I am not familiar with the Optibeam but had a similar problem with my
antenna. To make a long story short, one of the loading coils (open air
winding) was positioned very close to a bolt used to hold the driven element
to the center insulator. The coil would intermittently short out against
the bolt causing resonance to rise on one side of the element. To correct
this, the coil was repositioned (bent) to provide a bit more space from the
bolt.
73, Keith NM5G
-----Original Message-----
From: towertalk-bounces@contesting.com
[mailto:towertalk-bounces@contesting.com] On Behalf Of Gary.Stone@med.va.gov
Sent: Wednesday, December 14, 2005 8:15 AM
To: towertalk@contesting.com
Subject: [TowerTalk] Beam toubleshooting problem - help please
Hi to all,
I have a problem with a yagi and I am seeking your assistance and opinions.
I have a couple of people coming out to the
station and I hope to gain some views here to share with them. First, the
basic problem. On 40 meters
the SWR will sometimes jump to about 3 to 1 from flat. I can make a few
contacts and this does not happen but it
will eventually. A quick turn of the antenna in any direction drops it back
to flat. I have made as many as two or three
10 minute QSOs before the problem hits again. Now, my setup:
RG45G Tower, 80 feet.
Optibeam OB17-4 right above top of tower (3 ele on 40, 4 on 20 and 4 on 15
and 6 on 10 for 17 ele total - 39 foot boom square boom)
The 3 elements on 40 meters have an 'air coil' but the rest are all full
size elements.
12 feet further up on mast I have a Force12 WARC7 with 3 ele on 12, 2 on 17
and 2 on 30 mtrs.
Both antennas are fed with LMR400 Flex to a point down the tower and then
continues with LMR400 into a coax switch (Array Solutions Rat Pak) at
the base of the tower. Then a single LMR400 into the shack.
OK, some facts that hopefully help with the troubleshooting.
* This installation has been completed about 7 months and I have made a
little over 800 qsos on 40 meters before the problem started.
* I can't detect any change on the other 3 bands. NOTE: The OB17-4 has
square tubing for the feedline to the 4 driven elements for the
4 bands. The SWR on 20, 15 and 10 seems good and consistent.
* Turning the beam correcting the problem certainly seems to point to
something loose in the 40 meter section of the beam. Well, we
did find a loose bolt on the end of one side of the driven element for 40
mtrs. Corrected that (that took awhile and two people on the
tower) and then the problem continues.
* The change that most recently occurred before the problem was that we have
had trouble with the feedline loop for the rotator off and
on. And recently the coax got caught in something and became damaged. So,
the piece of LMR400 flex from the OB17-4 to to a place down
the tower was replaced with another piece of coax cut the exact same length.
The coax was then rerouted to try a better way to make
the rotator loop so that nothing was binding. I know this is the recent
change just prior to the problem but it is difficult for me to think
there is a feedline problem with the coax because the same run of LMR400
feeds the other 3 bands on this beam and they are fine.
* Connected an swr meter and radio directly to the coax at the base of the
antenna with same results (sometimes flat and then
sometimes shoots to 3 to 1 SWR). This bypassed the coax switch at the base
of the tower and all parts going to the shack.
* The OB17-4 has a rectangle "termination stub" that is about 1 foot x 2
feet and it serves to match the SWR for 40 meters.
OK, since turning the beam corrects the problem temporarily (every time - 10
our of 10 times) I would conclude a loose element or a cracked
or otherwise damaged termination stub? We had hoped the one loose element
piece solved the problem. Without taking down the driven
element the driven element appears to be tight at all other joints. My
question (among others) is "Can a parasitic element (perhaps with a
loose connection in one of the telescoping sections) cause this problem? If
(for example) a telescoping piece of one of either the reflector or director
had an intermittent loose connection would that cause an SWR spike. (I sure
hope that is not the case as that would mean a bucket truck
or a crane !!). I am thinking the next step would be to completely take
down the driven element (do able) and make sure all connections are solid
and then put it back and make sure the termination stub is good too. We
have also thought about taking a long pole (would have to be 20 feet or so
like bamboo) to the top of the tower and tap the reflector and directors to
see if we can get it to screw up consistently. Of course we can do this
with the driven element also and the termination stub. Fortunately the
termination stuff is easily reached from the top of the tower.
So, thanks so much TT pros and any thoughts greatly appreciated. I am
thinking it must be something limited to the 40 meter section of the
OB17-4. Am I missing something?
Gary, N5PHT
Gary Stone, BA, LCDC
Case Manager, Bonham DSAP
Sam Rayburn Memorial Veterans Center
(903) 583-6411
_______________________________________________
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
TowerTalk@contesting.com
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.
_______________________________________________
TowerTalk mailing list
TowerTalk@contesting.com
http://lists.contesting.com/mailman/listinfo/towertalk
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