loop antenna question...
 

greetings...

I have been a ham since the mid 60's and the main antenna of choice appears
to be dipole... However, over the past few months, or so, I have been
reading more and more about this loop antenna.....

I am reasonable sure I can get more information about it so I won't bore you
with how to build one, etc, .. my question is more of why or how... Why has
this antenna become so popular over the past little while and how did it get
this popularity....What charateristics, besides seemingly not needing as
much geography to need to put up, does this antenna have that make is so
appealing?

I might consider it...for 80 or 40...

Larry ve3fxq

On Sat, 10 Sep 2005 08:46:24 -0400, "larry" wrote:

greetings...

I have been a ham since the mid 60's and the main antenna of choice appears
to be dipole... However, over the past few months, or so, I have been
reading more and more about this loop antenna.....

I am reasonable sure I can get more information about it so I won't bore you
with how to build one, etc, .. my question is more of why or how... Why has
this antenna become so popular over the past little while and how did it get
this popularity....What charateristics, besides seemingly not needing as
much geography to need to put up, does this antenna have that make is so
appealing?

I might consider it...for 80 or 40...

Larry ve3fxq



Depending upon the size of the loop in wavelengths and depending upon
where in the geometry it is fed and depending upon its height, it
might very well be a good cloud warmer - in which case it would
probably give excellent performance in the immediate area (as opposed
to DX).

People that I QSO with on 75 seem to do very, very well with loop
antennas in the evening over a range of up to 500 - 700 mi.

Hi Larry

You are talking about a shielded magloop rather than a full wave length
single quad loop...

They exhibit reasonable performance for their size and are very quiet.
ie the signal to noise is often better on the magloop than a larger
antenna. I have seen claims that the signal strength is maybe 2 S points
lower but the noise is 5 S points lower.

I have heard of configurations where a dipole is used for TX and a
magloop for RX.

The at antenna tuning makes for less chance of a strong nearby signal
desensitizing your RX. The downside of that is that you have to have
some method of doing this remote tuning. Bandwidths in the order of
20-30khz on 80m are not uncommon.

They can be built for 2:1 or 3:1 frequency coverage range. I have no
idea how the performance changes at the far end.

I actually havent built one weak grin

Hope this helps.

Cheers Bob W5/VK2YQA

larry wrote:
this popularity....What charateristics, besides seemingly not needing as
much geography to need to put up, does this antenna have that make is so
appealing?

larry wrote:
I might consider it...for 80 or 40...

How long is the loop in wavelengths?
--
73, Cecil http://www.qsl.net/w5dxp

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How long is the loop in wavelengths?
--
============================

.. . . . . and is it in the horizontal of vertical plane?

HI Cecil and the others...
How long ....
This is one of the things I would have to research if I was interested on
one...
In my imagination.. I have been thinking of these loop antennas as being
sort of a very large coil of wire as the input tuned circuit of the first rf
amplifier...
As the coil gets larger, in diameter, you need fewer turns.... So, by
extension, if you had a large diameter tuned circuit, let say 10 feet, you
would need very few turns... Unfortunately, .as well any tuned circuit, you
need a parallel capacitor... As you changed frequency, since we normally
change the capacitor, you have to remotely change the parallel capacitor...
Ok.... So you have this rather large loop, inductor, and you have it's
parallel, capacitor you have to peak them (I believe I have already said
that).. Now. since our cable usually comes in 50 ohm impedances.... you have
to transport this weak signal to the radio, you have to use a split
capacitor.... the full capacitor is in parallel with the coil.... The ground
capacitor, of the two series capacitors, is adjusted to give the proper
match to the cable and the series capacitor is the parallel to the coil, you
have a very marvellous network of two capacitors, needing separate tuning to
keep this overall loop tuned...

A lot of work.... I just hope this loop antenna system can be used in both
transmit and receiver... (just my initial opinion)....

Any further thinking on this subject... you have my attention...

Larry ve3fxq



"Cecil Moore" wrote in message
...
larry wrote:
I might consider it...for 80 or 40...

How long is the loop in wavelengths?
--
73, Cecil http://www.qsl.net/w5dxp

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larry wrote:
A lot of work.... I just hope this loop antenna system can be used in both
transmit and receiver... (just my initial opinion)....

Full wave loops are quite different from small loops but both
can be used for both transmit and receive. As usual, transmit
is the challenge. A one-turn small loop works best if resonated
by a vacuum cap.

Any further thinking on this subject... you have my attention...

The ARRL Antenna Book has some good information on the subject.
Single turn loops work well for transmitting. Multiple turn
transmitting loops have a nasty habit of disappointing the user.
--
73, Cecil http://www.qsl.net/w5dxp


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It should be borne in mind the best form of radiator is just an
isolated straight length of wire. But curved or bent wires do almost
as good.

Loop antennas are nearly always single turn conductors and provide a
means of getting as much wire into space as possible with the largest
possible linear dimension being dependent on the size of your back
yard. Or your attic. Loops are space savers.

Subject to the space available, for a given perimeter, the larger the
area enclosed by a loop the better are its radiating properties and
its power efficiency. However, when an antenna is already 80 or 90
percent efficient, there is seldom any point in attempting to improve
it. 100 percent is impossible. And to improve it to 95 percent
results only in an entirely imperceptible 0.2dB or 1/20th of an
S-unit.

Again considering space requirements, the physically smallest loops
are descibed as Magloops. Magloops, not difficult to design, are the
most efficient of all small antennas of any type.

When the perimeter of a Magloop is only 1/3rd of a wavelength at the
operating frequency, efficiency can approach that of a full size
1/2-wave dipole. The disadvantage of a small magloop lies in its
restricted range of operating frequencies. Three adjacent HF bands at
most. The cost of a vaccuum variable capacitor cannot be neglected.

But even at frequencies as low as 1.9 and 3.6 MHz, magloops with
perimeters as small as 1/15 wavelengths, diameters as small as 1/50
wavelengths, are quite usable in small backyards and attics. Expect
signal strengths 2 or 2.5 S-units worse than a full size half-wave
dipole. There are reports of improved signal to noise ratio on receive
although I have not experienced this myself.

For design of magloops at any HF frequency and any reasonable size of
loop, download program MAGLOOP4 from website below.

For performance of much larger horizontal loops, for use in your
backyard, download program RJELOOP4
----
Reg, G4FGQ.

A follow-on from the previous message -

As usual, I forgot to mention the website from which the programs
could be downloaded. It may be something to do with a bottle of fine
vintage Port I have recently opened.

Download MAGLOOP4 and RJELOOP4 from website below and make what sense
you can out of them.

Incidentally, the number 4 in each name is an indication of previous
programs of similar names through which programs have been improved or
diverted.
----
.................................................. ..........
Regards from Reg, G4FGQ
For Free Radio Design Software go to
http://www.btinternet.com/~g4fgq.regp
.................................................. ..........

Single turn loops work well for transmitting. Multiple turn
transmitting loops have a nasty habit of disappointing the user.
--
73, Cecil http://www.qsl.net/w5dxp

===================================

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.

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.