Topband: Buffaloed by a bias tee

[Tom W8JI]

Hi Pete,

You are going to have to trust me on this one.

You really should ***NOT*** be measuring the input of the bias T with the 
MFJ 259 B analyzer with the configuration you have.

You can damage the 259 unless you use a smaller series cap and a shunt choke 
to protect the 259. The most important point I am trying to make is 
***NEVER*** connect a bias T without a shunt choke, especially one with a 
large series coupling cap, to the 259 input port. The 259 uses 10 volt rated 
microwave diodes, and the charging current of the cap can cause that much or 
more voltage to appear across the diodes.

Also, if you have a relay outdoors or somewhere, the back EMF from field 
collapse can kill the diodes.

I say this all with significant experience on the 259B design. The MFJ 259 B 
is not like a regular receiver or transmitter. You are, in effect, charging 
a .1uF cap to 12-15 volts through the input port of the 259.

It's your analyzer, but I can tell you I would not allow anyone here to do 
what you are doing with my network analyzers, vector voltmeters, or my 
259B's. I have lost $30K network analyzer diodes that way, vector 
voltmeters, and MFJ259/269 diodes that way.

Also, your test does not prove a thing at this point. It does not prove the 
inductance is changing. It does not prove the inductance is not changing 
either.

The reason it does not prove anything either way is the MFJ is sensitive to 
ripple and noise from power supplies that are coupled to the input port. 
When you change the supply loading, you also change the ripple and noise.

So you could be measuring the choke and the choke could be changing, or you 
might be measuring the PS ripple or some other change. But this is secondary 
to the fact you are connecting a bias T without an input shunt choke to the 
259.








----- Original Message ----- 
From: "Pete Smith N4ZR" <n4zr at contesting.com>
To: "Richard (Rick) Karlquist" <richard at karlquist.com>; "topband reflector" 
<Topband at contesting.com>
Sent: Thursday, January 23, 2014 11:07 AM
Subject: Re: Topband: Buffaloed by a bias tee


> Hi.  Well, it does indeed seem to clinch it - when I power the tee but 
> don't draw any current, the impedance measured by the MFJ does not change. 
> So now to find some of the right sort of RFC.
>
> 73, Pete N4ZR
> Check out the Reverse Beacon Network at
> http://reversebeacon.net,
> blog at reversebeacon.blogspot.com.
> For spots, please go to your favorite
> ARC V6 or VE7CC DX cluster node.
>
> On 1/22/2014 6:52 PM, Richard (Rick) Karlquist wrote:
>>
>>
>> On 1/22/2014 12:32 PM, Pete Smith N4ZR wrote:
>> . 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.
>>
>> Possibly the current through the choke is saturating it.
>> If you connect the power supply but disconnect the load drawing
>> current, does the impedance go back to normal?  That would
>> clinch it.
>>
>> For the choke, be sure that you are NOT using a toroidal choke.
>> It needs to be a solenoidal type wound on a ferrite rod.
>> Also, do NOT use "shielded" inductors.  Ferrite beads will
>> also saturate.  Most chokes you come across are the wrong
>> kind.  I just bought some chokes today.  They only had two
>> bins of suitable ones, out of several thousand bins of inductors.
>>
>> 45 uH is a little marginal, but doesn't explain your problem.
>> 100 uH would be better.
>>
>> Rick N6RK
>>
>
> _________________
> Topband Reflector Archives - http://www.contesting.com/_topband
>
>
> -----
> No virus found in this message.
> Checked by AVG - www.avg.com
> Version: 2014.0.4259 / Virus Database: 3681/7027 - Release Date: 01/23/14
>