Possible antenna connection problems - any help please?

[shaplandhill]

I have recently fitted an Icom M421 VHF/DSC to my boat. This powers up fine. I connected the antenna and have tried to do radio checks but no joy. I believe I may have connected the coax cable at both ends incorrectly. Could somebody talk me through the procedure in idiot proof language so I can check to see if and where I have gone wrong. (my local boat yard have said they can do it but not for about three weeks !!!)

Regards

Rob

 

[Salty John]

The radio end will have a PL259 plug. This should be connected to the coax cable as per the example on my site under cruising resources. Depending on what type of aerial you have it may also have a PL259. It is possible that the aerial has a different type of connector, including a push/twist fit. Try to establish that it is not shorted between centre core and braid of the coax. To check the cable run you need to have each end of the cable disconnected - from the radio and from the aerial. Check that there is no continuity between the body and pin of the PL259. Then check that there is continuity between the centre pin at one end and centre pin at the other end of the cable, and also between the body at one end and the body at the other end. If the aerial is up the mast, this is not practical.
I hope this helps.

 

[thomashoebus]

Without the antenna connected you should be able to receive and transmit to a nearby vhf. I tested this with a handheld vhf. This way you can see if there is a problem with the vhf or the antenna.

good luck

Thomas

 

[lutraii]

CAUTION - DONT TRY IT ! It's possible to damage a radio by transmitting without an antenna connected. I suggest you check your user manual. The manuals for both my Icom radios (M21 and M601) warn against transmitting with no antenna

 

[lutraii]

You can check that the radio works by making your own temporary antenna, which could also be used in an emergency. Proceed as follows:

Take a few feet of 50 ohm coaxial cable and connect to a PL259 coaxial plug. The braided outer connection is usually made by screwing the cable into the plug body, the inner by soldering into the hollow pin. Check that there is no short circuit between the inner and outer conductors.

Strip the outer insulation off the other end of the cable for about half a metre.

Pull the insulated inner conductor through the braid where you cut the outer insulation and bend the braid and inner at 90 degrees in opposite directions so you have a 'T' shape.

This gives a dipole antenna of 1 metre which is about right for VHF channel 16.

Plug the antenna into your radio and hang the other end of the antenna from some convenient support so that the dipole is more or less vertical.

Good luck!

 

[William_H]

Hello Lutra. A far more practical way to do the antenna is to fold the braid back over the supply coax for the required 1/4 wavelength distance leaving the 1/4 wavelength of unshielded centre conductor ou the top. The whole can be shoved into plastic tubing or an old fishing rod to make a fine antenna. My calculations give a little over 17 inches for each leg. (less than your 1/2 metre0 olewill
PS if the connector at the antenna is BNC small 1/2 turn type then ensure that the centre pin can just be touched with a finger nail. They can withdraw into the connector too far giving no connection.