Wednesday, December 28, 2011

50Mhz Halo Test

The final piece in the antenna jigsaw, putting up the 50mhz Halo.

When I sent for the antenna I wasn't sure just how big this antenna would look and also how it will perform. Assembling the halo was quite easy, but the instructions didn't mention how to tune this thing apart from the small variable capacitor on the connector block where the coax connects. The halo frame is adjustable and on the drawing it shows 4 inches as the extension length from the adjuster. The delta match is also variable in length, so after first assembling the unit I connected it to my FT817 to adjust for best vswr. After a lot of fiddling I got it down to 1:1. Now it was ready to fix up on the mast. The halo came with a good quality clamp and was easy to fix on the mast, but was very close to the satellite dish.

When I tested the halo on 50Mhz I was a bit disappointed. At first I couldn't hear the Buxton beacon, it is drowned by another carrier on top of it. Eventually, I managed to find it which although stronger than it is on the discone antenna, I would have thought it would come in stronger. When I transmitted on the halo the vswr went up very high, something is obviously wrong. I now need to get the ladders out again to maybe move the halo away from the satellite dish and re-tune the antenna.

I will reserve my judgement until later when I get chance to optomise the antenna and try it again.

Latest update - 28th December 2011

Today I took down the halo to make some adjustments and measuring. If you buy one of these from Moonraker (or anywhere else for that matter) the rather vague instructions that come with it are wrong! It shows a measurement for extending the square of the halo and shows it as 4 inches. The halo is tuned far too high in frequency and I found that I had to extend the square and delta match right to it's limits. Even at this, it tunes best at the top end of the band.
My halo is on a mast outside on the side of the house. I spent a long time at the top of a ladder fiddling with it and eventually giving up! After some thinking time it occured to me that a halo is of course just a 'bent' half-wave dipole, so I took a tape measure and measured the length excluding the insulated part. It turned out to be far too short for a half-wave at 50Mhz. I then adjusted the overall length to as near as I could get it. Even with everything set at it's extreme, it was still slightly short. After that I was able to tune the VSWR with the trimmer.

So now that I have the halo tuned I used the antenna during the 50Mhz contest to see how it performs now. VSWR is now down to 1.5 :1 and I must admit that every station I heard I managed to work even though conditions on the band were poor. It was a little anoying when other stations were using yagis and my halo is omni-directional. I would feel more in control to have some forward gain. I am also not happy with the position of the halo, it is screened in some directions because it is sandwiched between the houses. Would probably work better in the clear up on the chimney perhaps. I could hear the Buxton Beacon at s9, but I still think that it should come in louder than that. overall verdict on the 50Mhz Halo... It is adequate for general use on 50Mhz, but I would prefer a 2 ele HB9CV similar to my 70Mhz. I am limited for space though, so the Halo is a bit of a compomise.


Anonymous said...

My instructions say 3" giving overall length of 30". Never got it to work because of swr issues , will try extending to full length. Thanks for that G8TIS

Anonymous said...

Further update. Now tunes at the bottom of the band. All 4 adjustments at max length but then the bolt is too short. Replaced it with a longer length of M5 threaded rod from the local diy place. G8TIS