High Impedance Traveling-Wave Antenna Preamp

This project is for a differential input shaped-gain preamplifier. Using a preamp with gain that increases with frequency can be important  to reach the "quiet rural" ITU propagated noise level target. Aat the same time protecting a receiver from too-strong low and mid frequency signals and energy is particularly necessary for broad-band receivers such as SDRs. Shaping the gain between the radiation resistance of an antenna, particularly a wide-bandwidth one, may be required to avoid overloading an SDR's ADC at the low frequency end while still maintaining enough signal and noise levels to maintain their dominance at the ADC and detector.

This preamp is particularly useful when following a balanced traveling wave antenna that is being operated over a significant bandwidth rather than a lower impedance narrower bandwidth and higher Q 'resonant' antenna. When used with an additional input transformer to provide  transformation from its 100 ohm input impedance to perhaps 600 ohms and also act as a balun to maintain or provide symmetry it becomes a good candidate for use with an LoE or Beverage antenna.

The gain sloping circuit used here is described by Clint, KA7OEI, and should be read and understood. Because in the end, every receiving system and location is unique, understanding this makes the path to tailoring the gain shape for a particular situation easier. The values used in this kit may be altered at production time by simply selecting a different JLCPCB C# when a fabricated and assembled PCB  is ordered.  Of course, larger changes in the shaping circuit and the whole design may also be accomplished by using the Hardware Source code to create an end result even more tailored to meet one's needs.

Please read Clint's original description and a follow-on article from his blog.

It's also possible to remove the shaping and obtain a relatively flat response with higher maximum gain by eliminating or bypassing shaping components. Maintaining differential input with high performance amplifiers can still provide low IMD and broad frequency coverage which may be desirable for other applications.

See also the Hybrid Antenna System overview and the comments and links there.

The schematic follows but for better viewing of the design, download the Kit file from the Material List below , unzip it and drop the .sch or .pcb file onto kicanvas from a web browser.

Gain is adjustable, but the shape is shown here:

PreTest PreampB

Upon receipt of the assembled Shack and Preamp PCBs from the manufacturer,  simple preliminary testing should be performed.  This should include the absence of shorts on the power supply input and the output of the 7805 LDO. The board will already have been flying probe tested at manufacture and physically examined after all components were soldered but one more test after delivery is good insurance. 

Once there is confidence that everything is OK with the Shack and PreampB PCBs, 12VDC power can be applied to the Shack PCB and 8V output voltage measured and the adjustment set for 8V.   If possible it is a good idea to provide the 12VDC from a current limited bench power supply as extra insurance.  Normally PreampB current should be no more than .5A  and PreampB so setting a current limit point slightly higher than this will ensure that if something has gone wrong, excessive current can't flow and damage a component. 

RF testing is best done using a scalar or vector network analyzer such as the nanoVNA. These are 50 ohm unbalanced measurement devices and since the preamps have balanced inputs and outputs a 1:2 transformer such as the Mini-Circuits T2-1 is useful to create a differential 100 ohm test environment.  As an alternative, the binocular core from a 300:75 ohm FM/TV transformer can be rewound with 3:2 turns. These measurements are not meant to be precise, only functional so the impedance error is not critical. 

Noise testing is best done with a spectrum analyzer (SA) or broadband SDR having continuous coverage of .01 - 30 MHz.  Either a TinySA or a KiwiSDR works fine for this. Broadband sweep is important since it can identify problems that wouldn't be obvious with banded ham-band-only receivers.

Rough gain measurement should show on the order of 10 dB gain for PreampA and around 20 dB for PreampB. PreampA should show relatively flat gain while PreampB's upward gain shaping should be obvious.

Because the preamps are both fixed gain, when used with a calibrated SA or SDR both the total gain and noise relative to the antenna connections is determinate. Both have spectral shaping  that will affect the measured response as well as the final response when used in the intended dipole or traveling wave antenna system.  The PreampA system can be modeled from theoretical antenna impedance however PreampB which uses the LoE is poorly defined. The transfer function from its radiation resistance to the preamp output is less well known due to the variability in the type of soil over which it lies.