Thanks Reg, printed out your info and in the process of reading, looks like
i will have lots of info and reading to do
all the best of weekend and cul.
73 de Jean-Guu/ve9buf


"Reg Edwards" a écrit dans le message de
...

"Jean-Guy" wrote -
Hello group. I am in the beginning of research for a magloop antenna as
due
to restrictions here.
Wonder what can be said of them , pro's and con's.
Also any idea for good articles about them and how to go about building
them.?

Let me know what you think.

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

Jean, look no further.

I assume your 'restriction' is the amount of space available, and the
consequential environmental clutter which doesn't help.

From the important point of view of power radiating efficiency the magloop
is the most efficient of all the small antennas. Much more efficient than
most.

Small means small in relation to free-space wavelength. A loop diameter
less
than about 1/8 of a wavelength can be considered to be small. But loop
antennas with a diameter as small as 1/80 wavelengths are quite usable
when
installed in a fairly clear and unobstructed environment. But don't let
the
prospect of using one in your bedroom or attic put you off.

With a wide range tuning capacitor a magloop can cover several bands, eg.,
160 to 40 metres, or 20 to 10 metres, whereas other very small antennas
are
single band only.

Assuming you are not going to buy one.

A magloop consists of a square or hexagonal or circular loop of a LARGE
diameter (one or two inches or more) conductor. Or a broad, flat strip.
The
actual shape of conductor doesn't matter all that much. A circular
conductor is marginally the best. What matters is the enclosed area.
Copper
is marginally better than aluminium.

Construction of the loop itself is a straightforward mechanical task. The
main difficulties lie in connecting the ends of the loop to the tuning
capacitor. All connections MUST be of the same substantial construction
as
the loop itself.

The variable tuning capacitor, a vital component, for the highest
frequencies (14 to 30 MHz), can be self-constructed. For the lowest
frequencies variable capacitors (the vacuum type) as large as 1000 pF are
needed, and these can only be purchased. They are expensive but worth the
money. They have a resale value.

For single band working a high-value fixed capacitor can be be used with a
low-value home-made or purchased variable capacitor in parallel.

All capacitors must have high working voltages. For 100-watt transmitters
working voltages of 5000V are needed. The smaller the loop diameter, the
lower the frequency, the higher the working voltage.

The only other component is a small internal coupling loop which lies in
the
same plane as the the main loop and, to match to a 50-ohm line, it has a
diameter about 1/5 of the main loop. It is a valuable property of a
magloop
that the diameter of the coupling loop remains approximately constant from
one band to another, the only variable quantity needed to maintain a match
to a 50-ohm line is the value of the tuning capacitor. (All other antenna
matching circuits need at least two variable components.)

The coupling loop can be just a self-supporting circle of wire of no
greater
wire diameter than that of the inner conductor of the coaxial cable which
feeds it. Ignore old wives' tales about Faraday screens. But if you feel
like it, by all means extend the coaxial braid and jacket around the
length
of the coupling loop.

Especially at the lower frequencies, a very slow-motion control is needed
for the variable tuning capacitor. This is necessary because of the
extremely high Q and selectivity of the loop + capacitor tuned circuit.
You
can't have high efficiency without a high Q with magloops.

Well, I've given you the essentials. You will find on Google the various
ways of making your own tuning capacitors complete with drawings - the
most
awkward part of the whole business. The first thing to do is obtain the
values of the variable capacitors. See computer program below. Being
lazy
and rich enough I've always purchased mine.

To design your own magloop, or, once you have the dimensions, to compare
the
performance of one manufactured magloop with another, download in a few
seconds program MAGLOOP4 from website below and run immediately. Program
MAGLOOP4 has been written to professional standards.
----
.................................................. .........
Regards from Reg, G4FGQ
For Free Radio Design Software go to
http://www.btinternet.com/~g4fgq.regp
.................................................. .........