[Antennas] pseudo-doppler DF antenna(s)

[refmon]

Hi,

This is a receive-only inquiry regarding Watkins-Johnson-type pseudo-Doppler
direction finding antennas.  I am having some difficulty locating one of the
several 8971A compatible antennas, although I might be getting close.  In
the meanwhile, I am building what I hope will be a useable antenna set from
an assortment of Home Depot and Leows parts.  The general geometry of the
assemblies is understood...that is, dipoles must be at 90 degrees, etc.

These antennas typically consist of a lower frequency band and an upper
frequency band antenna subsystem of varying scale (part of the prime
question) mounted small-over-large on a central vertical mast.  These each
subassembly consists of qty 4 vertically oriented dipoles mounted at exactly
90 degrees around the central mast.  These are similar to the marine DF
units seen on many yachts.  Consider for the purpose of the primary
question, the larger, low frequency segment.

The WJ catalog lists 4 antenna sets in great detail, including size, weight,
array diameter, length of each vertical dipole, frequency ranges of hi & low
subassemblies...enough info to build a non-lab grade workable antenna.

Each subassembly feeds a 4-way pin diode selector switch unit driven by the
WJ-8971A 4-count ring counter...when running, this simulates a high speed
spin on the antenna...the 8971A calculates the arrival phase of a signal at
each of the four dipoles and displays the resultant heading.  The hi/low
frequency selection signal is issued by the receiver.

Question 1:  In the experimenting sense, within reason, if one increases the
diameter of the array, will this increase the Doppler effect?  Is there a
rule-of-thumb relationship between subassembly frequency range and array
diameter?  Back-figuring the catalog data does not reveal a general trend,
but I do note that in all cases, high frequency segments are smaller in
diameter than low frequency segments...I expect the Doppler effect is
optimal only within some range of wavelength percentage that I have yet to
realize.  Also, on the high frequency segment, which can be fairly small
diameter, thus close to the central mast, is there a minimum mast-to-dipole
clearance?

Question 2:  The WJ unit is apparently designed for relatively limited
deployment in the field or in-close fixed installations...the standard
antenna control cable (not coax...this carries the Doppler switch drive) is
25 or 50 feet.  Considering most installations, this does not get the
antenna very high.  We're going to digital here, so feel free to send me to
the "pin diode switch driver list" if needed.  If I want my antenna at, say
50 feet, I'll need a cable at least 80 feet, given the constraints at my
shack.  I expect that will be beyond the drive range of the TTL outputs for
the drive pulses.  Is it feasible to A) add a very low impedance drive stage
at the shack end (keep TTL levels, but buffer the pulse drivers) and B)  add
a recovery pulse shaper up at the antenna (run DC, etc to support the
circuitry).  I know the basic exercise is feasible, but I'd expect a couple
of potential anomalies...the duty cycle of my recovered pulses may be
incorrect due to varying TTL trigger on the degraded pulses arriving at the
antenna.  Might I be better off to shift levels to, perhaps 12 volts low Z,
then readjust at the antenna to drive the pin diode switch?

I realize that some pretty simple units can be built, but I expect the
WJ-8971A is fairly sophisticated and may be rather demanding on these
points...anyone out there with experience or views?

with thanks in advance

John Collins
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