which is a better way to couple a small magenetic loop antenna ?
Peter Shintani
Thanks for reading this posting.
I am seeking advice.
Which coupling method offers a wider matching range for a small magnetic
loop, a gamma match or a just a loop ? I heard that the coupling loop
diameter should be about 1/5th the diameter of the main loop.
I have seen both on magnetic loops. I think the commerical antenna
seem to prefer a loop for coupling the coaxial feed to the small
magnetic loops. Some QST articles refered to a gamma match. Any advice
would be appreciated. My loop has a 9ft circumference, and is made out
of 3/4` ID copper tubing. I have 10kV 5~100pF vacum variable cap waiting
to be connected.Please advise.
Peter
Dave R
Peter Shintani wrote in message <362822C7...@funtv.com>...
I've built one using the coupling loop method and it worked real well the
band width with either method is only going to be on the order of 5 to 10
khz (2:1 points) that's about what your looking for , because if your
getting much wider bandwidth than that it's usually do to R losses.. this
type of an antenna is a very Hi Q system.. but a resonance it will work
surprizingly well for it's size..
73 DAVE KC1DI
Peter Parker
loop is that it worked on one band and not on the other (my loop is for
80 & 40).
I replaced it with a gamma match-type arrangement (no capacitor) and
could get 1:1 SWR on both 80 and 40m when the loop was tuned properly.
Full constructional info on my loop appears at the URL below.
73,
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Reg Edwards
you say, it is roughly 1/5th of the diameter of the main loop. A single,
self supporting coupling wire loop is all that is necessary. No need for a
screened loop as is sometimes advocated by Guru's wishing to advertise the
vital fact that they have heard of Faraday screens. Michael must turn over
in his grave.
A small coupling loop has the important advantage that the coupling
coefficient required between small and main loops tends to be independent
of the frequency band. Avoid gamma matches - they do not isolate the feed
line from the antenna. So the feedline is also inclined to radiate (and
receive).
The gamma-match also tends to change with frequency band. It is
inconvenient to re-adjust. Whereas the small coupling loop is extremely
simple to adjust - if it should be necessary, just rotate it with respect
to the plane of the main loop. Can be done with two fingers.
The diameter of the internal coupling loop can be computed by program
RJELOOP1.exe available by downloading it from my website. Takes only a few
seconds to download. Not zipped-up. Can be used immediately. It provides a
full analysis of any regular shaped loop from triangular, via square, to
circular.
Program RJELOOP2.exe does the same for rectangular loops.
Both programs provide full analysis of loops having tiny circumferences up
to loops half-wave all the way round at which point the tuning capacitance
is zero pF. (That is a good way of testing other computer programs.)
By the way, forget all about the nonsense spread around by Guru's who know
nothing except the existence of loss resistance due to soldered joints in a
loop. It is equivalent to an extra 1.5 inches of copper. Deary me, all
those extra milli-micro-ohms !
--
Regards, Reg G4FGQ Click below.
http://www.btinternet.com/~g4fgq.regp
Tony Fegan VE3QF
I have been experimenting with Small Loop Antennas for over 8 years. I
have tried shielded loop, open loop and "gamma" types of matching. If you
have no more than a 2:1 frequency range then a shielded loop type of feeding
will work adequately. I myself much prefer the gamma (no capacitor) type of
feed. The secret is to use large diameter tubing and close space it to the
loop. You should be able to achieve less than 1.5:1 over at least a 3:1
frequency range. I find that a high Q loop system is harder to match with a
loop type feed but is OK for lower Q loops using plate type capacitors.
The vacuum variable capacitor will let you build a small loop antenna
with quite a high Q if you are careful with the construction. For the gamma
match I find that 1/2 inch id. soft copper tubing spaced 1/4 to 1/2 inch
internal and in the plane of the loop works well. For testing I squash the
end of the gamma tube to fit snugly over the 3/4 inch loop and use a hose
clamp. The coupling can be adjusted by altering the spacing or altering the
tapping point. Have a few different lengths of tubing handy for the gamma
match so that you can find the best combination. If you can borrow an MFJ
antenna analyzer for this, it will save you hours of cut and try. The
tapping point for the gamma will probably end up between 1/4 and 1/3 of the
loop circumference.
The vacuum variable capacitor handles the voltage OK but I am not sure
how much current those wafer thin plates (tubes) can take without
distortion. A vacuum is not very good to conduct any heat away. Most of my
loops used trombone type capacitors.
Hope this helps rather than confuse.
73,
Tony Fegan VE3QF
ve...@amsat.org
pup...@netpar.com.br
shin...@funtv.com wrote:
> Hi:
> Thanks for reading this posting.
> I am seeking advice.
> Which coupling method offers a wider matching range for a small magnetic
> loop, a gamma match or a just a loop ? I heard that the coupling loop
> diameter should be about 1/5th the diameter of the main loop.
> I have seen both on magnetic loops. I think the commerical antenna
> seem to prefer a loop for coupling the coaxial feed to the small
> magnetic loops. Some QST articles refered to a gamma match. Any advice
> would be appreciated. My loop has a 9ft circumference, and is made out
> of 3/4` ID copper tubing. I have 10kV 5~100pF vacum variable cap waiting
> to be connected.Please advise.
> Peter
Hi, Peter, I finished a small octogonal loop for 80m last sunday, and I got
a simple way to couple it to the 50 Ohm coaxial. I connected the braid at the
center of the lower side (horizontal) of the octogon and connected the center
of coaxial with a straight copper wire (#19) to the bottom of the vertical
side. A frontal view of this arrangement resembles a triangle which the sides
are: L/2, L, and 1.4L , where L is the side length. The connection point with
the vertical tube was selected to get 50 Ohm, no reactance , using a noise
bridge connected directly to the antenna. I got 1:1 swr near the resonance,
this way.
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py2...@unetsul.com.br
In article <01bdfa8e$83eba340$ba5dac3e@default>,
"Reg Edwards" <G4fgq...@btinternet.com> wrote:
> There is nothing better than a small coupling loop inside the main loop. As
> you say, it is roughly 1/5th of the diameter of the main loop. A single,
> self supporting coupling wire loop is all that is necessary. No need for a
> screened loop as is sometimes advocated ...
Some years ago I was playing with such loops, square and octagon shaped and
asked to some mailing list reflectors abt such "screening properties" and I
collected the following information (true or not, I don't know):
1) The "shielding" helps to decrease the influence of metal/handling. So the
receiveing only shielded loops that can be handled without de-tuning (?). 2)
The "shielding" acts like a "low pass filter" or better, a frequency
dependent coupling circuit(L/C?), helping to "broad" the freq. response of
such coupling loop.
Shielded or not, the electromagnetic changes at the wavefront causes a
voltage swing at the main loop "terminals". Voltage at such loop/resistance
creates magnectic fields that are induced at the secondary (coupling loop).
So, doesn't matter if there is an "eletrostatic shield" or not.
Sounds reasonable?
73
Marcus Ramos
PY3CRX/PY2PLL
running a 1m dia loop from 10 to 30MHz (SWR < 1.8:1)... one day 160 meter
too!!!
Reg Edwards
the 160m band. You should construct it from 150mm (6-inch) outside diameter
copper tube. And I hope your bank manager will provide the loan for
purchase of the vaccuum, variable, EHT working voltage capacitor, the size
in nano-farads being computed with program RJELOOP1.exe, downloadable from
my website in a few seconds and immediately ready for use because it is not
zipped-up.
Good DX ;o)
--
Regards, Reg G4FGQ Click below.
http://www.btinternet.com/~g4fgq.regp
py2...@unetsul.com.br wrote in article <70ifts$sag$1...@nnrp1.dejanews.com>...