"Reg Edwards" wrote in
:


"Dave Oldridge" wrote in message
9...
"Lee" wrote in
o.uk:

Is there any optimum height for a 3ft dia magloop, or is it
dependent
on frequency??.

I have mine planted in the lawn with the top about 6ft from the
ground
(on a rotator!)
and was curious whether to work it against ground or put it at the
top
of the tower,
or have i completely lost the plot???, Heh heh........

Surprising how cheaply a magloop can be built for and after 12
months
it still works well!!.

What band is yours on? I have a 55-inch diameter coax loop that I'm
using
on 75, but I'm seriously thinking of working up a multiband design
in soft-
drawn 3/4" copper pipe. It would be six feet in diameter (mainly
due to
space considerations).

Dave Oldridge+
ICQ 1800667
========================================
Dave,

Magloops with copper pipes are the most efficient of all small
antennas.

They should be rotateable through about 45 degrees.

Changing from coax loop to 3/4" or 1" copper pipe loop will be a
considerable improvement on both transmit and receive. Especially if
it is not too low above ground and is not in a cluttered environment.
The idea is to keep ground loss less than conductor loss.

Actually, considering the costs, I'm thinking perhaps just 1/2 inch pipe.
I did some modelling and the difference between 1/2 inch and 2 inch pipe
is really only 6db and I can get nice flexible 1/2 inch pipe.

For multi-band operation you will be best with a motor-driven vaccuum
tuning capacitor. With a 1000pF capacitor and a small loop the 160, 80
and 40m bands can be covered. It depends on the minimum capacitor
setting and loop perimeter.

I'm thinking more on the lines of switching capacitors in and out. I
also don't care if the feedpoint SWR goes up fairly high, as long as my
tuner can flatten it. I can make some pretty good fixed capacitors from
heliax.

There is no noticeable difference between aluminium alloy and copper
pipe. Aluminium alloy is lighter in weight and stands up to the
weather better but is more difficult to bend.

A square loop is not noticeably worse in performance than a circular
loop and may be easier to construct.

This is true given equal areas.

Download program MAGLOOP4 from my website. It may provide food for
thought.

I have it, thanks. I also have a spreadsheet that I made up that does
all the calculations for copper loops.

--
Dave Oldridge+
ICQ 1800667

On Fri, 12 May 2006 23:26:10 GMT, Dave Oldridge
wrote:

I did some modelling and the difference between 1/2 inch and 2 inch pipe
is really only 6db and I can get nice flexible 1/2 inch pipe.

Hi Dave,

Only 6dB, hmmm? Considering the following:

I'm thinking more on the lines of switching capacitors in and out.

the switch contact resistance will probably swamp out any perceived
efficiency gain achieved by added pipe diameter. You may be lucky and
have them fuse themselves closed - luck could run the other way and
they fuse open.

73's
Richard Clark, KB7QHC

On Fri, 12 May 2006 21:27:35 -0700, Richard Clark
wrote:

On Fri, 12 May 2006 23:26:10 GMT, Dave Oldridge
wrote:

I did some modelling and the difference between 1/2 inch and 2 inch pipe
is really only 6db and I can get nice flexible 1/2 inch pipe.

Hi Dave,

Only 6dB, hmmm? Considering the following:

I'm thinking more on the lines of switching capacitors in and out.

the switch contact resistance will probably swamp out any perceived
efficiency gain achieved by added pipe diameter. You may be lucky and
have them fuse themselves closed - luck could run the other way and
they fuse open.

Even if they don't fuse (gee Richard, its only ~100A RMS loop current
at 10W EIRP (assuming Rr of about 0.001 ohms)... but it will be hard
to achieve such high EIRP), it is some more loss resistance to drive
efficiency even lower.

I wondered about the effectiveness of the switching arrangement given
the very narrow bandwidth of the antenna, it might take a hundred
discrete tuning combinations to effectively cover the 80m / 75m band.

Owen
--