The day I rediscovered radio.

Well, it's been a true 'junk box' end to the week, when the increasingly rare leftovers from other projects have been recycled into an analogue of the 'Alex Loop' (£300) magnetic loop antenna.

The RG213 'junkbox' loop, proving definitively that solder and food do mix!  Note the 'sausage' shaped primary loop, which also extends a bit outside the main loop.  This gives the best coupling to the secondary loop, and is a form recommended by many experienced loop users over the years.

 

For the most part, this is an exercise again to show newcomers that magloops can be made very cheaply, are not as mysterious and impossible to build as some make out, and are effective solutions to multiband operation, needing hardly any physical space.

The leaning tower of Pisa is about 60m tall.  A loop just 1m in diameter sends out waves that long!

 

Well, to test things out to the extreme, I located my magloop, made of RG213 cable, in the kitchen, and went 5W /QRP for real FT8 QSOs - on 60m!

As predicted, I did have to change the capacitor for a wider-space version (a Jackson Brothers 15-500pF unit).  Tuning by hand at 60m is fairly easy and reliable, but gets progressively 'twitchier' as we go up the bands, and becomes impossible by 20m.  This can be rectified with a simple PWM motor, or a different capacitor range. 

New, wider-spaced capacitor.  If you build a loop, don't space the SO239's as closely as I have here (I was in a rush to test the cap), because they affect system capacitance too much.  Space them 10-15cm apart, and tuning will be a bit less 'twitchy'.

The first thing to note is that a magloop calculator, such as this one from 66 Pacific, will throw out an efficiency for this particular loop of about 3%.  With 5W going in, this should mean my real output signal is a mere 150mW.  

Is this calculation correct?

Almost certainly not.  The reasons why are eloquently set out by Professor Mike Underhill, G3LHZ, in a chapter entitled The Truth About Loops in the International Antenna Collection book by the ARRL and RSGB.  It is well worth a careful read, notably the parts about the persistent misinterpretation of the Chu-Wheeler criterion.  Prof. Underhill concludes that, at lower frequencies, the conventionally-computed efficiencies (lower, very faint curve in plot, below) for magnetic loops are up to 30dB in error - something he terms "a scandal".  A loop-related paper by Underhill can be found at Scribd.

Returning to my tests, I wouldn't say that QSOs are impossible at 60m with 150mW.  But they might be very few, and weak.  

This was most certainly not my experience this weekend!

For example, I was very happy to make a good QSO with my friend PE4BAS, who kindly then sent me a screengrab of the current reports:

For sure, I was very happy to see these kinds of reports from several locations across Europe, and I was effectively working a slow pileup on 60m for a long time on Saturday afternoon.  The use of QRP power certainly seems to bring out the best in the radio community, with people making a genuine effort to support a low-power operation.

In the morning, I received a -15dB report from K1HTV, though he seemed to be only receiving at the time.  I was also especially happy to make a QSO with TF3DT.

60m FT8 activity with my 5W and loop, past 24 hours.

There aren't so many international WSPRers on 60m, but the pattern (using 1W) reflects the FT8 activity:

I havent' enjoyed radio this much for a while.  Operating on 60m ordinarily would need a dipole of 30m length, or a vertical of 15m.  Those are not possible for many people.  So getting on that band, let alone operating there effectively at QRP levels, from the kitchen, really is quite an achievement!