[TowerTalk] Phasing Multiband Verticals

[Tom Rauch]

> Date:          Sun, 19 Apr 1998 13:22:14 -0400
> From:          "Scandurra,Anthony" <AnthonyScandurra@FirstUSA.com>

> Does anyone have any experience on phasing multiband verticals such as
> the Cushcraft R7000, R6000, or HyGain DX-77 ?

It is possible to design a single phasing system that functions 
correctly over octave bandwidths, but there are problems.

Here's how you do it...

You simply invert the phase of one feedpoint (one way is with a broad 
band phase inverting transformer at one feedpoint, another is by 
flipping one element upside down--sorta like in a log periodic). 

Then you use equal length cables back to the phase delay line, which 
is fed at one end or the other, and is equal to the electrical 
spacing of the antennas, and has the same impedance as the antenna 
opposite the fed- end presents

This gives you a unidirectional pattern in the direction of the 
element nearest the "fed end" of the phasing line, and the delay line 
will be exactly the correct length on ANY frequency where the 
antennas are 1/4 wl apart or closer! The same delay line will be 
exactly correct for 100 KHz, 1 MHz, 10 MHz, or 30 MHz if the antenna 
spacing is less than 1/4 wl at 30 MHz!!

The big rub is the delay in the line will not be correct unless 
the line sees it's characteristic impedance. Now that's the major 
design problem. 

(I use just such a system on all my phased arrays..even my four 
square for 160 meters. The delay line is correct on any band, or over 
the whole band, without changing delay lines. My phased receiving 
loops cover 100 KHz to 5 MHz with no change in the basic directional 
pattern, or adjustments of any kind.)

Another smaller rub is the antennas, if identical, require equal 
CURRENTS (equal power is absolutely not correct) for maximum F/B 
ratio. One way to provide equal current is to feed the elements with 
1/4 wl long lines (or odd multiples) that have equal VOLTAGE applied 
at the source end.   

The final rub is any shortened  "no ground" vertical is somewhat 
lossy (inefficient) compared to what the same length and loss 
antenna would provide over a good counterpoise system (I've measured 
one very popular "no radial" vertical to be about 14% efficient on 7 
MHz) .

If the spacing is 1/4 wl the array efficiency will equal that of one 
vertical, but if spacing is closer than 1/4 wl efficiency will be 
lower than that of the element alone (that's why close spaced 
elements usually have less gain than 1/8- 1/4 wl spaced antennas in 
the real world, even though lossless or very low loss elements would 
have MORE gain as spacing decreases). 

What this hundred word answer means is yes, it is possible to design 
and construct a multi-band reversible phased array. It could even be 
a single antenna that could be used on multiple bands at the 
same time! 

It would, however, be a true "design project". The off the shelf 
antennas you mentioned would probably not be good choices for high 
performance.
73, Tom W8JI
w8ji.tom@MCIONE.com

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