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Author Topic: HF Loop Antennas for Mobile Use...?  (Read 1806 times)
VK5CQ
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Posts: 115




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« on: December 08, 2009, 04:51:25 PM »

I'd like to experiment with a homebrew HF Loop.

Where are comprehensive articles (on-line) on the topic, eg,with design formulas, construction tips, discussion of trade-offs & radiation patterns, for various sizes & heights?

Perhaps you've got some tips to share?

I suppose they hear well (so they ought to send at least as well, right?)

How to design multiband HF Loops?

How to make them withstand winds (while stationary) & wind resistance (while the vehicle is in motion)?

How does their performance compare to "short" loaded whips (eg, CODAN, Yaesu, ICOM, etc.)?

What are some of the modestly-priced sources of the wire needed to make them, and about how much to buy?

TIA
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WB6BYU
Member

Posts: 13353




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« Reply #1 on: December 08, 2009, 07:19:11 PM »

You can find most of the information by doing a search for
"magnetic loop antenna", though "magnetic loop" is not
really a good description of how the antenna works.

There are several links to the subject on the late AC6V's
antenna links page here:

http://ac6v.com/antprojects.htm

Presuming that you want to transmit on it, the most
common implementation is to use a fat, high-conductivity
single-turn loop that is tuned to resonance with a variable
capacitor of some sort opposite the feedpoint (though there
are some variants with the capacitor at other locations.)
Radiation resistance is very low, so currents and voltages
are very high even at modest power levels.  One of the
early military loops was made from 8 sections of aluminium
tubing, and they found they had to gold plate the joints to
keep the losses low for good efficiency.  Many loops are
built from copper water pipe or tubing, sometimes 5cm or
more in diameter to reduce losses.

You can build a single loop and use a variable capacitor to
tune it to multiple bands, but the required minimum /maximum
capacitance ratio required to do so (as well as minimum vs.
maximum usable lengths in wavelengths) usually make it
impractical to tune a frequency much greater than 2 or 2.5 : 1.
(You might squeeze 30 to 10m into a single loop, but not down
to 40m without a serious loss of efficiency.)

SWR bandwidth is very narrow - perhaps 5 to 10 kHz on 80m
without retuning.  That is why many designs use motorized
capacitors for remote tuning, which adds an extra level of
complexity to construction.

You can see some photos of Russian military mobile loop antennas
at the Antentop Radio Magazine site here:

http://www.antentop.org/004/starec.htm
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K6PCW
Member

Posts: 11




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« Reply #2 on: December 09, 2009, 08:03:24 PM »

Just for fun, check these out!

http://www.star-h.com/products/nvis.html

http://www.stealth-us.com/

http://www.stealth-us.com/Demonstration/DemoVideo.wmv
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N5EG
Member

Posts: 250


WWW

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« Reply #3 on: December 09, 2009, 09:30:39 PM »

Hi 'C'.

Here's 4 photos of HF small mobile loop by JL1BOH:

http://www.aa5tb.com/jl1boh_01.jpg
http://www.aa5tb.com/jl1boh_02.jpg
http://www.aa5tb.com/jl1boh_03.jpg
http://www.aa5tb.com/jl1boh_04.jpg

Long list of links to the subject:
http://www.aa5tb.com/loop.html

-- Tom, N5EG
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WA3SKN
Member

Posts: 5499




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« Reply #4 on: December 10, 2009, 05:17:52 AM »

Small loops simply do not radiate all that well. (see Kraus's book on Antennas).  And these are feuding words to be sure.
But, there is nothing wrong with experimenting!
In a mobile environment, the physical mounting and strength is more important than the electrical characteristics... and most loops are quite a sail, so build it strong!
Anyway, good luck with the project!
73s.

-Mike.

ps: what type vehicle are you mounting it on?
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K0BG
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Posts: 9880


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« Reply #5 on: December 10, 2009, 06:19:28 AM »

It might be a fun way to waste time, and you might be pleased. However, a very good quality HF mobile antenna, properly mounted, will have an efficiency of about 4 to 5%, perhaps a little more. The loop will be considerably less, and won't be much better than a hamstick if that good.

Alan, KØBG
www.k0bg.com

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N1LO
Member

Posts: 1039


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« Reply #6 on: December 10, 2009, 10:12:03 AM »

I don't want to squelch your experimenter's spirit. It's fueled by wanting to make something better.

The mag loop doesn't seem like the way to go for reasons already mentioned. Let me give you another idea.

To increase HF mobile efficiency and bandwidth on the lower bands, You've just got to have significantly more length than 8'. Look into an inverted L design to get the length without the height problem.

Mark
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