VHF radio problem/question??

[flattie]

I have a Kenwood vhf radio that came with the boat. Its connected to a shakespeare antenna (not sure of model but it has the very wide extension mast - maybe 3'' diameter) about 5' long into which the you screw in the 5' whip). Last weekend when trying to transmit on high power all that happened was a rapid clicking noise whenever I held down the transmit button. Switching to low power I was able to transmit clearly. I verified this with a buddy who was fishing about a mile away and on my back up Standard handheld. Needless to say theres no manual for the radio and a search of the web doesn't even turn up the fact that Kenwood ever made a vhf. The coaxial connection at the radio has some corrosion on the male plug. I'm wondering if this would cause the problem with being unable to transmit on high power?? Any help suggestions would be much appreciated.
flattie

[jstout1]

It sounds like a protection circuit is kicking in. In the high power mode, if the radio gets too much power reflected back to it from a bad cable/connection/antenna it will cut back to reduced power to prevent damage. Once it is in low power mode the sensors will let the radio go back to high power and then it will trip the protection circuit again. Hence the clicking.

If you use the low power setting the reflected power will not be strong enough to trip any protection circuits and enough power will be radiated to travel a short distance.

To ohm out the cable, disconnect both ends of the antenna cable (radio and antenna), short the center conductor and shield together at the radio end of the cable (just stuff some alluminum foil or something inside) and then check to see if you have a short between the center conductor and shield at the antenna end. If you do, take out the alluminum foil and then check to see if you now have an open at the antenna end.

If it ohms out okay, I would suspect the antenna.

Good luck.

[flattie]

jstout1,
Thanks. To be clear here 'cause I'm a little dense. I disconnect the coax cable from both the antenna base and the radio. I check the resistance on the center pin with a ohm meter with one lead from the ohm meter going to one end of the coax and the other lead from the meter going to the other end?? Then once I know the coax's resistance I put a piece of tinfoil in one end of the coax (the end with the male plug going to the radio) to short across the center pin and the shielding and recheck the resistance as described above - if the numbers are the same it means I have a short in the coax and it needs replacing if not the most likely culprit is the antenna itself?? Thanks again jstoudt1 as you can see I'm a little dense when it comes to electronics...
flattie

[jstout1]

I'm used dealing with long cable lengths that you can't reach both ends so I use the short one end and look for the short at the other then unshort the end and look for an open method.

If you can reach both ends check continuity between the center pins. Then check the continuity between the shields. Then make sure the shield and the center pin are a complete open.

Basically you have two wires, center pin and shield. They should be totally isolated from each other but be as close to a short center to center, and shield to shield.

Wow a picture would be worth a thousand words. Hope this helps.

Let me know.

Joe
Wicked Good

[Kamper]

If you dont feel comfortable with the process to reconnect a cable to the antenna, buy one of those small "emergency" antennas and compare your functionality. *

You can also pull your VHF, make up some power leads and test it with*your friends antenna. This is probably the most time-consuming technique but is technologicaly the simplest diagnostic method, aside from cleaning/replacing the plug.

Good luck!*

[flattie]

My next step is going to be to use the 6' emergency antenna I have and hook it up. I suspect its the coax connector as its somewhat crudded up.