Topband: Buffaloed by a bias tee

[Tom W8JI]

The quickest way to blow up any test equipment, from a MFJ259 up to an HP 
network analyzer, is to use a bias T without input safeguards on the test 
gear.  :-)

> 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.
>

Is the supply regulated? Is it filtered? Does it have ripple? Do you have a 
shunt DC bypass on the MFJ meter side?

> 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.
>

The MFJ, like many sensitive RF measurement devices, has diodes on the 
antenna port. The charging current for the .1uF  goes through those diodes, 
as will any hum or noise, or any transients as you switch. While that stuff 
does not bother receivers, which have filters and other protection and 
pretty rugged parts, it can really tear up RF measurement stuff.

If it doesn't damage it, it at least throws the readings off if the supply 
has any noise or hum.

You should use the .01 as a coupling cap and the .1 as a bypass if that is 
all you have. :-)

The RF choke may also be changing inductance from DC current, but first you 
need to make life easier on the 259 by changing the caps around and by 
bypassing the RX port with a choke.

73 Tom