Topband: Ph.Verticals

[Tom Rauch]

Hi Martin,
 
> 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
> lines....

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
w8ji@contesting.com


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