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Message |
    
Kid_vicious
Senior Member
Username: Kid_vicious
Post Number: 1356
Registered: 9-2004
| | Posted on Monday, March 27, 2006 - 10:47 pm: | 
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i thought i'd start a new thread for this since it seems to be unrelated to my other thread on this antenna.
so, i've got the antenna mounted right in the middle of the roof. the trunk and hood are both bonded with copper braid.
radio is a 148gtl, going to KL-300p, to radio shack SWR meter, to antenna.
jumper from the radio to amp is about 9 feet.
jumper from amp to meter is about 6 feet.
antenna connected to output of meter.
cable from antenna is LMR240.
jumpers are both RG8mini.
PROBLEM: with the amp on, the SWR jumps to over three.
with amp off, SWR is 1.1 to 1 (ch.20)
jumpers have been switched. no change.
new PL-259 and UG-176 reducer put on (done right!)
no change.
checked antenna mounting. all good. well grounded.
no change.
dummy load attached to output of meter. SWR 1.1 to 1 with amp on.
so the problem is no narrowed down to the antenna itself or the cable coming from it.
decided to put the dummy load on the antenna base instead of the antenna.
if you are familiar with the wilson mounting system, then you know that it teminates to an SO-239 connector that the antenna coil screws onto.
its pretty cool!
so i used a double male barrel connector, and put the dummy load on the antenna base.
SWR was 1.1 to 1 AMP ON!
so now ive got the problem narrowed to the antenna itself.
it used to have an SWR of 1.7 to 1 with amp on when mounted on trunk, and didnt vary at all.
so im thinking that when i screwed it onto its base after mounting it on the roof, that i somehow broke a solder connection inside the base. (low power, SWR great, high power, SWR BAD!)
does anyone know how to take one of these things apart?
am i missing something else here?
thanks for any and all help. this is a first for me,
matt |
Marconi
Advanced Member
Username: Marconi
Post Number: 631
Registered: 11-2001
| | Posted on Tuesday, March 28, 2006 - 12:02 pm: |
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Well Kid, I would think that something is wrong with your ground plane for this antenna. It really needs a suitable GP in order to work right. The dummy load does not require a ground plane in order to function and likely accounts for your difference.
You may think you have a good plane for the antenna, but what counts here is what the antenna sees and not what you see.
If it is possible to get to the ground from the roof side of this antenna mount, then try attaching a 9 foot radiator (wire will do also if straight) in the horizontal plane as a ground plane. See if that makes any difference when loading the amp. If you can, just attach the element to the base which should be grounded using some vice-grips maybe. |
Kid_vicious
Senior Member
Username: Kid_vicious
Post Number: 1357
Registered: 9-2004
| | Posted on Tuesday, March 28, 2006 - 7:18 pm: |
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while i agree that i may have a groundplane issue, and will try the 9 foot radiator idea; remember that this same antenna, same linear, and same coax, (except the LMR240 that i subbed in) used to have an SWR of 1.7 amp on or off, when mounted on the trunk lid.
the trunk lid was grounded to the main body of the car which includes the roof which the antenna is now mounted on.
so i dont think its really possible that i have a worse groundplane than i did before.
it actually seems much better with the amp off.
right now the antenna is on the center of the roof, which is grounded with braid to the trunk lid and the hood, so i know ive at least got those working in my favor.
one thing that i thought of after posting this is that the dummy load is ALOT more broadbanded than my antenna is, so if the amp was producing a second harmonic, then the dummy load would still be resonant but the antenna would not.
i think i need to try a low pass filter after the amp, or maybe a choke balun.
not sure yet.
here's something else to consider: the wilson 1000 comes stock with 18' of coax. i am hoping that this length has nothing to do with the tuning of the antenna. (it better not!)
i replaced it with about 10 feet of LMR240.
also, today i grabbed a cheapie 3 foot jumper from the junk box in the way out of the house.
i hooked it up going from the amp out to the meter in, and antenna coax to the antenna out of the meter.
the old one was aobut 6' long.
the SWR dropped from just over three across the band to just under two across the band.
so coax length is making a difference in my SWR.
i dont like it, but i guess thats the way it is.
i guess i'll try some different jumper lengths and see what happens.
any ideas or suggestions will be given the serious consideration that they surely deserve!
matt |
Chad
Advanced Member
Username: Chad
Post Number: 753
Registered: 11-2004
| | Posted on Tuesday, March 28, 2006 - 9:29 pm: |
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I would HIGHLY suggest trying a different meter. I was having the same problems wiht the radio shaQ meter and switched over to an Astatic 600, problems went away, Honestly I don't think they were there in the first place... False readings.
Same EXACT symptoms as yours.
Chad |
Kid_vicious
Senior Member
Username: Kid_vicious
Post Number: 1358
Registered: 9-2004
| | Posted on Tuesday, March 28, 2006 - 10:33 pm: |
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thanks chad, good to know.
i have another meter at home to test it out with, but its an old micronta!
maybe they used to be better than they are now.
wouldnt be too tough!
matt |
Chad
Advanced Member
Username: Chad
Post Number: 754
Registered: 11-2004
| | Posted on Wednesday, March 29, 2006 - 9:39 am: |
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Lemme guess, slowly inch the power up and BAM it pegs within a few watts of increase? I ws soooo stumped too, posted here a little over a year ago but since the amp really did not pull more current as the SWR suddenly jumped I was rather suspicious of a bad meter. Mine started to jump at about 40 watts and was pegged by 50, yes, I re-calibrated. THE RS meter seems OK for lower power or maybe it just hates the KL300P?
Chad |
Kid_vicious
Senior Member
Username: Kid_vicious
Post Number: 1360
Registered: 9-2004
| | Posted on Wednesday, March 29, 2006 - 10:58 pm: |
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still not quite sure what im dealing with here, but using a different meter,(an old micronta three window) the SWR is 1.5 across the band.
no dip in the middle though.
it seems strange to me that the antenna shows no "curve".
with the amp off, the antenna shows a curve that seems to be inline with what it was designed to do.
1.3 on the band edges and 1.1 in the middle.
amp on, 1.5 across the band.
still have to hook a low pass filter up after the amp and see if that changes things any.
for right now though, im happy!
i can run the amp and everything sounds good.
no more noise in the system.
i'll post if anything changes. |
Chad
Advanced Member
Username: Chad
Post Number: 757
Registered: 11-2004
| | Posted on Thursday, March 30, 2006 - 9:15 am: |
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All meters are different and few affordable ones are truly accurate. I don't know how broad banded the wilson is but the Imax 2K will show almost identical SWR on 1, 20, and 40, it's just broad banded though. Glad you feel better about it though. It sure did have me puzzled for quite some time!!!
Chad |
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