SWR Warning Light

[Smokn_X2]

I have a Cobra 29 with a very active antenna warning light. It wants to come on a lot after it warms up I guess. I have been over and under my truck, replaced the coax, stud, mount and tried several different antennas, and still I cant get it right, Other radios work fine with this particular set up by the way.
But...with a meter in line with the coax the SWR light never comes on and seems to be pretty low although it does vary from time to time.

Anyone got any suggestions? Thanks

 

[555]

is the swr below 1.5 on your antenna system

most are set at 1.5
someone might have turned the wrong adjustment and its just flickering

you can set it a 1.2 swr anything higher and it will flash

 

[theshortcircuit]

VR6 is used to adjust the threshold for the warning lamp. If the swr is ok, it may be as simple as tweaking this a smidgeon.

 

[Bionic Chicken]

A lot of times if the radio has been tuned (and/or using an amp) the SWR led will flash even if the antenna is properly tuned. It sees the increased power and if it was set on the threashold initially it will blink. If the antenna is tuned properly just move the adjustment just so the led will go out and then you still have the AWI operational in case you have a problem. Or..............you can put a piece of tape over it.

BC

 

[Smokn_X2]

The SWR is 1.4 on channel 1
It is 1.2 on channel 20
and slightly less than 1.1 on channel 40

after it has been on for about 10 minutes or so channel 1 goes up to about 3 as does 20 and 40 just not quite as high.
The antenna is a 4.5 foot K40 fiberglass trucker type antenna with the little tuning whip at the top.
555, even if VR6 had been tinkered with, would that make a difference with a separate meter?
I have used three different meters and they all read the same give or take a point..
The radio has been slightly modified, but the guy who did it is strictly by the book and didn't give it but a few watts if any. I'm thinking it will dead key 4 and swing up to 12.
But I did not know that about VR6, thanks shortcircuit for the Info, about where should this be set at?
Thanks...

 

[theshortcircuit]

...But I did not know that about VR6, thanks shortcircuit for the Info, about where should this be set at? Thanks..

I've found it best to connect the radio to a 50 ohm dummy load with obviously the speaker cover off. While keying the radio, adjust VR6 clockwise to establish the threshold you desire for the SWR warning lamp. I suppose this could also be performed on the proven (confirmed with meters) low SWR antenna system with the same success; if you do not have a dummy load.

 

[Smokn_X2]

Thanks again shortcircuit.

 

[freecell]

the swr meter function in the cobra 29 is adjusted from the factory so that the warning light is activated as swr reaches 3:1. you cannot perform the calibration without a load resistor that simulates this swr. a value of resistance between 16 - 17 ohms or 150 ohms connected directly at the output of the transmitter allows the calibration to be performed properly as both of these values will produce a 3:1 swr at the output of a 50 ohm source.

 

[Cityboy]

I have a dummy load of 50 ohms; so what is the easiest way to build a 16-17 ohm or 150 ohm dummy load. I have never built one of my own. Could I take a piece of coax and attach a 1w or larger resistor of the proper value and do it this way. Point me in the right direction for a diy on this dummy load.

 

[Moleculo]

You could replace the 50ohm resistor in the dummy load with a 150ohm with appropriate power handling.

An alternative way to do this is to just leave your dummy load alone and instead insert a small antenna tuner in between the radio and the dummy load. Adjust the tuner until it reads 3:1 SWR.

Simple.