CAVEATS / DISCLAIMER :
There is a slight chance that this Doppler will not operate properly
with some particular type of receiver... most likely an "economy" receiver, with
a very wide ( or very asymetric ) passband, in the I/F strip. There is very
little mention of this in any other Doppler D/F literature, so it is probably
very rare... but I have seen it.
Many years ago, I built one of these gadgets and attached it to an old Bearcat
BC-210 scanner, with disappointing results... the display was hopelessly "locked"
at one bearing, which never changed. After some considerable investigation, I found
that the I/F passband was severely asymetric, with a very "steep" skirt on one side,
and a very "gradual" skirt on the other. This was ( apparently ) done to overcome
a design deficiency in the synthesizer... it was only capable of incrementing in
steps of 5 KHz. This was fine for VHF "low band" ( 30 - 50 MHz ) and "high band",
( 144 - 174 MHz ) but the UHF band ( 450 - 470 MHz ) typically uses channels that are
spaced at 12.5 KHz intervals. This means that SOME of the channels would ( if selected )
be 2.5 KHz "off channel" (in the I/F strip) at the input of the FM detector.
Re-designing the synthesizer for 2.5 KHz steps was probably not acceptable, because
it would double the "lock time" for the PLL, and ( therefore ) cut the channel "scan rate"
in half. It appears the problem was solved by deliberately "de-tuning" the I/F passband,
( on one side only ) so that signals which were 2.5 KHz "off channel" would still be
FM - detected properly. This worked fine for "real world" radio signals which were
limited ( at the transmitters ) to 5 KHz of peak FM deviation, but the extra receiver
bandwidth yielded a Doppler signal which deviated outside the linear detection range
of the FM detector. ( see the section called "PULSES" for an explanation )
This could have ( probably ) been solved by reducing the antenna radius to a smaller
value, which would reduce the peak Doppler shift ( hopefully ) to a value small enough
to be properly detected. At the time, I didn't know about this option, so I never
tried it...
Anyway, I wanted to mention it...
ANTENNA ISSUES
It is worthwhile to mention that the antenna switching unit is the weakest part of
this DF design... I have found that reciever sensitivity ( at 144 MHz ) is reduced
about 10 db, due to switching noise caused by some of the parts employed.
The original idea was to design a DF that used ordinary / common parts that could be easily
obtained, almost anywhere in the world... that precluded the use of PIN diodes, which are
( probably ) abundant only in the U.S., Europe, and parts of Asia. There are ways to
improve the performance of the antenna unit, involving the substitution of PIN diodes,
and other parts. See the section titled "ANTENNA" for a more complete explanation of
the problem and alternatives.
It was difficult for me to decide if I should mention this "flaw".... the original
DF design was intended to serve as an "introduction" project for people who wanted to become
involved with transmitter hunts. Within that context, this design serves that purpose well,
but the original DF has evolved ( by the addition of sophisticated options ) to the point
where many people might ( reasonably ) assume that this was a "first class" design, in
all respects. Eventually, the antenna switching circuit will be re-designed for better performance, across a
wide bandwidth. ( hopefully to 1.0 GHz ) For now, be aware that there is room for improvement.
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