[TowerTalk] Phasing Help

[Jim Lux]

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
>
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