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
Check out the Reverse Beacon Network at
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For spots, please go to your favorite
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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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