Testing the quality of VHF antenna and cable

[foojin]

The masthead VHF antenna on our boat does not provide a strong signal to our radio.

A second antenna which the PO installed on the pushpit (with cabin toggle to switch between them) is superior.

Is there a simple way to test if the weak signal from the masthead is the result of a faulty/weak antenna or a problem with the cabling up the mast?

[tkeithlu]

The most likely source is corroded connectors along the way or simply the long coax up your mast, so step one, and the cheapest, is cleaning and reconnecting, checking for corroded wire and corroded wire in connectors as you go. If that doesn't do it, then yes, the easy way to isolate the problem would be to take your second antenna, which you know is good, and switch it with your suspect one - two cheap tests - does the masthead antenna now work well, and does the pushpit antenna now not work well? If you replace the cable up the mast, spring for the good stuff, RG-8 or RG-213; manufacturers often use the cheaper RG-58.

[smac999]

test it with a swr meter. though unless you put a dummy load on top you won't know which one it is. just that one is bad. probably do both at the same time anyways if they are old.

I would also try it with the swtich removed and just one connected direct. it might be the problem.

[PommyDave]

A cheap ordinary Vswr meter is far from ideal because those are made for HF/shortwave frequencies (between 2 or 3 and 30 MHz) while VHF Marine is between 156 and 162 MHz.

If you can get to both ends of the coaxial cable, then you can test for continuity and poor connections. Also test for any short circuit as well (center to braid).

An antenna analyser (such as from MFJ or anything better) should show the antenna as being resonant somewhere close to 157 MHz.

If you still want to check for Vswr, then anything below 2.0 to 1 will be fine. (It is a newbie myth that you have to have a 1:1 "SWR".)

[michaeld]

The problem you are likely having is attenuation from your existing coax. Use LMR400 coax. It is superior to anything you can get at a marine store.

[Scott Berg]

A good tech will have both an antenna analyzer like the MFJ259 and a time domain reflectometer (TDR).

They can give you a great sense of the system from your transceiver location.



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[tkeithlu]

I think Foojin's need is to deal with what is likely a simple problem by inexpensive means. Marine VHFs are not rocket science (or even antenna science). They work well with good antennas, good coax, and neither grounding shorts nor bad connections along the way. I like the points of bypassing the switch and checking for a grounded antenna, since they are possible sources of the problem, but all the likely sources of the problem should be possible to test/eliminate with nothing more than a multimeter.

That said, I did stump two radio techs with my VHF not transmitting until the second one called a friend, and the friend's questions led to the realization that I was leaving my sat phone on top of the VHF, and its charger was blanking transmission.....

[Littlechay]

Like others have said. Check the basics first; connectors, corrosion, the switch, the antenna, holes (chaffed areas) in the shield of the coax, perhaps a short to the mast at a chafe point.

Removing the switch is a good idea. It is always best when working on faults to remove as much unnecessary equipment from the equation as is possible. If you make the antenna reach directly to the radio then connect it directly (or move the radio). Once you have the basic system Radio - coax - antenna working then add the other components back in.

I disagree with PommyDave: you can get plenty of cheap VSWR meters that cover about 120Mhz to 220MHz that will give you a good indication of what is happening in your antenna system. Plenty on EBay for less then 40 USD. I have an old twin dial one that I carry on the boat that has helped several people avoid expensive repairs now.

When buying coax, if it comes to that, be very careful. It's not just the type RG-8, URM-67, RG-58 etc. that you should look at. There is huge variation in the quality of covering material, density of the braid, whether the inner and braid are tinned or not - in short you get what you pay for with coax cables.

[capngeo]

Originally Posted by Littlechay

SNIP

I disagree with PommyDave: you can get plenty of cheap VSWR meters that cover about 120Mhz to 220MHz that will give you a good indication of what is happening in your antenna system. Plenty on EBay for less then 40 USD. I have an old twin dial one that I carry on the boat that has helped several people avoid expensive repairs now. SNIP

^^^^^ Good Advice^^^^^

[foojin]

Thanks for the advice and suggested remedies. Much appreciated.

Clearly need to educate myself on SWR meters and the like.

Back to the boat soon, after housesitting for friends, and will get on to finding a solution.

Cheers