHF interferers at different frequencies it may be possible to make a sort of "multi-band trap dipole" similar to what is used to match a transmitter to a dipole at HF. In this use it would be used to provide maximum attenuation and poorest match to the source at the dipole tips. This has not been tried but should be relatively easy to implement.
Notice that a coil of monopole conductor, in fact any air wound "inductor", may also be thought of as a length of 377 ohm SWTL line when the inter-turn capacitance is low- when there is wide turn spacing compared to the conductor diameter. In that case the transmission line is predominantly SWTL - it operates only in the TM mode. When its impedance is reduced by inter-turn capacitance, e.g. "squeezing of the air coil" the impedance becomes a combination of TM and TEM modes. This SWTL understanding might be used to provide an analytical model for any air-wound coil, something that seems to not have previously existed. It also explains the practical advice to someone seeking to create a high impedance (self resonant) choke. "Start with a quarter wave of straight wire and wind most of it into a coil. Then pinch the turns to tune it to the frequency of interest". See A New Antenna Model for more detail.
The two kinds of trips in the following picture each show >10k ohmsimpedance at 100 MHz and have similar effect. They are 8 turns of AWG #25 ona ~29 mm diameter form, spaced 2m anda commercial 3.3uH self-resonant inductor (TDK B82144F2332K000). The latter is small enough to fit inside the preamplifier enclosure.
In general, sites with very low ITU broadband regional noise, say Quiet Rural, but also near to sources of very strong signals, especially VHF ones, are the most difficult to optimize for dynamic range. Fundamental limitations of even the best components and designs can be seriously challenged to achieve the best results in situations of this sort.
The following plot is from an RX888 being examined by ka9q-web to demonstrate the limitations to the system sensitivity. It is from a small residential community with regional noise that is likely between ITU "Residential" and "Quiet", perhaps about 10 dB higher than "Quiet Rural".
It may be useful to consider what is being attempted and what work may remain.
Each of the above items may change at any time. Because perfection has not been achieved, there are likely items which could benefit from further attention. There are many factors affecting maintaining or improving the overall performance which need to be monitored and verified.
The 6m dipole size and accompanying preamp has been targeted to create a system capable of achieving ITU Quiet Rural performance in the best of regions. Modifications to meet the requirements of particular locales are possible. Dipole length, conductor diameter, height and polarization may all be varied. However, the interactions among these variables are complex. It is to be expected that making the dipole either longer or using larger conductor diameter will not create a better result. But in some circumstances making the dipole shorter may prove to give a net improvement in terms of visibility and strong signal handling, two name two. This may result in no loss of performance in locales that don't meet the Quiet Rural level since even though the Single Antenna system noise floor may increase it may still remain below that of the region.
Increasing the diameter of the conductor should similarly produce no significant improvement and may unbalance the antenna. The approximately .5mm diameter was chose as a compromise between low interaction with the CAT5 feedline cable and the ability to survive wind and ice loading. Because the preamp is a high impedance device, very little current flows in the dipole conductor so there is very little loss that larger conductor diameter might noticeably reduce.
Polarization can be varied. Vertical polarization was chosen to provide the lowest practical beam for long distance ionospheric propagation but rotating the dipole horizontally may reduce noise while increase ground absorption and take-off angle. This may be a good solution for NVIS propagation in those portions of the spectrum where it is possible.
Mounting location is a significant variable. Particularly in locations limited by unwanted local noise sources, moving the antenna horizontally or vertically may have a dramatic effect on delivered SNR. It may be that a shorter dipole moved upward can still potentially deliver the regional ITU noise performance by reducing coupling to local near field noise. But moving vertically may also increase the level of strong signals present at the preamp input so risks the system entering into overload. As an example, a 3m dipole at the top of a 10m mast may deliver better SNR than a 6m dipole at minimum height on a 7m mast while also having better immunity to overload.
Generally speaking as applied this is a low Q non-resonant antenna. No single parameter is sensitive or has a large effect on performance. Reducing the length by a factor of two can be expected to reduce signal level about 6 dB where the antenna is electrically short since the recovered voltage is reduced by two. At shorter wavelengths where the dipole is a half-wave or longer the maximum signal level will increase slightly even at very large electrical size while the pattern will vary wildly as mentioned as a possible means to mitigate strong unwanted VHF signals.
Modifications of this system may greatly improve the performance but interactions among the variables must be studied and controlled to achieve maximum benefit from any change. The overall environment may change in a multitude of ways so constant monitoring and verification, comparison of performance with other stations and general observation are necessary to maintain the best performance.
This project is unending