VHF Testing

[fiddle]

Last weekend my vhf would'nt transmit. I could recieve ok. This was first time out this year and I called for a radio check on 16. To find out if its the set or the antenna can I use the swr meter that I bought in the CB radio days? I know I can take my set to a friends boat, connect it there to work out whats at fault but I have this swr meter and , well if that will do it!!
Is calling on 16 still the way to call the coastguard, or are they only using that for distress and are encouraging us to use a different channel to call up. I do get the feeling that Ive missed something recently!

<hr width=100% size=1>

 

[jimi]

why do'nt you try calling up your friend using the emergency aerial on an intership channel?

<hr width=100% size=1>

 

[aztec]

as far as i'm aware the swr meter just compares the radiated signal against reflected, so should work ok with any RF signal. this would indicate whether the antenna was of the correct electrical length, but actual output won't be measured. to see if the thing is transmitting go to a working channel for a test, maybe the local marina? just in case there's someone calling for help.




<hr width=100% size=1><A target="_blank" HREF=http://homepage.ntlworld.com/s.amos/index.html>http://homepage.ntlworld.com/s.amos/index.html</A>

 

[tome]

Hi Fiddle

CB radio is 27MHz so make sure your SWR meter can also handle marine VHF (156-163 MHz).

I recently fitted a DSC set. When I made a radio check on Ch16 I was loud and clear, but when I did a check via DSC to the local CG they came back and told me my DSC was strength 2 (1= poor, 5=best). I investigated further and discovered that I had a power supply problem - the circuit breaker was the cause. I could receive fine but the set voltage dropped to 7 Volts when I transmitted and I was putting out less than 0.25Watts (should be 25).

I fixed that problem but the SWR gave me a reading of 3. Further checks showed that an in-line connector had corroded and when I replaced that the SWR dropped to 1.1

So, calling the coastguard ch 16 is fine but not as good as a DSC call where they can see your received signal strength. If your SWR meter covers the marine band, this is a great tool for checking your antenna installation and I'd aim for a reading of less than 1.3 which corresponds to around 1% reflected power. Anything higher suggests a poor connection somewhere.

Regards
Tom

<hr width=100% size=1>

 

[brianrunyard]

The tuned circuits of CB SWR meters are seldom capable of handling the VHF frequencies.

<hr width=100% size=1>Brian
http://homepage.ntlworld.com/brunyard

 

[Evadne]

Hi Fiddle,

I may be telling you something you've already tried but it is a more common problem than most appreciate. It's a tip that has got me a free pint int the past, "fixing" a neighbour's radio in 10 seconds, so it is worth passing on.
Check the in-line fuse for corrosion. Tx takes a lot more power than Rx and if you've a high impedance in your power line (e.g. green gunge on the fuse) it will kill your radio on Tx although it will Rx perfectly. Fuses have tinned copper contacts, and salt air corrodes them. Just polish it with crocus paper or similar and spray with WD40.

Dave

<hr width=100% size=1>

 

[tome]

Dave

Good point about the fuse but don't use WD-40 (it's an insulator!).

Regards
Tom

<hr width=100% size=1>

 

[aztec]

point duely noted, thanks brian.

regards, steve.

<hr width=100% size=1><A target="_blank" HREF=http://homepage.ntlworld.com/s.amos/index.html>http://homepage.ntlworld.com/s.amos/index.html</A>

 

[Strathglass]

I would be most surprised to find tuned circuits within an SWR meter. They are intended to be non-resonant devices.

CB SWR meters should not be totally discounted.

Some 27Mhz SWR meters are surprisingly well engineered and they often have silver plated internal cavities. They are capable of of measuring SWR with surprising accuracy at VHF frequencies. Certainly accurately enough to tell if a serious problem exists.

If there is a power reading facility on a 27Mhz SWR meter this will not read correctly at VHF.

<hr width=100% size=1>

 

[Birdseye]

I have frequently used an swr meter that i bought in cb days. it might not give the correct figure of swr, but all you want is something that will tell you that your aerial or its connections are bu****ed. it will most likely do that - mine does.

alternatively, borrow someone's emergency aerial .

apart from the fuse issue, it really is most likely to be aerial problems with corrosion on the conectors, water in the coax braid etc. someone once told me (cant vouch for it myself and therefore cant recommend it) that you could get rid of coax cable water by putting the cable in the microwave and boiling off the water! would be interested to know if that is true

<hr width=100% size=1>

 

[duncan]

voltage meter on the radio's power supply when you push the transmit button will help discounting power. However this seems a quite common problem. 2 weeks ago my radio refused to come into life when switched on. 12.8 odd volts at the PS; must be the radio I thinks. Take it back to the sparky and he says it happens and he will get it fixed but lets check anyhow - connect it to a battery and it fires up no problem. Back at the boat reconnect radio adn voltmeter across power leads - turn unit on and voltage drops to 3.4v; it doesn't power up. Couldn't find any lose connection etc - 2 feet of cable and 2 crimped connections - so I just started again with a new bit of wire and bingo - all working fine.
Sometimes 12v isn't really 12v!
good luck

<hr width=100% size=1>

 

[ean_p]

ah there go those crimped connectors again..............

<hr width=100% size=1>

 

[fiddle]

Thanks all for your advice. It was a new aeriel 12 months ago. I will check the feed first. I must confess that I thought that if its getting power, its getting power! Again many thanks.

<hr width=100% size=1>

 

[brianrunyard]

I'll take your word for it that you know of some CB SWR meters that work accurately at VHF.
I wouldn't rely on a CB meter myself, I've got 3 CB SWR meters and neither of them give readings that agree with my "Pye Reflectometer type RFL 5", this is a professional instrument (I used to repair VHF radios).
It's specifications are:-
68 to 174 Mhz with power ranges 15, 75 and 150 watts
400 to 500 Mhz with power ranges 7.5 and 75 watts.
So you will see that SWR meters do have a set frequency range, wish they didn't, I could have saved money.

<hr width=100% size=1>Brian
http://homepage.ntlworld.com/brunyard

 

[tome]

Ean

I'm beginning to believe that you're a bit of a solder purist, a Trappist monk worshipping at the altar of lead and tin amongst the many interconnection heathen.

What's the betting that the crimps were born of one of those lovely cheap Halfords tabernacle tools of worship?

Regards
Tom

<hr width=100% size=1>

 

[tome]

Having read my SWR meter manual I believe that Ian Simpson is correct in his post that the meter doesn't have a tuned circuit. Almost all SWR meters double as RF power meters and it is in this mode that the circuit is tuned and therefore frequency sensitive. If you can make your SWR meter calibrate it will probably give you a reasonably accurate reading.

Meters designed for high power RF probably aren't sensitive enough to achieve a satisfactory calibration on 25W. All IMHO.

<hr width=100% size=1>

 

[gtmoore]

After reading this thread I took my old CB SWR meter down to the boat to see what happened when I connected it up. Well...I hope it's wrong cos its showing well over 10 which would definitely have been a no no when I was playing around with CB kit when I was younger. I seem to be transmitting OK. When I put my multimeter on the end of the antenna coax at the mast I get nearly (but not quite) 0 ohms resistance. Is the actual antenna connected to both the core and the braiding or do I have a problem with a short somewhere?

If I hadn't flapped around like a headless chicken disconnecting coax and checking plug connections when I got such an awful SWR reading I would have had the sense to try the same experiment using the emergency antenna but I guess that will just have to wait until next weekend.


<hr width=100% size=1>Gavin

 

[ean_p]

ah! ye of little faith...verily i say unto thee, that there is more rejoicing in heaven that one sinner hath seen the light .........

<hr width=100% size=1>

 

[tome]

A multimeter supplies DC current to measure resistance, and at DC the impedance of the antenna will be close to zero so this doesn't necessarily indicate a problem.

Were you able to get the meter to calibrate before you read the SWR? If not, you won't get a sensible reading. An SWR of 10 indicates a serious problem.

In my case, the problem was in the coax from the set to the mast base where the mating BNCs were badly corroded. I replaced them with new crimped (!) BNC connectors and that cured the problem.

<hr width=100% size=1>

 

[tome]

Ah but verily it is easier to pass a camel through the eye of a well crimped terminal than to lose a poorly soldered connection at the gates of hell...

<hr width=100% size=1>