Dick Green WC1M wrote:
> Hi. I'm considering buying a used Cal-Av 2D-40 2-element 40m beam and
> would
> like to hear some opinions on performance and installation.
>
I'm not an owner, but I have done a lot of modeling and building of 2
element driven arrays like this one. The bandwidth and front to back
ratio are considerably better than parasitic arrays. There is
more involved in tuning however. A parasitic array will always
work--at some frequency-- and it is a simple matter to retune
the working frequency to where you want it by adjusting the parasitic
element. The VSWR can then be tweaked by adjusting the driven
element. Maybe fool with the gamma match a little. Completely idiot proof.
A driven array requires that both elements be tuned to the correct
frequency. It is may also be necessary to have a "coupling" adjustment,
depending on the design. Additionally, there could be a "matching"
adjustment. If there are enough degrees of freedom,
you should always be able to get the beam tuned
up. However, you need the right algorithm. What I do is find
the frequency where the F/B ratio is best, then optimize F/B ratio
at that frequency. Then move the frequency in small steps to
where I want it. Then touch up matching. I suggest you read the manual
and evaluate whether you can follow their directions. They may have their
own algorithm. There are lot of things to go wrong in the design
and tuning of one of these arrays, which is probably why they
are not common.
I had an homebrew array that lost a tip from an element, but I didn't
know it (it was mechanically there, but not electrically connected).
I tuned up the array anyway using my usual algorithm, but it had
little or no gain. Later, I discoved by close visual inspection
what had happened. This is the type of thing you could get into
with a used (or even new) antenna. You might want to tune it up
while pointed vertically so you can be sure to easily get access
to everything.
Rick N6RK
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