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Author Topic: how do you know when an antenna trap is bad??  (Read 2592 times)
FOXBAT426
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Posts: 274




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« on: May 19, 2010, 07:24:48 AM »

greetings, i'm currently running a cushcraft MA5B mini beam antenna with my set-up. on 20 meters i have a strange problem where when i key up, the power jumps and gradually fades down to zero, even while i talk. when i release the PTT and push it again the same thing happens - the fwd power meter initially jumps up and then gradually fades - also my SWR needle does not move. this does not happen on 17, 15, 12 or 10 meters - all seem fine. when i disconnect the antenna and connect a dummy load 20 meters acts fine as do the other bands - i get approx 100 watt ionto the dummy load on all bands. i'm guessing the 20 meter traps are bad - has anyone seen this with a trapped antenna?? thanks!

john - ki4ucw
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N3JBH
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Posts: 2358




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« Reply #1 on: May 19, 2010, 08:22:21 AM »

Cushcraft had a problem with these traps on older models. I have a older model the replaced my traps free of charge. I do not know now that they been taken over by that other company if there still as service oriented or not but it is worth a shot. Jeff
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W4DRR
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Posts: 82




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« Reply #2 on: May 19, 2010, 08:57:16 AM »

20 Meters is the lowest band.  There shouldn't be any traps for 20.

73,
Bob
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73,
Bob
N1LO
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Posts: 1039


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« Reply #3 on: May 19, 2010, 09:32:15 AM »

Hi John,

I can only add that I have seen this symptom twice in other antenna systems and the cause was a poor connection somewhere in the feed line or antenna component.

 GL OM,

 --··· MARK·N1LO ···--

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AH6RR
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Posts: 803




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« Reply #4 on: May 19, 2010, 09:39:05 AM »

20 Meters is the lowest band.  There shouldn't be any traps for 20.

73,
Bob

 Bob the MA5B is a mini beam and it has traps for 20 I ran one for 2 years until I got my SteppIR 3ele. The problem could also be in the Balum box that feeds the 2 driven elements check that also. If your radio has a swr meter built in test it with that also. Better yet find someone with a antenna analyzer to see if there is a problem with the antenna/coax etc.
The MA5B is a OK antenna I got my first 75 DXCC countries with it.

Roland AH6RR
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WB6BYU
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Posts: 12980




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« Reply #5 on: May 19, 2010, 09:46:12 AM »

Quote from: FOXBAT426
...also my SWR needle does not move...

Does this mean that the SWR needle does not move AT ALL (which means either the SWR is
always perfect, or the meter isn't working) or that the reflected power doesn't CHANGE over
time after keying the transmitter?

This is a critical piece of evidence for determining where the problem is.

If there is a fault in the antenna that is causing the transmitter power to drop over time,
it would most likely show up as a high SWR at the rig.  If the SWR meter is working, but
not showing a high SWR, then we'd have to find some other explanation for the problem.
(Perhaps RF feedback.)  But if the rig is cutting back power due to a SWR that drifts higher
over time, that could be due to a poor or intermittent connection in one of the traps
(likely the output side of the last trap) or somewhere else in the antenna.

In that case, it probably wouldn't hurt to take the covers off the traps in the driven element
and give them a visual inspection.  If it is a corroded joint it may show up with an ohmmeter
check, but being intermittent you can't be sure you'll catch it.
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K1WJ
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Posts: 450




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« Reply #6 on: May 19, 2010, 12:21:02 PM »


If the SWR is not rising & power is going down - the radio would be suspect #1 in my view. 73 K1WJ  Cool
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N4CR
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Posts: 1650




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« Reply #7 on: May 19, 2010, 04:03:56 PM »

20 Meters is the lowest band.  There shouldn't be any traps for 20.

73,
Bob

 Bob the MA5B is a mini beam and it has traps for 20.

Well, it might have a loading coil for 20, but it's most certainly not a trap.
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73 de N4CR, Phil

We are Coulomb of Borg. Resistance is futile. Voltage, on the other hand, has potential.
FOXBAT426
Member

Posts: 274




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« Reply #8 on: May 19, 2010, 04:52:59 PM »

thanks everybody - it was a bad connection from the matching network box to the antenna. a little sandpaper did the trick - the 4 connections from the matching network to the antenna were a bit tarnished, but did not look suspect. i am grateful for everyone's help. a few here actually called a bad connection. glad to see it wasn't a trap or the radio! will be checking those more often.    regards and 73, john Ki4ucw
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