Topband: Buffaloed by a bias tee

ZR zr at
Wed Jan 22 16:23:29 EST 2014

That gives only a 509 Ohm reactance at 1800 Pete. Going to an oft suggested 
100uH raises that to 1131 which is possibly sufficient. I use 220uH for a 
2488 XL simply because I have a bunch from a hamfest and they work fine for 
me in similar service.


----- Original Message ----- 
From: "Pete Smith N4ZR" <n4zr at>
To: "topband reflector" <Topband at>
Sent: Wednesday, January 22, 2014 3:32 PM
Subject: Topband: Buffaloed by a bias tee

> I'm still wrestling with the bias tee for my 1-of-8 remote beverage 
> switch.  If I use a cliplead to connect a 270-ohm dummy load, bypassing 
> the relay, and connect an MFJ-259B to the receiver port on the controller, 
> the impedance looks completely reasonable - with a 3:1 binocular 
> transformer, 89 ohms R  and X=5, measured by the MFJ.  However, as soon as 
> I connect a 12V regulated supply to the bias tee - one of the little radio 
> shack variable wallwarts - the measured R drops to 5 ohms and the X goes 
> up to 19.
> My history major's diagnosis is inadequate isolation between the DC supply 
> and the RFline, but why?  The series RF choke in the DC line is 7 turns on 
> a ferrite core, measures 45 uH at 2 MHZ, and the bypass capacitor is a 
> 0.01 uF disk, on thesupply side of the choke.  Theseries cap between the 
> Antenna and the RX jacks on the controller is a .1 uF disk (it was what I 
> had).  I do not yet have a safety choke between the RX side and ground, 
> but will add one before I deploy it, if I can ever figure out what's going 
> on.
> I'd really appreciate some ideas of what to try.  Thanks in advance!
> -- 
> 73, Pete N4ZR
> Check out the Reverse Beacon Network at
> blog at
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> ARC V6 or VE7CC DX cluster node.
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