Emory,
That's got to be frustrating! I have a couple of those antennas and got them
working OK, so I wonder what's up with yours. Some thoughts in no particular
order:
What else could be going on? Connect the roof end of your feedline to a 50
ohm dummy load and check the swr in the shack. If it is 1:1 then you know
that this coax and connectors and swr meter are OK.
The gamma aluminum tubes are not supposed to make physical contact. The
small one should have some insulating plastic material on it. They provide a
variable capacitance by sliding one tube in or out of the other.
If you are using a meter at the antenna to adjust it, use a coax jumper that
is close to a half wavelegth long (physical length times velocity factor to
equal a half wavelength). Some meters don't read right if right at the
antenna connector because of the strong field there. A few feet back is much
better, but if that jumper is a quarter wave or odd multiple therof the
readings can get screwy as you play with the gamma.
With a problem like this it is easier to work with it on the ground if that
is still possible. Point the antenna straight up with the director a few
feet above the ground. Lash the boom to a pole or suspend the antenna from a
tree branch or something. Start out with everything reset to the factory
dimensions. What I do is keep the "tuning strap" tight enough to make
contact but loose enough to still be able to move the larger tube of the
gamma arm in or out. Mark the position of the tuning strap on the driven
element tubing. Measure the Swr, and then move the larger tube in a bit over
the small tube and its insulator without changing the position of the tuning
strap on the driven element and see if the swr got better. If it did, go a
little more, if not go out instead. Write down these readings, especially
the lowest one. Then change where the tuning strap is on the driven element
by a half inch or so and repeat the process of fiddling with the gmma tube.
Were you able to get the swr lower? If so keep going in that direction with
the tuning strap, if not then move the tuning strap back the other way. I
think you get the idea. But if you have to go way different on the factory
lengths, then something eles is wrong.
When you get the antenna set right, the swr will be good over a large
portion of the band, like under 1.5 to 1 from 50.0 to 50.8. If the swr is
only good in a very narrow portion, something is still not right.
Good luck and let us know what you find.
73
Chet, N8RA
-----Original Message-----
From: vhfcontesting-bounces@contesting.com
[mailto:vhfcontesting-bounces@contesting.com] On Behalf Of ejh
Sent: Monday, March 05, 2012 3:50 PM
To: vhfcontesting@contesting.com
Subject: [VHFcontesting] Need help with Cushcraft A50-3S - high SWR
I have a new Cushcraft A50-3S, 6 mtr beam, and I am having trouble getting
the SWRs down.
I measured multiple times the element lengths and set them to the 50 Mhz
sizes.
I then set the gamma match to the exact sizes shown in the manual, (set and
X) but even installed on my roof away from all objects the SWR was very
high, way too high for my transceivers. I kept adjusting the gamma match and
the only way I could get SWRs down low enough to work with my rigs was to
make it as short as possible with the small rod pushed all the way inside
the larger rod. But even then the SWRs were well above 2 at 50.125 mhz? I
have worked with it for many hours and I am unable to figure out how to
adjust the gamma match correctly.
This is replacement for a damaged A50-3 I had for years, did not have
problems with the old one. The gamma match on the new one is different, on
the new gamma match the small rod does not make contact with the larger tube
it slides into. I tried two different, new lmr-400 coax runs, no change.
I sent email to Cushcraft/MFJ but they have not answers previous emails, so
don't suspect I will hear from them. Any help would be appreciated.
Thanks
Emory
WM3M
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