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 Post subject: 75 Ohm to 300 Ohm balun
PostPosted: Nov Mon 20, 2017 6:17 am 
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Location: Costa Mesa, California
This was an idea suggested by Glen Zook. I gave it a try this weekend and I like the performance. It seems to work well for boat anchor receivers--at least as well as other hook-ups and maybe better for some radios. The balun is a simple TV antenna coupling. I have a few left over from the years when I had outdoor TV antennas. The connector is from RF Adaptors. Even though it is a 75 Ohm input, it is close enough. The jumper is disconnected on the back of the receiver and the balanced antenna inputs are used.

Norm


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 Post subject: Re: 75 Ohm to 300 Ohm balun
PostPosted: Nov Mon 20, 2017 2:43 pm 
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Most of them seem to work pretty well down to the lower end of the shortwave range and some are good down into the broadcast band. If in doubt you can use your signal generator and scope (or RF voltmeter) to check its abilities at the lower frequencies which were outside of its original intended range but seem to be within the capabilities of the typical TV balun.

With the RF noise environment as bad as it is today in most environments a key to decent reception is to get the antenna into the "cleanest" space possible and use a feed setup that won't pick up noise along the way. The old standard of running a single wire out the window doesn't work so well today in most environments and the ability to use coax and a balun to feed vintage receivers can go a long way towards providing better reception.

Twin lead can provide good immunity to outside noise pickup but only if it is VERY carefully run and running it near even small conductive items (like fasteners in siding) can greatly reduce its balanced nature while coax is very forgiving of routing. Good balanced feed installation works amazingly well and I was amazed by some of the tests run on the solidly mounted high power balanced feed used at one of the VOA installations where radiation from the feed was nearly below detectable level. You won't get that sort of performance from carelessly installed Radio Shack twinlead but in a carefully configured installation it can be impressive.

Rodger WQ9E


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 Post subject: Re: 75 Ohm to 300 Ohm balun
PostPosted: Nov Mon 20, 2017 3:53 pm 
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Location: SW WA state
Norm,

I had the same idea years ago, and tested the Radio Shack 4:1 TV baluns on a sweep generator/ spectrum analyzer and found that they do indeed work well down to about 5 M/c.


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 Post subject: Re: 75 Ohm to 300 Ohm balun
PostPosted: Nov Tue 21, 2017 12:31 am 
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Joined: May Sun 22, 2011 11:27 pm
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Location: Dallas,TX
Norm,

Can you point me to the correct adapter from RF Parts.

Thnx, Mike

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 Post subject: Re: 75 Ohm to 300 Ohm balun
PostPosted: Nov Tue 21, 2017 2:16 am 
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RF Parts does not seem to carry this adaptor. I got mine from RF Adaptors who sell on eBay.

https://www.ebay.com/itm/UHF-SO-239-Fem ... 2749.l2649

Norm

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Last edited by Norm Johnson on Nov Tue 21, 2017 3:48 am, edited 1 time in total.

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 Post subject: Re: 75 Ohm to 300 Ohm balun
PostPosted: Nov Tue 21, 2017 3:01 am 
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For some reason twin-lead must be run, use standoffs with the correct plastic bushing at least 4" away from wood and 8-10" away from metal siding and wire lath (stucco).

For each run between standoffs, twist the twin-lead 2 turns/ft. reverse the twist in the next run. This does two things. Any electrical unbalance is "normalized" by exposing each side of the twin-lead. Wind forces are also balance, less whipping of the twin-lead. By reversing the twist, twists do not accumulate at the end of the final run between standoffs.

If an "F" connector fitted balun is used a short run of coax and an "F" feed-through or lightning arrestor can be used outside the home to ground the coax.

Some 1-4 (4-1) baluns are designed with capacitor feed to block any DC or other small potential from the coax. The caps are generally too small to pass sufficient signal at BC frequencies. Other 1:4 baluns have DC coupling so a power supply can feed an amplifier at the antenna.

IMHO I suspect the DC coupled balun may pass greater signalat BC frequencies.

YMMV

Chas


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 Post subject: Re: 75 Ohm to 300 Ohm balun
PostPosted: Nov Tue 21, 2017 3:41 am 
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Joined: May Sun 22, 2011 11:27 pm
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Norm,

Thanks and 73.

Mike

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