VHF Signal weak but readable

[lumphammer]

After fitting new antenna and coax cable from SaltyJohn of this forum. I'm still only getting a weak but readable response from a radio check to Solent coastguard from Northney Marina on Hayling Island on high power.

I would have thought it should have been better than that as my mast is 14m above sea level

What I have checked so far

Power supply reads 12.8v at the radio when transmitting on full power.
Using a Shakespeare VSWR meter I'm getting a VSWR of 1.4 - 1.5
The meter is only indicating a transmit power of 13-15W.
I can receive traffic from Portsmouth e..g. QHM and ferries very clearly

I have tried connecting to my AIS aerial and I have slightly better VSWR but the transmit power is still less than 15W

Is the low indicated transmit power because of the VSWR or is it possible I have a faulty radio?

 

[Giblets]

Looks like 1.4 - 1.5 is not too bad from this article.

http://www.electronicspoint.com/threads/re-how-to-use-a-simple-swr-meter-and-what-it-means-to-your-vhf.158105/

 

[Salty John]

The VSWR and power figures seem normal. Where is Solent Coastguard relative to you, and what is in the way?
VHF range is line of sight - hills, tall buildings, towers etc. will block the signal.

 

[lw395]

I don't know where Solent's antenna is, you could be in a bit of a hole. There are no hills on Hayling, but trees, buildings etc might create a null.
Even other masts close to your aerial can do odd things.
15W might be a bit low, but with calibration errors it is not proof of anything terrible.
You could check the volts spec at the radio, it may need 13.8 to achieve specified power?
I assume you receive them well enough?

Move the boat and try again!

 

[lumphammer]

The VSWR and power figures seem normal. Where is Solent Coastguard relative to you, and what is in the way?
VHF range is line of sight - hills, tall buildings, towers etc. will block the signal.

It could be the marina offices and a big hotel otherwise its pretty flat between the north end of Hayling Island and the Solent. Another boat I know was in Emsworth was getting "loud and clear" just after me.

I will try again when we next go out and see if there is any difference

 

[jac]

Also - you might want to consider using NCI calshot on 65 as they do radio checks now, freeing up HMCG for other tasks.

 

[Len Ingalls]

You may want to try another mike.Yours may be "muffled" due to age,dampness,etc.

 

[JVL]

Easy route is "borrow " a radio off a neighbour and try that, I would be very suprised if it's salty John s metz kit.

John

 

[Martin_J]

Hi

I would have expected your power meter to have shown more than 15W on the power setting.

Bear in mind also that the SWR reading will also only be comparing your reflected vs transmitted power at the point of insertion of the SWR meter.

I assume that you have a short patch cable between the VHF output and the input of the power meter... Are you sure this patch cable has been made up perfectly?

Any reflection or bad connection in that patch cable could be presenting a bad impedence to the radio and the radio could therefore be reducing the output power to protect itself.

I would have expected the meter to read somewhere between 22W and 25W... Remember the radio is not permitted to Tx at more than 25 watts so will usually be slightly under but never above 25W output.

I'd try a new patch cable to the meter (or check shield to shield and center to center resistances) in the first instance.

SWR of 1.4 isn't too bad. Out of interest, did you get a standard RG58 cable or something less lossy? With a 14m mast and probably at least another 6m to the rig you do find quite a power loss although this won't show in the meter and you should still see the full 25W power reading.

 

[William_H]

What OP needs to do is to try to arrange to listen to his own transmitter with someone else transmitting. As said it could be a poor audio power on what is a strong RF signal. The coastguard man will only be able to comment on how he hears it. VHF FM has the ability to sound strong even when signal is quite weak. As it gets even weaker it starts to have a lot of noise on the audio (voice). If the receiver has the squelch open ie continuous noise when you transmit it should go very quiet until you speak. Wheras a weak signal will give very little quieting of the background noise.
So get in a position where the signal will be strong ie another boat nearby and compare voice loudness of your radio compared to another radio another boat or a hand held.
I have a 25w radio I picked up at a trash market. Works well but microphone audio is weak. OK if I shout.
Maybe OP is simply not speaking loud enough and directly into the microphone. good luck olewill

 

[MM5AHO]

Its a fact that Coastguard radio reception reports are not all that helpful.
"Weak but readable" helps, but it doesn't give any idea if its the RF signal that's weak or the audio modulation of it. Not many marine VHF receivers measure the incoming signal strength, so they can't report that.
"Weak with interference" is another heard sometimes. But was that interference just others transmitting at the same time, or perhaps a buzz on the transmitted signal?
I once heard "strong but unreadable". I wasn't sure what they meant, as I heard the signal, and it was weak for me, but readable.
In diagnosing transmitter problems, we could all help more by reporting more of what we hear - describing the problem in a few more words could enable faster resolution.

 

[Martin_J]

Exactly.... I have had a transmitter with a faulty output stage. . It was probably only outputting a few hundred milliwatts.

Trying to diagnose the issue by listening on a hand held nearby the issue was only apparent if you knew what you were listening for.

The receivers these days can make a really weak signal sound clear.

As MM5AHO says ... reception reports from an untrained ear are not always that helpful.

 

[Burnham Bob]

A friend had similar issues but we found that - as a performing musician - he expected the mic to pick up from a bit of a distance. Holding the mic to his lips cured the weak transmission problem

 

[Len Ingalls]

Its a fact that Coastguard radio reception reports are not all that helpful.
"Weak but readable" helps, but it doesn't give any idea if its the RF signal that's weak or the audio modulation of it. Not many marine VHF receivers measure the incoming signal strength, so they can't report that.
"Weak with interference" is another heard sometimes. But was that interference just others transmitting at the same time, or perhaps a buzz on the transmitted signal?
I once heard "strong but unreadable". I wasn't sure what they meant, as I heard the signal, and it was weak for me, but readable.
In diagnosing transmitter problems, we could all help more by reporting more of what we hear - describing the problem in a few more words could enable faster resolution.

+1

In my 30+yrs as marine electronic tech-I learned that I had to ask the RX party the correct questions in order to diagnose a VHF TX problem.
My Bird wattmeter quickly diagnosed TX RF problems(transmitter or antenna,SWR,etc.) I found that there was not much,if any,difference reported by the RX station whether I output anything from 1 watt to 25watt into a good antenna over 5-6mi. distances.Luckily,I had a very helpful block of customers who kindly listened & helped.
I would generally ask for a radio check from my position to a customer who was as far away as possible.
I found that Coast Guard were a poor test RX,since they have multiple repeater stns with directional ant.They can hear weak signals that normal vessel stn. may not.

Diagnosing audio problems,without carrying my shop deviation tester,is much trickier.
Audio problems are generally due to the mike,as it is a wear item,taking the most abuse due to cord stretching,dampness in "voice coil" & switch corrosion/wear.
What I found helpful was to ask the RX stn(as far away as possible) to go to a quiet channel.I would then start asking questions re-cutting out,scratchiness,weird noises,etc.,while talking continuously for 10-15 seconds & stretching the cord,partially relaxing/increasing the pressure on PTT swx,etc.
Scratch & cut out usually meant either the cord or PTT swx. "muffled" usually indicated the voice coil.Either report indicated replacing the mike & cord assy.

Hope this helps
Cheers/Len

 

[haydude]

After fitting new antenna and coax cable from SaltyJohn of this forum. I'm still only getting a weak but readable response from a radio check to Solent coastguard from Northney Marina on Hayling Island on high power.

I would have thought it should have been better than that as my mast is 14m above sea level

What I have checked so far

Power supply reads 12.8v at the radio when transmitting on full power.
Using a Shakespeare VSWR meter I'm getting a VSWR of 1.4 - 1.5
The meter is only indicating a transmit power of 13-15W.
I can receive traffic from Portsmouth e..g. QHM and ferries very clearly

I have tried connecting to my AIS aerial and I have slightly better VSWR but the transmit power is still less than 15W

Is the low indicated transmit power because of the VSWR or is it possible I have a faulty radio?

VSWR < 2 is considered a good match to the antenna impedance. Total Radiated Power is reduced with high SWR, but in your case your transmitter might have an effective TX power lower than nominal which can happen if the components are loosely tuned or if they have been previously damaged transmitting with a poorly matched antenna. However I would not trust the SWR/Power meter 100% as they tend to have high error rates. Components (in particular the TX transistors) get damaged by overheating when the power they produce cannot be radiated by the antenna due to poor impedance matching and thus it is reflected back (Standing Wave) to the transmitter. Once they overheated their efficiency can be reduced or they can stop working altogether.

From your message it is unclear if "weak but readable" is the report from the costguard relative to your transmission or your reception of the coastguard. Reception in general is not influenced by the transmission components.

Bear in mind also that there is very little perceived difference in reception from a transmitter bradcasting at 15W or 20W on the 2 metres band if TX and RX antennae are in line of sight. In general the antenna gain is what influences most the radiated power from a transmitting station and thus reception of the signal on the receiving station.

 

[haydude]

Any reflection or bad connection in that patch cable could be presenting a bad impedence to the radio and the radio could therefore be reducing the output power to protect itself.

I am not aware of radio that reduce power when they detect high SWR. Those that can detect it will cut off TX completely rather than reduce power. Those that do not, will overheat the TX transistors and cause the more damage the longer is the transmission and the higher the SWR and temperature.

 

[MM5AHO]

I am not aware of radio that reduce power when they detect high SWR. Those that can detect it will cut off TX completely rather than reduce power. Those that do not, will overheat the TX transistors and cause the more damage the longer is the transmission and the higher the SWR and temperature.

I think that more transmitters reduce output power on detecting high SWR than cut off Tx altogether.