Wednesday, 24 February 2010

The mystery of the upside down antenna....

Last summer I made up a half wave dipole out of some wire and strung it up vertically in the roof space of the house. The dipole was for 4 metres (70mhz).Some years back I had purchased a couple of second hand, ex PMR 70 mhz sets, since moving to this location I hadn’t used them and so I thought I would dig one out and put it back on the desk. Added to this there are a couple of stations here locally that use 4 metres FM and I thought it would be nice to chat on this band. All went well with my wire dipole, I was able to work my local friends and during a surprise bout of “lift” conditions I even managed to work S51DI in Slovenia, on 4 metres FM with my wire attic dipole.
Inspired by my success,( and the fact that the wire dipole was fragile and was always getting knocked around when I went into the loft to store or retrieve things!) I purchased a rigid aluminium dipole.
I adjusted for the correct part of the band, and it works as well as an indoor antenna can. However there is one strange thing….
I have to mount this dipole “upside down”- as I said it is vertically polarised, but the only way I can get it to match properly, with a decent SWR is to have the radiating element of the dipole at the bottom, not at the top as it should be.With the active element at the top there is a very high VSWR. Swinging the dipole into the horizontal plane wont work either it results in an SWR in excess of 5:1. Now this doesn’t really matter, the antenna works and I can use it for local contacts as I intended. It is however a puzzle as clearly something in the loft is affecting it. Incidentally I have a 6 metre dipole mounted horizontally in the same roofspace and that works fine!

The Answer is of course to get that dipole up outside. With my current setup the only way I could do this would be to mount it horizontally beneath my VHF/UHF vertical. Signals should of course be better if the antenna is outside but the drawback is that most FM stations on 4 metres are vertically polarised. Maybe what I gain with the antenna outside will be lost in the effects of cross polarisation?

1 comment:

  1. About of disadvantages of horizontal outdoor mounting - I'm not think so. I've used a horizontal polarised 2m beam antenna for many years and that antenna worked much better, than, say, same antenna, vertical polarised, but mounting indoor :-)
    Polarisation plays a great role in the direct vision zone, but after the first jump you can't find any noticeable differences. Mostly, of course.