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From: myers@hpfcdj.HP.COM (Bob Myers)
Newsgroups: sci.electronics
Subject: Re: Antenna Publication
Message-ID: <17660024@hpfcdj.HP.COM>
Date: 3 Oct 89 17:58:38 GMT
References: <4918@cbnewsm.ATT.COM>
Organization: Hewlett Packard -- Fort Collins, CO
Lines: 61

>Is there a recommendation for a recent publication on antenna
>theory and use. My friend had built a transmitter for use in
>model rockets that he uses. It works fine but he would like to
>make sure he is getting the best possible range per ounce of 
>transmitter payload. Please email any responses. Thanks!

Well, the good ol' ARRL Antenna Handbook is still probably the best 
introduction to the subject for a beginner, without getting into the
fairly heavy-duty math of serious antenna theory/design.  However, I think
that I can provide some additional help in this particular case.

The problem your friend's facing is one of providing the strongest signal
AND an omnidirectional pattern, with an antenna which I assume must be kept
as small and as light-weight as possible.  (By the way, knowing the intended
frequency of operation would help a LOT.  For this application, you really
want to be in the VHF or UHF range.)

If an antenna is properly "matched" to the transmitter, such that we can
assume that all of the power makes it into the antenna, then the only way
to provide actual "gain" is to make the antenna directional; that is, to
squirt more of the signal in one direction, at the expense of having less
power radiated in others.  This is the principle behind "gain" antennas such
as the Yagi beam or the log-periodic (which actually is a compromise between
gain and a wide bandwidth).  Omnidirectional antennas MUST (to qualify for the
name!) radiate equally well in all directions, and there's only so much
signal available to begin with.  This sets a limit on the signal power
which can be radiated in a given direction.

BUT, you say, what about these "omnidirectional" antennas which claim "gain
of Xdb over a quarter-wave whip" (which is itself an omnidirectional antenna)?
Well, consider that the radiation pattern acutally extends in three
dimensions, not two.  If a perfect "isotropic", or "point" source were 
possible, it would radiated equally well in ANY direction - in other words,
a plot of equal-signal-strength points would be a sphere.  Real antennas -
such as vertical whips - do not do this, but instead have a radiation
pattern which is roughly toroidal.  The "flatter" the toroid, the greater
the signal strength in that *plane*, at the expense of signal strength in
other directions.  The usual means of getting the pattern "flatter" (besides
moving the antenna up away from the ground) is to make it *longer*, and thus
a 5/8 wave vertical antenna can be said to have "gain" (toward the horizon!)
over the shorter quarter-wave whip.

For a model-rocket based transmitter, though, we have the restriction of
having to keep the antenna short and light, plus we cannot count on any
particular orientation with respect to the ground-based receiver.  A small
whip antenna is probably the best that can be done here, and likely this will
take the form of a helically-wound whip (or "rubber duckie") for size and
ruggedness reasons.  The key, then, is to provide the best possible *receiving*
antenna; if you have someone to track to rocket, a dish antenna provides
tons of gain, but a narrow "beamwidth" - you've got to aim it pretty
well.  A good choice might be a small Yagi on a hand-held mount; check the
amateur radio handbooks for suitable designs for the VHF frequencies.  (And
again, exactly WHAT frequency did you have in mind?  Gentle reminder: are
you *licensed* to use the frequency in question?)

Good luck! 


Bob Myers  KC0EW   HP Graphics Tech. Div.|  Opinions expressed here are not
                   Ft. Collins, Colorado |  those of my employer or any other
myers%hpfcla@hplabs.hp.com               |  sentient life-form on this planet.