Topband: Buffaloed by a bias tee

[Pete Smith N4ZR]

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 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
>>>
>>
>> _________________
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>>
>>
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>
>