# RF Choke and Radio Telescope help!

Discussion in 'Wireless & RF Design' started by quantumryan, Dec 19, 2010.

1. ### quantumryan Thread Starter New Member

Dec 19, 2010
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0
Hello,

I just posted this on the Radio and Communications forum, but this particular forum looks to be highly viewed and relevant.

I've recently began trying to construct the Itty bitty Radio Telescope, I have the base, coax cables, lnbf, dish and the strength meter. The only problem I have currently is the RF choke. As seen in this tutorial of the telescope http://www.aoc.nrao.edu/epo/teachers...procedure.html there is an RF choke which needs to be sodered to a stripped coax. I'm not completely sure what an RF choke is, and in specific the one theyre using. They call for a 1 - .1 MHz rf choke in the list of materials.

Thanks

Ryan

2. ### Kermit2 AAC Fanatic!

Feb 5, 2010
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That would be a unit of 1 (each) .1 MHz rf choke.

More info is needed. Chokes(inductors) are specified in units of henrys. Usually millihenry or microhenry.

For the value of the choke we need to know the input design of the circuit and whether the choke is in series or parallel. The other thing we must know if the value of the choke is not given anywhere, is what value of capacitance is the choke 'in line' with.

For example, a freq of .1MHz and 100 pF would resonate with a choke of 25 millihenry, but if the capacitance were 10 pF it would resonate with a 250 millihenry choke.

You see the problem? Same freq. with two common, but different capacitance values and the choke value changes by an order of magnitude.

3. ### thatoneguy AAC Fanatic!

Feb 19, 2009
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RG-8 cable has a capacitance of 20pF/ft. Unsure of the input to the LNB.

Something around 9-11 uH would be an effective choke for < 100kHz at the other end of the coax from a napkin math, assuming 50Ω Termination.

A google search for 100kHz choke yields values from 2.2uH to 470uH, as the value is dependent on the antenna and LNB. From reading the assembly instructions, it would seem the exact value isn't extremely important and can be changed out as needed to filter out terrestrial signals.

4. ### Kermit2 AAC Fanatic!

Feb 5, 2010
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Whew!

Glad to know such things aren't important anymore!

and I really should have put micro and not milli.

5. ### thatoneguy AAC Fanatic!

Feb 19, 2009
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They are still important, but looking the instructions given, it doesn't seem "critically important", at least my interpretation of them. They should have used the inductor value in uH, or at least the Radio Shack part number.

It seems it is only to block noisier bands though I'm not sure why 100kHz was chosen.

Apr 5, 2008
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Hello,

The RF-choke is to block the rf signals to the powersupply.
In the attached PDF from page 8 and on the RF-chokes are mentioned.