VHF expert required

[santavey]

please can someone explain the following.
Havings lots of trouble with VHF. Suspected masthead ariel at fault. Replaced with pushpit mounted one. Works OK but with poor range. Also picks up lots of noise that has to be squelched out almost to the point where no signals recieved. Worst on Ch16 especially with engine running. Got fed up. Replaced masthead ariel. Still as bad, worse than pushpit ariel. Replaced lead (RG58) between new masthead ariel and radio using barrel type connector. The spare wire from the new ariel left coiled up under the headlining. Searched for VHF problems on forum. Bought SWR meter. Connected to pushpit ariel. Tested same. Now this is where I really begin to loose it!
Ch M Forward 30 watts, reflected <1, SWR <1
I thought it was limited to 25 watts.
Connected SWR to masthead ariel without using the new internal cable.
Ch M Forward 14 watts, reflected 1.7, SWR 1.4
Repeated test with different radio. Very similar results.
Same test on Ch 77 gave Forward 20 watts, reflected 2.8, SWR 2.2. Why different results on different channels?
Ran test with masthead ariel & new internal cable.
Ch 77 Forward 18 watts, reflected 1, SWR <1
Am I messing with things I really dont understand or is there a simple answer?
Any advice received with thanks.

 

[ShipsWoofy]

Easy one first

[ QUOTE ]
Same test on Ch 77 gave Forward 20 watts, reflected 2.8, SWR 2.2. Why different results on different channels?

[/ QUOTE ]

Different frequency. Your set up is a compromise to work well over a band, it is best practice to tune the circuit to ch16, but also worth checking the other channels to ensure they are not too badly affected..

Noise. oh dear, why and where are you getting this noise from, do you have a fridge, is the windex close by,

Try turning everything off but the VHF, if this is not possible, turn off the master switch and use a jump lead to power up the VHF. If this clears up the noise you need to start turning things on one by one until it returns. At this stage use logic before jumping too deep into cables etc. As an example, when my windex is turned on my hand held will not rx ch0, I can live with this, but this is an example.

From your results it appears you may have two problems, a) the masthead aerial has a high VSWR and b) the noise. I presume the pushpit aerial came with a factory fitted cable?

Your results on the masthead suggest a little cable cutting or aerial tuning might help.

But before doing this, are you by any chance using a CB 27Mhz SWR meter?

How far from the boat are you, for this kind of fault we really need to work a stage at a time.

Can you bring the radio and pushpit aerial home and run it off the car battery?

Who fitted the new aerial cable?

Lots of things to try.

 

[lenseman]

Hello Solway and welcome to the forum.

On the 18 July 2007 at 10:32, I wrote a long article to assist a forumite (PatrickB 02 - Practical Boat Owner's Reader to Reader) with similar interference problems and the answers I gave might be helpful to you. It was listed under "VHF Interference".

Regarding your Standing Wave and reflected signal from the Tx to the Antenna. Firstly, I very much doubt if your SW meter is that accurate and that the 30 watts is probably a little ambitious.

Secondly, your antenna is (will be) 'cut' for a particular frequency inside the marine band. This is the frequency at which your particular antenna will resonate correctly. This is when you should get minimum reflected and maximum radiation from your antenna. This assumes a correct installation?

As you 'tune' the transmitter to a lower or higher frequency (change channel), the antenna will not be able to resonate correctly on this different frequency and you will get more reflected signal returning and this is what you are measuring in your SW meter. Again, I am assuming everything else is correct, co-ax termination, ground plane at the antenna, no water ingress etc?

Get back to me if you need further advice?

 

[Salty John]

Looks like you have eliminated the radio as the problem.
The SWR test on the masthead, repeated with a different radio and on different channels look OK at 1.4:1. 2.2:1 is high, and under 1 is impossible. 30 watts is unlikely as a power reading.
I think you should thoroughly check your connections. You should have no conductivity at all between outer shell and centre pin with the cable disconnected at each end. It is easy to get a single strand from the inner core bent back and touching the braid if you are not careful when assembling a PL259. Check for breaks in the cable cover and corrosion of the braid. If you are completely happy with all the connections and the radio is ok, and you have the same problem with two antennas, it has to be an on-board 'noise' problem. Or your expectations are too high. Or you're in an area where reception is bad anyway.
Good luck, I know how frustrating it can be tracking down these problems!

 

[MarkGrubb]

Hi Solway,

Radio problems can seem like a bit of a black art. All good suggests so far, recheck the installation and that your SWR meter covers the VHF frequency range.

My comment is to ignore the power reading on your VSWR meter. Unless SWR is very low, the power reading will be unreliable. These meters are usually designed to give the correct power reading when the output is terminated with a 50 ohm load. If your SWR is high, then the impedance of the antenna is not 50 ohms.

If you want to check output power, a 50 ohm dummy load can be bought from most shops that sell amateur radio equipment, try maplin. Or you can make one by using resistors (but not the wire wound type).

As has been mentioned the antenn should have been supplied tuned to the correct frequency, so check coax and antenna installation. Try removing any metal near the antenna, such as a windex, as this may affect the frequency to which the antenna has been tuned, and then recheck the SWR. Use a multimeter to check there is no short between the core and shield of the coax. If you can connect a 50 ohm load to the end of the coax then this should give a SWR to 1:1 and prove the coax is OK. If water has got into the coax then this will cause problems.

Cheers,

Mark.

 

[EDM]

Soloway
" Also picks up lots of noise that has to be squelched out almost to the point where no signals recieved."

Simplest way is to use (as previously pointed out) a "clean Power Supply" i.e. Bring on board a small 12v Battery and run your VHF from that - with EVERYTHING turned off.

If your VHF still sounds terrible and you are having to use excessive squelch possible problems now are
a) The Set is knackered (A Technical term for non radio people)
b) There is some external Interference
i) Are you alongside in a Marina - maybe some heavy power cables

If you have a Bad Antenna connection you should get poor reception of everything .... not bad signals and great interference.

Can you make a comparison with a Handheld VHF ????
If both radio's sound the same - then external issues rather than internal are almost 100% to blame.

That hopefully should give you some idea as to where to look... please feel free to get back to me if you need further assistance.

 

[santavey]

Many thanks to all for the advice. I will try all the suggestions. In answer to some of the questions, the new masthead aerial is a glomex whip rated at 50 ohm impedance. The SWR meter is for marine frequencies. Putting an ohm meter across the internal cable shows no short but across the aerial cable shows a short. I was reluctant to chop off the spare cable but perhaps I should. I have learnt one thing, power out is a function of frequency & aerial type. That explains why distant stations cant hear me. I am not transmitting at anything like 25 watts on certain frequencies. One further question. How do you tune a system for maximum performance?

 

[fishermantwo]

[ QUOTE ]
Also picks up lots of noise that has to be squelched out almost to the point where no signals recieved. Worst on Ch16 especially with engine running.

[/ QUOTE ]

Replace brushes in engine alternator.

 

[EDM]

"How do you tune a system for maximum performance?"

If you are using a commerically bought Marine VHF, which has not bee messed around with - then you will be hard pushed to improve all around performance. Should you wish to have the worlds most efficient antenna on say CH 16 - then you can start fiddling, but the upshot of this is when the other station says tun to channel X - your antenna will perform badly. A marine VHF should radiate in all directions, and whilst you can easily improve direction and gain (power), this comes at the expense of omni-radiating signals.

So best leave it's length (which is the frequency it resonates on - or at least a fraction of the frequencies wavelength) alone.

Err one slight thought ... you are using Marine Grade Coax ?
And the Coax does say 50 Ohm on it ?

You say "I was reluctant to chop off the spare cable but perhaps I should"

If you have 30 meters of excess cable then you should reduce it to 3-4 meters.... if however you have only 3-4 meters that is perfectly acceptable. Note: Very long Coax runs do loose power unless you are using very expensive Coax.

I've known people to save a few quid use Sat TV coax (70 Ohm).

Keep us posted how you get on...