VHF radio - permanently high swr reading - causes?

A

andyp

Well-Known Member
Joined
22 Oct 2009
Messages
88
Location
Gosport
Visit site
Hi,

One of our yachts has a problem with her VHF radio (ICOM IC-M503).

The symptoms are poor transmission range and when I checked with an SWR meter the reading was high (1:3 or greater) whether I used the masthead antenna, the stern rail AIS antenna or an emergency handheld antenna.

I don't believe a fault in the transceiver itself would cause this (apart from a breakdown of the TX/RX switch inside the box) so I wonder whether the root cause may be poor grounding of the transceiver itself?

Any advice welcome.

Thanks
 
Send it back to Icom to be checked. A friend had similar problems with an Icom, various antennas gave high VSWR although his suffered with reception too. Icom fixed it for about £50 if i remember correctly.
 
andyp said:
Hi,

One of our yachts has a problem with her VHF radio (ICOM IC-M503).

The symptoms are poor transmission range and when I checked with an SWR meter the reading was high (1:3 or greater) whether I used the masthead antenna, the stern rail AIS antenna or an emergency handheld antenna.

I don't believe a fault in the transceiver itself would cause this (apart from a breakdown of the TX/RX switch inside the box) so I wonder whether the root cause may be poor grounding of the transceiver itself?

Any advice welcome.

Thanks
Click to expand...

Handheld antenna really shouldnt need the unit to be grounded.
If its possible try a different unit and compare SWR for a particular antenna.
personally I wouldnt go with your "I don't believe a fault in the transceiver itself would cause this (apart from a breakdown of the TX/RX switch inside the box)". I would most certainly be pointing the finger at the box, theres a bucket load more than TX/RX switch that can do this.
 
As a radio ham I regularly check fellow boat systems with my antenna analyser, saves loads of time and over the last few years never been an issue with a modern radio., swr meter don't really help, most high swr issues are cable to antenna. Would suggest call you local radio club and for a pint you should get someone to check it for you. Be a little careful checking high swr with a radio as it has to transmit to do this and high swr or cable fault can blow the output transistors and then you will have a broken radio. One good thing about Icom is they are very good at fixing stuff, used them many times for amateur radio gear
 
I did my C&G amateur radio course in the 1980s and I've not been active for many years and I've forgotten most of it so I may be talking bolox ......but

When testing a vhf, I'd always set to low power and use quality patch coax leads through an SWR meter into a dummy load. I'd hope to find values of around 1.2:1 across the frequency range. Then repeat the test into the antenna system.

Valve power amps are more tolerant of high SWR and I recall using long wire antennas, through an ATU quite successfully at around 3:1 (albeit there was a risk of interfering with neighbouring TV sets). Solid state ttansmitters are much less tolerant of high SWR and risk damaging output stage. A high SWR reading on a marine vhf radio should be investigated.
 
andyp said:
Hi,

One of our yachts has a problem with her VHF radio (ICOM IC-M503).

The symptoms are poor transmission range and when I checked with an SWR meter the reading was high (1:3 or greater) whether I used the masthead antenna, the stern rail AIS antenna or an emergency handheld antenna.

I don't believe a fault in the transceiver itself would cause this (apart from a breakdown of the TX/RX switch inside the box) so I wonder whether the root cause may be poor grounding of the transceiver itself?

Any advice welcome.

Thanks
Click to expand...

Crikey we dream about getting the SWR down to 1.3 on most of our installs.
First are you using a proper VHF meter and not a cheapo CB unit.
Ahem..and vast numbers of VHF installations are soldiering on with 3:1 and much much worse.
New equipment is vastly more robust than the old stuff.
As you are complaining about Tx ( and not RX) range would first suspect power output, hopefully you are not checking with channel that automatically switches to low power usually 1 watt.
There was a problem with Icom stuff where a filter in the receive circuit made the set deaf.Cheap and simple fix especially if contact ICOM direct.:)
Are you reaching the distant station...but unable to hear them ...a common cause of thinking you are not transmitting correctly.
A meter that measures power would be handy or if you are any good with a multimeter see how much current is being drawn in both high and low power.
...and 99% of problems are corrosion in the power line most usually either an inline fuse or earth connection.
The connections on the glass fuses corrode and fall to bits,the blade fuses simply weld themselves into a sold mess,aluminum just adores a salty enviroment.
 
Last edited:
Just to add that the SWR meter if used does need to cover VHF as lots of common ones around are ex CB and cover lower range nearer HF, unfortunately an analyser will cost about £200 minimum, mine was £450 but does the job very well. Also to add my swr measures 1.15 : 1 but that is using an antenna I've made at home for less than £10
 
You do not clarify where you have connected the swr meter and the alternate antennas. If you have connected the SWR meter and its patch lead to the back of the radio then the fault is either the radio or the patch lead or SWR meter if you are connecting the alternate antennas to the swr meter. If you are using the deck fitting then the fault could be the deck fitting or cable to the radio from deck fitting.

Do you have a switch to select stern rail or masthead etc?Is any of this in the circuit when testing?

As others have said you should use an SWR meter which is calibrated for VHF. Most CB meters are for HF.
 
Thanks for all the help and tips.

When I swapped the suspect set to an identical yacht the SWR performance "improved" to less than 2:1 (was 3:1, not 1:3 as I originally typed) so I believe the set is basically OK. All tests have been made with the SWR meter patch lead on the rear of the radio set and the antenna plug (PL259) directly connected to the meter. There are no antenna selection switches in the circuit.

That leaves the aerial connections or the power supply arrangements. I know the set is only drawing about 3A when transmitting on a 25W setting and I think this should be nearer 6A so that may be part of the issue - low current = low power = shorter Tx range. I have remade the power connections and removed the in-line fuse temporarily.

I checked my SWR meter (which was not expensive) and as pointed out by other formites it is indeed an HF/CB version, optimised for 28 MHz! Obviously not the correct piece of kit to test a VHF 156.8 MHz waveform but by using the same meter for tests in different boats and on different radios, I believe the difference in result can be used to narrow down the fault.

We are at sea next week so will try for some more definite measurements of effective range. Thanks again.
 
andyp said:
Thanks for all the help and tips.

When I swapped the suspect set to an identical yacht the SWR performance "improved" to less than 2:1 (was 3:1, not 1:3 as I originally typed) so I believe the set is basically OK. All tests have been made with the SWR meter patch lead on the rear of the radio set and the antenna plug (PL259) directly connected to the meter. There are no antenna selection switches in the circuit.

That leaves the aerial connections or the power supply arrangements. I know the set is only drawing about 3A when transmitting on a 25W setting and I think this should be nearer 6A so that may be part of the issue - low current = low power = shorter Tx range. I have remade the power connections and removed the in-line fuse temporarily.

I checked my SWR meter (which was not expensive) and as pointed out by other formites it is indeed an HF/CB version, optimised for 28 MHz! Obviously not the correct piece of kit to test a VHF 156.8 MHz waveform but by using the same meter for tests in different boats and on different radios, I believe the difference in result can be used to narrow down the fault.
....
Click to expand...
Using a 28MHz SWR meter at VHF is asking to damage your transmitter.
There is no basis to assume that comparisons between two transmitters using a mismatched meter will tell you anything useful.
You could get a wildly different reading just due to the phase of the source impedance being a little different.
If your swr is anything like 2:1, there is no reason for that to cause the power drawn to drop.
 
andyp said:
I know the set is only drawing about 3A when transmitting on a 25W setting and I think this should be nearer 6A so that may be part of the issue - low current = low power = shorter Tx range.
Click to expand...

6A sounds pretty high. The manual may say it as a max, but 25w is about 2A, plus a bit for running the set. Would be surprised if you need another 4A for pumping out 2A.

When you say TX range is short - what order of short do you mean - a few yards or miles?
 
Pye_End said:
6A sounds pretty high. The manual may say it as a max, but 25w is about 2A, plus a bit for running the set. Would be surprised if you need another 4A for pumping out 2A.

When you say TX range is short - what order of short do you mean - a few yards or miles?
Click to expand...

The current consumption going into a transceiver has no direct correlation to the transmitted power. The 25W refers to the transmitted amount of RF energy leaving the set. Any transmitter is nowhere 100% efficient. My 100W transceiver draws 20A on full output, say about 240W DC input. However I would expect more than 3A on transmit, more like 4.5 /5.
Perhaps you loosing some volts before it reaches the set ?
I would be very dubious of trusting a CB type SWR meter on the frequencies we are dealing with here.
Good luck.
Richard
 
With smaller output power high swr will have a large effect on the transmitted power output, 25watts it not much in the radio world. As said before you can spend lots of time testing but your really using the wrong equipment to test the antenna. If you send the radio to Icom and they tell you it's ok you are no further forward. Where is your boat located?
 
I can't imagine that a radio problem is a diy job. As I offered in a previous post, I still think it best to test SWR into a dummy load while on low power. If the result is around 1.2:1, the problem is not with the radio. Check the coax, joints and antenna. If the SWR is high, send it for repair.
 
davidjackson said:
I can't imagine that a radio problem is a diy job. As I offered in a previous post, I still think it best to test SWR into a dummy load while on low power. If the result is around 1.2:1, the problem is not with the radio. Check the coax, joints and antenna. If the SWR is high, send it for repair.
Click to expand...

Using a dummy load should give 1:1 or close and with a good accurate power meter test output, the swr test is for antenna effencency, two separate tests, either not reliable unless you have the gear, which will cost more than a marine radio and experience to really test and be confidant of the results. Different story for Ham gear when a transceiver can be over £1000
 
Looks like the problem is on the boat and not in the radio.
Using a CB VSWR meter at low wattage has always given me good results in fault-finding as it is only a ratio we are measuring, doesn't have to be watt-accurate. ( I have a shed full of bigger stuff, but not for the boat)
I guess 99 % of small boat VHF transmit problems are connectors, saturated cable, or seagull-attacked twigs.
 
You must log in or register to reply here.

Share this page

Top