[Antennas] Re: Antennas digest, Vol 4 #137 - 9 msgs

2003年5月26日 19:51:16 -0400

Phil Florig, W9IXX, asked about a "tab antenna" described in
Electronic Desigh News magazine for the 915 MHz band.
>http://www.e-insite.net/ednmag/index.asp?layout=articlePrint&articleID
=CA296495
>> Thought maybe some of the experts could look at and see if
> can be used on the lower bands. I am just starting to look
> at modelling and this looks too complicated for a beginner.
> Maybe a great test for the more advanced. HI....

Different from the traditional microstrip patch antenna
designs often used at microwave frequencies (which are also
small), this antenna still uses dielectric loading (via the
printed circuit laminate) to shorten its physical length.
Building such an antenna is pretty trivial, but the modeling
is certainly not. The author of the article discusses how he
simulated the distributed capacitance and losses of the
laminate by using a number of discrete loads. This sort of
modeling is generally NOT for the beginner.
The antenna can be scaled for lower frequencies, but you will
run into difficulties with size and thickness if you try to
use this antenna design below the 440 MHz band. You will also
run into difficulties using more than a dozen watts or so of
power as the board traces are small and dielectric heating of
the laminate will become a problem. This antenna, as the
author noted, does have reduced efficiency. Its only real
advantage is a reduction in size over a conventional antenna,
and even here the reduction is not that much. I suspect the
author designed this antenna for use with low-power wireless
devices where size was more important than efficiency.
If you are looking at reduced-size high frequency antennas
that still have reasonable efficiencies, I suggest you dig up
the articles published by Jerry Sevick, W2FMI, in QST a few
years ago. Properly done, a reduced-size antenna can be quite
efficient - BUT at the expense of bandwidth.
For example, I built a three foot long CB antenna for a friend
back in the early 1970's. It far out-performed any shortened
commercial CB antenna on the market, and almost duplicated the
performance of a full-size whip. But it was rather narrow-
banded! It had a 1:1 VSWR on Channel 19, a 2:1 VSWR on either
channel immediately above or below, and was totally useless on
other channels. Being passed by another car slightly detuned
the antenna, and a big truck nearby could change the resonance
by at least 10 kHz. If this antenna were mande of lossier
materials, its bandwidth could have been much better. But the
perfomance would have suffered greatly.
 73, Dr. Barry L. Ornitz WA4VZQ [email protected]