Thursday, October 01, 2009

The G5RV jr on steroids?

A few years back I only had a very short garden (Even shorter than the one I have now!). I guess thats what comes of living in London and not being a millionare. I opted for a 6BTV as it didn't have a large footprint.

Of course one neighbour took exception to this and instead of talking to me went to the authorities to waste some time & money. I decided it wasn't worth the effort fighting my corner so I replaced it with a GRV junior. Stringing it from the roof I could just fit the short 5RV in the garden. It sloped, and the last five foot of feed line needed supporing but it was better than nothing. Actually on 40 M and 10 M it seemed to outperform the 6BTV.

Thus began a long love hate relationship with the G5RV jr. when I moved house I installed the G5RV between my dormer window (3rd floor) strund to the top of my 6BTV and then down to the garage. This worked reasonably well but I sooon found the high impedance it presented to my tuner and loss in feeder meant that things started to heat up running QRO. Also the plastic insulators became slightly conductive after a time.

I looked around for an alternative antenna and tried the short Carlina Windom. For all the hype it didn't perform any better in the same position, the SWR drifted all over the place on 15M. and was difficult to support. Over a number of months testing and upgrading I settled on a modified version of the short G5RV. These are the changes I made -
  • Replaced all insulators with glass/ceramic
  • use open wire feeder with spacers instead of the 450 Ohm ladder line
  • Increased the open wire feeder length to improve SWR on 15 and 40m (
  • replaced all the antenna wire with flexi-weave (I only did this as the original stuff wouldn't solder) The length is 7.844M each side
  • Installed an 8 turn high power 1:1 balun at the feedpoint.
  • Replaced the coax feedline with 10M of Andrews 450
  • Increased the centre support by 1.5 m (Part of changing the 6BTV into an 80m inverted L
The finished antenna works well on 40 and 10m, ok on 30, 20 and 15M and although its a high SWR tunes on 17 and 12M. The SWR doesn't drift on any of the bands whilst running 400 watts RTTY so whilst things may be warming up in the system or tuner its not enough to be a problem.

Here is a photo of the finished antenna -

And the balun -

The balun is 4 inches in diameter and constructed form RG393 which is a large as RG214 but has a double silver plated screen and conductor + PTFE dielectric so its a bit more stable.

I constructed the open wire feeder using spacers from Sandpiper Aerials. Very inexpensive but you have to thread them on individually which is a bit of a chore.

This is looking up the ladder line. One side of the antenna runs trough an insulator attached to the vertical.

Originally I modelled the antenna on MMANA which indicated that it would resonate at about 28.1Mhz. In actual fact it came up at about 27.78Mhz which may be due to the height? Either way I found shortening it slightly brought it into resonance on 28.1Mhz? Most importantly it doesn't look to be a horrible match on 20 or 40m so I assume the loss in the coax isn't to high. I did consider running a tuner at the balun, I may experiment with this in the future to see if it makes a big diggerence. Does anyone make reasonable 500 Watt auto tuner?


Blogger Steve Nichols said...

I think Mmana-Gal has come up with a higher frequency resonant point than reality due to plastic-coated wire. This invariably slows the EM wave down a little and I find I have to adjust real lengths down by about 5-7%.

Steve G0KYA

12:43 AM  

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