Supplying B+ remotely to an antenna amp
Mark Connelly, Gary Thorburn
A while ago I picked up a GaAS-FET preamp for 1-30 MHz, made by Advance Receiver Research. I found it on a ham flea junk table, and have never even tested it. The item can be seen at the following URL, but perhaps you are already familiar with it, or have even used one.
Assuming it works, I'd like to mount it at the antenna, and feed its 12-v power from the shack thru the coax. I need some ideas on how to properly do this.
I can isolate the DC from my receiver easily enough with a 0.1 uf cap between the center of the coax and the receiver ant input. But I suppose I also need two RF chokes: One to keep the RF signal from shorting through the battery (a choke in series with the battery at the receiver end) and one to keep the RF signal from shorting through the amp (a choke in series with the B+ input at the amp end).
What size chokes do I need? I looked at some of your circuits for examples of chokes positioned in power supply circuits, and I find examples in the 2000 uH range, but also in the 2000 mH range. A 2000 mH choke is a pretty big beast isn't it? I think a MW-range antenna coil is typically only about 300 uH, right?
Any suggestions you can offer would be very welcome.
The reactance of the DC-feed choke at the lowest anticipated frequency of operation should be about 10 times the characteristic impedance of the feedline: e.g. XL about 500 ohms for use with 50 ohm coax. If you're covering a very large frequency range, you might want a large value choke in series with a small value one that's about 1% of the large value (since a large value choke may not look purely inductive above its self resonant frequency).
Sometimes paralleling the choke with a resistor in the 1K range "de-Q's" it and prevents weird interwinding-capacitance resonance effects from causing amplifier oscillations or unintended peaking of VHF (TV/FM) signals on the B+ line.
The XL formula is:
(2 * pi * F * L) where F is in MHz and L is in uH.
I often use 2200 uH which offers 500 ohms of XL at 36 kHz so, yes, you can go a lot lower in inductance without worrying. I don't recall ever using anything larger than 10 mH (10000 uH) in any of my designs. 470 uH to 4700 uH is the typical DC-coupling inductor range I use.
If you go with 270 uH, that would give 500 ohms XL at 295 kHz. Since your amplifier only works down to 1 MHz anyway, that would certainly be fine. Remember to take the choke's DC current-carrying specification into account. Give yourself a safety margin of 2 to 1: If your remote amplifier needs 100 mA, select a choke rated for 200 mA or more.
Mouser is the supplier I use the most for inductors, though Digi-Key, Allied, Newark, Ocean State, and several other vendors are worth looking at also.
You can also make your own inductors with toroidal cores. For very high current applications, this may be the only feasible solution. You need to know a given core's "AL" value to allow the calculation of the number of turns required. For instance, an FT114-61 core has an AL value of 79.3. If you need a 270 uH inductor, the formula would be
which gives 58 turns.
Return to BADX Home.