Thu, 2 Nov 2000 11:33:02 -0500
> Experiments this year with two 1/4 wave verticals, 1/4 wave spaced and fed
> in phase or 180 deg out phase, failed because I was never able to get the
> same impedances (out by 25 ohm) in the two positions. Phasing was done by
> switching an electrical 180 deg line in or out the two 1/4 wave feed
I'm assuming the system correctly had the same impedances to
each element, but the different was different in the two cases. The
impedance's at the feedpoint will always vary because the patterns
are greatly different, the end-fire pattern is much more directive and
so will have a lower impedance.
Out-of-phase the feed impedance of each vertical is around 16
ohms (perfect ground).
In-phase it is about 55 ohms. Any reactance will also change and
you will have almost no gain. Combining two antennas spaced less
than 1/2 wl apart almost never results in useful directivity increase,
either for receiving or transmitting.
> Searching for the answer on what could cause this "little" problem, I came
> across an artical in the ARRL Antenna Compendium, Vol 2; "Steerable Arrays
> for the Low Band" by W5AH This design uses a hybrid power divider and an
> 2:1 unun to match the divider to 50 ohms. The divided outputs can be used
> to either feed in phase or with the help of delay lines the phase delay
> one wishes.
I don't have the Antenna Compendiums, so I can't comment on that
specific article. I can make two general comments:
First, a power divider is a waste of time. It generally does nothing
for the system. You can get the same results by simply parallelling
the feedlines. If the phase is not zero or 180 degrees and the
elements are current-fed, you do NOT want equal power to the
elements. You want equal currents which is NOT equal power. A
power divider, if the impedances presented to the combiner are not
equal, doesn't provide equal voltages or equal currents any more
than parallel the two lines would do.
Second, the impedances will still change at each element if you
change pattern types. That change will have to be compensated for
elsewhere in the system.
I'd read some of the articles on phasing by Roy Lewallen, W7EL.
Roy understands phasing and the effects of mutual coupling.
73, Tom W8JI
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