Monday, October 12, 2009

Dud Diamond

As regular readers will know, I recently bought a Yaesu VX-8E with GPS and APRS which I use so that my XYL and others can see where I am when I'm out with a radio. Because the rubber duck antenna is a bit long for convenient use when the radio is clipped to my belt or in a pocket, I thought I would try one of the very small stubby antennas. There is not much choice of alternative HT antennas from suppliers in the UK and the prices of those that are available are absurdly high, but I saw a Diamond SRH-805S being sold for the bargain price of £6.99 including postage from an eBay seller in Hong Kong, so I decided to order one.

I put the antenna on the VX-8E and went for a stroll into town half a mile away as the crow flies but with a lot of buildings in the way. With the radio in my top pocket running 5W my position beacons were received back in my shack quite well except for when I was inside a shop, so I was fairly pleased. However on Sunday I took the radio up the local SOTA summit Watch Hill and was unable to access a single repeater. A repeater that was end-stop on the rubber duck was about S3 on the stubby.

Back in the shack I investigated further, which resulted in a "shoot-out" between all the portable whip antennas I have. I used my K3 with transverter as the test receiver, as it has a direct dBm display of received signal strength. As best as I could measure, the SRH-805S is 13 - 15dB down on the VX-8R rubber duck. I expected it to be worse, but not that bad!

I then attached the antenna to my RigExpert AA-200 antenna analyzer and found that the SRH-805S was giving an apparently infinite SWR! After some tests to check that the antenna was actually making contact with the centre pin of the connector, I had the idea of increasing the sweep range and then saw that the antenna did have a sharp resonance, but it was centred at about 169MHz!

I wasn't able to check performance on 70cm as there is no 433MHz activity round here and my antenna analyzer only goes up to 200MHz. But I tried the SRH-805S on my FT-817 and on both 2m and 70cm the radio displays the warning HIGH SWR! So this stubby antenna is completely useless on either of the two amateur bands it is supposed to cover.

It's quite annoying as the antenna obviously is resonant, just on the wrong frequency, and on that frequency it might actually work quite well. But there isn't much I can do. It would have been better if I had bought this antenna in the UK because I could have sent it back to the retailer as "unfit for purpose." Instead I ended up with a £7 piece of junk.

At least my tests produced one useful result: I found that the rubber duck on the VX-8E works just as well as any of the longer SMA whip antennas I have including a 19 inch telescopic whip. So I can save myself the bother of carrying those around.

1 comment:

Barra said...

Can there be somting said about original Diamond 2/70 HT antennas and the Stub variant? Did you measure those?