vhf radio recieving

[Guest]

This may sound stupid but when listening to vhf radio communications is one supposed to hear both sides of the conversation? Listening in the marina to channel 80 I could only hear the marinas responses. Is this normal or do I have a problem with my set or aerial.
Thanks

 

[Guest]

range

I think its just that the marina's have more powerful transmitters which you can hear from a long way away but the boats have smaller transmitters. You should be able to hear a boat speaking to the marina when you are nearer each other.

The squelch button will increase/decrease the amount of traffic you pick up.

However, if in doubt, join the rest of the people out there and ask the coastguard for a radio check!

 

[Guest]

The marina almost certainly has an antenna mounted high up. It has a large VHF range (L km). The two boats (you and the unheard boat) will have smaller VHF ranges (R km). You will only hear the other boat when it is within 2R km of you. You and the other boat will be able to communicate with the marina at a range of L+R km.

David

 

[Guest]

It\'s not about range

If you don't know the answer to this question then you should probably not be using the VHF. There is a mandatory licence required to operate marine VHF that should cover this.

What you will learn is that there are two different types of VHF channel. All intership channels(e.g. 16, 67, 06, 13...) are simplex. This means that both parties transmit and receive on the same frequency. Therefore anyone listening to that frequency will hear both sides of the conversation (range permitting).

However a number of channels designated for ship-to-shore communcations are duplex - this means that you transmit and receive on different frequencies. This includes Ch 80 (and Ch 12 - I think). In this case if you are on a 'ship' you will only hear messages transmitted from 'shore' and vice-verse. Hence you can only hear one side of the conversations.

 

[Guest]

not sure you are right

Firstly, you dont have to remember stuff like that to have a license. You remember it to pass the exam and afterwards, the important thing is how to USE it. Better you know how to do a mayday relay than the difference between duplex and simplex!

I agree on the duplex/simplex thing but are you telling me you've never heard a ship calling a marina for a berth ? or visa versa ? I have.

I was out last weekend and as I approached Cowes I heard ships calling and the marinas replying. I heard the marinas well before the boats due to its greater range.

Explain that.....

 

[Guest]

Re: not sure you are right

Ditto. I can certainly hear requests for berths and responses on CH80.

 

[Guest]

I agree

A more likely explanation is that the boats calling on ch80 are using low power and the reply from the marinas is on full power so it is a matter of range.

Pete

 

[Guest]

It depends on the channel

Different marinas use different channels. The most common ones in UK are (I believe) 80, M1 & M2. Of these 80 is duplex and M1 & M2 are simplex. (For more details see e.g. http://www.wibble.co.uk/links/ukspectrum/spectrum.html
). Which channels were you refering to?

The original question related to Ch 80 and the reason is almost certainly the one I gave.

It is theoretically possible to receive both sides of a duplex channel on a VHF set - but I know my (admittedly rather elderly) set does not, and I was not aware that any 'leisure marine' sets have this capability.

However I am not an expert in this area - and this opinion is worth what I was paid to give it! (that is nothing!).

 

[Guest]

I think the reason you CAN hear both side is..

that with duplex, a marina has two ariels so it can listen and send at the same time. However, us mere mortals have sets that use 2 different frequencies for sending and receiving (pressing and releasing the button changes the frequency) but EVERYONE uses the same frequency to send and EVERYONE has the same frequency to receive.

Consequently you can hear all of it

 

[Guest]

Let\'s throw some light on the subject

First of all, think of your antenna as a light globe, it throws light all around, and like a VHF signal it is more or less "line of sight". That is, it will not bounce off the atmosphere like 27 Mhz or CB radio will, sometimes called a skip.
Getting back to the light bulb, the further you are from the globe the less light you see, (less signal strength}. if the light is on or over the horizon you may not see it all, (Or hear it). an antenna at the mast head will outperform one located on the transom because it will (See) pick up signals (The light bulb) that can't be seen from a lower point. Most Marinas have their antenna mounted as high as possible to improve performance (View).
So, if you hear only the Marina (or another yacht close by) but not the caller, it will be because your antenna is lower / smaller and can not (See) pick up the other radio. Happy listening. Old Salt....

 

[Guest]

Re: Let\'s throw some light on the subject

I'm the person who first asked the question. I have just completed the new radio course and believe me there were no questions about simplex duplex or anything particulary technical. Merely how to make calls especially mayday.
I dont think my problem is range because I cannot hear the one side of the conversation even when the boat is next door. This is why I was asking because I have asked the marina for a radio check and they recieve me loud and clear.

 

[Guest]

Check this link...

Check out the following link

http://www.radio.gov.uk/publication/ra_info/ra290.htm

After all they should know!

In particular notice the difference between Duplex and Simplex operation - and especially the fact that it is easier to use M and M2 (the Simplex channel) than Ch80 (duplex).

Since the query related to Ch 80: "This is a duplex channel using two frequencies; one to transmit to the ship station and one to receive from the ship station"

 

[Guest]

My apologies...

...for accusing you of not having a licence; Simplex/Semi-Duplex/Duplex was certainly covered in my VHF course; I assumed it was a standard part of the syllabus (or perhaps it went out with DSC/SRC?). It certainly should be covered since it is quite important to understand.

Most of the comments made above about range are half correct - and would be accurate IF you were talking about a SIMPLEX channel. However Ch80 is DUPLEX (check the link in my posting above for confirmation). This means that
(i) ships can talk to shore stations
(ii) shore stations can talk to to ships
(iii)ships cannot talk to other ships
(iv) shore stations cannot talk to other shore stations

What you experienced was totally normal and there is no fault with your set. You were not listening to the frequency on which the yachts were transmitting and so did not hear them.

(To be pedantic most leisure marine VHFs are only capable of "Semi-Duplex" operation).

 

[Guest]

Re: My apologies...

Thanks, I think my mind is at rest. Perhaps I need to get myself a good practical book.

 

[Guest]

Re: range

The fact that you can only hear the marina Channel is quite easily explained.
Transmissions from any set will only go in a straight line. With a maximun distance of line of site to the horizon which is approx 19miles max.
Quite a lot of marinas have there arial tip considerably higher than most yacht arials.
There are exceptions to this rule as the coastguard covering liverpool has there arial on the top of Snafell so they cand relay a transmit further than there base station. The other factor to consider may be that you are within the marinas transmission area and that you yourself could very well be outside the other stations area, or you could be also masked out by Land or a large vessel.
Hope I have been off assistance in answering your query!
Kind Regards
Stuart.

 

[Guest]

Re: It\'s not about range

Starting again at Bedouin, he is correct in his statement about Ch80 being duplex. Macmillans give it as Tx161.625, Rx 157.025. ChM Tx/Rx 157.85. ChM2 Tx/Rx 161.425.

The answer about marinas having two aerials, may well be correct - about having two aerials, but they do not leave the VHF on Tx when not Tx'ing. So there would be no way that a boat calling the marina could be heard by another boat.

Bedouin was correct all the way through, but others did not believe him. The thread about distance would or could relate to simples operation, but not in the case of Ch80. Some small harbours use duplex - Hayle in Saint Ives Bay for example, so you would not hear the boat calling, only the reply.

Talk about confusing answers!

 

[Guest]

Duplex/Simplex

Channel 80 is Duplex so you only hear one side of the conversation.
M and M2 are Simplex, so you hear both sides, if in range.

 

[Guest]

Cor, that caused a debate..

As you say Bedouin was correct all along. It's perfectly normal to hear only the shore station on 80. Strange though, that last Saturday, I could hear some fairly clear (slightly broken) transmissions calling the marina, who usually use 80. Unless they were using M, I think it could have been atmospheric conditions causing breakthrough. Could be the reason others think they've heard it.

 

[Bilgediver]

That is quite normal when listening to stations using semi duplex....ie the Marina is transmitting on a different frequency to the boat...You all hear the marina but only the marina hears the boats..

HOWEVER
if the marina was using full duplex with transmit and recieve activated at the same time then due to feedback in the handset you would hear both sides of the conversation as the marina would retransmit the boat conversation....

You often hear this on SSB where although the ship working the shore station is on another frequency its content is retransmitted inadvertantly so you hear both sides if eavesdropping.


John

 

[brianrunyard]

Re: I agree

As has been stated before, it's nothing to do with power. Ch 80 is a Duplex Channel, therefore your ships VHF is tuned to recieve the frequency the marina transmits on Ch 80, which is differnet from the frquency a ship transmits on Ch 80, hence you do NOT hear the ship.