Wednesday, 22 November 2017

Triple Magnetic Loop Array vs. Low Half Sloper - WSPR Test

Ken Franklin, G3JKF, is a good man.  Good, because he is an experienced, skilled and and diligent person who has developed his triple magnetic loop array carefully, over many years. 

It is almost a fashion amongst many operators to dismiss magnetic loops as 'useless dummy loads'.  Whilst it may be a fashion, it is absolutely and certainly wrong. 

Firstly, regular readers may recall my tests in early 2017, of a single loop against many full-sized wire antennas across the UK.  The outcome blew any notion that a magnetic loop is necessarily worse than a full wire antenna out of the water.

Over the past few days, I was again in contact with Ken by e-mail.  We decided to run a 1W WSPR test of his triple loop array, which would fit in just about any UK garden, against my very low 80m sloper, which is fed against a 6m-high tower with a 3-element LFA Yagi on top.  The tower is relatively low by most operator's standards, because it is already on top of a 15m-high ridge, 100m above sea level.  This low additional tower height, for my Yagi, permits high levels of extremely low-angle gain afforded by the terrain to be exploited.  But that is another story!

Remember that I have already said my sloper is very low.  It was only put up to get some signal on 80m because it was convenient, and the band is not really of much interest to me.  If you have a 30m-high tower, then a half sloper against that is likely to be a very much better performer.  So this is not a 'slopers are useless' sort of article.

Well, the results speak for themselves!  A small loop array (details available freely here, and images here (all protected by copyright)), beats the absolute socks off my wire sloper. 

Whilst my wire gets reasonably good comparative distances, its signal strength is poor - down by a whopping 14.6dB overall - as this distribution plot of simultaneously-received spots across all distances shows:

The only saving grace for my poor sloper comes from considering relative received signal strengths beyond about 1200km, where the difference falls to a somewhat less embarrassing 7.4dB in favour of the loop:

Here's how the map of spots works out: 

Clearly, my antenna was failing entirely to reach the US, whilst the loop was managing to make it across the pond.  A couple of spots, such as from EA8 and UA3, showed a 2 and 4dB difference, respectively, but again in favour of the loop.  The consolation offered by them is slight, to say the least!

Even with lots of additional radials and careful attention to my sloper, I doubt I could get much more than a couple of extra dB out of it.  The height is simply too low.

The real message is this: loops work.  Loop arrays work even better.  Loops are very small in relation to operating wavelength.  They are far less prone to high winds, notwithstanding Ken's occasional experiences with broken timbers, as we all have from time to time!  Loops will fit in most UK gardens, which are typically very small.  Even better, small loops are unlikely to feature prominently in the list of priorities or concerns for planning officers and, in most cases, likely to avoid any planning problems at all.

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