If you're wondering whether a magloop is worth having, I can say that it is - provided you don't spend too much money on a commerical unit.
 |
| Magloops. I've built a few... |
I say this because, if you can solder and build an antenna carefully, then there is nothing a commercial maker can provide that will be any better in terms of performance. Indeed, with homebrew, you can knock-up multi-loop arrays that will probably be better than a commercial unit, and use wider bores to maximise efficiency at lower frequencies.
The only advantage you get with a commercial magloop is the remote tuning mechanism which, of course, is quite a big advantage over manual tuning, especially if the antenna is outdoors! Many of you will be able to make stepper motor drives and so on, which is not that difficult, but was a challenge for me.
This week, I decided I would clear out my magloop constructions. All of them 'worked' in as much as they matched up nicely to 1:1 (and quite easily, using vacuum capacitors). But none of them make you feel they are worth the effort and money put in - unless your situation leaves you with no alternative.
The reality is that, whilst a magloop will indeed give a dipole a run for its money, and the likes of G3JKF's three loop arrays might even beat a dipole sometimes (the environment makes a huge difference), they are a complex and expensive alternative to a simple arrangement of an end-fed wire or dipole. At the moment, no worthwhile permanent installation magnetic loop is available commercially for less than about £500. The most expensive is over £1000. Neither will 'work' any better than some copper tube bought at your local DIY shop.
Once, when I started my ham career, a small antenna for a wildly windy location was very desirable and, in winter, almost essential. But, with a small tower to which I can attach a half sloper, this requirement has now passed; the simple wire covers 80-30m, no matter what the weather. In the absence of a tower, a strong timber pole or tree could have provided an alternative in the shape of an inverted-L (WSPR shows the performance of the two to be almost identical.) For almost all paths, the wire usually beats the magnetic loop quite easily. And I speak of a QTH which is undeveloped, open, and elevated. It has a copper mine for ground!
 | ||
| From Russia (via Ukraine), with love. |
If you are stuck up on the 35th floor of an apartment building, or have a postage stamp front yard, then a magloop is a very good compromise that gives you near-dipole performance across several bands, allows access, without too much scaling up, to the lower bands, and lets you work the world whilst avoiding planning controls - at least in the UK.
Here are my findings from the years of soldering:
(1) A three loop array, as used by G3JKF and replicated here, works very well and, when installed at about 2-3m high can match or beat a low-ish wire dipole (almost all 80m dipoles are low, relative to the wavelength.) Your environment is critical to how well - or not - your particular antenna will work.
(2) A Russian vacuum capacitor of about 7.5-350pF and a roughly 1.2m square loop will yield coverage from 60-20m, perhaps also taking in 17m. If you can only afford one capacitor, buy a 7.5 (or 10) - 1000pF, which should cater for all loop designs and ham bands. You need at least 2.5m square to cover 80m. Ukrainian sellers are usually very reliable and offer caps for about £100, sometimes a bit less.
(3) Efficiency is helped by wider bore tubing. It's uneconomical to go beyond 28mm, which is also about the largest commonly found tube in DIY stores, and where 90-degree elbow joints cost over £5 each! At 20m and above, 15mm pipe is much cheaper, and has high efficiency of about 95% or more.
(4) I've never noticed any meaningful difference, when connecting the loop to the capacitor, between using Flexweave wire, braided copper strap, or solid copper strap in the performance (using WSPR) of a magloop. My final arrangement was solid copper strap clamped with stainless hose clips to the vacuum capacitor. Some aluminium conductive grease (used to preserve Yagi joints), was added between the strap, clamp and cap, to ensure a corrosion-free connection.
(5) Air capacitors are fine, and need less torque to tune. But, except for spot frequency (e.g. WSPR) use, you will definitely need gearing to achieve fine tuning, even when using a stepper motor. A vacuum capacitor can be tuned accurately by hand and ear, and is much more stable than an air capacitor. Vacuums also, of course, take very much more power than an air spaced cap in most cases.
(6) Performance indoors can be surprisingly good. But watch the power, as the field intensity can reach high levels. Not much of a worry for many photon-hardened hams, but worth considering (and avoiding) when there are kids about.
(7) I quickly moved away from using Faraday primary loops. These are mechanically unstable in many homebrew cases. Much better stability and consistency is found by using a 4:1 balun, with one side of the output connected by a short, stout wire to the centre of the lower tube section (not the top - this is less efficient), and the other to a gamma wire or rod (a 10 sq. mm earth cable is good for this), connected to roughly 2/3 the way up one side. You can also ditch the balun, which may or may not extend your loop's tuning range slightly.
(8) For 'spot' frequency use, which is useful with a high-Q, narrow bandwidth antenna like a magloop, you can use an antenna analyser to accurately tune up outside or away from the rig. Spot frequency use is now much less of a problem with the JT modes, which you may never in fact need to stray from!
Here's to the coming of the next 'magloop moment'!
No comments:
Post a Comment