[TowerTalk] PHASED VERTICAL ARRAYS

[Jim Lux]

At 12:57 PM 4/6/2006, K6XN wrote:
>Team
>
>I have had excellent success on DXpeditions by simply spacing a pair of
>verticals a quarter wavelength apart and connecting them with a 3/4
>wavelength (at the frequency of interest) 50 ohm coax. Please note that a
>length of coax which produces only one electrical 1/4 wave length wont work
>because its *physical length* in the real world will be less than the 1/4
>wavelength real world physical length needed to actually connect to the two
>verticals. The shortest length of coax in the real world that will work for
>a pair of verticals spaced 1/4 wavelength apart and provide the needed 90
>degree phase shift is 3/4 wavelength. This wasn't obvious to me 40 years ago
>until I actually tried it  :-)

This will make a cardioid pattern, sort of.  You won't necessarily get a 
real sharp null, because the element currents won't be exactly phased 
right, but getting 10dB F/B wouldn't be all that tough, and there's no 
question it will be obviously directional.

The ARRL antenna book (page 8-23 in my edition) doesn't give data for that 
precise feed scheme, but for a variety of other combinations of two 
feedlines that are 90degrees different in length, and for 75 and 50 ohm 
cables, the phasing looks like it's around 100 or so degrees instead of 90, 
and the magnitudes are within 10% of identical.  A 10% magnitude error, 
assuming perfect phasing, would give you a null of 20dB (i.e. 
20log10(1-0.9).  Similar sorts of effects from 10-20 degree phasing errors.

The other thing is that if the takeoff angle is very far off horizontal 
(which might be the case with verticals and a close in skywave signal), the 
antennas are actually closer than 1/4 wavelength.  If the elevation angle 
is 30 degrees, the antennas are electrically 78 degrees apart, with respect 
to the incoming wavefront.

>In the most simple phased vertical set up wherein the two 1/4 wavelength
>spaced verticals are excited at one vertical and both verticals are
>connected together with a 3/4 wavelength coax they will produce a cardioid
>pattern in one direction along the line formed by the verticals. When the
>array is fed from the other vertical it will form a cardioid pattern in the
>opposite direction. If both verticals are fed with equal lengths of coax and
>the 3/4 wavelength phasing line between the two is disconnected then the
>array will exhibit a figure of eight pattern with the lobes orthogonal to
>the direction of the cardioid patterns. This simple setup worked great for
>me when I was chasing European DX from California on 80M years ago and I was
>primarily interested in one direction.
>
>There are better (but more complicated) ways to feed and to change phasing
>for a pair of driven phased verticals using relays, multiple phasing lines,
>tuned elements etc but the above technique is pretty simple and has worked
>well for me to get enhanced propagation in desired directions and to reduce
>interference from undesired directions.


I agree entirely.  It's a system design decision.  There's a lot of virtue 
in simple implementation, without worrying about getting that last dB of 
F/B.  I think it's  especially so in a portable style operation, where 
there's enough other uncertainties in the system that you won't see much 
improvement in performance unless you go to a LOT more sophistication. (for 
example, computer controlled LC networks, network analyzers, etc.)

Jim