Monday, 4 December 2017

Should you pay £850 for a magnetic loop?

Have you seen the MLA-T and similar series magnetic loops?  They offer a good, portable solution to multi-band operation, sitting nicely on a table whilst you work the world.

But the price! The MLA-T is currently (December 2017) selling for about £850!

OK, it comes with a stepper motor but, so far as I can see, the tuning is not automated beyond a couple of warning LEDs.  In other words, you only get a multi-turn loop, a capacitor and a stepper motor for manual tuning for your money.  Some say the manual tuning is difficult, others say it's quite simple.  Being very high-Q antennas, magloops always need a steady hand and tiny steps to tune manually (a vacuum capacitor usually makes it somewhat easier).

Now, the MLA-T gets good build quality reviews, and WIMO doesn't make any wild claim that this antenna is meant to match or beat full-sized antennas.

But let's consider, for a moment, the facts about all magloops, based on my own extensive and careful builds and testing.

First, any magloop is definitely a compromise antenna.  You can pick and choose what, exactly, is being compromised, but the usual criterion is: physical size.  Where more than 50% of the world's population live in cities, any smallish antenna that offers reasonable performance from indoors or a small outdoor space is clearly something of great value.

But it's doubtful anyone would choose a magloop as a first choice if they had a large field and no neighbour or local authority problems to deploy a large wire antenna.

Second, the materials used to make something of the size of a MLA-T antenna (not allowing for the remote tuning), are very cheap.

Let's say you build a sqaure loop version from 10m (4 x 80cm diameter loops, as in the MLA-T) of 22mm copper tubing.  This would cost, including all the 90-degree elbow fittings, about £60 from any number of outlets.  You could use an air-spaced capacitor, which you can pick up, with a bit of perseverance (they are widely sought-after), for about £10 from online auctions or a junk sale.  A plastic box (if you really need one) and some other sundry bits - maybe some wire for the feed and timber to make a stand, might cost you another £30.

Incidentally, according to Underhill  - and my practical experience - there is no difference in performance between shapes, provided the total perimeter is the same.

So, we're looking at no more than about £100 to build your own DIY version of an MLA-T, but without the remote tuning.  For those adept at modestly advanced electronics, the tuning is also easy and cheap to build.

Even if you buy a good vacuum capacitor, you are still only looking at about £200 total.  That's a long way - less than a quarter of the way, in fact, from £850.

Providing the DIY loop is made carefully, the same laws of physics will apply to both it and the MLA-T (or any other antenna).  It is unlikely, therefore, that there will be any real difference in performance between the two.

Leaving aside the debate about just how well you can get out with a magloop, the real question for anyone thinking of using such an antenna is: do I really need to splash out four times more money to get one than if I built one myself? 

1 comment:

  1. Anyone can buy mine for half the price. You can work with it on a table. It has a unique isolated reduction to control the varicap. It also has a advanced tuning indicator built inside so a SWR meter is not needed. And I add a high quality and shielded type coaxial cable to connect a radio to the antenna. I even send it free, no postage! Oh, before I forget. It is suitable for HAMradio and SWL ;-) 73, Bas