[amsat-bb] Re: bias tee circuits and components for 2.4 GHz downconverter

[G0MRF@aol.com]

Hi Michael.
 
I see a lot of things written about bias tees, saying they are complicated  
or they need particular values of exotic inductor or coupling / decoupling  
capacitors.....but all the ones I've 'thrown together'  have all worked  
perfectly with any converter with a VHF output. (The same is not true for  UHF)
 
General rule is to use a choke that is a minimum of 10 x the impedance of  
the line. So for 50 Ohm cable that's 500 ohms or about 700nH.    Generally don't 
go above 2000 ohms as you can run into unwanted resonances in  the choke.  
But that's a huge range, in practice 470nH to 4.7uH should be  fine.  - Your 
1.5uH is within the limits. I've used iron dust cores from  Amidon without 
problem.
 
For the 2 capacitors required, the coupling capacitor and the decoupling  cap 
to the DC supply, almost anything will do as long as it has a low impedance  
at 122MHz. The coupling cap can be 1n to 10nF ceramic.  The decoupling  
capacitor can be the same value / type. Personally I always add an electrolytic  as 
well for the DC connection.  1uF to 47uF will do.
 
Regards
 
David
 
 
 
In a message dated 11/05/2008 18:23:12 GMT Standard Time,  
mat_62 at netcommander.com writes:

I have  a  Pacific Monolithics surplus MMDS downconverter here that I would 
like  to get up and running on S-band. It has an 122 MHz IF and I want to whip 
up a  bias -tee to power it. I've looked at the circuits in the Satellite  
Experimeters handbook and my problem is finding an rf choke of the right value  
that is readily available in my area. Shouldn't I be able to just wind   
something with an air core out of magnet wire or something similar that will  do the 
trick? According to the circuit I have I'd need about a 1.5 uh choke  for 146 
Mhz and it isn't critical so that's probably close enough for my  
downconverters IF frequency. I hate to spend shipping charges to get one  little choke or 
ferrite core for winding a choke on here. Most places have a  minimum order 
and I don't need anything else at this time. I'd also welcome  links to any 
other circuits that might be out there.
Thanks,
Michael,  W4HIJ