Saturday, 25 November 2017

15m Magnetic Loop Tests

Love them or hate them, magnetic loops remain a fascinating antenna concept.

Over the years, I've built quite a few magloops, more recently being inspired by the work of G3JKF and other, careful experimenters.  I did go through a period of disillusionment with loops, but came back to them after I realised that some of the less impressive results were simply the result of too much operating them indoors.

Since the advent of the digital weak signal modes, the need to 'QSY' - or change frequency to make different contacts - has, in effect, been eliminated.  You just dial up the mode's standard frequency and shift the audio tone up or down.  So the traditional 'problem' of loops being very high Q, narrow bandwidth antennas has also, as a result, become less of an issue.

To reduce costs and space requirements, I built a small loop for the 15m band yesterday.  Instead of the usual copper pipe, I used a spare, 1.5m-long piece of thin brass sheet about 100mm wide..


The advantage of sheet is that it is flexible, and can conceivably be 'unlooped' and removed from the support structure in larger antenna builds where car transport to site might be a consideration. Other than potentially being a bit too much like a wing in windy weather, I wasn't sure if the sheet would display any disadvantages. 

I ran the loop on WSPR using 1 Watt, though because of cold, windy and very wet weather, I kept the antenna and its unhoused air spaced capacitor inside my kitchen-cum-shed-cum-shack.  You have to take this into account when evaluating the outcome.

First, here's the distance plot of my loop, indoors, against an M6 station with a Cobwebb antenna at 200mW on top of a high rise building, with a nice, semi-rural aspect all around, in Kent.



Looking across without much analysis, and taking into account the different power outputs, the signals appear, on average, broadly the same.  Let's look at how the simultaneous spots results come out:




Unfortunately, there are not very many simultaneous spots.  But there are just about enough to make the outcome somewhat reliable.  The difference in favour of my antenna is 5.5dB, which supports the subjective assessment of the distance plot as being similar, because the M6 station is putting in about 6dB more signal into his antenna (which, remember, is outdoors on a high-rise building).  Also, the magloop is directional (I was running the main, vertical lobes in a NW-SE direction).

When looking at the geographical plot of spots, it does seem that my loop is able to take advantage of the good ground gain afforded by the local environment.  One spot (highlighted) shows an enormous, 16dB stronger signal for my loop over the Cobwebb.  And the only other US spot, from Florida, was of my loop - the Cobwebb didn't make it.  Whilst these are very few spots on which to make firm conclusions it does, at least initially, look quite good.



The following day, I was pleased to see G3JKF had joine me on a rather dead 15m band.  An EA8 station provided the only spots for most of the day, which at least made comparisons simple!  Later, I heard a 9L station but, unlike G3JKF, did not receive a spot from him.

Overall, the results are again quite encouraging - only a 1.5dB advantage for G3JKF, with a triple array in a good outdoor environment.




If you're new to loops, you'll notice that G3JKF and my loops never use the primary, 'Faraday' loop feed mechanism.  Those can lead to trickier matching.  We both use what could be described as a pseudo-gamma match - a mixture of direct and induced coupling, and usually running through a 4:1 balun.

You can find plenty of good images if you search 'magnetic loop gamma match' online.  But I would caution that all my loops show best matching when the gamma match is connected a good 1/2 to 3/4 way up one side of a loop, rather than the much shorter feed shown in most images.  An antenna anlayser makes progress infinitely quicker than being without one.  Use a crocodile clip on your centre pin matching wire for initial investigations of the best point for your feed to attach, then you can mark and solder the wire permanently, without the clip.


Over the weekend, I made this four parallel loop array, nominally for the 15m band.

Today, I'm running the new, four loop array for 15m made from 10mm microbore copper tube, but now outside, despite currently experiencing some windy, very cold weather from the Arctic!

Dry. if cold weather permits some WSPR testing at 21MHz


The results, running against G3JKF's triple-loop array now follow.

First, the plot of distances achieved, remembering that 15m is quite quiet at the moment, and that for much of the day, only one station was hearing us (EA8):


This result was fairly poor relative to G5TA's multiband vertical, though one has to remember the loop is only the size of a supermarket shopping bag:



Compared to G3JKF's loop, the outcome in terms of signal strength was, allowing for the low number of spots, the dramatic, rapid swings in propagation on 15m, and a difference in latitude that generally favours southern England over north west Wales, my loop is doing reasonably well and perhaps much the same.  For about an hour in the morning, my unweatherproofed loop and vacuum capacitor was also soaked by a big shower, which impacted its matching to the extent of raising the SWR from 1:1 to 1.5:1 until the cold Arctic air dried it out.


Overall, I can certainly say my loop is working almost or as well as G3JKF's loop, which is reassuring in terms of build quality.  My loop could be a bit higher, but that can't be addressed for the moment.

Unfortunately, 15m is too variable a band at the moment, and too quiet overall to permit a good set of data to be collected that yields a robust comparison with either G3JKF or much of anyone else. 

That said, I did run a one-after-the other series of tests over a couple of hours, comparing the loop with my vertical delta loop for 15m (via a 2:1, corner fed, SWR = 1:1).  The result?  Lots of QSB that made comparisons tough even on a 4 minute timescale.  But a figure of about 7dB in favour of the delta loop eventually emerged from the noise. 

7dB against the magnetic loop will be a killer for many - hence the 'dummy load' claims that are made.  But nobody, graced with lots of space and no neighbours to complain would use a magnetic loop as a first choice.  Magloops are for those with much less space, perhaps wanting some noise reduction, and the ability to use an antenna across multiple bands, perhaps from indoors.  With an indoors 1.2m-square loop, I've had good SSB QSOs as low as 60m using just 35W.  With digital modes, the possibilities are quite good.

Still, now you know that a reasonably effective 15m loop can be built from 10mm microbore copper tube, merely by cutting four loops out of the coil of tube that comes wrapped in plastic in a shop and soldering them to two pieces of 15mm copper tube or stiff copper strip - no bending required!







2 comments:

  1. "You just dial up the mode's standard frequency and shift the audio tone up or down."
    By doing that you ARE changing the frequency. You just have multiple signals within the passband of your receiver. It is the receiver's detector that generates different tones, and the transmitter's modulator which generates different RF frequencies based on the tones.
    If everything were on one frequency your receiver would hear all of them as the same tone.

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    1. Of course. The comment is meant to convey the idea that QSOs take place within a narrow frequency range, relative to modes like SSB. That's why I didn't say there is no frequency shift - only that 'in effect' - there is usually no need to change the radio's VFO dial to make contacts with different stations.

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