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Subject: Re: Radio Direction Finding
X-Newsreader: Microsoft Outlook Express 6.00.2800.1106
Date: Thu, 07 Nov 2002 04:58:51 GMT
NNTP-Posting-Date: Wed, 06 Nov 2002 23:58:51 EST
You can phase a couple of loops (or verticals or dipoles or double-extended
zepps for that matter) with a single phasing line and get a more-or-less
cardiod response with a pronounced null as long as the spacing is less than
half a wavelength. The calculations for the spacing and phasing are covered
in "Antennas" by Krause in the section on "Arrays of Point Sources".
Matching the phasing line so its phase shift is really what you want is
non-trivial. Some of the ARRL publications show examples (e.g., the ZL
special, for example) of phased arrays.
"Frank Raffaeli" wrote in message
> Keith Wootten wrote in message
> > Wireless Wizards,
> > Is there any way of making a unidirectional or cardioid antenna which
> > uses only the magnetic component of the EM wave? I know about
> It would help to understand why you have this constraint.
> > synthesising a cardioid pattern using a loop and a whip, but I don't
> > want the whip. It has to be much smaller than the wavelength, but
> > efficiency isn't much of an issue.
> How much smaller?
> > I can't think of a way. Maybe there isn't one?
> > Cheers
> Without more details on your application, it's hard to develop a good
> design assumption; however, I suggest you look at TDOA (time delay of
> arrival) and doppler methods. A TDOA system can work very well with
> antennas smaller than 1/4 electrical wavelength, at the expense of
> efficiency. A TDOA system can be implemented by switching two antennae
> / RF amplifiers at the reciever, and using the output of the receiver
> phase detector to adjust a servo that dynamically adjusts antenna
> position. There is no ambiguity but the drawback is that the antennae
> rotate via servo. For a stationary, but slightly more complicated
> solution, use the doppler.
> Frank W. Raffaeli
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