Electronics > RF, Microwave, Ham Radio

Active Loop Antenna Power Feed


I want to build this loop antenna for shortwave SDR listening: http://www.lz1aq.signacor.com/docs/wsml/wideband-active-sm-loop-antenna.htm. As described, it uses two twisted pairs of an Ethernet cable for the output signal and power. I am a bit confused about exactly what is happening with the power and output signal in the circuit. The power supply appears to be floating relative to the output signal, and flows through a current balun, which I believe is there to stop common-mode currents from flowing. What bad things would happen if common-mode current did flow in the power supply leads? Does the power supply need to be floating? The output signal is coupled out of a transformer (L4, L5, L6) resulting in a balanced signal, which goes through another current balun to convert it to an unbalanced signal referenced to ground. Why is this necessary? Couldn't you just attach ground to one end of L6 and the other end to the other twisted pair?

When I build this circuit, I would like to use a coaxial cable to bring power to the antenna and take the output signal out, using a DC bias network. Would this result in worse performance than the arrangement shown in the circuit as it is?

You can convert it to use coax with power up the coax, and still have reasonable results. Simply use a 100nF capacitor in series with L6 to isolate the DC feed from the output, and connect the 13V supply wire to the centre core instead. You still need the balun on power, but connect the -13V5 side to the shield instead.

Bottom end you will need to feed in the power using another balun, basically replicating the input side in reverse, and feeding in your 12-15VDC supply to the new inductor, and connecting the ground lead to the shield. 100nF capacitor to isolate the DC from the antenna input and you are done.


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