Sunday 13 June 2010

Magnetic Loop Musings

The trouble with all the recent fun and games with beacons (Blogs passim) is that the station antenna tends to be monopolized - with the result that "ordinary" operating suffers. I decided I needed to think about a dedicated beacon antenna...

Recently I read a nice nugget of wisdom concerning antenna design: you can have any TWO of the following desiderata, but not all three...
  • small size
  • high efficiency
  • wide bandwidth
This set me thinking - I don't need any bandwidth for an antenna dedicated to 30m beacon working - it is almost transmitting a monotone!

After recent success with the Walkabout base-loaded whip, I thought about making a shortened dipole with two similar elements. However, I realized there's an important category of antennas of which I've no experience - magnetic loops. These can follow the rule by being small, efficient and (as they're deliberately tuned to one frequency) narrow bandwidth. I set about making one as a learning exercise...

The prototype was made with "available materials"; 8mm microbore copper pipe, scrap timber, odd pieces of clear sheet polystyrene, a few cable clips and an old 115pF variable capacitor from the junk box...


Here's a close-up of the driven element...


and here's the tuning capacitor...


I could study the tuning of the experimental antenna in two ways; sweeping it with my g3wpo dipper (fast and approximate)...


or via estimates of relative input impedance using my Return Loss Bridge (seen at the right hand side of this photo - VERY slow yet precise) ...


The antenna did, indeed have a single resonant frequency and it was amazingly sharp! Unfortunately, the resonance was too low (running from just below the 40m band to around 9 MHz). If I had been bothered to do the math before throwing the prototype together, I would have been able to predict this...

The inductance of a single, circular turn is a standard result (Claudio, in3otd, has an on-line calculator) and the variable capacitor had "115pF" conveniently penciled on one of the vanes. My prototype loop (which felt as if it ought to be "about THIS size") turned out to have a diameter of 44 inches (1.117m).

Using the equation...

...predicts a resonance at 7.89MHz (I've no idea of the provenance of the 115pF label on the cap, and I haven't accounted for parastics, mutual inductances, etc). The predicted result agrees pretty well with observation - the tuning is "good enough for jazz".

Reducing the value of the tuning capacitor increased the observed resonance in the expected way, but the Q started to drop as I went over around 9 MHz - so I'll try making a smaller main loop. I need something that will go through the hatch into the attic after all HI HI.

...-.- de m0xpd

1 comment:

  1. As usual, you have managed to make a much more professional looking effort than mine. In part due to my use of junk box coax lengths the justification of which is that its cost was zero.

    One thing to consider related to putting it in the loft is that the tuning is so sharp even temperature changes or possibly rain on the roof can change the tuning. I've noticed the SWR of my attic MFJ loop change quite substantially from one operating session to another, though that's easily taken care of using the remote tuner.

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