Wednesday, December 02, 2009

Unexplained behaviour

This morning I tested another VHF/UHF short stubby antenna, this time a genuine one from a Japanese manufacturer. Not a Diamond SRH805S but a Comet SMA501, which is currently being sold by Radioworld for £19.95.

The first thing I did was check it on my antenna analyzer, and I was satisfied with the SWR curve which you can see in the chart below. The antenna is pretty narrow band, as would be expected being very short, but it was tuned almost spot on 145.000 MHz, which is the centre of the 2m band over here. The Comet SMA501 is also specified for use on transmit on 70cm, but my analyzer doesn't cover that band so I couldn't check it so easily.

After that I put the antenna on the VX-8E. My crude test was to transmit on low power and watch the S meter on the FT-817 on the other side of the room. I had to disconnect the antenna and all cables from the 817 to get the meter to read below end-stop. In fact the reading was still end-stop using the standard Yaesu rubber duck. The SMA501 gave a reading corresponding to about S9. I then put the "dud" Diamond SRH805S on the VX-8E and you could knock me down with a feather! The signal strength reading was exactly the same!

For a comparison, I put my Elecraft DL-1 dummy load on the VX-8E. This is a dummy load that is not particularly well screened, comprising several resistors on a printed circuit board. The signal did not raise any bars on the FT-817ND S-meter at all. So both antennas are definitely radiating - the signal wasn't being picked up from the radio itself.

When I tested the SRH805S I noted that the SWR was near infinite, and the SWR curve showed the antenna was tuned for around 169MHz. The SMA501 is near perfectly adjusted. At 3:1 its SWR is a bit high but nothing a handheld transceiver shouldn't be able to cope with. I wasn't comfortable about the high SWR of the Diamond antenna and was sure that this was why it performed so poorly. In fact I suspected it of being a cheap Chinese fake.

Yet according to my FT-817's S meter both the fake Diamond and the real Comet radiate just as poorly as each other. Of the two I'd obviously be happier using the Comet which would be reflecting less power back into the VX-8E's PA. But I would have expected that to make the Comet more efficient and it doesn't seem to. I can't explain this.


Paul PC4T said...

Hello Julian, I don't expect effective radiation which such a small antennas. They look very small on photo. How long are they?
73 Paul

g4ilo said...

This Comet antenna is 45mm long. Of course, it will be less effective than the rubber duck or quarter wave telescopic but it should be better than it is. After all, we use electrically very short antennas on HF such as mobile whips for 80 or 160m which are proportionally as small as this 2m/70cm antenna is.

LY2SS said...

If weather permits I would try this test outside home.

Best 73 and thanks for good time reading your adventures.

Zilvis, LY2SS

Big Gun DXer said...

Small antennas such as these are very difficult to measure accurately. The radio/analyzer/whatever they are attached to is an important part of the antenna. It is the counterpoise. This may explain why your faux(?) Comet does not show a good VSWR when you measure it. What does it look like to the radio? Hard to say. The simple field strength test you performed indicates that it's not a dummy load anyway.