The XM240 reflector being insulated from the boom was a concern for me
also, not only for static discharge due to lightning, but also for
arcing, if someday I decide to shunt feed the tower. With the
reflector insulated, the separation for the reflector consists of
several 1/8 inch gaps around the U bolts, through the slots in the
insulators(which are now filled with spider webs). I guess this does
make a pretty good spark gap, but I still don't like it. Gaps like this
eventually end up shorted or intermittent due to the repetitive spark
vaporizing metal and depositing the metal on the insulator surface.
I also grounded the reflector to the boom with a 2 inch long aluminum
strap and clamp, but I am also concerned that I don't recreate a
problem, which the manufacturer went to some expense to correct. Since
they added parts, I'm sure there was a good reason for the change.
I cannot understand why they insulated the reflector. Simulations show
there is absolutely no difference in the pattern with the reflector
connected to the boom or insulated. It doesn't even change the pattern
if the reflector is connected not at its exact center. You indicated
they made an electrical improvement (improved pattern?). It sounds like
this may have been a cover-up answer. Since I can detect no difference
when modeling the antenna, I can only speculate as to the reason for the
change.
As for guessing: If in the predecessor 40-2CD, they used the same large
U channel support to tie the reflector onto the boom, and the reflector
was bolted to this U channel at several places, this would have caused
the reflector to resonate at a lower frequency due to the large diameter
of the U channel, compared to what it would be if it was insulated.
This would have affected the pattern (if they had not recognized this
and compensated for it). It is easier to calculate performance when both
the driven element and reflector are identical except for length, and
insulating the reflector would make them both identical. Another guess
would be intermittent contacts at the multiple U bolts, which could have
intermittently caused the same effect (also would cause noise if they
were intermittent). One good way to eliminate intermittent contacts
would be to eliminate the contacts.
These are just some guesses on my part. Just trying to avoid recreating
a problem. Anyone have any other speculation? Are there any other
antennas that have the reflector insulated? Anyone know of any reason
why it might be good to do this?
Jerry, K4SAV
reheinri@rockwellcollins.com wrote:
>Mike,
>
>I have experienced exactly the same thing with my XM-240 here in Iowa. I
>could hear the static jump during winds and passing storms. Here are a
>couple of other observations:
>
>1) I use an Ameritron Coax Switch and the feedline is grounded. What you
>are hearing is the reflector arcing to the boom across the small insulator
>rings in the U-bolts.
>
>2) I modified my XM-240 after having two of the loading elements blow up
>in near lightning hits. (And I mean blow up. I have aluminum spatter and
>the wire was tangled. The heat shrink tubing was blown away. And that was
>from a near hit. Happened twice in two consecutive summers.) Here's how I
>modified it. I grounded the reflector to the boom and I added two of the
>Wireman's "porcupine" static discharge devices on top of the tower where
>the phillystran relief cable attaches to the mast.
>
>A bit of history is in order. The XM-240 was both a structural and
>electrical improvement of the original 40-2CD. The electrical change was
>to insulate the reflector to improve the pattern. Apparently there was
>some imbalance and by insulating the reflector it cleaned up the pattern.
>But the capacity hats with their nice pointy ends form nice discharge paths
>for static build-up. So when a storm passes or there is precipitation
>static they fire form the ends of the capacity hat. I just put the
>porcupines up to help provide an alternate path for the discharge.
>
>Seems to work. Haven't had a problem in three years since the change.
>
>For reference my XM-240 is at 82 feet above a Cushcraft X-9 on top of a
>Trylon self supporting 72 foot tower.
>
>Hope this helps.
>
>Rick N0YY
>
>
>
>
>
>_______________________________________________
>
>See: http://www.mscomputer.com for "Self Supporting Towers", "Wireless
>Weather Stations", and lot's more. Call Toll Free, 1-800-333-9041 with any
>questions and ask for Sherman, W2FLA.
>
>_______________________________________________
>TowerTalk mailing list
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>http://lists.contesting.com/mailman/listinfo/towertalk
>
>
>
_______________________________________________
See: http://www.mscomputer.com for "Self Supporting Towers", "Wireless Weather
Stations", and lot's more. Call Toll Free, 1-800-333-9041 with any questions
and ask for Sherman, W2FLA.
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