[TenTec] New Jupiter on its Way _ A Few Questions

[Rick - DJ0IP / NJ0IP]

One addition to what Stuart has written.

Though we would like to have as large of magnetic loop as possible in order
to get good performance on the low bands, large loops do not work well
indoors.  The need to have a distance equal to their radius away from the
floor, walls, or ceiling. This effectively limits the loop's size to about
one meter (3 ft.) in diameter.

I had a very expensive commercial loop which was 1.7m in diameter.
I could get it to work halfway decent indoors on 40m but it just would not
find a good match on 80m.
I moved it to the balcony, pushed it up such that it was above the edge of
the roof a little and it worked great on all bands.  Much better than when
indoors.

Then I moved it to the front yard and raised it such that its lowest part
was 10 ft. in the air.
I compared it to a 40m dipole (40 ft. up).  There was basically no
difference, except for some stations, I could rotate the loop and then it
was a few S-units stronger than the wire dipole.

Don't jump to any false conclusions.  My bet is, most loops do not work like
they should due to poor construction techniques.  You have to use the best
quality of parts and take much care to keep the resistance between various
components as low as possible.  Otherwise the loss will be too great.

If you open the MFJ mag loop, you'll see that its variable capacitor is
wielded to the aluminum loop.  That's the best way to keep the losses low.

For 80m indoors, I would suggest a 2 or 3 turn loop.  40m works best with a
2 turn loop.  It will also work on 40 with a single turn loop, but not as
well.  

One of the guys in my local club lives in a townhouse in the city, with a
postage stamp size back yard.
He bought a VERY expensive commercial loop ($1700) and mounted it about 10
ft. high on his patio.
His results have been nothing but astonishing.  
He is as loud as anyone else running similar power, all over Germany.

Again, if you can place it outdoors, it will work a lot better.
But indoors it will be better than other wire antennas or sticks, "if" you
build the loop with low loss.

73 - Rick, DJ0IP
(Nr. Frankfurt am Main)


-----Original Message-----
From: TenTec [mailto:tentec-bounces at contesting.com] On Behalf Of Stuart
Rohre
Sent: Thursday, April 24, 2014 11:25 PM
To: tentec at contesting.com
Subject: Re: [TenTec] New Jupiter on its Way _ A Few Questions

Based on an article on loaded antennas in the latest QEX magazine from ARRL,
I would say a magnetic loop, if larger than 0.1 wavelength diameter, might
have either the same signal as a loaded dipole, (short dipoles as I imagine
you mean), or the magnetic loop will have BETTER signal to noise on receive,
which is very valuable in the indoor environment.  Gain in the receive, or
low noise, may be better than more signal on transmit.

Both types of antennas would give better results if placed in an
unobstructed attic, (wood house), or placed at least, on the edge of a roof.
A larger than 0.1 wave loop could be hidden in a fenced back yard, and even
put on a pulley system that would raise it up to hang in a back yard tree
for after dark operations.

Being indoors as now; you are in a cage or cobweb of electric wires,
possible metal plaster lath, and obstructions from large appliances on the
other side of a wall.  You are subject to causing TVI to electronics inside
the home at least, or next door, or the next apartment.

Of all the indoor operating means, the loop offers the best results,
especially on receive, if shielded, and made of low loss construction.

Use large diameter loop material and low loss connections.
One of the best loop designs for HF used copper tubing of 1 inch diameter or
larger, and was octagon shaped, because angled fittings were available.  The
joints were silver soldered for lowest loss.

The capacitors on the end can be trombone type made from sliding copper
pipes, with teflon tape as spacer and insulation.  Or use surplus vacuum
variables which turn up at swap meets.  Or even large metal air spaced
variables.  while you might spend some money up front on material,
(pipe) and capacitors, those parts and the loop can be used for ANY future
locations, Field Days, or even mobile on a van.

Locally, Bob, WB5AOH had one on his van that worked HF great on low bands
like 40 and 75.  He would still have it but for a low hanging tree limb!
(With big mobile antennas back in bicycle days, I learned you have to drive
both the vehicle and the antenna!)

-Stuart Rohre
K5KVH
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