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Author Topic: 1:1 Choke for 160m Marconi Twin Lead Inv. L Antenna  (Read 1809 times)
WY0Q
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Posts: 26




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« on: October 22, 2010, 09:01:57 AM »

I have been using this antenna for two years with great results. I usually run about 350 watts on this band. I have 12  ground radials of various lengths and also tied into a 75 ft. wire fence.  Sometimes I have RFI  which knocks me off the internet.  I was considering using a 1:1 choke of 18-21 feet of coax on a form, (plastic coffee Can) at the feed point.  Not sure if this will work well on 160m though. Anyone tried this, or have a better idea I can try?
« Last Edit: October 22, 2010, 11:28:43 AM by Bernard Paul Dyl » Logged
K4SAV
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Posts: 1825




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« Reply #1 on: October 22, 2010, 07:06:04 PM »

20 ft of RG213 on a 5 inch diameter form should give about 15 turns.  That should produce a coil with a low frequency inductance of 17.9 uH, a distributed capacitance of 12 pf, a self resonance of about 10.7 MHz, and a loss resistance of 0.3 ohms at 1.8 MHz.  Unfortunately the impedance at 1.8 MHz is only about 208 ohms.  That's not enough to do much good. 

If you doubled the number of turns you could get about 514 ohms and that might show a little difference but again, it might not.  You really need something much higher to be significant.

A better choice would be one of the large ferrite toroids and use miniature high power coax so that you can get several turns on it.  Some people use the one inch beads that will fit over RG213 but this takes a lot of beads and can get expensive.

Even if you do install a choke, it may not solve the problem.  Your computer may be picking up the radiated signal out of the air.  In that case you will need some beads on the computer wiring.  That should be a cheaper solution and would be this thing I would try first.  I have a bunch of 2 inch snap-on ferrite beads I use for this purpose.  Put as many turns as you can thru the bead.

Also make sure all the computer cable connectors are tight, and any other TVs, VCRs, etc, that are sharing the video cable have good tight connectors.  You might want to temporarily disconnect those other things just to make sure they aren't causing the problem.  I had an old VCR once upon a time that was sensitive to common mode currents and would generate TVI due to RF overload and that caused all kinds of problems.  Most of the newer video recorders don't do that.

Jerry, K4SAV
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WY0Q
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Posts: 26




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« Reply #2 on: October 23, 2010, 07:52:32 AM »

Thanks Jerry; that's  sort of what I figured.  Your suggestion of toroids on the coax would be better as would beads on the computer cables.  I'll add a few more on those.  Thanks.
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G3TXQ
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Posts: 1464




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« Reply #3 on: October 23, 2010, 01:28:44 PM »

About 18 turns on a FT240-31 would be useful on 160m: >8000Ω and resistive.

Look at the 4th line on this chart:
http://www.karinya.net/g3txq/chokes/

Steve G3TXQ
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G8JNJ
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Posts: 473


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« Reply #4 on: October 24, 2010, 09:03:28 AM »

Hi,

Some notes at the end of this document relating to RFI and broadband routers.

http://g8jnj.webs.com/EMC%20and%20Amateur%20radio.pdf

Regards,

Martin - G8JNJ

www.g8jnj.webs.com
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