6m, 50MHz 'Magic Band' Livens Up!

I noted a few days ago that 6m sporadic E was livening up, and now it's a near-daily occurrence (mid May).

Regular readers may recall I tried to build a convoluted loop for 15m some time ago, which initially was a disappointment but on re-examination, worked just fine.  It wasn't an antenna that I was using much, though, so recycling time was around again!

So I had the support structure made of the old 15m beam PVC lying around (as you can see from the algae in the photo!)  I decided these were ideal to run a full wave loop for the 6m band, approximating a circle and so having a greater capture area than any other shape.  I threw together a reflector to make a 2-element beam yielding perhaps as much as 7dBi gain.  The spreader elements are hooked up to the boom using cable ties, as it's a purely seasonal antenna, for me at least, so can be pulled apart very easily later.  I melted holes using 3/4" copper tube in a blowtorch, and then glued PVC support cross arms to which the main elements are attached.

Runs beautifully at 1:1.2 SWR.  All-PVC construction, including the 36"-long boom.  This was spare material; I recommend you use fibreglass spreaders for your antenna, as PVC deteriorates in sunlight.

I later found the PVC, having lain outdoors for nearly two years, had become brittle and prone to breakage; the tee and elbow bends are particularly badly affected.  I've since remade the antenna with an aluminium tube boom and metal brackets (exhaust clamp and 90 degree aluminium type) supporting light fibreglass spreaders - the bottom sections of 3m poles are ideally sized and of the right strength for this quad.

That's much better!  Fibreglass spreaders and exhaust clamp/angled aluminium supports on an aluminium boom.  It catches very little wind and so is extremely robust.

I referred to this fine site, and by keeping to the measurements for the two element version (plus the 'extra ' bits for trimming) indicated there, I found I had an SWR of 1:1.2 without any cutting or adding at all.  How often does that happen?  Connection is via a quarter-wave stub (allow for the velocity factor, assumed to be 0.66 here) and then to normal 50-ohm coax; again, the stub measurement is correct on the site link above.  A ferrite or air-wound balun is needed (air wound is fine in most cases).

The image below is detail of the bracket and aluminium spreader supports.  It's easy to do, but positioning the bracket is a bit of a brain scrambler in that you have to incorporate an offset due to the fact the 90 degree aluminium passes on the outside of the boom, not though it.  You can only realise and deal with this fully in the flesh.  When I was done, I drilled a single hole to join the two supports at the centre with a single stainless bolt and nylock nut, making the whole thing more stable.  You can use good quality cable ties or jubilee (hose) clips to secure the fibreglass arms; I prefer cable ties for their ease and lack of rust!

UPDATE ON PERFORMANCE.

In the few days since making this beam, I've worked all over Europe and into Iceland on 6m sporadic E using no more than 100W SSB and some 25W PSK-31.  I am very happy indeed with the antenna, not least because it cost me virtually nothing!  Please don't make your antenna out of PVC because it, and especially the connecting bits, becomes surprisingly brittle after a while outdoors.

A year on, and having lain on the ground over the wettest winter on record, I've just reinstalled the beam for the 2014 Es season.  I replaced the junk RG58U with RG213 (don't forget enough coax to make 4-5 turns for a choke balun!), and connected-up.  Result?  The SWR had gone to 1:1.5, but that was only because I had a wire 6m beam nearby.  Pulling that and a 2m antenna down restored a 1:1 SWR and it's currently WSPRing as I write. 

Here's the SWR and impedance plot as taken by a SARK-110 analyser 3 years later.

The stub is made of about 40 inches of RG6 75Ohm 0.66 velocity factor satellite coax; you can cut it to the right length using analyser - the impedance of an open-ended piece of coax drops to zero at the 1/4 wave length.

Perfect matching across the whole 6m band.

Here's the quad full-face into 50mph gusts.  Rides them with ease:

6m Quad in 50mph Gusts from John Rowlands on Vimeo.