As Joel says it is indeed exciting to go into another
season on 160 meters.
In regard to portable antennas I must comment that
one soon tires of laying out radials for temporary
antennas. I have been using portable antennas for 6
years now, and that is why I have designed my
receiving antennas to work with high impedance
amplifiers. They require no radials. As a matter of
measured fact, radials can adversely affect their
performance.
Speaking of performance, there are a couple things
about the 8 element circle arrays that I would like to point out.
It takes a simple Eznec model to find out that the tuning
of the loaded elements in a passive array is much
more important to performance than most realize. A 10%
change in reactance in the tuning will shift the element output
voltage phase by more than 13 degrees.
Ten percent load reactance change is barely noticeable
when looking at SWR. It will shift the output amplitude by
more than 20%. These large values of change unaccounted
for will certainly change the performance of an array. The
high impedance amplifiers can easily maintain phase
stability within 2 degrees and 2% amplitude. These numbers
have been verified measuring actual over the air signals
using precision equipment.
To make matters worse, the combining scheme used by
most 8 element array makers simply parallel coaxial cables.
Coaxial cable phase delays will not be correct when both ends
of the cable are misterminated. This causes additional phase
shift errors in a system. (Caveat, yes at 90 and 180 degree
multiples the phase will be correct). I have spent years of
research on large arrays and abandoned the passive
approach a long time ago. For me it was just too unstable.
I now use 2 ea. active 8 element portable arrays among others.
One interesting thing about these arrays is you can make
concentric 80 meter and160 meter antennas. With the
essentially ungrounded short elements, there is virtually no
damaging mutual coupling.
There is no doubt in my mind that the 8 element circular array
may be the best bang for the buck in top-band receiving. The low
angle response is stellar. The 8 element arrays have quite an
advantage in terms of Cost Of Ownership over other large receiving
antenna arrays. This is especially true when it comes to real estate.
For the people contemplating constructing one, I encourage you
to look at everything you can find. When the RDF and directivity
of an antenna exceeds about 10 dB one must verify that the
signal processing circuitry actually combines signals properly. It
is ALL about correct phase and amplitude. One of the tools I use
is to inject a fixed signal into each antenna of a system. I measure
the phase shift and amplitude levels through the entire system.
I then plug these response numbers back into Eznec as a Source
and then run the plots. One easily spots errors in a system this
way. Of course the test equipment must be suitable for resolving
1 degree phase and 1 percent amplitude. All of this precludes any
irregularities in the radials affecting the signals. I have done some
work with measuring received signal phasing that indicates radials
have an affect on signal phase as well. More to come on this later.
I have yet to see actual numbers for one of these arrays based on
passive elements. It would be interesting to know if the array could
be improved. The high impedance amp version here works quite
well.
Lee K7TJR OR
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