Tuesday, 12 June 2018

Magnetic loop - careful what you believe!

A long spell of warm, dry and calm weather has been blessing north Wales recently. Prime time for magnetic loop testing where the tuning capacitor isn't weather proof!

Like all good antennas, it doesn't have to look good.  It just has to work!  Note the non-primary loop feed mechanism.

I've had very good results with the magloop in earlier tests, where the WSPR outcome matched many full-sized wires.

I decided to have another go at 14MHz with my 1.2m per side, square, 28mm copper pipe loop.

Here is the pattern of the vertically polarised radiation, according to MMANA-GAL:
Vertical radiation pattern, magnetic loop, looking from above.

There is also a horizontal radiation component, 3dB weaker than the vertical component:

Horizontally polarised radiation, magnetic loop from above.
 
For all of you who slavishly repeat the primary 'Faraday' loop feed mechanism, you may well wish to abandon it.  The very few serious magnetic loop researchers I know of use a pseudo-gamma match.  This is best explained with a simple schematic, where the solid blue depicts the loop, and yellow the feeding. 

This is simply a triangle of wire from one side of a 4:1 balun, soldered about 3/4 the way up one side (best position found by experiment).  The other side of the balun is just connected in the middle of the lower tube.  The capacitor (not shown) sits at the top.  This 'pseudo-gamma' matching system is a combination of direct and induced feed.

For this week's run, the matching omitted the hypotenuse of the triangle, using just an 'L'-shaped feed running close to the the loop.

Back to the WSPR run.

The matching was 1.09:1 at the WSPR frequency, and the tuning capacitor a 25-165pF wide-spaced air butterfly.  All data is for 36h of spots.

I compared with G8LIK, who runs an efficient, large horizontal wire loop from a good, relatively open site. Because WSPR inevitably means comparisons of other people's antennas, it's important to note that none of this blog aims to be mindlessly critical of others' antennas.  Indeed, I am very grateful to all comparison stations, all of which have been carefully chosen for the quality of their antenna installations and long-term, proven WSPR efficiency.

And for those who may ask why I am not comparing with, say, a vertical antenna, the reason is, frustratingly, that it is very hard to find a good quality antenna in a good environment and that is operating on WSPR for more than a quick test of an hour or two!

SNR across all distances.

I've managed to crop the reported comparison, but my magloop was 3dB weaker across all spots as compared to G8LIK's full loop.  In itself, that's a very good result for such a small antenna that is also completely outside any town planning restrictions in the vast majority of installation situations.

Now, I became a little disappointed when I looked at how my magnetic loop compared with reports from beyond 2400km - what you might call 'DX' distances:

SNR beyond 2400km.

I am now 13dB below the full wire loop!  I can't remember things being that bad in any other test.

Something is wrong!  Let's look at the graph of distances achieved over 36h:


No doubt at all that my magnetic loop matches the full wire loop during the day, and considerably outperforms the wire loop during the evening.

Looking at the spot reports, it becomes clear that I was only 13dB weaker to one station - EA8BFK - who is in the sharp null of my NW-SE aligned loop, and obscured by my house:


As I've reported previously, when you run WSPR tests and use data analysis tools, you have to consider all the analyses, not just some of them.

I could have concluded my magnetic loop was useless for DX, unless I looked at the distances achieved and, more clearly, the map of all spots for both stations.  Rather obviously, my small magnetic loop reaches far into north America, whilst the full horizontal wire completely fails to get across the Atlantic. 

All spots, 36h of data.

For good measure, I include VK7JJ's web-based analysis of my spots, which reports around 36,000km per Watt equivalent SNR for my magloop, compared with just 14,000km/Watt for G8LIK.


How about a comparison with a half-sized G5RV firing E-W (confirmed with the operator, G4WIM, 11/6/2018), to address the hardcore naysayers?  This plot is for a week, with a gap where I was charging the USB battery.




On long-haul DX, beyond 5000km, the half-sized G5RV is 3dB stronger than my magnetic loop.  That's still a very good result for the loop:

SNR comparison with half-sized G5RV beyond 5000km.

In terms of spot 'quality', G4WIM's half sized G5RV comes in at around 41,000km per Watt equivalent, some 4000km per Watt above my magnetic loop:

I'm not sure why I keep repeating this experiment, other than to keep providing data to those who claim magnetic loops are 'just dummy loads'.  WSPR has no opinions or human bias.  It just reports the truth.  Clearly, magnetic loops work.  It's just a shame that trying to make a weatherpoof, micro-adjustable tuning capacitor is a bit tough, at least for people like me who are not very good at electronics!



No comments:

Post a Comment