VHF antenna amp.

[Lake-Effect]

One year I visited a friend who was bragging about how he had run fairly heavy coax through his mast. At the dock he was flipping through all the available WX channels he could get... then i showed him that I could get more WX stations on a cheap handheld. Turns out that he hadn't wrenched up the new connectors tight enough.


Moral - check the obvious stuff first.


And yes, a VHF VSWR meter inserted after the radio is the best check on the overall state of the antenna system from radio to antenna.

[model 10]

One more question, is there a way to ohm out a connector and a section of coax with a regular meter?

[Captain Bill]

Originally Posted by Ecos

One more question, is there a way to ohm out a connector and a section of coax with a regular meter?

It will tell your if there's no connection but unless it's really really bad you probably won't be able to tell with an ohm meter.



As for the 1 watt linear amplifiers on ebay they are designed to amplify weak TV signals. As soon as you hit the transmit button they will fry. You cannot simply run the signal through them. Also they are likely 75 ohm as they are designed for TV and not compatible with your 50 ohm VHF system. It would seem that you asked for advice here because you thought some people might know more than you. You are getting good advice from people who know something. Why are you arguing with them?

[Wotname]

Originally Posted by Ecos

One more question, is there a way to ohm out a connector and a section of coax with a regular meter?

No, not really.
Of course an ohm meter can tell you if the connector has become open circuit or has high resistance or if the inner and outer of the coax is shorted etc and therefore faulty.

However there is a multitude of situations (like a poorly assembled connector or kinked coax) which will cause serious RF problems yet not show up with an Ohm meter.


Although sounding like a broken record, the SWR meter will pinpoint most (almost all) problems in the transmission path.

EDIT: Captain Bill is faster on the keyboard

[model 10]

[QUOTE=Captain Bill; You are getting good advice from people who know something. Why are you arguing with them? [/QUOTE]


Wow dude, Why don't you just disregard me? Or do you have some need to sound superior? If you are going to try that, do a better job of explaining things.
Besides that, people are using amps even if you think they don't. So how about an answer that does not include your opinion of my psychology?

[Wotname]

Originally Posted by S/V Illusion

Just one point for clarification - an SWR meter doesn’t indicate antenna efficacy. For example, one could attach a light bulb to the end of the coax (or dummy load) and see a 1:1 SWR ratio.
............

Yes, I didn't elaborate on this aspect - mainly because I was trying to keep it simple but your clarification is important to remember. Thanks for highlighting it.

Originally Posted by Wotname

...........
Please remember this is the simple explanation and not the truly technical one but it should give you the general idea of how the SWR meter can be useful.

Originally Posted by S/V Illusion

........
An SWR meter ‘sees’ only what’s happening at a given point in the coax. An antenna analyzer can do what you suggest but that’s another animal entirely.
............

This is also correct and raises the question of how to use an SWR meter. A big subject but suffice to say to the OP that judicious placement of an SWR meter at various places along the transmission path (i.e. back of the radio to the antenna) and an understanding of the readings will pinpoint the locations his problems.

The big plus of a decent antenna analyser and /or a time delay reflectometer (TDR) is that most of the time, all the analysis can be done from the comfort of the cabin (and for the professional, quickly and simply) . Mind you, if the fault is aloft (and it often is), someone still has to go up the mast to sort it.

But I getting away from the OP's basic problem so let's return to that -

All you really need to know is how much of the 25 W from the back of the radio is reaching the antenna. . Find someone with a suitable RF power meter, measure the output power at the radio and then measure it at the antenna connector (i.e. the far end of the coax). Or even better, flog off the linear amp and buy a SWR meter - they are less than $100. They are reasonably easy to use and there is plenty of help on CF if you get stuck

You do need to ensure the meter is suitable for the VHF frequency range and can handle 25 W. You will need a few coax adapter connectors and a 50 Ohm dummy load but soon you will the local antenna guru!

[stormalong]

Originally Posted by Ecos

Is been this way for 12 years. No better, no worse over the years. The radio was new at the that time and the antenna and all the connectors to the base of the mast were new last year. From the base of the mast to the radio is the 1970's coax which is a much larger cable than the rg-58. There is also a new connector at the radio. I have never tried an SWR meter.
The radio works just fine out in the open and up close, but during the morning cruiser net's we often don't hear everybody in the area.
This new amp has a peak power of 30w, so I guess it could handle the 25w from the radio. I don't understand the 30 db gain part. What does that mean?
As for being illegal, there is a boat here in the jungle with an amp and he sounds great.

You did not say if the problem is receiving or being heard. In either case 1970's coax is 40 to 50 years old. In a marine environment it is past due for replacement. And you said you have RG-58 from the mast to the radio. More connectors to corrode. A proper connection at the base of the mast would have almost no loss. However, in a marine environment connections corrode. It is best to have a single run of cable with no intermediate connections.

You can use an ohm meter to "guess" at the condition of the coax but to do it you have to disconnect the antenna, test for conductivity from shield to center - should be open circuit. Then short the connector at the masthead and the resistance should be close to zero. Any other results it's a bad cable but passing this test tells you nothing about the cable's performance at VHF frequencies. An antenna analyzer is the only way to test for sure.