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Old 30-07-2017, 15:44   #1
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Checking VHF radio

My Icom M504 vhf is does not open a Sea Tow automated radio check point approximately 5 miles away. Previously I could use that one and another one probably 12 or more miles away.

I used a Shakespeare ART 3 meter to test system. SWR is 1.5 ,transmit power shows about 12 watts,when using the receive test on the radio I am not hearing the tone they describe. Also when transmitting and watch the ammeter on the boat it looks like the radio is using about 3 amp when transmitting.

I have a Metz antenna on a 54 ft mast.

Not sure if this data from the ART 3 is indicating a problem any help would be appreciated.
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Old 01-08-2017, 12:09   #2
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Re: Checking VHF radio

I now seem to be getting decent reception and transmit range from my VHF. But am not sure what to make of the Shakespeare ART 3 only showing 11 or 12 watts of output. If that is accurate I would assume my transmit range is being limited. Any input would be appreciated.The radio is a Icom 504.
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Old 01-08-2017, 13:21   #3
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Re: Checking VHF radio

Have you checked all / any plugs / connectors and the antenna connector at the masttop?

With vhf it is like with any other wiring - connectors tend to allow moisture and the metal inside degrades. This leads often to slowly degrading transmit power. If there is any slack, the cable can be shortened about 1 inch and then a new connector soldered on and sealed (I use the butyl tape).

This 12W sounds lowish. I think the specs call for 25W but I am not sure how this measurement is done.

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Old 01-08-2017, 23:13   #4
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Re: Checking VHF radio

12w is low. You should see 20-23 or so. (25 is legal limit so most radios fall under to be safe)

Normally if only 12 is going out it's because a bunch is comming back. But not if you are reading 1.5

I wound make sure you are using it correctly. (You need to set the dial)

If you are. I would measure the radio voltage at the input plug. It could be a big voltage drop issue.
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Old 02-08-2017, 12:10   #5
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Re: Checking VHF radio

Quote:
Originally Posted by smac999 View Post
12w is low. You should see 20-23 or so. (25 is legal limit so most radios fall under to be safe)

Normally if only 12 is going out it's because a bunch is comming back. But not if you are reading 1.5

I wound make sure you are using it correctly. (You need to set the dial)

If you are. I would measure the radio voltage at the input plug. It could be a big voltage drop issue.
I did do the dial set procedure, as I review what I did I may have zeroed to wrong side of the specified mark. At any rate assuming I did SWR is probably not more than 1.75.

Upon returning to the boat I will recheck SWR and try and check voltage at radio input. Any tricks to testing voltage looks like good insulator connections, I kind of hate to comprise them!

Also when one does have a high SWR the power shown going into the antenna would read low? Would radio be drawing enough power from input line to produce the 25 watts, if so where does the excess energy go?
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Old 02-08-2017, 14:50   #6
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Re: Checking VHF radio

I believe energy actually removed from the source but not radiated out (as radio waves) will be radiated out as heat. Either the unit would heat up or else the antenna would - depending on where the problem (the resistance) is located.

Energy is never lost so if it is actually removed from the battery (amp meter) then it must go somewhere. If not radio waves, then I think heat.

You should be able to run a comparative test by deploying another antenna of known quality on a short piece of vhf cable. This will also give you an emergency / spare antenna / cable for your boat.

If your boat is visible from any landmark remote enough, you can then make a real life test with a handheld unit and a friendly soul operating at one end.

Let us know what ensues.

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Old 02-08-2017, 18:23   #7
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Re: Checking VHF radio

Don't just check the voltage behind the radio, check the voltage UNDER LOAD. Key the transmitter, or use a 20W light bulb on the power cables, and make sure you see full battery (or alternator) voltage when there's a LOAD on the wiring.

If you've got some bad wiring or a corroded contact, it is very possible to see 13 volts when there's no load, but 11 under a high load. Which would be a problem.

If the power is good, then it is time to check everything else.
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Old 02-08-2017, 21:45   #8
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Re: Checking VHF radio

Quote:
Originally Posted by Sea Lyon View Post
I did do the dial set procedure, as I review what I did I may have zeroed to wrong side of the specified mark. At any rate assuming I did SWR is probably not more than 1.75.

Upon returning to the boat I will recheck SWR and try and check voltage at radio input. Any tricks to testing voltage looks like good insulator connections, I kind of hate to comprise them!

Also when one does have a high SWR the power shown going into the antenna would read low? Would radio be drawing enough power from input line to produce the 25 watts, if so where does the excess energy go?
Yes high swr will read low power output. Because the rest is reflecting back into the radio. And damaging it. A better meter actually only measured power going out, and power comming back in. And you have to calculate the swr yourself. And then you'll see like 12w out, 10w in.
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