Topband: Buffaloed by a bias tee

Thu Jan 23 14:18:20 EST 2014

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 at contesting.com>
To: "Tom W8JI" <w8ji at w8ji.com>; "Richard (Rick) Karlquist" 
<richard at karlquist.com>; "topband reflector" <Topband at 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 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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