To: <towertalk@contesting.com>
> 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
--
FAQ on WWW: http://www.contesting.com/towertalkfaq.html
Submissions: towertalk@contesting.com
Administrative requests: towertalk-REQUEST@contesting.com
Problems: owner-towertalk@contesting.com
Search: http://www.contesting.com/km9p/search
|