A few notes about my Mag Loop for 40 mtrs. It can perfom very well indeed but has been a bit temperamental lately. It is really a "work in progress". At its best it can hold its own well within an S point of my very low -SWR trap dipole.
That claim is confirmed from several contacts I have had on 40 mtrs.. It is also deceptive by way of appearing to be a very "quiet" aerial. Yes - it picks up less noise, and if local interference is a nuisance then rotation can usually make a useful improvement. In other words, my observations are much in line with most others.
The Plan initially was to make the loop as big as possible for the 40 mtr band where it should be at best efficiency, then one day tune it on 80mtrs for local 'rag chews' perhaps. Using software from W2BRI's Magnetic Loops pages I came up with the following. In practice the numbers seem spot on. My loop needed 40pf to make 7.1 Mhz.
The loop is 3mtrs dia across the flats.
Click to enlarge.
Construction and mounting.
An octagon is much easier to make than a circle and incurrs only minor losses.
Elbows are available for joining the 20mm pipe and a map gas torch allows enough heat for soldering the joins. The $2 shop chopping board once again comes to the rescue along with a plastic T section to bridge the open ends and to hold the capacitor. My butterfly capacitor straddles the two ends of the pipe, one stator connected to each side with copper strap.
I have pinched the G400 rotator for my hexbeam!
This variable capacitor was far too big. I could not turn it slowly enough to tune the loop. Only a10pF variable was needed and works well. A 300mm length of URM67 made a stable fixed 33pF capacitor. Button 6Kv caps were hopeless as the value changed with transmit power at 100Watts.
The feed loop is about 1/5 the size of the main loop. It is sized and positioned to bring the SWR down to 1:1 at loop resonance. So far I can only achieve that by squashing the feed loop into an oval but it works very well.
Once set the SWR holds good for any frequency the loop is tuned to.FORGET about antenna tuners, they are not helpful here.
Above - a slotted extension of the center pipe engages with the G400 rotator below it.
The copper loop and all its bits and pieces literally hang from a wooden gallows. It is easily raised or lowered with the rope and pulley at the top. A loose plastic collar acts as a guide and keeps the "array" lined up with the rotator. No weight at all is on the rotator, it simply engages with a flange. To lower the loop is simplicity itself. Undo the coax feed from the small loop and free the guide collar. Then hold and lift the bottom of the loop whilst walking outwards and lowering on the rope/pulley as you go. The bottom of the loop you are holding can be placed on pre positioned trestles.
It is useful to do this. With the loop horizontal and about two foot above the ground it can be tinkered with and tuned quite effectively.
Remote tuning is accomplished with a 3 volt Tamiya motor. The torque from these little motors is incredible. All it needs is 3V(2 x AAA) and a two way two pole switch wired to reverse/change polarity. The motor is mounted just above the feed loop near the guide collar. A wooden dowel rod, covered with heatshrink, connects to the rotor of the butterfly capacitor above.
Now here is my first problem.
The first capacitor I made, to stand the 4000 volts, was about 80pF as a butterfly cap. The tuning rate had to be VERY slow in order to get a perfect SWR when changing frequency. I ended up with about 7 sections of planetary drive gears on the Tamiya to give a 40,000 to 1 ratio.
Yes, forty thousand to one!!. It almost did the trick and was tunable but engineered another problem. The beautifully made Tamiya gears do accumulate a play and this results in a considerable time lag when changing tuning direction. It was taking several seconds to catch up.
Whilst there are ways to combat this it was clear that my lovely tuning capacitor was far too big a value.
A fixed capacitor made of a length of RG213 was used and trimmed down to about 35pF. A two plate butterfly cap' was made and the motor gearbox reduced to 4 or 5 sections only. It was a vast improvement but another problem soon surfaced. The high voltage generated at the open ends of these loops is no joke. I found that the RG213 forming my fixed capacitor had arced across at it's open end and turned things to carbon. Back to the junk box - found some 6KV button type caps and put two of these in series to give the required value.
The 6KV caps proved useless. Clearly the heat generated by nearly 3000Volts changed the value, the SWR shifted with key down.
The coax however is the answer. Just peel back the copper braid about 20mm to prevent the arcing and seal it from the rain.
This arrangement works very well. I do have to squash the feed loop into an oval and after that it was easy to tune the loop for a perfect SWR across the 40mtr band. The sharp tuning, the Q of these loops, is amazing. When they are tuned "on the nose" they really do perform well.
Meantime I have a hexbeam to play with and last night I got a 5/9 from Brazil on 20 mtrs to encourage me.
Sunspots may be creeping up.......so slowly.
Don't be put off by my problems. I am convinced Magnetic Loop aerials work extremely well when you know how.
30th July 2010 - At last I have a usable mag loop on 40 mtrs which will tune across the band to a perfect SWR. At best it is about an S point down on the dipole but can do better if the direction and frequency favour it. The dipole of course is fairly narrow band, the loop is tunable across all of it.
My variable capacitor is 3 to 10 pF and the fixed capacitor is 35pF of URM67. (approx 100pf per meter)
The open end of that coax has the braid peeled back for 20mm to prevent arcing and then sealed.
I have an old truck windscreen wiper motor with a worm drive, it will once again make a fine rotator, this time for the loop.
We only have to switch the polarity across the windings to change direction - 'works well.
This antenna is used exclusively with my old TS530 and 100 Watts is all I need for a lot of contacts.
Even the rain makes little difference to operation now.