At 09:57 AM 6/6/2005, Steve Katz wrote:
>::The June 2005 issue of CQ Magazine reviews a new Comet HF vertical antenna
>that's only 20-30 dB down from a real antenna! That was quite an
>eye-opener. Evidently, there is such a thing as a multiband E-H. -WB2WIK/6
One doesn't want to really look at SWR range as an evaluation of antenna
performance.
Take ANY antenna, put a 7 dB attenuator on it, and the worst possible SWR
will be 1.5:1 (For example, a T pad with 19 ohms in each series arm, and
56 ohms in the shunt arm).
However, that antenna may not be very efficient.. whatever power is
intercepted by the antenna may just be reflected back by the mismatch at
the feedpoint.
Example.. Transceiver, 7 dB pad, resonant dipole antenna, with the
feedpoint shorted. The transmitter will never see worse than 1.5:1, but
the performance could be a LOT worse than 7dB down from the dipole's 2 dBi
normal gain. Transmit into this interesting system.. you won't get much
reflected power coming back (14 dB less, in fact), but, you also won't be
radiating much power either.
This interesting observation has been the basis of many "miracle multiband
antenna" designs. It's not quite as obvious as the classic "a dummy load
has a perfect match" argument. Start looking at all sorts of matching
networks at the base or in the middle of an antenna (especially using
parallel small wires or transmission line segments) and the losses can
start to accumulate, and you get to the equivalent of a 5 dB pad (max 2:1
VSWR) in the transmission line. It will still radiate and receive, and in
many, many, many applications, nobody would ever notice that 5,6,7 dB
degradation in receive level. Especially on HF links, the propagation is a
much bigger factor. Someone listening to signals that are fading up and
down 15 dB isn't in a position to accurately estimate average received
field strength. It might just appear as if the fades below the noise floor
just last a bit longer.
>I'd look at QST and other reviews of Gap antennas where they
>are compared to trap verticals.
>
>There is a HF Vertical Comparison sold by Champion radio
>that agrees with the reviews, and my own measurements found
>the Gap really very poor on lower bands compared to a trap
>vertical. Of course for receiving that does not matter, but
>then the Gap has major issues with common mode on the
>feedline and that's bad for receiving.
>
>Since this is just a spotting application, DX Engineering
>just released a high quality active receiving antenna. It
>has the advantage of being able to be used as a dipole or
>vertical, and uses a CB whip(s) for the element.
Active receiving antennas are very attractive, assuming you can deal with
the dynamic range issue on the amplifier.
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