Cal & Swr readings with Antenna warning light

Zachary,
You've got a couple of issues going on, and I'll address the ones I have enough info for.
One, which you discovered by getting the external SWR bridge, is that your antenna is too short. Adding a spring will help; the 3- or 4-inch medium duty ones will be fine. They have a piece of braid on the inside so that the electrical length doesn't change as the spring flexes; otherwise the antenna would seem longer when it flexed.
Once the spring is installed between the mount and the antenna, check SWRs again. If they're lowest around Ch 20, you're golden. Even at 2.0:1 (please note that it's a ratio, ALWAYS expressed as "to one") you're only losing 11% of your signal, and any normal final stage can handle that.
Except that yours has been doubly handicapped. You mentioned that you paid for a "peak and tune", which usually entails clipping the audio limiter (thankfully not in your case) and squeezing another watt or two out of the transmitter. The factory tuneup calls for adjusting for peak output, then adjusting down to 4 watts without modulation, where the final amplifier transistor is not straining. Guess what happens at a "peak and tune"?
Yup.
And nobody will ever hear the difference, until your final transistor dies an early death from the added heat it makes from making that extra power, and you're off the air.
The antenna warning light ("AWL") just samples a small portion of your power output, kinda like the RF output meter on the panel, and the SWR function that's built in -- this just samples the "reflected" side.
A by-product of turning up the output power is that, even if the SWR is mostly OK, there's still more power being reflected. Under normal circumstances (with stock tuning) that would mean that the antenna system had failed. In this case, the AWL circuit just sees more energy and turns on.
Remember all that heat that the final is making? It's supposed to be dissipated by thermal grease between the mounting tab and its insulator, and between the insulator and the heat sink. In the Cobras made since the late 80s, the factory no longer puts grease on the heat sink side.
So even without the peak and tune, the transistors die early -- just usually not until the warranty period has expired.
So -- (finally!) you can adjust the AWL threshold by turning VR6 while transmitting with your antenna hooked up, *once it's tuned*. But make sure it still comes on with no antenna hooked up.
A lot of shops don't even have proper signal generators, so they don't even touch the receiver tuning.
FWIW, I use a 2-ft Barjan on my service vehicle so I can fit into the garage, and typically get 10 miles with a barefoot Cobra 29 that's been correctly aligned. Out on the road, out of town, I switch to my sidemounted stainless 102" whip & spring.
Hope that helps,
-- Handlebar --
P.S. I've been doing this over 30 years

 

Zachary,
By the way, as antennas go, the longer and higher mounted, the better. Think of the antenna as a sail trying to catch wind.
And the shorter the antenna, the bigger the difference a fixed length change (like a spring) will make, and the tinier pieces you have to nibble off a fiberglass antenna's windings to make.
-- Handlebar --

 

Way cool! I've just found this forum, and it looks like every once in awhile I'll find a post where I can contribute a little something for the other users here.
Please let us know how it works. When you adjust the VR6 after tuning the antenna (nice!), turn it till it just goes out on transmit with normal voice. Then disconnect the coax from the radio, transmit briefly, and confirm that the AWL comes on again. A few seconds at a time into an open circuit won't kill anything.
See ya,
-- Handlebar --

 

Stop messing with the fiberglass antennas first thing and get yourself a metal antenna. With a fiberglass antenna, they can be a real pain in the butt to get set right when your running a 4 foot antenna but with a full size Wilson you have more of a tuning range and therefor are easier to set,although you may have to clip the length of the steel whip a few times to get it right. They also perform much better than a fiberglass antenna as well. You only want to balance the SWR reading from ch 1, 20 and 40,this way you dont have to check each channel.

You also must understand that the red light and SWR/cal on that radio can be adjusted inside the radio and can be off,causing the warning light come on.

I have seen guys mount a 102 inch steel whip to the front of the bumper,on the very bottom of the bumper on either side. Those antennas are very hard to bend so they take more abuse than any other antenna and work just fine. Just run as much coax as you need to get it to your radio, as long as you do it in 3 foot lengths,like 18,21,23 and so on.

As far as your SWR readings. Anything under 2.0 is completely fine. I know you may not believe me but its true. The people that are listening to you talk can not tell the difference between a 2.0 SWR versus a 1.1 SWR so chasing the perfect match is a wast of time and neither readings will your radio. It's only when you get above 2.5 that your start running into trouble. Check this out. lets says you are using RG-8 coax with an SWR reading of 2.0 on 27.205 ( channel 20 ) with a 30 watt radio your losing 3 watts. with an SWR of 1.1 your losing 2 watts so you can see that the 1.1 match is a wast of time. You can go to this website and see for your self the affects SWR plays on the ammount of power that passes out your antenna with different SWR readings and different coax. http://www.ve3sqb.com/hamaerials/ocarc/

You will also not hear a change in the way your radio receives with a 2.0 vs a 1.1.

After saying all of that i think a better choice for you would be a Wilson mag mount antenna. Just sit in the very center of you roof,set the SWR by raising or lowering the whip or even clipping some of the length of the whip off and forget about it. The Magnet draws its ground from the body of the truck so you dont have to run any ground wires at all. If you cant get the SWR down in the center of the roof then just move the mount forward or backwards on the roof until you find a location that the antenna likes. They work great as well but they can be a little high in price.

 

OH......and dont use a spring. The only time you may need to use one is if you use a 102 inch steel whip. For some reason these antennas like the 6 inch spring and can be easier to tune your SWR in with one. When you use a spring on any other antenna it changes the total length of the antenna and will make getting your SWR down a real pain in the butt! When it comes to an antenna,they are built with the correct length in mind and if you add a spring to one you throwing that off which is why SWR problems are hard to solve.

Also,a different antenna other than a fiberglass antenna may solve your grounding problems as well. Different antennas require more or less ground than others.

 

Zachary,
I just reread the thread, and noticed you'd ohmed out the mount to ground, etc. How about from the stud to ground? When you put the mount together, is there any chance you used a metal washer between the stud and the bracket on the top of the bracket? Here's what the mount should look like. That nylon washer has to be where it is, or the antenna will be shorted dead to ground, and you'll get high SWRs regardless of where else you put the intended grounds.
Just a thought,
-- Handlebar --