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Friday, 4 May 2012

Why is this just so insane?

Well, in a recent post I mentioned that the 2m/70cm beam antenna here had decided that once horizontal it didn't want to work so well. It was OK when I used it for satellite operation at 28 degrees, but when we put it back to horizontal the SWR went through the roof. I managed to take the head of one of the grub screws that allow you to adjust the gamma match - so that wasn't too handy either. Anyhow, as an antenna for conventional SSB working it's not going to be much cop, as it's a real compromise for both bands.... so......

I bought a 7 element ZL special from moonraker for 144MHz:

and it duly arrived a few days later.

Now I am no real antenna expert; my understanding of antennas is that you attach a piece of wire to the inside of a length of coax, place the wire on something nearby that's handy - maybe a hedge or something - then you work the world on HF.... not so on VHF!

There were two confusing aspects to this antenna for me; firstly there are two driven elements (which are actually folded dipoles) and the directors (elements) are not ordered smallest to biggest from the front of the antenna to the dipoles. Hmmmm.....

The two dipoles are fed at 180 degrees out of phase with each other, here you can see the main feedpoint which contains the coax from the radio, a piece of 300 ohm ribbon cable which feeds the other folded dipole and a tuning "stub" - this is in effect an open length of RG58:

These two folded dipoles are sightly different sizes, presumably this offers a broader bandwidth to the antenna tuning point.

Having struggled the pole back down, me and the ever patient Vince G0ORC, we had the new antenna mounted ready to go back up in the air:

You can see the 6m beam and also the new 2m ZL special:

We tuned the antenna on the ground; you chop small bits of the end of the open "stub" for best SWR. It seems to be quite straightforward, once up in the air the SWR was perfect in the SSB portion of the 2m band:

So, the antenna is back in the air, the SWR is near perfect and we could hear PI7CIS all the way from JO22, the directivity of the antenna seems quite sharp, so something is working OK!

Fun, egh?

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