Topband: Buffaloed by a bias tee
zr at jeremy.mv.com
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 contesting.com>
To: "topband reflector" <Topband at contesting.com>
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
> I'd really appreciate some ideas of what to try. Thanks in advance!
> 73, Pete N4ZR
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