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[TowerTalk] Beam toubleshooting problem - help please

To: towertalk@contesting.com
Subject: [TowerTalk] Beam toubleshooting problem - help please
From: Gary.Stone@med.va.gov
Date: Wed, 14 Dec 2005 08:15:12 -0600
List-post: <mailto:towertalk@contesting.com>
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

 

 

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