The Hammond is 400 mA and probably OK, I never used one.
I use something like these 100 uH from Mouser Electronics:
542-73F104AF-RC
or depending on current
542-5250-RC
----- Original Message -----
From: "Pete Smith N4ZR" <n4zr@contesting.com>
To: "Tom W8JI" <w8ji@w8ji.com>; "Richard (Rick) Karlquist"
<richard@karlquist.com>; "topband reflector" <Topband@contesting.com>
Sent: Thursday, January 23, 2014 1:45 PM
Subject: Re: Topband: Buffaloed by a bias tee
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
http://reversebeacon.net,
blog at reversebeacon.blogspot.com.
For spots, please go to your favorite
ARC V6 or VE7CC DX cluster node.
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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