VHF voltage drop when PTT

[santavey]

Trying to sort out poor VHF performance.
Discovered voltage at VHF drops from 12.4 V to 12.2 when PTT on 1 watt.
But a massive drop to 9.6 V when PTT on 25 watts.
What should I expect to see? No drop, some drop?
Batteries OK. One is new. Set is Simrad non DSC wired through switch panel.
Would the VHF operate with only 9.6v i.e. not sufficient power to “push” the signal along 60 ft of co-ax and out the other end?
Is the voltage drop simply caused by inadequate or poor DC wiring or could I have a set problem?
Does the voltage (or power) required to “push” the signal along a a longer co-ax have to be greater than for a short co-ax. If it does that might explain why the set gives adequate but not fantastic performance when connected to a secondary antenna on a short co-ax run.
Lots of questions I know
Thanks in advance for any advice.

 

[barrow_matt]

From my amateur radio days I do remember seeing voltage drops on the power supply when transmitting. Have you ever started your car with the lights on and noticed how they dim.

What gauge are your power wires, bigger is better! I believe the more resistance you have in the wiring the bigger the effect will be.

 

[noelex]

That voltage drop is way too high and will stop the VHF transmitting on high. You should be seeing something less than 0.5V on full transmit power.
With that massive voltage drop, it is almost certain you have a poor connection somewhere.
Measure the voltage drop along the connections to determine where the voltage drop is occurring. Don’t operate the set for long on high as these low voltages could do some damage.
You can try connecting the set directly to the batteries ( with a fuse) to check the set is OK, but its likely to be fine.

 

[3663Ian]

Tricky and lots of differnt quastions here....

The VHF is probably spec'd down to 10.8 Volts. It may work a bit lower. Please tell us the model number. Any way it's not good so we need to find out why.

The lenght of the coax should not be a problem (it's condition might be)

I am assuming that the radio is connected to the domestic battery.

I suggest a few simple tests.

Check the voltage both at the battery connection posts and near the radio (off, Rx, and TX)

Repeat this with the radio connected to the emegency antenna.

If the voltage drops on TX with both antennas.....then fix this first.
If the votage drops at the battery connection posts then its a battery or battery charging problem
If the voltage only drops at the radio its a wiring problem.
Once that is resolved retest the transmission.


If the voltage only drops on the main antenna then we will pick up form there.
Let us know how you get on.

 

[savageseadog]

Attach a long wire to the battery negative and put a decent digital meter negative clip onto it. Measure voltages at the radio while on transmit, the negative and positive should not change by more than 0.125-0.25V. Work back through wiring, connectors and fuses to find the cause.

 

[Bilgediver]

Lots of questions I know
Thanks in advance for any advice.

If the battery is OK and starts the engine and shows over 12 volts even when the Ptt is pressed then if the voltage at the radio is low there are faults in the power feed to the radio.

The radio will take a little over 6 amps on full power so make sure the the supply cable is OK and check all the connections including the insides of the inline fuse holders as any corrosion in these places can cause a voltage drop.

You could check the radio itself by connecting direct to the battery and seeing if it works OK.

The radio should work down to a voltage of around 10 volts but below that it will probably cut out.

The radio supply should be adequate to supply all the power the radio needs even when connected to a mismatched antenna and there should be no appreciable voltage drop at the radio.

 

[santavey]

Set is Simrad/Navico RT1400
Couple of years ago replaced antenna and new co-ax down mast 'cos I thought that was the problem.
Didn’t work.
Last year replaced co-ax from base of mast to set.
Didn’t work either.
Went back to using antenna mounted on pushpit.
That works but doesn’t give the range to other boats.
Seems OK when contacting Liverpool on Ch 16
And thats the bit I dont understand. If set cuts out on low voltage it should cut out regardless of the antenna connected or would it?
Cant get back to boat for a few days but will try all suggestions.
Many thanks.

 

[ccscott49]

Set is Simrad/Navico RT1400
Couple of years ago replaced antenna and new co-ax down mast 'cos I thought that was the problem.
Didn’t work.
Last year replaced co-ax from base of mast to set.
Didn’t work either.
Went back to using antenna mounted on pushpit.
That works but doesn’t give the range to other boats.
Seems OK when contacting Liverpool on Ch 16
And thats the bit I dont understand. If set cuts out on low voltage it should cut out regardless of the antenna connected or would it?
Cant get back to boat for a few days but will try all suggestions.
Many thanks.

Just an aside, DO NOT transmit without an antenna connected, you will burn out the output transistor. I also agree with the others, its a wiring problem, no way should the voltage drop that low.

 

[srm]

Had a similar problem with a fridge - finally traced it to a corroded aluminium bodied fuse!
but only after I had checked every other connection and replaced some of the wiring with heavier duty.

 

[Bilgediver]

Had a similar problem with a fridge - finally traced it to a corroded aluminium bodied fuse!
but only after I had checked every other connection and replaced some of the wiring with heavier duty.

That was why I reminded OP to look in the inline fuse holders....Hidden away in a damp spot who knows the condition.

 

[elton]

It sounds like you've got an inadaquate power supply to the VHF. Try connecting it directly to the battery with heavy duty cable.

 

[Boathook]

I would carefully check the cable size. This has caused more me problems than corrosion over the years. Becuse of the cable run length on my boat I have used 6mm2 wire which is the biggest that the standard yellow crimp fittings take.