A little 6m work.

Well, it's not much, but it works fine enough.  The wire length was 1.37m per side for a SWR of just under 1.2:1.  Not that there was much to work - only Ireland and the Isle of Man today, with one PA station heard in rapidly changing, brief Es.

A multi-element beam and mast is on the list...

This is the HFTA gain prediction for the simple dipole, together with the terrain profile to the east.  Note the deleterious effect of the higher ground between me and the sea on low angle radiation:

At 4m above very good ground, the pattern, which is almost entirely horizontal radiation (blue), as you'd expect, is plotted below. MMANA-GAL reaches much the same conclusion about peak gain as the HFTA model does.  More height is needed, but it's fine for casual working, not least because 6m propagation is often binary: it's either strong or dead!

Standing the test of time...

A quick review of antenna durability for the late edition post today.  I thought this would be useful, because there are plenty of antenna designs out there, but very few that report back some years later, to say what is left of them!

Yesterday, during rather cold weather, I decided to do some maintenance on my 6m 2-element quad.  This was unashamedly based on this web site, way back in around 2012.  Since then, it's been a superb antenna for me - so much so that I have always resisted the temptation to buy an all-aluminium quad to replace it.

I immediately knew that making the antenna out of timber would not last very long in our windy, wet environment.  So I made a spider support from aluminium 90 degree pieces, and (eventually) the arms from spare fibreglass elements from a 7m fishing pole.

The good news is that, if you make the antenna as I did, plus a few minor alterations, then this will last you many years without trouble.  Mine is going into its seventh year now, still with the original components.  The feedpoint has had to be redone a few times, but only because I used TV 75Ohm coax with an aluminium outer for the 1/4 wave stub; this reduced to dust in our salty air in no time.  It now has an all-copper, heavy-duty coax stub.

Originally, for a short while, I recycled a Pfeiffer, 15m quad experimental frame in PVC tube to hold out circular wire loops for the 6m, 2-ele quad.  Of course, it worked very well, with maximum enclosed area.  One day, I hope I can try the linear-loaded Pfeiffer quad at 15m again, because, as you can see, the saving in size is quite considerable:

Early days of my 6m quad.

The trouble with the PVC is that is has a high wind loading area.  Worse, the glued 'tee' pieces very quickly deteriorate under UV light, to the point that they fail in about two years or so outdoors.  Despite this, you see an awful lot of US-based antenna designs using PVC tube.  My advice: avoid it completely!  And for intolerant neighbourhoods (my neighbours know to leave me alone!), that bright white plastic is not very stealthy!

I then made the quad look like this:

Second iteration.

Since this photo was taken in early 2013, the only change I've made is moving the coax down from the boom (which then stupidly ran down the lower driven element spreader again) so that it simply feeds directly the same lower corner (horizontal polarisation) from the mast itself.  This reduced the interaction between coax and antenna considerably, and reduced the length of feedline by nearly 3m.  You can see the arrangement in the last image of this post.

Here's the quad spider detail.  Tape and cable-ties were OK for those nearly 7 years, provided one changes the cable ties every two years or so, but the top arms slip through eventually.

What you  need to do - and what I was doing yesterday, it cut notches with a rotary tool in the 'L'-shaped aluminium, so that the cable tie (or, preferably, stainless steel hose clamps), make better contact with the fibreglass arm.  Typically, the arms are slightly smaller than the depth of the 'L' bed, so there is only slight contact between them and the securing fastener if you don't make notches.

Also, it's a good idea, especially for the topmost bracket, to drill a hole and fix a stainless steel bolt, to physically prevent the upper spider arm from slipping down.  You can do this for all arms if you like, as the tension of the antenna wire does tend to pull all of them inwards.

Also cover the exhaust clamps in plenty of grease to make them last longer.  In an ideal world, I would suggest you use stainless clamps for a square aluminium boom, not a round one.  This avoids slippage and rotation as one could get with round tube, though it has not been a problem for me.

When you think this antenna has gone through nearly 7 years of very strong sunlight and even stronger winds, then, for the peanuts it cost to build, it really is a true winner!

Serviced and ready for another several years.

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.