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Old November 8th 03, 04:06 PM
Mike Coslo
 
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Default Magloop woes

I've finished my magloop, and have a few questions.

I made the antenna according to Reg's magloop program.

Octagon loop, 7.3 meters diameter

19.05 conductor diameter

100 watts output

3 meters above ground (though not for testing)

average soil

The diameter of the coupling loop is .420 meters in diameter. Since I
saw this param changing all over the place as I changed frequencies in
the program, I just chose a diameter from somewhere in the 40 meter
band. The loop is done up as a Faraday loop from coax, circular, and
placed in the traditional position. Continuity has been measured.

All seems to be well, but when attaching the antenna analyzer to the
thing, I get infinitely high SWR in most places (understandable) with a
foray into around 100-300 ohms on what I think may be the tuned frequency.

Anyone know why my measurements could be so far off? At first I thought
the loop could be cut wrong, but just using the program shows a wide
variation in loop sizes, so I doubt that is correct.

TIA - Mike KB3EIA -


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Old November 8th 03, 05:11 PM
Roger Adam
 
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Mike,

I don't see any mention of a fixed or variable capacitor. I take it that
there is one fitted!

Just an observation,

Roger, G7JAQ,


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Old November 8th 03, 06:16 PM
Reg Edwards
 
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"Roger Adam" wrote -

I don't see any mention of a fixed or variable capacitor. I take it that
there is one fitted!


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

Neither is there any mention of frequency. Where is the antenna analyser
connected? What parameters are being measured. How long is the feedline,
etc. ?

Does the antenna work when connected to a transmitter or receiver? If it
doesn't what are the symptoms?

A magloop has an extremely high Q and narrow bandwidth. Behaviour
approaches that of a quartz crytal. If you don't get the expected
measurement results then the most likely reason is the limited capabilty of
the analyser. A precision, scientific-grade instrument is needed.

Reg.




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Old November 8th 03, 07:21 PM
Richard Clark
 
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On Sat, 08 Nov 2003 16:06:36 GMT, Mike Coslo
wrote:

Octagon loop, 7.3 meters diameter

....
...somewhere in the 40 meter band.


On Sat, 8 Nov 2003 18:16:19 +0000 (UTC), "Reg Edwards"
wrote:

Neither is there any mention of frequency.

....
A magloop has an extremely high Q and narrow bandwidth. Behaviour


Really? You two are talking past each other.

73's
Richard Clark, KB7QHC
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Old November 9th 03, 02:25 PM
Mike Coslo
 
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Roger Adam wrote:

Mike,

I don't see any mention of a fixed or variable capacitor. I take it that
there is one fitted!


Of course! I have a 4 element trombone cap, powered by an electric
screwdriver.

- Mike KB3EIA -

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Old November 9th 03, 02:56 PM
Mike Coslo
 
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Reg Edwards wrote:

"Roger Adam" wrote -

I don't see any mention of a fixed or variable capacitor. I take it that
there is one fitted!



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

Neither is there any mention of frequency. Where is the antenna analyser
connected? What parameters are being measured. How long is the feedline,
etc. ?


The frequency I chose for the *coupling* loop size was around 7 mhz in
the 40 meter band. I chose this because of the wide variation in loop
sizes called for by the program.

the antenna overall is designed to hit 75 meters, and at the other end,
17 meters. But 40 meters is the mainstay of this antenna.

The analyzer is connected to a piece of coax that is connected to the
coupling loop, which is a Faraday loop.

The feedline for the testing is around 5 feet of 50 ohm cable. I don't
doubt that my proximity to the antenna might have some effect, but I
don't expect it to be that extreme.



Does the antenna work when connected to a transmitter or receiver? If it
doesn't what are the symptoms?


Haven't connected it yet. I wanted to see if I was in the ballpark.
Initial tests suggested I probably aren't.


A magloop has an extremely high Q and narrow bandwidth. Behaviour
approaches that of a quartz crytal. If you don't get the expected
measurement results then the most likely reason is the limited capabilty of
the analyser. A precision, scientific-grade instrument is needed.


Now I am really perplexed! If I need an instrument such as you
describe, how will I ever get this thing to tune with my lowly IC-745?
It certainly isn't a scientific instrument. What I used to test the loop
was a MFJ-259 analyzer. Not "lab grade", for sure, but not at the bottom
of the barrel either. It has worked competently for everything else I've
used it for.

- Mike KB3EIA -

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Old November 9th 03, 03:08 PM
Mike Coslo
 
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wrote:

On Sat, 8 Nov 2003 18:16:19 +0000 (UTC), "Reg Edwards"
wrote:


"Roger Adam" wrote -

I don't see any mention of a fixed or variable capacitor. I take it that
there is one fitted!


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

Neither is there any mention of frequency. Where is the antenna analyser
connected? What parameters are being measured. How long is the feedline,
etc. ?

Does the antenna work when connected to a transmitter or receiver? If it
doesn't what are the symptoms?

A magloop has an extremely high Q and narrow bandwidth. Behaviour
approaches that of a quartz crytal. If you don't get the expected
measurement results then the most likely reason is the limited capabilty of
the analyser. A precision, scientific-grade instrument is needed.




I've read a little about magloops (I'm a total dunce as far as aerials
are concerned). It always mentions the fact that losses should be
minimised on aerials made of copper tube by making sure that all the
joints are well-made. Would it not be better to get a long piece of
pipe and a plumber's pipebender and make the loop out of one piece of
pipe to avoid joints?


You are right, but.....

If we look at the "best" setup, it would probably be one very wide
diameter (not talking about the perimeter of the loop itself) silver
loop. But these things work even with soldered copper pipe.

Plus with the perimeter of my loop (7.3 meters or almost 24 feet!)
getting one solid piece of pipe would be pretty hard, let alone forming
the loop. So I do a few tradeoffs.

- Mike KB3EIA -

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Old November 9th 03, 06:27 PM
Reg Edwards
 
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Mike,

Ignore the old wives. The extra resistance due to soldered joints around a
copper octogon is unmeasurable. Wipe off any visible surplus solder while
hot or use fine glass-paper. I finish off with metal polish and a coating of
clear yatch laquer. A beautiful, long lasting appearance .

The usual conductor material in commercial magloops is an aluminium alloy.
Its lighter, cheaper, weather resistant and you can't tell the difference in
radio performance. By far the best way of improving magloop performance is
to increase the circumference of the main loop. Double the circumference,
half the capacitance and the difference will astound you. However there's
nothing more to be gained after the circumference has increased above about
3/8ths wavelength at the frequency of interest.

Just make the main loop diameter, capacitor value, conductor diameter and
coupling loop diameter according to program Magloop4.

Ideally, there should be no direct connection between coupling loop and main
loop. Otherwise RF currents will flow on the coax feedline and can make a
mess of the null in radiation pattern. A few ferrite sleeves over the
antenna end of the coax will help.

The coax used in the coupling loop can be junked. The Faraday thing is a
waste of time and material. Just use a length of wire stiff enough for it to
be self-supporting. It can then be easily adjusted for diameter.
Alternatively, if made a little oversize, it can easily be rotated a little
relative to the main loop to vary the coupling coefficient.

If the loop diameter is adjusted for the 80m band it will be near enough for
the 160m and 40m bands too. If you do nothing with the program except
change frequency you will see the diameter of the coupling loop changes very
little from one band to another.

But if you have some wierd, bulky, home-brew contraption for the tuning
capacitor then in practice things may behave less conveniently.

You won't need an antenna analyser. Just connect a transceiver with its
ordinary SWR meter to the antenna and it should work right away. Don't take
undue notice of the SWR meter till exceeds 1.5 : 1.

It is the transceiver with its own SWR meter which has to be kept happy -
NOT the analyser.

If you've never used a magloop before then it may take a little time to get
used to the VERY narrow bandwidth especially on 160m and 80m.

Just go ahead, finish construction and stop worrying.
----
Reg, G4FGQ




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