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Author Topic: Mobile HF loop  (Read 2309 times)

Posts: 159

« on: January 05, 2013, 01:35:12 PM »

thinking of a loop, maybe 5 turns of wire around the perimeter of 12" fiberglass upright rod supports on each luggage rack corner, creating a roughly 5' x 3' horizontal loop, fed with a SG-237 auto coupler. Thoughts?

Posts: 17476

« Reply #1 on: January 05, 2013, 02:14:01 PM »

What bands do you plan to operate?

You likely will be better off with a vertical loop rather than horizontal one
due to the expected angle of radiation at low heights above ground.  (And
the roof will act as part of the ground in that case.

And I expect that you'd get better efficiency using all 5 turns in parallel
(to make a single turn loop using effectively fatter wire) rather than in
series due to the current distribution around the loop, but I'll have to
wait until I get a chance to model it to see.

With 16' circumference it would work in "small loop" mode on 20m and
lower frequencies, where the height above ground will be the least
in terms of wavelengths.  Such a loop would have maximum radiation off
the edges, but the ground reflections will cancel a lot of that because it
is horizontally polarized.  It would be approaching a half wave on 10m,
where the feedpoint impedance would be quite high (basically a bent
dipole fed at the ends.)

Posts: 17476

« Reply #2 on: January 05, 2013, 04:34:11 PM »

I built a model of a 3-turn loop for starters, 3' x 5' and fed in the middle of the
long side.  (I had to get a bit creative in the model to make sure the wires
didn't intersect while maintaining proper segment lengths for close-spaced wires.)

Using 3 turns of #8 wire (1/8" in diameter), the losses in free space are about
16dB just due to the wire resistance.  On 20m it's about 7dB, again just due
to the resistance of 1/8" wire.  Larger copper tubing would help, but my model
used 1" spacing between turns, and I didn't want to rework the turns spacing
to handle thicker wire.

Feedpoint impedance (in free space) is less than 1 ohm resistance except between
11 and 17 MHz, where it peaks to a high impedance (my model says 1200+j100,000
ohms at 13 MHz) and I'm not sure how efficiently (if at all) the tune will be able
to match it on most bands.  Certainly with the very low impedance I'd be worried
about the switching relays in the tuner handling the voltage and current:  on 15m
my model suggests about 9kV @ 10A at the feedpoint with 100W, and about 6dB
loss due to wire resistance (so 25 watts radiated.)

Putting such a loop at a low height above ground increases the losses even
further.  My version of EZNEC won't easily handle the combination of the car and
surrounding ground, but just mounting the loop about 6' over ground increases
the losses 5dB or so on 15m, and more so on the lower bands.  A center-loaded
whip appears to be more efficient on all bands I've checked.

So there are a number of learnings from this.  First, if you are going to use a
loop at low heights, it should be in the vertical plane rather than horizontal.

Second, any such small loop (less than about 1/4 wave in circumference)
may very well be outside the range of many commercial autotuners, even
if it uses multiple turns to add conductor length.  Not that it's impossible
to make such a loop work, but generally it requires a dedicated matching
network designed for that purpose that can handle the high currents and
voltages encountered in such a loop.

It is possible to build a loop about 3' x 5' in the conventional manner for
such loops and match it on 40, 30 and 20m, and possibly 80m if you have
a big enough variable capacitor.  Typically you'd use copper tubing (as
large as you can afford) and the efficiency would be considerably better
than the numbers reported here.

Posts: 328


« Reply #3 on: January 08, 2013, 10:06:39 AM »

I worked a Ham on 40 meters once who was mobile, in a tractor trailer, using a big rectangular loop, on his own trailer! His goal was a high angle 40 and 75 meter signal only, but he said he had some success with it on 20 meters ?
I lived back in Seattle at that time, can't recall his Callsign.
But, he was always heard in the northwestern states, back in the 80's.


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