Radio Coax

[Howard Armstrong]

i replace the coax on Angelina because of a digrading signal. Lightning strike. The radio and antenna have already been replaced. i still have low signal and can only talk about a mile. the SWR is high or low, whatever is the wrong one to have. Does anyone have any ideas on diagnoising where the problem may be. what i intend to do is check ohms in the cable, for high resistance. does the antenna mount need be grounded or isolated?
thanks for any help
Howard Armstrong

[rdavison]

Howard,
I agree with Ron that the problem is probably in the connections you made.  The SWR should be low.  1 to 1 is optimal - anything under 2 to 1 is OK.  Either an open connection or a short from center conductor to shield will cause a high SWR.  

If the antenna is a metal whip type, the shield probably is grounded at the mounting bracket.  

If you don't have easy access to both ends of the coax, it's very difficult to tell what's going on with an ohm meter.  The difficulty is that the matching coil at the antenna base may or may not show a low resistence across the coax.  I use both types of antennas for amateur radio.  The SWR bridge is your best bet for determining if you have trouble or not.  

If the SWR bridge shows a high reading (a short or open will probably show 8 to one or higher on new coax), you will have to gain access to both ends of the coax to find the problem.  If you are using a bosuns chair up the mast, I would have one person use the ohm meter at your radio installation while you go up the mast. Unscrew the coax from the antenna.  Have the person on the ground hook the ohm meter between the center conductor and shield (while keeping their fingers off the connections!  This is important because a newbie can misinterpret body resistance as a short).

With the coax dangling at the top, the ohmmeter reading should be infinity.  When you short the coax at the top with a wire or screwdriver, the reading should be under a couple of ohms.

If this doesn't happen, cut off the base connector and redo it.  If that doesn't fix the problem, you'll have to replace the upper connector.  The final passing test is always that the SWR is low.

I prefer soldered connections too but if you do use crimped connections, be sure the coax is not hanging from the antenna but is instead tethered to the mount or other screw with a cable clamp or wire tie.

I'm assuming in all of this that you are using a good grade of new marine coax.  Old coax becomes very lossy at vhf.

Hope this helps.

[rdavison]

Howard,
Forgot to say...no matter what type of connection you use at the antenna, you should wrap it with Coax Seal, available at radio shack or your marine electronics dealer.  it's black tape-like stuff that you wrap over the coax and connector right up to the base of the mounting bracket.  Then you mold it with your fingers so it is a tight seal.

[howard armstrong]

I found the problem. i had replaced the coax with a new good quality coax and had had the end installed by the provider. We started checking with the ohm meter and got what you said we should get. then what we finially got to was the two year old antenna was full of water. i learned a lot about radio diagnosees so even though it was a pain in the stern, it was a good experience. thanks for your posts and i will put the information in my notebook.The end result was a 1.1 on the swr meter. i did use soldered connections i dont like crimp on. also the antenna manufacture said that the antenna should read infinity with an ohm meter also so in my understanding you shoud be able to disconnect the coax from the radio and get an infinity reading also.
thanks for the help.

[BillR]

Howard,

I had a similar problem in which the antenna was shorted to ground.  Whether it was due for a change after 14 years or was subject to lightening (as I live in Texas) is unknown.  Investigation started the same way with very low output and reception.

I noticed the comment from Ron Hill as to the use of an antenna separator.  I take it that is a potential source of trouble?

[rdavison]

Howard,
Glad you found the problem.  Knowing that the antenna should be an open circuit will let you find coax shorts without getting to the other end but won't let you confirm center pin connectivity.  Fortunately, shorts from center wire to shield are the most common problems - usually from losing control of one of the little shield wires while soldering on the connectors.