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Author Topic: Minimum wire gauge for a QRP random wire antenna  (Read 3341 times)
KATEKEBO
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Posts: 117




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« on: December 07, 2010, 12:58:41 PM »

Does wire gauge count for a portable QRP random wire antenna?  What would be the minimum wire diameter than can be used for 5W QRP operations?
73
S. Bucki
KD8KQH
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WB6BYU
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Posts: 13353




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« Reply #1 on: December 07, 2010, 01:06:43 PM »

Once you get down to #32 or so the birds can't see it in time to avoid it, and you have to replace
it more often.

It really comes down to mechanical issues more than electrical ones - primarily the strength of the
wire.  You can use #40 magnet wire if you have some way to hold it up.  I find #28 or so is about
the smallest that is convenient for me to work with.  For portable operation where I'm going to be
putting it up and taking it down, I'd use stranded, insulated hookup wire:  it is much easier to work
with than magnet wire (especially with regard to kinks).
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KATEKEBO
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Posts: 117




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« Reply #2 on: December 07, 2010, 01:15:02 PM »

So wire gauge does not affect performance?  Would a 22-gauge wire be as efficient radiator as, let's say, 18-gauge?
S. Bucki
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K7KBN
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Posts: 2817




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« Reply #3 on: December 07, 2010, 01:21:13 PM »

So wire gauge does not affect performance?  Would a 22-gauge wire be as efficient radiator as, let's say, 18-gauge?
S. Bucki


Efficiency isn't going to play any significant part.

The size of wire you'll want to use depends a lot on the frequency you're planning to use it with, and the type of feedline, and just how you plan to deploy it.  An 80 meter dipole, center fed with coax and supported only at the ends, is going to have to be made of heavier wire than, say, a 20 meter dipole with all the other things being equal. 

If you have a center support, so much the better.  Now you can use lighter wire.

But as I said, the "efficiency" of #12 copper wire compared with #20 copper wire is negligible in this sort of application.
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73
Pat K7KBN
CWO4 USNR Ret.
WB6BYU
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Posts: 13353




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« Reply #4 on: December 07, 2010, 01:29:59 PM »

Quote from: KATEKEBO
Would a 22-gauge wire be as efficient radiator as, let's say, 18-gauge?

For a standard dipole you probably couldn't measure the difference in efficiency between them.
For some antennas (especially those with a very low radiation resistance) you can.

At #40 wire you probably can measure the difference in efficiency, but it might only be 1dB or so,
depending on the application.  That's not enough of a difference to make an antenna unusable,
and may be a worthwhile tradeoff in some applications.

For an end-fed "random wire" antenna, if it is over 1/4 wavelength long you probably won't notice
much difference in efficiency over the range of reasonable wire sizes.  If your "random wire" was
only 7 feet long on 80m, you might, though your tuner losses are likely to be higher than your
wire losses, making a careful measurement difficult.
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WA3SKN
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Posts: 5499




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« Reply #5 on: December 07, 2010, 01:53:19 PM »

At RF, "skin effect" comes into play.  And at our typical frequencies of operation... support is the major issue.  I have used 64 gauge wire, but do not recommend it.
The length/diameter can affect the bandwidth somewhat.  The power is not really an issue.
Have you tried stainless steel fishing line?  And metal foil tape sticks to most objects with ease.  Just some "stealthy" thoughts!
73s.

-Mike.
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G3TXQ
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« Reply #6 on: December 07, 2010, 02:00:31 PM »

To put the efficiency issue in perspective, on a 40m half-wave dipole the difference between #12 wire and #40 wire is equivalent to less than 1dB.

73,
Steve G3TXQ
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WX7G
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Posts: 6146




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« Reply #7 on: December 07, 2010, 02:32:35 PM »

An example: A 40 meter dipole made of #32 wire vs. #18 wire. Dipole is 30' above average GND.

Gain difference is 0.6 dB.  

« Last Edit: December 07, 2010, 02:36:29 PM by DAVE CUTHBERT » Logged
K5LXP
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« Reply #8 on: December 07, 2010, 02:36:59 PM »

Piling on a few more- your ground, feed and match losses typically far outweigh antenna conductor losses. 


Mark K5LXP
Albuquerque, NM
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KATEKEBO
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Posts: 117




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« Reply #9 on: December 07, 2010, 03:30:05 PM »

Many thanks for all the answers.  I will use #22 hook-up wire which  have sitting in my drawer.
S. Bucki
KD8KQH
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AD4U
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Posts: 2174




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« Reply #10 on: December 07, 2010, 03:46:55 PM »

I never tried it, but I imagine you could run 1500 watts SSB and probably CW into a dipole made from #30 gage wire.   Except for the feedline such an antenna is practically invisible.

When somebody posts a question about installing a decent antenna in antenna restricted neighborhoods etc, I always recommend an invisible dipole over some small, short, heavily loaded, high dollar, poor performing, piece of junk.

A stealth or "invisible" dipole antenna will almost always perform rings around all these antennas, and it costs many times LESS.  But lots of this high dollar "junk" is sold mainly because of the advertising hype and the hams who do not understand what is really going on.

I guess there are situations where a person can only install a mobile antenna on a balcony railing or a several hundred dollar loaded small loop etc in the flower bed.  These people WILL make SOME contacts. But it has been my experience that many, if not most, hams running these antennas quickly get frustrated.  There are much better alternatives.

Dick  AD4U
« Last Edit: December 08, 2010, 08:23:31 AM by DICK WHETSTONE » Logged
N3OX
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« Reply #11 on: December 07, 2010, 05:52:37 PM »

A stealth or "invisible" dipole antenna will almost always perform rings around all these antennas, and it costs many times LESS, but lots of them are sold due mainly to the advertising hype and the hams who do not understand what is really going on.

Yep.  I used an approximately hundred foot "invisible" doublet when I lived in an apartment.  Couple random legs sling-shotted into nearby trees, fed with a 1:1 current balun and remote tuner.  Power level was 100W.

No way I could have equaled that with anything that was confined within my balcony.  Had it up for several years, and mechanical strength was by far the biggest problem.  Even then, I only had to replace something half a dozen times in a few years, usually one leg at a time.  It was enameled magnet wire, and I think it was 30 gauge.
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73,
Dan
http://www.n3ox.net

Monkey/silicon cyborg, beeping at rocks since 1995.
K0ZN
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Posts: 1553




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« Reply #12 on: December 07, 2010, 06:21:05 PM »



 The ONLY consideration is mechanical strength and how invisible you want it to be.

73,  K0ZN
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WA3SKN
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Posts: 5499




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« Reply #13 on: December 08, 2010, 12:23:34 PM »

"I will use #22 hook-up wire which  have sitting in my drawer."

An excellent choice!  There is nothing wrong with using something that is already available.  Get it as high and in the clear as practical, and start making contacts!
73s.

-Mike.
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K1CJS
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Posts: 6046




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« Reply #14 on: December 08, 2010, 12:44:00 PM »

What you really want is a heavier gauge wire so as to be able to stand up to the weather, blowing winds, snow and ice, (if you live in a colder area) and so on.  It stands to reason that those vonsiderations should be looked at first.  Other than that, for a random wire, just about any gauge will do.
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