At 02:41 PM 5/10/2005, Keith Dutson wrote:
>I need some help with phasing two antennas in a stack. The antennas are
>Force12 C-3 and C-4XL. The C-4XL is essentially a C-3 with 2-el added for
>40M. Since element spacing and feed method (current balun) is the same for
>both on 20M, I think I only need to compensate for the difference in
>distance of the balun to center of tower between the two antennas. This
>distance is 15 inches for the C-3 and 98 inches for the C-4XL. All coax is
>Belden 9913F7 which has a velocity factor of 84%.
>
>My first thought was to just make the coax from the C-4XL 83 inches longer,
>but now I am not sure. Maybe I need to multiply this by 84%? Then last
>night I was reading the ARRL antenna book about phasing different antennas
>and got totally confused. Apparently I need to model the two antennas to
>find the difference in phase angle in degrees, then add the appropriate
>amount of coax to offset the difference. Needless to say, I do not have the
>models or experience to do this.
Exactly...
You've got two problems. The first is that the "phase center" of the
antenna is not necessarily where the feedpoint is. The second is that the
feedpoint impedance will affect the amount of phaseshift resulting from a
length of coax.
Phase center is hard to describe in words, but here goes. Imagine that the
antenna radiates a nice spherical wavefront (like ripples spreading out on
a pond). The amplitude of the wavefront will vary (if the antenna has any
directivity, and ALL physical antennas do), but the ripple should be
symmetrical. The phase center is where that wavefront appears to emerge
from (where you dropped the rock in the pond). In reality, most antennas
do NOT have a nice spherical wavefront, although things like dipoles have
fairly smooth phase patterns, and many gain antennas (horns, for instance)
have fairly smooth phase patterns in the main beam. Important to remember
though.. the phase center is an imaginary point, and does NOT correspond to
a physical location (except for a trivial antenna). Furthermore, the phase
center varies with frequency.
Identical antennas, though, will have identically placed phase centers, so
if you want antennas to be placed so that their phase centers are 10 meters
apart, you just have to put the antennas 10 meters apart.
(HFTA, in the ARRL antenna book, does NOT allow for phase center
displacement, even though it allows different antennas at different
heights, it assumes the phase centers are stacked on top of each other).
So, you need to model (or measure!) the phase pattern of your antennas. It
doesn't have to be a real fancy model, probably.
--
Then, there's the feedpoint impedance interaction with the transmission
line problem. There are a number of programs around that will help you
figure this part out. TLW (in the antenna book) does NOT calculate the
phase shift, although it does calculate the transformed impedance. The
Excel spreadsheet XLZIZL from AC6LA does do this calculation, as will my
phased array spreadsheet.
In theory, you also need to know what sort of mutual impedance there is
between the two antennas, which in turn will depend on the stacking
separation (and the antennas).
Then, you have to figure out what lengths of coax will get the right
currents into the two antennas.
All of this is non-trivial (which is why people stacking use identical
antennas, etc.... it makes life easier)
However, realistically, probably almost ANY combination of different
lengths for stacking will help, since what you really want to do is move
lobes and nulls around. There ARE some pathological ones, like exact
multiples of a wavelength.
--
The question comes down to are you trying to figure this out for planning
purposes (in which case you can run one set of assumptions), or, are you
stacking existing installed antennas (in which case you can actually
measure the mutual impedances, and the relative phase center displacement).
Measuring the phase center displacement works by putting a big resistive
pad in both lines (to swamp impedance effects), then using two different
length transmission lines to a matched power combiner. You then look at a
strong source that has a known angle of arrival to the antennas. (like a
ground wave?) 75 ohm coax and power combiners (as used in cable tv) can be
used to good avail here.
>I would appreciate any help in understanding how to properly phase these two
>antennas.
>
>73, Keith NM5G
>
>_______________________________________________
>
>See: http://www.mscomputer.com for "Self Supporting Towers", "Wireless
>Weather Stations", and lot's more. Call Toll Free, 1-800-333-9041 with
>any questions and ask for Sherman, W2FLA.
>
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_______________________________________________
See: http://www.mscomputer.com for "Self Supporting Towers", "Wireless Weather
Stations", and lot's more. Call Toll Free, 1-800-333-9041 with any questions
and ask for Sherman, W2FLA.
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