LOST JEEPS
http://www.lostjeeps.com/forum/phpBB3/

CB Antenna Tuning Quirks/Tips
http://www.lostjeeps.com/forum/phpBB3/viewtopic.php?f=45&t=6013		 Page 1 of 1
Author:  Kugellager [ Tue Feb 21, 2006 1:35 am ]
Post subject:  CB Antenna Tuning Quirks/Tips
Ok so today I finally got around to tuning my CB antenna after building my lightbar/antenna mount and discovered something odd I did not expect.

I hook up the SWR meter and remove the protective rubber cap at the tip of the antenna in preperation for removing some of the wire coil at the end to adjust the SWR. I get the SWR to between 1.0 and 1.5 for all channels and place rubber cap back on and tighten the antenna one last time. Once everything was reassembled I tested the SWR one more time. To my surprise the SWR had jumped back up to just over 2 at channel 40.

The reason for this I discovered was caused by me replacing the rubber safety cap on the antenna. I went up an took it off and sure enough the SWR went back to the levels measured before the rubber tip was on. In order to compensate I had to remove the rubber cap and cut off quite a few more of the wire coils to get the SWR correct once the rubber safety cap was installed. The antenna is now tuned to just over 1.0 for channel 1 and to just about 1.4 for channel 40.

Has anyone else observed this effect with the rubber cap? I find it strange as the rubber should have no effect (electrically) on the antenna signal.

I would suggest that anyone who has tuned their antenna with the rubber cap off re-check their SWR once the cap is on if you have not done this before. The difference in the SWR with the cap on and off is considerable.

John
];')
Author:  corwyyn [ Tue Feb 21, 2006 2:37 am ]
Post subject: 
Only thing I can think of is that the rubber cap is compressing or in some way deforming th coils so that the SWR goes wonky. I have one of the Firesticks that has a screw in the end of the antenna - you turn the screw in or out to adjust SWR and there is a locknut to keep it set.. I may go out this weekend to experiment, but I haven't had any problems so far. As far as the original SWR goes, running 2:1 wouldn't have been a problem, you would have just lost a bit of emitted power. But hey, every little bit of power out helps...
Author:  Kugellager [ Tue Feb 21, 2006 1:21 pm ]
Post subject: 
I was thinking either what you suggested (though I tested that by moving the exposed wire around a bit...not it I think) or that maybe there is a conductive material in the rubber...It's black rubber and I know that sometimes pure carbon(good conductor) is used to make rubber black. If that is the case, the cap might be acting as a shield and reflects some of the RF back which might be why the SWR went up...as the meter measures reflected energy.

What do you think?

John
];')
Author:  dirtykj [ Tue Feb 21, 2006 6:01 pm ]
Post subject:  Re: CB Antenna Tuning Quirks/Tips
Kugellager wrote:
I hook up the SWR meter and remove the protective rubber cap at the tip of the antenna in preperation for removing some of the wire coil at the end to adjust the SWR. I get the SWR to between 1.0 and 1.5 for all channels and place rubber cap back on and tighten the antenna one last time. Once everything was reassembled I tested the SWR one more time. To my surprise the SWR had jumped back up to just over 2 at channel 40.


The tuning tips at firestik website explicitly mentions that the rubber tip should be kept on as part of the tuning, if you intend to keep the tip on during actual use.

here

:D
Author:  Kugellager [ Tue Feb 21, 2006 6:45 pm ]
Post subject: 
I definitely discovered that on my own...the question is...Why is it electrically longer?

Mine is a Tiger (Everhardt) antenna by the way.

John
];')
Author:  dirtykj [ Tue Feb 21, 2006 7:28 pm ]
Post subject: 
My antenna cap is vinyl. Since vinyl is dielectric, it could conceivably prevent some amount of signal from propagating.
Author:  Kugellager [ Sat Feb 25, 2006 10:22 pm ]
Post subject: 
dirtykj wrote:
My antenna cap is vinyl. Since vinyl is dielectric, it could conceivably prevent some amount of signal from propagating.
I think mine is actually vinyl and not rubber as well. That makes sense now...Didn't even think that it might be a dielectric...Probably making a capacitor between it and the wire.

John
];')
Page 1 of 1 All times are UTC - 5 hours [ DST ]
Powered by phpBB © 2000, 2002, 2005, 2007 phpBB Group
http://www.phpbb.com/