Antenna wire testing

[onestepcsy37]

i have a masthead vhf antenna. it has a long cable running from the masthead to the nav station. probably 60 feet. vhf reception and transmission is poor to nonexistent. i've tested the radio on another antenna and it's perfect. so it's the antenna or, more likely, the wire.

question. is there any way i can electrically test the antenna cable? i'm electrically challenged and don't really understand these things, but could i, for instance, put an ohmmeter on the antenna plug and get some kind of expected reading? how about shorting one end and then seeing if can get an electric flow through the cable with a battery powered line tester?

i'm looking for ideas from those of you who know more about this than i do...

[model 10]

I can't believe all these radio guys don't help out. You should have no continuity between the shield and core. You could check the voltage drop over the whole length and inspect all the connections. Not sure what the voltage drop would be for your cable. There is a chart somewhere. Probably a connection.

[Paul Elliott]

Your VHF antenna may normally look like a short circuit between shield and center conductor. Check the antenna specs if you can. I know that the spec will say "50 Ohms", but this is the Radio Frequency impedance, not the D.C. resistance you will be measuring. If this is indeed the case, with the antenna connected you should measure a very low resistance between shield and center conductor (measure at the radio-end of the cable). If you measure more than a few Ohms, there is an open connection, probably at one of the connectors.

If the antenna is normally a high-impedance, you should measure a virtually infinite resistance instead. If you measure something lower than a few thousand Ohms, then you have a short, again, probably at the connector.

Still can't figure it out? It's time to climb the mast and disconnect the cable from the antenna. You can again measure for shorts (center to shield) and opens. The easiest way to test for opens is to connect a cliplead between the center and shield, then measure the center-shield resistance at the other end of the cable. It should be no more than a few ohms.

Or, borrow a SWR meter, and jumper cable (since you are inexperienced with this, ask for help, or do some research on how to use an SWR meter).

There are more subtle failure modes, beyond shorts and opens, but these are unlikely. And don't assume the antenna is OK. It's more likely the connectors, but antennas do go bad.

Report back when you have done some of these simple tests, and if necessary we can walk you through any follow-on steps.

Here's a lively discussion about troubleshooting coax and connectors: http://www.cruisersforum.com/forums/...re-102997.html

[rwidman]

Originally Posted by Guy

I can't believe all these radio guys don't help out. You should have no continuity between the shield and core.

With the antenna disconnected, short the inner and outer conductors at one end together with a clip lead and you should read continuity between the inner and outer conductors at the other end. In this case, "continuity" could be a few ohms.

You could check the voltage drop over the whole length and inspect all the connections. Not sure what the voltage drop would be for your cable. There is a chart somewhere. Probably a connection.

There won't be a voltage drop that can be measured. An SWR meter would be one tool to check the assembled cable and antenna. A more practical approach for the OP would be to call in a marine radio technician and have him or her diagnose and correct the problem.

Or, buy a new antenna and new cable, pay a pro to install all the connectors, and then remove the old stuff and install the new.

[fairbank56]

Originally Posted by rwidman

With the antenna disconnected, short the inner and outer conductors at one end together with a clip lead and you should read continuity between the inner and outer conductors at the other end. In this case, "continuity" could be a few ohms.

If you read "a few ohms", you have a problem unless your using RG58 (would be less than 1 ohm), and if you are using RG58, you should replace it with at least RG8x (about .5 ohms) or preferably the larger RG8 "type" coax (about .2 ohms). These are the DC resistances you would read with meter on 120' (out and back on your 60' run).

Eric

[rwidman]

Originally Posted by fairbank56

If you read "a few ohms", you have a problem unless your using RG58 (would be less than 1 ohm), and if you are using RG58, you should replace it with at least RG8x (about .5 ohms) or preferably the larger RG8 "type" coax (about .2 ohms). These are the DC resistances you would read with meter on 120' (out and back on your 60' run).

Eric

I should have said one or two ohms, not "a few". That's what I was thinking, it just came out wrong. He has 60 feet of cable, that's 120 feet of conductor to measure. Even the leads of most meters will read a tenth of an ohm or more when held together.

Notice that I suggested bringing in a pro on this. To the average person, electricity is a mystery and radio much more so. He may try to follow advice given on a web forum, but much of it is bad advice and the average person is not going to understand what's being said, be it bad advice or good advice.

Unlike a TV set or entertainment radio, it's pretty important to have a VHF radio system that's working correctly and reliable.

[onestepcsy37]

i'm hoping that somehow i can take some simple measurements from the ground that will tell me that i won't have to go up the mast - or that i will.

sorry to report that so far, after reading all your posts, i know even less than when i started. worst case is i have to snake a new wire through the mast. not looking forward to that....

[model 10]

From the info you get here you may never know. One says you will get no voltage drop over 60 or 120' of wire...? the same guy or another says spend a boat buck or so to pull your mast and string new wire. You might be screwded!

[Paul Elliott]

Not to be a jerk, but I did give you some specific advice:
1) Find out if your antenna is supposed to measure open or shorted.
2) Measure the DC resistance between center and shield (measure at the radio end of your coax).

Depending on your antenna type, you now at least know if you have an open connection, or if you have a shorted connection. Your measurement may tell you more than this.

You can also try the SWR meter between your radio and the coax. This may confirm that you have a problem, but it probably won't tell you where it is.

Unless you discover that your radio-end connector is bad (or badly soldered), to narrow it down any further you will have to climb the mast and disconnect the antenna. You can now make the open / shorted measurements.

Be prepared to connect a temporary known-good antenna on the mast. If your cable and connectors are OK, it's going to be the antenna / antenna cable / antenna cable connector (these last two only if your antenna has a coax pigtail).

So start with the quick and easy tests. Let us know what you find. In this thread at least, I haven't seen any bad advice. Your other options are to hire a pro, or find a friendly ham.

[fairbank56]

Originally Posted by rwidman

Even the leads of most meters will read a tenth of an ohm or more when held together.

That's because meter leads are typically 20-22 gauge and cheap meters don't allow you to compensate. Good meters do and better meters do it automatically once you've entered the initial value.

Eric

[Lake-Effect]

The best single test of your antenna + lead-in is an RF analyzer or a VHF VSWR meter.

The tests generally go like so:
- if reception sucks and the radio receives well on another antenna and cable, you definitely have a problem, most likely with a connection.
- If reception is close to OK, an RF analyzer or VHF VSWR will confirm the functionality of the antenna plus cable as an RF system.
- antenna can be tested separately, the line can be tested by substituting a proper 50 ohm RF load in place of the antenna and repeating the VSWR tests

If electronics isn't your thing, I suggest first checking all connections, then hiring someone for RF tests.

[onestepcsy37]

i hope i haven't come across as being ungrateful for all your collective advice. but in view of my being an electronic simpleton even the simplest tests you've offered up seem to be beyond my understanding.

i have determined that with a good cable/antenna (friends boat) my vhf radio works great. my current antenna/cable is probably original, and 34 years old. i could hire a radio expert and i'm beginning to think he will tell me to replace the antenna/cable. so i'll save that expense and aggravation and just go ahead with planning a replacement. not looking forward to it but at least it's the last item on my must do list before heading out.

in view of that, i would like to ask another question. what is the best kind/size of antenna cable to use for a masthead mount with about a 60 foot run?

[model 10]

"The last item on your to do list" that is just about the limit.

[Jd1]

Originally Posted by onestepcsy37

in view of that, i would like to ask another question. what is the best kind/size of antenna cable to use for a masthead mount with about a 60 foot run?

RG8x .....

[rwidman]

Originally Posted by Guy

From the info you get here you may never know. One says you will get no voltage drop over 60 or 120' of wire...? the same guy or another says spend a boat buck or so to pull your mast and string new wire. You might be screwded!

A signal loss in a radio transmission cable would not be measured as a voltage drop.

I suggested a replacement antenna and cable as an alternative to hiring a radio technician to find the problem. Some people have an aversion to hiring experts, they like to do everything themselves.