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Re: Topband: Buffaloed by a bias tee

To: Tom W8JI <w8ji@w8ji.com>, "Richard (Rick) Karlquist" <richard@karlquist.com>, topband reflector <Topband@contesting.com>
Subject: Re: Topband: Buffaloed by a bias tee
From: Pete Smith N4ZR <n4zr@contesting.com>
Date: Thu, 23 Jan 2014 13:45:29 -0500
List-post: <topband@contesting.com">mailto:topband@contesting.com>
Tom, I appreciate your concern. I have made these changes - I am now using a shunt choke (homebrew,, measuring 45 uH) and a smaller series capacitor.. Now all I have to do is to find a source of some appropriate chokes for the final tee, and my problems may be behind me. I was just looking at some Hammond chokes, no. 1532h - 100 uH, rated for maximum DC current of 500 ma., solenoid wound, self-resonant frequency of 12 MHz. Sound reasonable?

73, Pete N4ZR
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On 1/23/2014 12:03 PM, Tom W8JI wrote:
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@contesting.com> To: "Richard (Rick) Karlquist" <richard@karlquist.com>; "topband reflector" <Topband@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


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