You seem to be exceedingly knowledgeable about loop antennas. I have limited
space available for a 160 meters antenna. What advice do you have about a
loop antenna for 160 meters constructed out of 3/4" copper pipe with an
octagon shape, and a perimeter of 100 feet. The plane of the loop would be
vertical with the bottom of the loop about one foot high. The loop would be
fed at the bottom, and the remotely tuned series capacitor would be centered
in the top side of the loop. Does this project seem doable and does it make
sense?
John, N9JG

"Reg Edwards" wrote in message
...
The reason a multi-turn maggloop of the same diameter would be
disappointing is because of increased loss in the loop conductor.

The ability to collect or radiate signals is a function only of the
diameter, ie., the longest linear dimension. Alternatively stated, a
function of the area enclosed.

With a single turn loop, conductor loss is due simply to skin effect.

With a multi-turn loop and the same amount of copper, proximity effect
comes into play and loss resistance increases faster than radiation
resistance.

Also, with a transmitting magloop, for the same power input, the
voltage across the tuning capacitor increases proportional to the
number of turns. Ridiculously high voltages appear. Although, the
value of the capacitor in pF is very much smaller, the physical size
of the capacitor remains the same.
----
Reg.

"John N9JG" wrote
You seem to be exceedingly knowledgeable about loop antennas.
===================================
It's just the way I write it.
===================================
I have limited
space available for a 160 meters antenna. What advice do you have
about a
loop antenna for 160 meters constructed out of 3/4" copper pipe with
an
octagon shape, and a perimeter of 100 feet. The plane of the loop
would be
vertical with the bottom of the loop about one foot high. The loop
would be
fed at the bottom, and the remotely tuned series capacitor would be
centered
in the top side of the loop. Does this project seem doable and does
it make
sense?
John, N9JG
=======================================

It makes good sense. Depending on your resources there will be
mechanical problems to solve. The weight of the large value,
motor-driven tuning capacitor, plus that of copper pipe will need
supporting. Plastic guy ropes may be needed. And ideally the whole
thing should be manually rotateable through 90 degrees.

Electrical comment : Do NOT cut the copper pipe to feed it at the
bottom of the loop. The balanced feedpoint input impedance will be
extremely small and impossible to match efficiently.

The best method of feeding is via a small loop of wire inside the main
loop in the same plane. The small loop is approx 1/5th of the diameter
of the main loop and is insulated from the main loop. The small
coupling loop is just a self-supporting wire between the inner and
outer conductors of a 50-ohm coaxial feedline and can be located at
the bottom of the main loop.

Compared with main loop diameter, the 1-foot height of loop above
ground is very low. Loss in the ground will be rather high. Try to
obtain a height of 6 feet for a not very great improvement.

With a perimeter of 100 feet, on 160 meters performance will be about
1 S-unit worse than a 1/2-wave dipole. On 80 meters performance will
be about the same as a full-size 1/2-wave dipole at the same height.
The perimeter is too long to work on 40m.

Further performance details can be obtained from program MAGLOOP4 from
website below.
----
.................................................. ..........
Regards from Reg, G4FGQ
For Free Radio Design Software go to
http://www.btinternet.com/~g4fgq.regp
.................................................. ..........

Thanks a lot for your advice. I would like to use the loop on 80 meters
also, but I thought that the self-resonant frequency of a 100 feet perimeter
loop would be too low for the loop to be resonated on 80 meters with a
series capacitor.

Also, from reading some material in the ARRL 20th edition Antenna Book, I
noticed that Ted Hart, W5QJR, used a gamma-matching arrangement to feed his
loop. If a small interior loop is used to feed the main loop, are capacitors
needed to couple the coax feed line to the small loop? Hopefully, these
capacitors would not need to be changed after the initial setup procedures
have taken place.

"Reg Edwards" wrote in message
...
[additional text]
The best method of feeding is via a small loop of wire inside the main
loop in the same plane. The small loop is approx 1/5th of the diameter
of the main loop and is insulated from the main loop. The small
coupling loop is just a self-supporting wire between the inner and
outer conductors of a 50-ohm coaxial feedline and can be located at
the bottom of the main loop.
[additional text]
With a perimeter of 100 feet, on 160 meters performance will be about
1 S-unit worse than a 1/2-wave dipole. On 80 meters performance will
be about the same as a full-size 1/2-wave dipole at the same height.
The perimeter is too long to work on 40m.

John,

The highest operating frequency of a magloop occurs when the tuning
capacitor is set to minimum or zero and the circumference of the loop
is approximately 1/2-wavelength. That's rather obvious.

100 feet circumference = 30 metres. So greatest working frequency is
150 / 30 = 5 MHz.

A small coupling loop is far and away the best method of feeding a
magloop. Nothing could be mechanically more simple. There are no
additional components and the loop is isolated from the main loop.
Furthermore its diameter for a good impedance match to 50 ohms tends
to be independent of operating frequency. The wire diameter of the
coupling loop need be no larger than that of the inner conductor of
the coaxial feedline to which it is connected.

I've never heard of Ted Hart. But, of course, a gamma match will
work. However the direct connection to the main loop unbalances it and
encourages radiation from the feedline. In comparison with a small
coupling loop it is also mechanically complicated.
----
Reg, G4FGQ

Thanks again, Reg. You can be sure I am printing out your comments and
storing them in my loop project folder. I will also be perusing your web
site http://www.btinternet.com/~g4fgq.regp/
I will need some info on size and placement of the coupling loop.

Regarding Ted Hart, W5QJR, there is some information about him at
http://www.nab.org/conventions/radio...o.asp?id=10459
A Google search on Ted-Hart Loop-Antenna finds about 100 hits.

"Reg Edwards" wrote in message
...
John,

The highest operating frequency of a magloop occurs when the tuning
capacitor is set to minimum or zero and the circumference of the loop
is approximately 1/2-wavelength. That's rather obvious.

100 feet circumference = 30 metres. So greatest working frequency is
150 / 30 = 5 MHz.

A small coupling loop is far and away the best method of feeding a
magloop. Nothing could be mechanically more simple. There are no
additional components and the loop is isolated from the main loop.
Furthermore its diameter for a good impedance match to 50 ohms tends
to be independent of operating frequency. The wire diameter of the
coupling loop need be no larger than that of the inner conductor of
the coaxial feedline to which it is connected.

I've never heard of Ted Hart. But, of course, a gamma match will
work. However the direct connection to the main loop unbalances it and
encourages radiation from the feedline. In comparison with a small
coupling loop it is also mechanically complicated.
----
Reg, G4FGQ

John,

In principle, the coupling loop can be located in the same plane and
anywhere within the main loop. So it is most convenient to locate it
at the bottom of the main loop within an inch or two of the main loop.

But make no direct connection between the two loops or between the
coax and the main loop. There has to be some sort of support for the
coax and coupling loop. So you can make use of the insulating jacket
of the coax feedline and sticky insulating tape. If any other
supporting insulating materials are needed any plastics will do.

You can, of course, make a beautiful engineering job out of it which
you can be proud of. But, radio-wise, the antenna will not work the
slightest bit better. ;o)

A comment about a motor-driven variable capacitor at the top of the
main loop. To allow the main loop to maintain self-balance about
ground, insert a multi-wire choke in the control wires and any other
wires feeding the motor. Or whatever form the motor may take.

Remember, the main loop should be allowed to take up its own natural
balance about ground to reduce "radiation from the feedline."

In the case of magloops, radiation from the feedline is a matter of
loss of radiation from the antenna. Waste not, want not!
----
Reg, G4FGQ

This information will same me a lot of effort and mental anguish as I
proceed with my loop project. Thank you for your help.
-- John, N9JG

"Reg Edwards" wrote in message
...
John,

In principle, the coupling loop can be located in the same plane and
anywhere within the main loop. So it is most convenient to locate it
at the bottom of the main loop within an inch or two of the main loop.

But make no direct connection between the two loops or between the
coax and the main loop. There has to be some sort of support for the
coax and coupling loop. So you can make use of the insulating jacket
of the coax feedline and sticky insulating tape. If any other
supporting insulating materials are needed any plastics will do.

You can, of course, make a beautiful engineering job out of it which
you can be proud of. But, radio-wise, the antenna will not work the
slightest bit better. ;o)

A comment about a motor-driven variable capacitor at the top of the
main loop. To allow the main loop to maintain self-balance about
ground, insert a multi-wire choke in the control wires and any other
wires feeding the motor. Or whatever form the motor may take.

Remember, the main loop should be allowed to take up its own natural
balance about ground to reduce "radiation from the feedline."

In the case of magloops, radiation from the feedline is a matter of
loss of radiation from the antenna. Waste not, want not!
----
Reg, G4FGQ

John,

I once had a magloop for 160 and 80 meters almost as big as the one
you propose. It worked fine. According to predictions. Eventually,
needing the garden space for other projects, I swapped it for a bottle
of New Zealand white wine.

But to reduce the weight of a 1.5-inch diameter copper-pipe conductor
plus the tuning capacitor, the conductor was made of thin-wall, hard
aluminium alloy known as Duralumin in these parts. (Pure aluminium is
far too soft and is useless.)

The loop was formed from three sections of tube which fitted into each
other to make a complete circle. The curvature of the sections had to
be formed in an engineering workshop.

It needed three guy ropes and could be rotated about an angle of 90
degrees, five feet above ground level, inside a 2-inch diameter, cast
iron tube sunk into the soil.

There was an aluminium alloy vertical supporting tube which extended
from below ground level almost to the highest point of the main loop
and the motor-driven tuning capacitor. A vertical conductor, running
diametrically across the main loop, has no effect on electrical
performance.

I had very good neighbours. Never had a single compaint about the
wierd contraption which soared well above the skyline from my side of
the 6-foot high bushes and trees which constituted the boundary
between my property and theirs. It never occurred to me to request
planning permission from the local authorities. With the assistance
of a local Black Country radio friend, G3VFF, a born mechanical
engineer, handicapped by an industrial accident, we just erected it!

It helped, to my satisfaction, to prove the veracity of some of my
radio software programs.

John, I wish you all the very best with your project.
----
Cheers, Reg, G4FGQ.