Half Sloper Success!

A few years ago, I was pretty active on 40m, using a simple end-fed multiband wire (Hawaii Emergency Radio Club) up a 10m fishing pole.  It certainly worked, and brought in plenty of DX.

The problem for an extremely windy site like mine is that keeping a wire up at 10m is pretty difficult - and impossible during the severe winter gales that reach in excess of 70mph, sometimes a lot more.

One thing that does take the battering is my modest, ex-lighting tower that props up my 12m LFA Yagi.  So it seems to make sense to use it as much as possible.

Having become a little irritated that I have done practically no 40m work recently, I mused over a half sloper.

So, out came the 40m dipole, and up a ladder I went to fix it as a half sloper - one end sloping down to the bottom of one corner of our garden, and the other just dangling down to the ground as a counterpoise.

Analysis with the SARK-110 showed that this wasn't a disastrous set-up - the SWR was about 1:2.8 which, at 40m isn't a big deal in terms of coax losses - 10W out still yields about 9W out, even at that SWR.  Here's the screen grab of the 'dangling-down' sloper:

A pretty good impedance, and the SWR isn't too bad at 40m.

The ARRL texts say that the SWR does tend to be quite high with a half-sloper, so the SARK confirms this for my set-up.

But wait!  I tried to extend the counterpoise, of which there were a couple of metres spooled on the ground, to see if the SWR would change.  It did - but in the wrong direction; it now sat at 1:3!  This was also the case, rather oddly, when I connected to ground via a copper rod.

Here was a clue: the SWR dropped again as I brought the counterpoise closer to the tower, which suggested some inductive effect was at play.

Simple!  Just a 1/4 wire at the operating frequency, and a short direct connection to the tower.  Drawing by K4EFW. 

So, up the ladder again with a short piece of wire in hand, and off came the dangling-down half of the 'dipole', to be replaced with a direct connection to the tower metal itself.

One wire off to the left, down to a 1.5m fence post, and the coax braid connected to the tower itself.

Now, the electrical continuity of a sectional, extendable tower can be dubious, so I connected to the topmost section, which is bonded to my Yagi, giving a pretty large lump of metal to act as a top counterpoise.  I used a stainless Jubilee (hose) clamp and some conductive grease (Innovantennas) to ensure a good connection and protection from our corrosive coastal atmosphere.

The result was really surprising, and way beyond my expectations, as the SARK reveals:

Direct bonding to the tower = good job!

I still can't quite believe that just bonding to the tower brings such a great match!  It could be improved upon, but for the moment, there is no real need to do so - it is performing far better than the ARRL texts say should be expected for this arrangement already!

So, with darkness falling, I set the antenna to WSPR.  The earlier, two-wire arrangement was pretty good, in line with most other 40m WSPRers.

But, with the tower-bonding, I was up there as the best, or with the best 40m WSPRers anywhere in the UK or beyond.  If this sounds like wishful thinking, here's this morning's plot of VK5MR's reception over the three hours prior to 08:00UT, taking in the all-important grey-line and long-path period:

You can't corrupt WSPR - the results speak for themselves!

Now, there are an awful lot of WSPR transmitters on 40m, and to be the only one over this important period to be heard in VK is nothing short of remarkable.  Further WSPR tests later on have confirmed the outstanding performance, and I'm very happy with it. 

A quick test with my simple RF meter showed that the field was very strong in the vertical plane, much better than the modest field measured in a diagonal plane with the earlier, two-wire arrangement.

So, accepting that the elevated position and outstanding ground conditions are vital factors, it does show that the half-sloper can be a world-class antenna!

One might wonder how this can be.  Curious, I modelled my terrain out towards North America, using a horizontal dipole as the reference, because HFTA doesn't handle verticals or slopers.

Looking at my results compared to an operator with a horizontal dipole just half a mile - but down hill from me - shows some interesting features.  First, let's look at a dipole at 6m, at 7MHz at my QTH:

Horizontal dipole at 6m (blue), terrain out to USA/NA, compared with a 10m dipole down the hill from me (red)

That already looks good, especially at low angles, and is streets ahead of the dipole at the bottom of the hill.

Now look what happens when the dipole is lowered to just 4m at my QTH:

Modelled dipole at 4m (blue), again compared to the dipole down the hill (red).

Lowering the dipole to 4m, which we can loosely accept for the sake of argument as an analogue to the average height of my sloper (not taking the vertical radiation into account), you can see the gain increases by well over 2dBi, yielding a total gain of nearly 7dBi - which occurs at a spectacularly low 1.5 degrees above the horizon.

This, together with the strong vertical component, explains why this antenna, simple and low though it is, is in fact ideally mounted for my terrain. 

Oh, and measurements on soil up here has recently showed a dielectric constant at low frequency to be - wait for it - 3500 within the copper mine proper, dropping to 885 at the QTH.  That is, also, pretty amazing.  A future blog post will focus on this soil work.