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.
The SA broadband antenna system has much in common with the SDR shown below it. 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 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 input exceeds some maximum, failure occurs. Also unlike previous analog receivers, this failure occurs suddenly rather than gradually. When an analog receiver experiences overload it tends to generate intermodulation distortion (IMD) which creates unwanted signals and noise in the area being received and detected. When a broadband, probe type receive system experiences too high input level from all signals within the entire spectrum it ceases to function instantly. That failure may occur with in the active antenna system portion or if the overload first occurs within the SDR, at its LNA preamplifier or the maximum tolerable ADC level may be exceeded.
The consequences 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, during the peak modulation of all these different contents may be very much greater than the carrier magnitude of any one of them. Even if there are only two equal size large signals having 100% modulation the instanteous 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 lower than this.
Also, for the broadband probe/SDR systems out-of-band signals may contribute to overloading. Even if a system is only being used to operate over 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 manifested differently it is essential to understand and recognize the failure modes and eliminate them or else the systems will not be usable.
Each location in the world has its own profile of signals and levels. These levels change with time of day, season and year for ionospherically propagated signals such as are used over all H F frequencies. Signals at VHF, UHF and even Microwave also are unique to each site and may similarly change. This situation makes selecting ideal component values and settings for a broadband SDR system particularly difficult. Even if distortion is recognized and understood, the causes can change over time so that providing an optimum configuration can be an ongoing task. This especially applies to the SA broadband receive antenna because making configuration changes may involve disassembling hardware, changing surface mount components (SMDs) and re-deploying.
Along with the necessity to eliminate overload within the SA preamplifier and Shack Board hardware, it is also important to select antenna size and components to optimize weak signal reception. This is an issue of optimizing an SA system to receive signals down to the propagated noise level at the antenna. This is why the measured ITU noise levels have been selected as targets for system design. Not every location has the same limitations. 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. If a particular locations ultimate ITU level is above 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 adding more ability to tolerate very large signals. Large antennas generally have larger signal levels at their terminals, up to about a half wavelength in size, so deploying a lesser system may have overall benefits.
Because of this need for special attention it is particularly important to make an initial assessment of any proposed site prior to deployment in order to best select antenna size and perhaps component values to give the greatest likelihood of providing the best strong-signal protection along with the highest system sensitivity. The topics below are an initial attempt to address this need.
Start 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 cover's channel. Silicone rrubber 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 day to fully cure. Note that this enclosure should probably be considered "water resistant" rather than "water proof".
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 your finger should result in significant and similar change in output observed for both of these 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 side of the dipole. These may be coiled prior to attaching the resulting assembly to the fiberglass mast 3m below the top with TyWraps.
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. 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 Preamp 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 connected to the RJ45 connector on the PCB. That cable will be curled 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 enough torque needed to close the cover gasket and 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 small 'ball' around the end arms.
Standard CAT5 cable pairing connections are used between the Preamp and the Shack Board RJ45s:
When complete, the antenna assembly should look like the picture and be ready for paint if it is desired. Spraying everything with flat green or brown camouflage paint is not required but makes the finished system much less noticeable.
With mast, dipole, preamp, CAT5 cable and Shack Board installed and operating, initial verification of the entire system may be done. This step confirms that the common mode rejection capability of the system is available. 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 buffer stage of the SA Preamp. 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 after the Preamp and all way to the 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.
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 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 available.
In this step it is possible that some unwanted responses may remain since near-field interference, sources of unwanted signals and noise, might 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 or perhaps even polarization to minimize the response in the system. If the source of the offending source is only quenched without changing antenna system siting then the susceptibility remains and may not be recognized when a another 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.
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 place 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.
It is important to understand that antenna size and component values on the preamp PCB only provide a compromise between maximum sensitivity and avoidance 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. 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 such strong local signals anywhere in the 1 kHz - 200 MHz spectrum it could be that intermodulation distortion within the preamp or Shack Board that will seriously degrade performance.
It is very much hoped that 'standard' component values and antenna size provided in the source and binary files will suffice for the majority of use cases and that much of what follows will not be necessary for most users. It is provided here to help those that desire to understand the compromises and be able to modify these and perhaps even change the design for special situations which require it.It's for the above reasons that using a Field Probe prior to deployment is 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 best be made ahead of final installation.
The following sections are an attempt to describe the degrees of freedom
for this kind of system optimization.
The four ITU noise regions are modeled along with the noise floor of a 6m SA antenna system:
The Output_noise_dBm plot modelss the full-size system with initial component values which appear 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 the SDR that follows, antenna size and 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 allow a degree level adjustment and of pre-filtering to reduce the potential for overload. Traditional LC filters cannot be 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 absence there should be about -6 dB gain through the Shack Board, from the differential CAT5 signal to an SDR with 50 ohm input. If these are jumpered the gain can be increased to 0 dB. This should rarely be necessary unless the SDR receiver or receive system is quite insensitive and needs high 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 altering HF performance:
Simply shortening the dipole from 6m to 3m can reduce levels across the entire spectrum with consequential degradation of the overall noise floor. This might seem drastic but at sites where the limiting broadband noise floor is significantly higher than "Quiet Rural" such as "Residential" or "City", there may be a lot of unreachable headroom. Raising the SA noisefloor and reducing the sensitivity may not have any significant adverse effect to the SNR of recovered signals.
All of these values are interactive so a model and simulator such as QUCS-S:qucsator is useful in selecting them once a particular target result is known.
As an example, the following spectral plot demonstrates two types of site noise. One type is a generally flat and unfeatured broadband noise floor with little variation. This corresponds to 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 - 5-10 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 which sets a limit for the 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 is present that drives the SA preamp into visible distortion 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 that causes noise floor lift across the entire spectrum. Here a little 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 day, season and even 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 up to 200 MHz or higher. A large local VHF or UHF signal might be causing overload yet be unseen.
A QUCS-S simulation of the SA preamp may be available to help with this but additional analysis including the entire RF chain all the way to the ADC of the SDR and/or the detector needs to be included.
Here is another spectrogram using a KiwiSDR showing the results of occasional overloading peaks in a receive system. These events can occur when the instantaneous sum of all signals becomes large enough to over-run the ADC within the SDR. They might also occur due to distortion generation within the SA preamp. Study and testing is necessary to determine which is the cause in order to determine a best solution.
In this case the problem is extreme when a 6m dipole is used but much less troublesome when antenna length is only reduced to 5m. This is the case because the signal levels are from local sources rather than ionospheric 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 takensoon 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 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 perhaps the strongest signals over the entire range. The large amplitude of these is partially because they are both close with line-of-sight propagation and very high power .They are probably contributing significantly to the peak instantaneous amplitude. These may not be detrimental if the SDR that is used 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 contribute to 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.
As configured, this system is operating very near to input buffer overload. The signals visible near 200 MHz are probably 2nd harmonic distortion of FB BCB signals. It would likely benefit only slightly from a somewhat shorter antenna, perhaps reduction of dipole length from 5m down to 3m. At lower HF frequencies this 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 even 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. This could have negative effect on the system noise floor and in an extreme case might limit the LF-HF performance and cause it to be worse than the regional ITU noise.
In general, sites with very low ITU broadband noise, say Quiet Rural, but also near to sources of very strong signals 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.