Thursday, 17 June 2010

More on the Mag Loop

A spell of summer weather has made it impossible to stay indoors - so I could re-visit work on the experimental loop antenna.

After the initial prototype, which tuned up only on 40m, I'd made v0.2 with a 0.7m diameter to tune on the 30m band...

In addition to the smaller resonant loop, I experimented with a few sizes of driven loop and came up with the optimal relative size you see above. I also added a long shaft to the tuning capacitor and a knob (seen at top left), so I could tune the system without too much trouble from hand capacitance effects.

I tried the QRSS beacon on the loop last evening, but was disappointed with the results. First problem wasn't the loop's fault, but it was an annoyance - a loss of frequency stability.

There was a 5 - 10Hz variation with something like a one minute period. I could see a few possible explanations for this...
  • poorer earthing than on the bench
  • thermal effects (heating from sun and shadow, cooling from breezes, etc)
  • sitting in the RF field of the magnetic loop antenna
...but I haven't yet had a chance to isolate the actual cause. Suffice to say that when I brought the beacon back into the shack, the frequency was rock solid again.

Aside from the irritating drift, the radiation from the entire beacon/antenna system was pretty disappointing, with only a very tenuous spot on Peter, pa1sdb's grabber. I was used to hitting most of the European grabbers when the beacon was driving the g5rv, so I began to suspect the magnetic loop wasn't up to much...

Today I had chance to make a few measurements on the antenna. I found the loop was tuned such that VSWR was about 1.25 at the 10.140MHz frequency the beacon had been working - that should have been OK. Mind you, all a VSWR reading proves (if it is done properly) is that power is going into a system - it doesn't confirm this power is being radiated out as nice radio waves! I decided I'd try to use the loop with my nearly-new FT817 on 30m CW, to find out if the loop could actually radiate...

I tuned the loop for 10.106MHz (by listening for the maximum noise output from the receiver, rather than the more formal tuning methods used before) and then measured the VSWR around this frequency...

All looks fine - but you can see just how narrow the tuning is.

The antenna clearly was working well on receive (I heard a SOTA activation by la8bca/p and a flurry of resulting traffic) and so I tried for a qso. Anders, sm2ecl, called CQ and he gave my answer a 549. Anders was using a loop as well - but he was driving his with 100 Watts as against my 5. I followed with a weak contact with Ludi, ok1has.

So the experimental loop antenna isn't just a dummy load, after all!

...-.- de m0xpd

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