Antennas Using Metal Roofs and Fences
In the Adelaide area and in many parts of South and Central Australia corrugated metal roofs and fences are very evident. What effects these have on small antennas was one of the questions after my small antennas talk to AHARS on 4th Feb 2008. How to achieve good performance on 160m and 80m in a small town lot was another question.
My first reaction was that iron and steel in the antenna environment can be seriously bad news for small antennas. But this is only true close to a mag-loop where the magnetic fields are strong. It is only the magnetic fields that couple into the high permeability and high loss of iron and steel.
So now after some cogitation I have had second thoughts. I would like to suggest that the two arrangements shown in figures 1 and 2 could well be worth a try. The basis for the proposals is that the ground conductivity is poor, as for a dry sandy soil. Then the fence can be connected up as a horizontal loop or used as a counterpoise for RF fed to the metal roof.
The connections are shown being made to a tuned loop as in the mini-midi loop arrangement that I now use on 80m and 160m most of the time. (The loop feed is not shown) But there are of course other matching circuits that do not radiate as the loop does. To use these, the feed-point impedance has to be measured first so that the matching network may be designed.
As a start the arrangement as shown is very easy to set up by trial and error. My preferred method is to tap the feeds on to the loop at points so that the loop Q is at least halved and the loop bandwidth is at least doubled. The SWR will rise to over 2:1. The loop twisted loop-gamma match can then be re-adjusted if desired. An easier alternate is to use a standard ATU to reduce the SWR from 2:1 to 1:1
Figure 1 Connection for vertically-polarised DX operation. Fence is a counterpoise for the roof considered as a patch antenna.
Figure 2 Connection for horizontally polarised local NVIS and medium range operation. The fence is connected as the ‘midi’ loop. Poor soil is an advantage – actually a necessity!
These arrangements should be suitable for 1.8 to about 10 MHz. For 14 MHz and above I would guess that an 80 cm loop mounted about a loop diameter above the metal roof will give reasonable to good DX performance. A rotatable loop is an advantage usually.
I would be very interested in hearing whether either of these arrangements turns out to be successful. In the UK we rarely if ever use corrugated metal for our houses or fences. Although some farm buildings do. So I will not be able to try it myself. Anyway UK soil is usually wet!
Mike Underhill – G3LHZ
25-02-2008