Re: [TowerTalk] 1/8 wave spaced 80m verticals

[Jim Lux <jimlux@earthlink.net>]

I think you really wanted to direct this to the original questioner, Colin, 
but, you raise some extremely important points..

At 01:07 PM 9/10/2003 -0400, Roger D Johnson wrote:
> Hi Jim....
>
>  As you can see by your replies, this is not a trivial exercise!
> Are you measuring front to back with a test oscillator on the
> ground or on received signals? This type of array, when fed with
> equal currents and one element at -135 phase angle, has a large
> high angle lobe to the rear. This can be very frustrating to
> someone who does not realize this as the F/B can be very good
> with test oscillator on the ground, while on air F/B can be much
> less. If you increase the phase angle, the high angle lobe shrinks
> and a low angle lobe appears. At about -147 degrees, they are the
> same amplitude and the low lobe grows until the array is bi-directional
> at -180 degrees.

This is why I'm not enamored of the current probe and model scheme for 
generating patterns... however, for lack of anything else (how many far 
field HF antenna ranges that duplicate your particular environment....), 
and, spending serious time with the models will give you a feel for the 
sensitivity of the adjustments (and bandwidths...)



>  Using short, close spaced antennas means the bandwidth will be
> very narrow. I think you will have to optomize the array for either
> CW or phone operation. Another ugly factor is ground loss. I have
> modelled several arrays such as yours and found the losses are so
> severe that a single element gives greater field strength. This is due
> to the mutual coupling lowering the impedance of one of the elements
> resulting in greater ground loss. I toyed with the idea of feeding
> the elements in phase for transmit and with the delay for receive.
> It made the system a bit more complicated but resulted in better
> transmit signal while still having the F/B on receive.


I think this is the general problem with closely spaced arrays of any kind 
(or, to be more accurate, physically small radiators... the radiation 
resistance is less, so losses become a bigger part of the picture)... Kraus 
talks about this at length in his textbook with analysis for W8JK arrays. 
It's been revisited in various ARRL articles as well.

The idea of using a different scheme for Tx and Rx has great merit, because 
the "figure of merit" is different... Tx you want to squirt as much power 
in the desired direction as possible, Rx you want to reduce the power from 
the wrong direction, but, losses aren't as big a deal..


>  I hate to rain on your parade but I've been there and done that. I
> think it's esential that you have a good modelling program to play
> with phase angles, ground loss, etc and some means of accurately
> measuring the various impedances in the system. Going at it "blind"
> will only result in frustration!

I'd never advocate going at it blind, but, on the other hand, I wouldn't 
spend all my time with the model... You need to model to the point where 
you understand the sensitivity to surroundings, and to understand when 
you've reached the limits of the model.  I keep waiting for someone to 
produce an accurate validated NEC model of a standard Southern California 
stucco covered house.  Just what sort of resistivity should I assume for 
the wires forming the walls, etc.




> GL and 73, Roger

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