How to check VHF cable and connectors

[Redline452]

An SWR meter would be best, but I'd start with a quick continuity check, given that most problems are corrosion in/at the connectors and cables (discounting the time lightning completely vaporized the masthead antenna).

An ohmmeter across the radio end of the cable will probably read short - most VHF antennas look like a short circuit at DC (don't even think about the "50ohm" cable - I could explain it but it wouldn't help). If you have a coupling at the mast base, open that and check for open circuit - if so, you mostly rule out the radio-to-mast cable run. Put a screwdriver blade between the center pin and the connector body, should read short or a very few ohms). Also check that the PL259 connector bodies (not the ring, the body) does NOT turn on the cable - it should be soldered to the braid. I've seen keel-stepped masts where the connector sat in the bilge and water had wicked up the cable turning the braid to power for several feet.

Next find someone to go up the mast (kids are light) to simply unplug the antenna. Cable should now read open. Bridge connector pin to shell, should read short.

If you need to replace a connector, consider doing the entire cable. If it's the masthead end, try a commercially-made pre-terminated cable, run down from the top so you don't have to solder at altitude (I once spent an interesting morning some 65' up at Staniel Key Bahamas reinstalling a corroded PL259. Tricky with the breeze, but a great view).

If you are replacing a connector (or cable and connector), make sure you have the correct connector for the cable type - insulation diameters vary.

Oh, and first off, measure the DC voltage right at the radio's power cord while transmitting. A bad connection or dirty fuse holder can deliver enough power for the receiver but cause a serious voltage drop in transmit.

[Robert Tilbury]

Hi all ,,, my vhf at 25 watts will pull 5.3 amps at the power cable ,,, if your transmitting at full 25 watts TX you should be using about the same amps at the 12vdc power cable ,,,
My HF at 100 watts pulls 19.5 amps TX
if its not pulling the full amps you may have a mix matched antenna and the roll back circuit will limit TX output ..
don't hesitate to PM me if you need help ,
Rob (VK2LOZ)

[bluewatervet]

A coaxial adapter to allow use of the handheld on the base station antenna is a very good suggestion. Also is a very good tool for emergency situations, allowing both radios to use either antenna. An old trick for troubleshooting is to utilize the squelch control as 'crude' test equipment. Easest to do if the radio(s) have rotary controls vs. preset via menu. Latter gives a better result to evaluate, but even menu based stepped presets can be useful. Tune each radio to a scratchy NWS broadcast frequency (or any other most times available marginal signal). Next, adjust the squelch so it just almost shuts off the receive. Then interchange both radios and antennas. If one antenna seems quite a bit stronger, therefore allowing a 'tighter squelch setting', then you have a open or shorted coax or connector within the weaker cable/antenna combination. If one radio seems fine on both antennas, the other radio may have a failing receiver section. You will need a coaxial power meter to check the transmit output levels, as noted by others' replies. Performing & recording this simple test & logging results while everythng is working well gives you a comparison benchmark for times of trouble. At marine VHF frequencies, long vs. short cable runs (when well under 100 foot) will have barely descernible signal level differences during this type of test. Antenna height above water is the most pronounced difference typically. The many other replies with ways to test are also great guidance. I especially liked the 'borrow a HAM one'.....

[Robert Tilbury]

Most Hams have antenna analysers ,, if you were close i could just connect mine up and see what the resonant frequency of your antenna is ,,, if its in the 156 to 174 MHz then its ok ,,,
See your local amateur radio club in sure someone would love to step on your SV and have a play ,

[hamburking]

Here is a simple test with your multimeter:

Undo any (every) antenna connection you can access. Use the "continuity" test setting (sometimes called zero resistance test). Touch one probe to the centre post and the other to the outer ring. They should be electrically isolated. If you find they are not, that is a big clue to your problem. Sometimes, when assembling the cables, there is a short. It can happen to anyone, or even prefab cables. This is a simple, basic test.

[hellosailor]

"Most Hams have antenna analysers ,,"
Maybe in Oz, but not in the US. Analyzers cost money, if there are two dozen hams in a club, one might have one.

Some of the easier alternatives: Buy an "emergency antenna". This is basically a 19" long whip with some cable, that you can place on the rail if you are dismasted, or aloft if the primary antenna fails. Useful to have just in case. Try your radio with that, and if it works, you've just confirmed that you have a good radio and a bad antenna.

Divide and conquer.

If your antenna and cable are the problem? Are they more than ten years old? Twenty? At twenty I'd say just replace it all, the odds are very high that it has failed over time. Antenna cables are rarely installed properly and they stretch (which changes the electrical properties) in the mast, from their own weight. And they take moisture damage. If you don't mind going aloft, replace the cable. If that doesn't fix it, replace the antenna as well. If you don't like making trips aloft, just replace them both while you are up there, if they are over ten years old.
With proper installation and waterproofing, then you can just forget about them both for the next ten years, or more.

[continuouswave]

I suggest this approach to the problem:

--disconnect the transmissions line at the base of mast;

--disconnect the transmission line at the radio;

--measure the resistance of the transmission line between radio and and base of mast; there should be infinite resistance between center conductor and shield, and very low, 0.1-Ohm resistance between the center conductors and the shields at each end;

--if any discontinuity is found in the transmission line between radio and base of mast, investigate the two connectors; repair any failed connections or short circuits; consider replace the entire transmission line and installing new connectors; repeat testing until the transmission line between radio and base of mast is proven to be good. Terminate the transmission line into a 50-Ohm load and test with a directional wattmeter inserted at the transmitter end of the line.

At this point you should have a verified good transmission line between the radio and the base of the mast. If there were a problem in the transmission line between radio and the base of the mast, you may have found the problem.

If there was no problem found in the transmission line between radio and mast base, continue as follows:

Due to the relatively high cost of sending someone to the mast head compared to the relatively low cost of the antenna and transmission line, you will discard all of the existing components and buy new ones.

--buy a new antenna for mounting to the mast top;

--buy new transmission line to be routed inside the mast; consider upgrading the quality of the transmission line to lower-loss cable;

--buy new connectors for the transmission line;

--install the new connectors on the new transmission line;

--connect the new transmission line to the new antenna;

--temporarily mount the new antenna at a location on the boat that is not too close to any other conductors but easily reached;

--bring the other end of the new transmission line to the radio via the most expedient route--this is another temporary connection;

--carefully and accurately measure and record the resistance between the transmission line center conductor and the shield at the connector at the radio end of the transmission line;

--connect the transmission line to the radio;

--test the antenna and transmission line for VSWR using a quality in-line directional wattmeter; record the VSWR across the VHF Marine Band

Assuming all of the above goes well and the radio and antenna work properly, then perform the following procedure:

--remove the connector from the transmission line at the radio end;

--carry to new antenna and new transmission line to the mast head;

--install the new antenna on the masthead;

--install the new transmission line through the mast by using the existing transmission line as a pull wire; cut off any connector on the old transmission line so as to reduce the diameter of the cables being pulled down the mast;

--pull the new transmission line down the mast;

--re-install a connector on the new transmission line at the base of the mast; compare new resistance readings from earlier data.

Now the only unknown component in your antenna system is the new connector you just installed at the base of the mast on the new transmission line. Connect the new transmission line to the radio via the existing transmission line from the base of the mast to the radio. Repeat testing.

For advice on how to take VSWR measurements using an in-line directional wattmeter, see my article on this topic at

Measuring Radio Antenna VSWR
Measuring Radio Antenna VSWR - CONTINUOUSWAVE

[continuouswave]

Regarding what sort of test equipment might be owned by a randomly selected licensed radio amateur and their competence in using it:

There are millions of licensed radio amateurs, just as there are millions of boaters. The notion that all licensed radio amateurs are competent to perform rigorous analysis of antenna and transmission line problems and own the necessary test equipment to accomplish that is perhaps the same as all boaters knowing how to perform celestial navigation with a sextant and owning also one.

[TreblePlink]

Originally Posted by continuouswave

[...]

There are millions of licensed radio amateurs, just as there are millions of boaters. The notion that all licensed radio amateurs are competent to perform rigorous analysis of antenna and transmission line problems and own the necessary test equipment to accomplish that is perhaps the same as all boaters knowing how to perform celestial navigation with a sextant and owning also one.

Unfortunately, very true. Due to loosening the testing standards, maybe 1 in 10 hams has a basic understanding; probably 1 in 100 really understand antennas and transmission lines.

[continuouswave]

Something happened with the link in my earlier posting regarding how to make VSWR measurements. Let me try again:

Measuring Radio Antenna VSWR - CONTINUOUSWAVE

[waterman46]

Originally Posted by smac999

not entirely. because low power output from the antenna (what the swr meter is showing) is not the same as low power output of the radio. they will only match with perfect SWR.

IE if you see 10watts on the SWR it doesn't mean you only have 10 watts coming out of the radio. it means 10watts is leaving the antenna. the rest is probably reflecting.

if you also measure 10watts with a dummy load and test cable then it's a radio issue.

There were many good suggestions on this thread, but don't try to figure out the post I quoted here. This mixed up terminology and errors in comprehension could be very confusing and the whole post should be ignored.

SWR is standing wave ratio, NOT any indication of actual power level. Basic SWR meter will not measure absolute power level. More expensive unit may have power meter function built in, but that capability must be clearly stated somewhere. TDR is great for locating faults but expensive, forget it unless you call in a tech who has that equipment.

Signal reports from outside stations can be completely misleading and not to be trusted too much. Use signal reports skeptically.

If you will be doing much radio work, a cheap SWR meter and dummy load of 25W or greater, will be very useful. Buy or borrow both of these items. You will also need to buy or make up a short patch cable with the UHF connectors on both ends, to connect the dummy load. And if you want to use the dummy load with the handheld (see one of the tests below) you will need a TNC to UHF adapter. If you are confused about all this, I suggest you take the two units into a local ham radio shop and explain the problem and what you want to do.

First check your antenna and cable with the SWR meter and see if you get better than 2:1. ' Better than 2:1' means like 1.5:1 or 1.2:1 for example. If bad mismatch like 3:1 or whatever, you will need to go up the mast and put the dummy load on the end of the cable up there and retest the SWR again, inside the boat.

You can also use the cheap SWR meter to compare the output power of your handheld to the output power of your 25W unit, using the dummy load, not the antenna, on the output (antenna) port of the SWR meter. For this test you will need an adapter to connect the handheld antenna port, usually a 'TNC' connector, to the UHF connector of the dummy load. SWR meter shows relative forward power (the good stuff) versus reflected power (bad stuff). But looking at forward power in this test, you should see much more forward power with the 25W unit (set to high power mode), compared to the handheld. Just a good way to check the 25W unit itself.

Get back to forum with the results. It will be interesting!

[hellosailor]

Something worth mentioning about an SWR meter.
Installing one behind the radio (i.e. separated by a foot of cable) and then leaving it permanently installed, can be a good thing. Instead of putting it away on a shelf, leave it there. Put a mark on the dial where "normal" is. If you are using the radio one day and you see the SWR has increased above the normal mark? Then you know there's a problem that needs attention, possibly the usual water getting into the antenna and cable.

FWIW.