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Message |
    
Jay_t
New member
Username: Jay_t
Post Number: 1
Registered: 3-2011
| | Posted on Monday, March 14, 2011 - 11:04 pm: | 
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Hi, I'M A NEWBE TO C.B. installation and tuning, so I will probably be asking a lot of questions. I bought a cobra 29 chrome and a Wilson 5000 trucker antenna. Initially I mounted it to a magmount but after reading wilsons recommendations on the 5000 trucker, I switched to a wilson heavy duty stainless roof stud mount.After swapping mounts,the swr reading on a radio shack 21-534 meter went to slightly over 2.5 but my cobra is reading nicely at 1.5. These meters were reading really close with the magmount,around 1.5 but when I switched to the stud mount,the radio shack meter jumped higher than the cobra. I should have a good ground at the mount because i sanded the underside of the roof in the stud area to bare metal. Even with the higher swr, I am sending and receiving better than I was with the magmount.Any thoughts would be greatly appreciated. |
Tech833
Moderator
Username: Tech833
Post Number: 1929
Registered: 8-2002
| | Posted on Tuesday, March 15, 2011 - 12:36 pm: |
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First of all, welcome!
There are lots of possibilities.
One is that the Radio Shack is putting out spurious emissions on frequencies outside the antenna's low SWR bandwidth and the Cobra is not. The spurs would show up as higher reflected power on your SWR meter. Since they would be much lower in power than the fundamental, they wouldn't show up as a huge SWR, but rather a slightly elevated one, like 2:1 or so.
Another possibility is that there is still a reactive component in your antenna that needs to be tuned out. If you had a perfectly flat 1:1 SWR on one radio and 2:1 on the other, this wouldn't be a possibility. However, since both are showing some degree of reflected power, this is possible. See if you can tune the SWR to less than 1.2:1, preferably a perfect 1:1. If the whip bottoms out, you can cut it. Then see what the Radio Shack radio says. Chances are good that it could also be out of calibration.
73 (Kindest regards),
833
Your radio 'Mythbuster' since 1998
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Dale
Senior Member
Username: Dale
Post Number: 1682
Registered: 12-2002
| | Posted on Tuesday, March 15, 2011 - 9:01 pm: |
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tech 833 is right. also need to know your
swr on ch1 and 40.this will tell ya to
to raise or lower whip.are there any other
devices hooked up other than radio,swr meter,antenna?set swr as follows
swr low on 1 high on 40=lower whip
swr high on 1 /low on 40=raise whip
ideally swr should be the same on both ends
or very close.anuthing uder 1.5 is ok
whip may need to be cut .i use bolt cutters
do this only if nessary.make sure all doors
are shut.as far as meters i had good luck with radio shack swr meters
dale/a.k.a.hotrod
cef426
cvc#64
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Jay_t
New member
Username: Jay_t
Post Number: 2
Registered: 3-2011
| | Posted on Tuesday, March 15, 2011 - 10:13 pm: |
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Thanks guys. Just to make sure we are on the same page, the radio shack that i referred to is a meter,its not a radio. The cobra is a radio with built in meter. The good news is that i had it checked today on another meter and although the readings weren't ideal, (around 1.4)they were lower than the radio shack was reading.I'm thinking that my next move will be to buy a good inline meter in order to fine tune. Any recommendations ? Thanks again. |
Tech833
Moderator
Username: Tech833
Post Number: 1931
Registered: 8-2002
| | Posted on Thursday, March 17, 2011 - 10:49 am: |
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Ahha. Thank you for the clarification.
OK, what you are experiencing is not unusual. Unless the external meter is connected directly to the back of the radio, they most likely will read different on an antenna with some reactance component at the feedpoint. If your antenna was a perfect 1:1, they would read the same (assuming they were accurate).
This is one of those cases where you no longer worry about it. If the meter in the Cobra is reading very low SWR, then call it good. Enjoy!
Your radio 'Mythbuster' since 1998
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Wetchicken
New member
Username: Wetchicken
Post Number: 1
Registered: 4-2011
| | Posted on Wednesday, April 06, 2011 - 9:23 pm: |
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Another reason you are having this is due to the phase angle at the antenna coax. Think of one wavelength being 360 degrees, and anywhere in between is 0-359° (or 1-360° lol). i.e. if the meter and radio are inline, the meter could be at 78° and the radio at 103° since the antennas coax is lengthened due to the jumper, thus giving the difference.
I also agree as long as the meters work in a similar fashion they would give similar results when attached directly to the antenna's coax. |
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