Topband: Buffaloed by a bias tee

Thu Jan 23 18:32:42 EST 2014

Mini Circuits makes a bias Tee box that has very low insertion loss down 
to 500Khz.  It is their model*ZX85-12G-S*. and as Tom pointed out there 
is a change with current throughput and you can view the chart  at 
http://research.physics.illinois.edu/bezryadin/labprotocol/ZX85-12Gplus.pdf 
I looks like it will do the trick on TopBand but the price of $99.95 
each is a bit discouraging.

Herb, KV4FZ

On 1/23/2014 3:18 PM, Tom W8JI wrote:
> 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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