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Author Topic: What balun for random wire  (Read 1059 times)
KK6CVE
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Posts: 10




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« on: October 30, 2013, 10:28:47 AM »

I use a random wire antenna to a tree while out in my RV. My question is what balun to use. 1to1, 4to1 or 9to1. I have been using a 1to1 with an insulated wire for the antenna and an uninsulated wire strung along the ground attached to the ground side of the balun. Wondering if the 1to1 is OK.
Thanks for any help as I have been doing a lot of reading on baluns and seem to be getting more confused.
Steve.
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K2DC
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« Reply #1 on: October 30, 2013, 01:57:59 PM »

With a random wire, the feed point impedance is going to vary significantly depending on the frequency, the length of the wire and the environment you throe it up into.  The 4:1 and 9:1 baluns are useful to transform the transmission line impedance to the antenna feed point impedance, and are not much help if you don't know what impedance to transform to.  Stick with the 1:1 to keep antenna currents of the coax shield (I'm assuming it's coax-fed) and you'll probably do as well as you can.

73,

Don, K2DC
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WB6BYU
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Posts: 13027




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« Reply #2 on: October 30, 2013, 02:24:58 PM »

It depends on how you are feeding it.

Generally when I use an end-fed wire I just tie one end of it to my antenna tuner and
match it.  No balun needed.  In fact, many tuners are more efficient matching high
impedances than lower ones, so work a bit better that way.

If you are running a length of coax from the feedpoint to the tuner, then one can make
an argument for using a current balun to reduce the RF on the outside of the coax
(which tends to act like a radial wire if you don't have a good enough ground system.)
In that case a higher ratio balun may give a better SWR and/or make the antenna
easier to match on some bands, depending on the lengths of the wire and coax involved.
(This is because the impedance at the feedpoint is transformed to some other value by
the coax when the SWR isn't 1 : 1, so it is difficult to generalize.)


You may find that using the frame of your RV for ground works at least as well as the
single radial ground wire.


Yes, baluns and long wires are often confusing, because many folks do things just because
they think they should (or because there are commercial products advertised for a specific
purpose) without really understanding and explaining why that is the best method, and
under what conditions it works.  As a result, many bad habits and lore get developed from
examples that don't even apply to their situation.
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KK6CVE
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« Reply #3 on: October 30, 2013, 03:05:14 PM »

Thanks for the input.  The feed line is coax as that is what the tuner I use requires. I'll give the body of the RV a try sometime and see if I notice any difference.
Again, thanks for the input.  Steve.
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WB6BYU
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« Reply #4 on: October 30, 2013, 03:36:05 PM »

You can use a short piece of coax (up to a couple feet long) to connect the wire to the
tuner, or put a banana plug on the end of the wire and plug it straight into the center
conductor of the coax jack.  (That's what I did for my first 75m antenna - a wire came
in through my window and plugged right into the coax jack on the SWR meter.)  I've also
found adaptors from a PL-259 plug to a single wire terminal.  There are lots of ways to
make the connection.  Just make sure that you also get a good ground connection to
the tuner.
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KB4QAA
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Posts: 2269




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« Reply #5 on: October 30, 2013, 06:30:19 PM »

If you are using a length of coax just short enough to bring it into the shack, the losses are really not big enough to require adding a balun.  In my experience that and a close ground/counterpoise were enough to keep RF out of the shack.
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KJ7WC
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Posts: 69




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« Reply #6 on: October 30, 2013, 07:30:26 PM »

I'd use an UNUN.
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GOUDURIX
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Posts: 204




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« Reply #7 on: October 31, 2013, 05:30:43 AM »

In theory an UNUN no? 4:1 or 9:1 which you can wire yourself easily. But this is meant for real longwires meaning several times the wavelength of your target frequencies to step down the high impedance.

If the so-called longwire is shorter than 1/4 WL on e.g. 80m then your antenna impedance will be very low and the 4:1 or 9:1 UNUN is a bad idea stepping down the impedance even further.

Jan
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