Why can't I transmit on some VHF channels?

Gubbo

Gubbo

Well-Known Member
Joined
17 Jun 2001
Messages
106
Location
Woodbridge
www.sailpast.co.uk
Why can\'t I transmit on some VHF channels?

I have just installed a brand new Simrad RD68 using my existing antenna set-up.

This radio has an "ANT" indicator on the screen which comes on instead of "TX" when the radio detects a fault. My problem is that it seems to work perfectly up to channel 8 (156.4 Mhz).

When I move up the frequency scale to 9, 10 or 69 it briefly flashes the ANT warning then settles to TX.

All channels above 10 (156.5 Mhz) simply produce the ANT warning locked on.

It is receiving fine and, much better than the previous set judging by the fact that I was picking up Harwich VTS (ch71) from Woodbridge. A test call to Coastguard on 16 didn't get a response.

I have checked and re-made the through deck connection and the masthead 1m stainless whip aerial was new last summer, as was it's cabling. I'm a little dubious about the cable run inside the boat from the radio to the deck socket because the innards were a showing some blackening when I cut it back to re-make the connection yesterday.

I intend to buy some more plugs and sockets to let me bypass parts of the system and try it with a separate aerial altogether.... but I need to visit a chandler for this and I'm working this weekend.

This is where my knowledge of radio runs out...... Do you think I'm right in suspecting the antenna system as causing this problem? Why should the higher frequencies be affected? Could the 12v power be the problem of it dips below on transmit? Could it be the radio itself?

I have called Simrad yesterday for technical advice but they haven't got back to me yet.... perhaps the forumites can do better /forums/images/graemlins/wink.gif
 
Re: Why can\'t I transmit on some VHF channels?

"I'm a little dubious about the cable run inside the boat from the radio to the deck socket because the innards were a showing some blackening when I cut it back to re-make the connection yesterday."

Quite likely thats where your problem is then. The set is showing 'antenna fault' because it is not connected to the antenna properly, as the black oxide coating of the wires is quite a good insulator.
 
Re: Why can\'t I transmit on some VHF channels?

It's quite possibly an antenna fault. If you can find someone with a VSWR meter, you can easily check this. It should be one suitable for VHF. Some CB radio types will be OK but there's no guarentee. If you know anyone who's a radio ham, he should have one.

A VSWR meter essentially measures the ratio of the power going up to the antenna & the power comming back. A perfect system has none comming back. A good system will read 1.1-1.3. Acceptable is generally less than 2. 3 means 50% is comming back. A short or open circuit will read infinite. What it doesn't do is measure any loss in a dodgy cable. However, most times this will lead to a high VSWR anyway.

Radios, will sometimes effectively measure this themselves & not transmit into too high a VSWR. VSWR high enough to trigger transmit protection doesn't necessarily mean poor receive performance. SOunds as if your antenna is on the edge since the lower frequencies will transmit OK but the higher ones will not.

If you want to get a meter, then somewhere like Waters & Stanton has simple ones that would suit from £35.

If after all of this, you decide to replace the antenna, then a quick trick may avoid it. Since the lower frequencies are OK & the higher ones not, you may find VSWR drops with frequency. All antennas will have increasing VSWR at the band edges. Try cutting a bit off the antenna & re-measure - 10mm at a time. Most marine antennas should be cut the the appropriate length anyway. Only do this if you've decided the antenna needs replacing & the VSWR is not a minimum mid band.
 
Re: Why can\'t I transmit on some VHF channels?

Your radio is protecting itself from transmitting into a very high standing wave ratio (SWR) antenna system and being damaged. It has an internal SWR meter.
The SWR is probably just at the point where the slightly higher frequency takes it over the internally set limit - perhaps 3:1. A good SWR is around 1.2:1 to 1.5:1.
You need to check cable and connectors - you may have a badly corroded section of cable or a badly fitted connector.
 
Re: Why can\'t I transmit on some VHF channels?

I suspect you are bang on the money with your theory. Not really my area of expertise, but I am sad to say I did spend a lot of my time in the early eighties messing about with CB radios. It is possible to ruin your set if you do not trim the aerial correctly, although it sounds like this set has an inbuilt protection circuit. I still have a SWR meter, which was working the last time I used it (1983/4!) Gubbo, you are welcome to use it, if you so desire.

My vague recollection is, as you transmit there is an element of your transmission which is absorbed by your aerial. If that element is too large then you could blow the RF transistors in your set. I personally would not use the set until you have checked the SWR, for fear of causing lasting damage.
 
Re: Why can\'t I transmit on some VHF channels?

[ QUOTE ]
I personally would not use the set until you have checked the SWR, for fear of causing lasting damage.

[/ QUOTE ]

Its sense not to use the set until you have sorted the aerial but dont worry overmuch about damage - the symptoms you describe are the radio protecting itself against a duff aerial. And dont mess around chopping bits off the aerial - thats interesting stuff to a radio ham but your aerial will already be the right length . The problem is undoubtedly the cable and it sounds as if you have water in it.

I once had a leak into the center of the boat that I struggled for ages to find. Turned out to be water dribbling down the inside of the VHF coax between the outer plastic and the inner. All the way from the masthead.

Incidentally, make sure you dont have a single strand of the outer braid bridging the plugs you have pout on. This can have very peculiar effects on occasions, and its easy to do
 
Re: Why can\'t I transmit on some VHF channels?

If it's OK at one end of the frequency spectrum, and trips at the other, the aerial could need trimmed. Quite often there is a grub screw at the base, holding the SS rod in place. This can be loosened and the rod length adjusted.
It may be that your radio is being too sensitive.
Start at chan 16 (156.00, centre of band) and work away in either direction. If it fails going up the way, it's too long, and vice versa.
Otherwise get a good quality aerial from the man a couple of postings up.
 
Re: Why can\'t I transmit on some VHF channels?

Many thanks for the replies so far.

I'm more confident that the cable replacement is the way to go.. at least as the next step. The antenna itself is a standard Glomex one so I trust its the right length.

I'm hoping that this will result in a good system, assuming that the previous radio was simply making do with the resistance in the cable and not being as "fussy" as the new one.

Gubbo
 
Re: Why can\'t I transmit on some VHF channels?

OK - my answer would be to try another antena direct, not through existing cabling. Do you have an emergency antena ?
Or simply unship radio - find another boat that you can temp connect radio and antena ... try again.

ok so you dont have friendly boater to try it out with different antena etc.....

Do you have a CB pal ? Now what I say will be poo-poo'd by some here - but it works. I know as it's what we did at the office for our "station".

Get your pal with his CB antena from his car to connect to your VHF instead of the set-up on board. Now try it. Radio will not be harmed (well ours wasn't .... ) you should get some life.

I know others have suggested antena cable - but I have doubts - I feel it may be radio. The frequencies are not that far apart that you should have such difference in use. IMHO.

I know you risk paying out - but is it better to have radio checked ?
 
Re: Why can\'t I transmit on some VHF channels?

Yes, I do have an emergency aerial set-up that I can use but just need to finish it off with another plug. I don't want to try it with any bodged connections or that may be misleading too.

I've also got a friend's boat next door to me on the hard and he's got a new antenna so should be able to try that way too.

I feel that I need to check out my installation before taking the radio back. That's a no cost option for me because it was bought locally brand new and is only a week old.

I'd better get the soldering iron on standby......
 
Re: Why can\'t I transmit on some VHF channels?

[ QUOTE ]
Yes, I do have an emergency aerial set-up that I can use but just need to finish it off with another plug. I don't want to try it with any bodged connections or that may be misleading too.

I've also got a friend's boat next door to me on the hard and he's got a new antenna so should be able to try that way too.

I feel that I need to check out my installation before taking the radio back. That's a no cost option for me because it was bought locally brand new and is only a week old.

I'd better get the soldering iron on standby......

[/ QUOTE ]

Excellent - so you have no-cost options at hand ... OK a little socialising expense to test and find out whether its radio or installation.

I use this method for many other boat items - echo-sounders, speed etc.

Bottle of wine in one hand ... item to test in other ... if you can't find a boat capable of connecting and testing ??
 
Re: Why can\'t I transmit on some VHF channels?

Do not use a CB antenna the ferquencies are completely different and will certainly cause damage, if it doesn't you will be extremely lucky

Re cabling is almost certainly the answer, if the cable showed oxidation then there is a short somewhere in the system. Re cable and use good quality Coax if you can use RG213 its thicker and won't loose as much signal strenth, but no critical on a short run like a boat
 
Re: Why can\'t I transmit on some VHF channels?

Which of those meters did you have in mind? There are quite a few there!
 
Re: Why can\'t I transmit on some VHF channels?

That looks excellent, Richard. That's what I'll order myself as the firm I was talking to seems to be pretty poor. You need to connect the downlead to the SWR meter, then using a patch lead, to the radio. I cannot conveniently plug the downlead into the SWR meter (then cable is too short) so I will need a 'back to back' to connect the downlead to the patch lead to the SWR meter. i.e. I need two patch leads and one back-to-back. I cannot see the back-to-back on their website, will have to phone.

The other thing worth having is a dummy load. A dummy load is like a perfect aerial. If you phone them they will tell you what you need. You can put a dummy load on the masthead instead of the aerial and make sure that all the cabling is fine -- e.g. make sure that you can obtain a decent SWR with the dummy load before actually connecting the antenna. Again, I cannot see it on their website so will need to phone.

I will be calling them on Monday, thanks /forums/images/graemlins/smile.gif
 
Re: Why can\'t I transmit on some VHF channels?

[ QUOTE ]
Do not use a CB antenna the ferquencies are completely different and will certainly cause damage, if it doesn't you will be extremely lucky

Re cabling is almost certainly the answer, if the cable showed oxidation then there is a short somewhere in the system. Re cable and use good quality Coax if you can use RG213 its thicker and won't loose as much signal strenth, but no critical on a short run like a boat

[/ QUOTE ]

Please explain why damage would result .... very interested to know the basis of that. Damage results when 25W output has nowhere to dissipate - NOT because of mis-matched antena length.

You may argue on this - but if you have sufficient cable length without loading coils - you can still operate a radio TX. That is info given by a manufacturer and not my own opinion.

Sorry but we are trying to find out what source of problem is and I still stand primarily on asking another boater to assist. CB was a last resort possibility - something we did here and worked perfectly - in fact Radio is still after 10 yrs in use even after antena took a lightning strike.
 
Re: Why can\'t I transmit on some VHF channels?

[ QUOTE ]

Please explain why damage would result .... very interested to know the basis of that. Damage results when 25W output has nowhere to dissipate - NOT because of mis-matched antena length.

[/ QUOTE ]

This is a mathematical subject so it isn't practicable to 'prove' anything to you unless your maths is currently at least AS Level +. Please take it as gospel. This isn't like global warming or the existence of God, this is fact.

The rules you learnt to allow you to work with electricity at mains (50/60 Hz) and dc don't apply above certain frequencies or with certain components. You only normally come across these cases with radio, TV, radar, and communications systems.

If you press transmit on your VHF radio without anything connected then all of the 25W is 'reflected' -- just like a mirror -- back into the transmitter. The transmitter output stage is two or four transistors and they already get quite hot in use. If the full output power is reflected back then they can fail -- the actual semiconductor usually melts and goes short circuit /forums/images/graemlins/frown.gif It is possible to put protection in place to prevent that -- in my opinion it is a crime that they don't -- but they don't /forums/images/graemlins/frown.gif
 
Re: Why can\'t I transmit on some VHF channels?

[ QUOTE ]
Unfortunately amongst various duties I also maintained radar systems.

I'll say no more. /forums/images/graemlins/tongue.gif

[/ QUOTE ]

If you are saying that you were right, then it is your public duty to say no more, but get into the loft and find some of your old training notes /forums/images/graemlins/wink.gif
 
Re: Why can\'t I transmit on some VHF channels?

Why doesn't it work? He already told us the most likely reason: "the cable run inside the boat from the radio to the deck socket ...... innards were a showing some blackening when I cut it back to re-make the connection yesterday."
 
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