Figure-of-Eight Magnetic Loop - WSPR Results.

Last week, I built a 75cm-diameter (per loop) 'Meight', or figure-of-eight magnetic loop, to see how well it would perform compared to other loops I've built.

The comparison antenna, as is usually the case, was that run by GI8YJV in Northern Ireland.  At least at 14MHz, this trapped dipole outperforms just about everybody else in the UK on WSPR, and almost exactly mirrors the performance of my very well-matched, twin-fed vertical delta loop.  The difference in latitude and longitude is also minor.

I also use G8LIK as a comparator, as he is using a large 'Skywire'-type, horizontal loop that also performs very well, albeit with clear signs of having more skyward radiation than low angle.

My homebrew figure--of-eight loop, graced by the Moon at sunrise, 29/10/2018.  Note the feed has now changed (below)

New feed, with wire external to and well away from the loop yields less minor SWR drift in rain.

 
Now, if you are one of those people whose mind is more fixed than flexible, than please do remember that this test is, and is only claimed to be, of a small transmitting loop antenna.  Its virtue lies in being small, works close to the ground, and is either outside planning regulations or very unlikely to warrant enforcement action in most countries.  It can be deployed indoors and on balconies.  This is not a comparison with a Yagi on a 100 foot tower, which is a huge investment of time, effort, money and space.

I have been quite pleasantly surprised with this antenna.  For one thing, it's very easy to build for very little money.  Secondly, it has proven to send out a good signal.  Mounted vertically on a low post (base ~1.5m), the loop was giving the GI8 dipole a good run for its money.   Even better, it consistently outperformed the large wire loop.

I felt the loop was working so well, even getting 200mW at good strength into west coast USA, that I hurriedly mounted it at a better height (base ~2.5m) for more tests.  This did yield a small but detectable increase in performance.  That said, the increase was not such that you would lose out in an appreciable way if you were restricted to using it on a lower, ground-mounted stand (which also has the advantage of being easily turned, for directivity).

So, let's look at about 3 days' worth of WSPR results.

First, the magloop vs. the trapped dipole at GI8YJV:

That's a pretty good result, considering the size of the magloop.  Now let's look at the simultaneous signals beyond 5000km, i.e. DX distances, to see how well each was being received by the same stations, at the same time:

The magloop is 5dB weaker than the trapped dipole.  It's about 6dB worse on local signals, but that's not usually of interest, at least to me.  That is pretty much what we might expect, and translates into just a single 'S' point down on the trapped dipole, if considered in the increasingly-irrelevant tradition of SSB signals.  In digital operating, 5dB is going to see you get through to almost all the places that the trapped dipole gets to, as the map of receiving stations illustrates:

Interestingly, the loop is able to match the dipole at times, even at the extreme DX range, though not necessarily at the same time:

When compared to the large wire loop at G8LIK, the magloop exceeds its performance by a very wide margin, most likely explained almost entirely by the better low angle, vertical radiation from the magloop, as evidenced by the big differences towards the late afternoon and early evening:

I'd like to show you other comparisons, but it's not easy to do.  First, you get a lot of people who only operate WSPR for a few hours, or a day or so.  Then you get people who operate continuously, but are anti-social in not publishing anything at all about their station, or providing a means to contact them to ask!  MX0PHX, a club station using a low, OCFD dipole, is the only other reliable station I can choose.  Whilst only at 4m up, the antenna is at least in an open, rural site.  The magnetic loop compares very favourably with this full-sized wire dipole:

In terms of signal strength at DX distances (set at >5010km), the magloop actually comes in at over 2dB better than the OCFD:

Whilst the 'Meight' loop is taller than a single loop, it is quite rigid, even in 10mm copper pipe.  The second loop adds another point of attachment at the top.  This is how strong this all is in the 100km/h winds of ex-Hurricane Oscar this morning (03/11/18):

One thing you do need to do is make sure that the capacitor and all connections to/from it, including those to the motor, remain dry.  My loop showed a small but definite SWR drift over time, which I eventually realised was caused by water getting in between the capacitor vanes!  Remarkably, this didn't seem to make an enormous amount of difference to the loop performance, though the SWR was far too high.  Sealing the cap housing ensured dryness, and now the SWR drift when it rains is very slight, remaining below 1.4:1.  There is probably more that can be done to prevent stray capacitance in wet weather, but it's not something to worry about at the moment.

All things considered, the 'Meight' is clearly an efficient antenna whose size, convenience and sheer possibility of being deployed when larger antennas may be prohibited, means this is very much worth building, regardless of whether you have a large antenna farm, or a HOA flat.

You can of course buy a 'Meight', which does have the advantage of automatic and manual remote tuning.  But, at £504, that's an expensive tuning circuit!  It's also the case that the commercial version comes delivered to you in sections.  Each connection will involve some loss, especially as they are not soldered, but mechanical compression fittings. 

UPDATE: 19 days of WSPR

All compared to MX0PHX (OCFD at 4m height, in open, rural area).

(1) Simultaneous spots, all distances:

(2) Simultaneous spots, DX distances (>5010km):

(3) DX Graph:

(4) Spots map:

(5) Magloop compared to GI8YJV(top WSPR-performing trapped dipole):