Wednesday, July 23, 2008

6M Swiss Style?

With only a sloop I thought it about time to put up a reasonable antenna for the 6M band. About a year ago I bought a 6M HB9CV which I had left in the garage until I felt ready to tackle it.

This was a second hand antenna and I have no idea who originally manufactured it. Initial inspection revealed that all the parts were there so I put it together on the garden table to see how it looked.

Apart from an end cap for one of the elements it looked okay. I noticed the matching section is kept away from the boom and elements by a set of grommets. That’s quite a neat idea as keeping the matching section the correct and even distance from the elements and boom is one of the common issues with this antenna and the idea appeared to work okay.

The antenna is driven through a variable capacitor and I was concerned that the one supplied with the antenna was a bit wimpy. Short term it needed to handle 100 watts pep & 40Watts on RTTY.

But I’m considering running a couple of hundred watts RTTY in the future and could already see the cap disintegrating in a molten glob ruining the antenna. As I had some ptfe coax left over from the trap project I decided this would make a useful capacitor.

I adjusted the existing capacitor for minimum SWR and then measured it on the MFJ analyser. It came up at 36pf so I cut an appropriate length of coax and trimmed it to the same capacitance.

Its worth trimming off about 3mm of screen at the far end so ensure the home-brew capacitor won’t flash over (I imagine this capacitor will need to work at quite a high voltage when running a few hundred watts through it!).

The finished coax capacitor was curled up and fitted to the antenna feed point. When complete the SWR was about 1.4:1 which was about as low as I could get it in my back garden. I have an ATU in the transceiver and it was capable of tuning that out so I decided not to fiddle further. Also experience has told me that you can adjust all day only to find things change once the antenna is installed.

The antenna is mounted below a small 2m beam on the back of my dormer window. It clears the rooftop and I can turn it using the Armstrong method. The other advantage of this set up is that there are no ladders to climb. All I did was assemble the antenna reach out and fix it in position.

I usually leave it pointing south east towards Easter Europe but I only have to reach out and I can rotate it towards W-land or wherever the DX is coming from. I can see a big improvement compared to the sloop with 59+20 signals form Italy and Hungary. The SWR is below 1.4:1 between 50.0 – 50.3 mhz so the rigs internal tuner easily takes care of it, most importantly testing it with 100watt carrier it appears quite stable. I’m sure this antenna can easily handle full legal, I wonder if the neighbours can?


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