OBSERVATIONS FROM SCOTLAND 28 May 2008 GM1SXX
Re-styling the loop.
In the interests of furthering my limited knowledge of mag-loops, I decided to 're-style' my 2*1M loop into a 1M square one to see how that size of loop performed. I dismantled the old loop with the help of my gas torch and cut the long vertical sides in half. I reassembled it into a loop half the size of the original but retained the 40pf motorised tuning capacitor. As luck would have it, I also came across a pair of brand new T200-2 cores that I've been looking for since I bought them! Out came the old dubious ferrite transformer and in went a new one, this time tapped a bit differently. By way of experiment I used 3+3+3 turns on it rather than the 5+5+5+5 I had used on the old one. To protect against the brittle core being broken )like the last T200-2!), I wrapped two layers of electrician's PVC tape over it so that if I dropped it, it might just survive!
This time, thanks to the larger core, I used a complete length of 15MM pipe as the bottom element of the loop rather than the 15mm-8mm-15mm soldered 'bodge' of my previous attempt. This also had the benefit of making the loop mechanically stronger. The 'transformer wire' was just good quality tinned-copper stranded hook-up wire. As before, I connected the co-ax to the transformer via a switch to select the tappings and slipped two ferrite sleeves over the co-ax where it went into the enclosure.
Failure.
My cut-down aerial was a complete flop. I couldn't believe it. I ripped
out the T200 core and put in a different core (ferrite)... still useless. In
sheer desperation to make it work, I ripped out the tuning cap and soldered two
croc clips to the ends of the arms. I tried various caps... all worked! My
original tuning caps was a dud! I can resonate it on all bands from 3.5 to
21Mhz using a 1000pf variable. 80M needs about 540pf.
I even tried a 500+500 but with only the fixed vanes connected. Nice trick cause
it shares the voltage across two air gaps and doesn't induce current flow
through the wipers at all. This is my claim to fame for tonight. Tonight I
'invented' this way of using a variable dual-gang cap at twice the volts and
half the capacity but with no wiping contact carrying RF. So there!
It's a cheap butterfly in essence.
I want 80/40/20 so a slightly bigger loop and a 500PF variable makes the most
sense.
Once I worked out that my original cap was a dud. I played a bit.
I have a carton full of compact fluorescent lamps in the back room. I squirted
in a guesstimated 3watts from the AA battery powered FT817 and it happily lit a
compact fluorescent lamp waved near the tuning cap! When I tried this simple
test, I did NOT expect it to work. Now I can imitate 'Uncle Fester' of the
Addams Family!
Any band 14Mhz and above will light the CF lamp quite brightly. It shows that
the loop does indeed radiate substantial amounts of RF power.
Remember, this is a 1*1M square loop. I'm thinking of re-rigging the whole
experiment, but with 22 millimetre pipework for lower losses / better efficiency
plus extra strength and a 1M wide by 1.5M high loop to favour the lower bands.
For me, that would make the most sense.
I have to say that G4FGQ's work is a masterpiece reference. His mag-loop
calculations are actually close to perfect, especially the capacitor values
needed to resonate the loop on different bands.
I really didn't think that 3W of RF would light a CF lamp. That was a big
surprise. Not full brilliance, but plenty to see with... and the lamp had a 15W
rating. Don't get in the way of the RF, even at QRP levels. That old
tuning cap is positively hooching with RF energy.
Oh, and the FT817 was perfectly happy with the SWR on all bands with a 3 turn
transformer coil. It has a happy/unhappy SWR indicator. This doesn't of course
mean that 3 turns is optimum and there's still scope for some changes there.
Hell, I need a quick portable version of this for use at work and out in the
hills!
73 Al.
GM1SXX
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