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, [email protected] 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