[MRCA] Midnight Oil Project

[Mark K3MSB]

Hi Breck

My LF Common Mode choke is 30 turns twisted pair on an FT-240-77 toroid.
 I measured about 5K at (about) 400 Kc.   The choke is 1.4 inch ID.

This is inserted between the Bias-T and the external antenna.

What is as important as the choke is the grounding of the coax to the
antenna.   Mine is grounded as it enters the house and at the base of the
antenna.

There are some web base Bias-T calculators available (ex:
https://3roam.com/bias-t-calculator/) that may help you.

If you are using a 117 VAC based power supply, you may want to use a choke
on the lead from the power supply to the Bias-T (for LF, I use
batteries...).   At the very least, look at the DC output on a scope to see
what's riding on it.   On some supplies I've had to add a 0.01 uF cap
across the output.

73 Mark K3MSB









On Mon, Mar 30, 2026 at 9:58 PM B. Smith via MRCA <mrca at mailman.qth.net>
wrote:

> You guys (Steve and Rob) are too sharp for me. Yes it is a circuit to
> provide DC power over the coax to one of my "Active" antennas. My main
> problem is not coupling my house noise to the antenna via the coax. Am
> working on power and decoupling via chokes. And the frequency of interest
> is below 500 khz -   ugggg   lots of noise.
> Thanks for playing.
> breck
>
> On 3/30/2026 5:45 PM, Steve Marrow wrote:
>
> Power over the coax to Rx amplifier or tuner.
>
> Steve
> KM4V
> ------------------------------
> *From:* mrca-bounces at mailman.qth.net <mrca-bounces at mailman.qth.net>
> <mrca-bounces at mailman.qth.net> on behalf of Mike Feher via MRCA
> <mrca at mailman.qth.net> <mrca at mailman.qth.net>
> *Sent:* Sunday, March 29, 2026 7:57 PM
> *To:* 'B. Smith' <smithab11 at comcast.net> <smithab11 at comcast.net>;
> 'Military Radio Collectors Association' <mrca at mailman.qth.net>
> <mrca at mailman.qth.net>
> *Subject:* Re: [MRCA] Midnight Oil Project
>
>
> SWR bridge with LED indicator? Blessed Easter to all – 73 – Mike
>
>
>
> Mike B. Feher, N4FS
>
> 89 Arnold Blvd.
>
> Howell NJ 07731
>
> 908-902-3831
>
>
>
> *From:* mrca-bounces at mailman.qth.net <mrca-bounces at mailman.qth.net>
> <mrca-bounces at mailman.qth.net> *On Behalf Of *B. Smith via MRCA
> *Sent:* Sunday, March 29, 2026 9:59 PM
> *To:* Military Radio Collectors Association <mrca at mailman.qth.net>
> <mrca at mailman.qth.net>
> *Subject:* [MRCA] Midnight Oil Project
>
>
>
> Last night's Midnight oil project.
> Z
>
>
>
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