VHF half'n'half

[muckypup]

VHF half\'n\'half

Had Harlequin's fixed VHF tuned to channel 18 today listening to the lock to try and learn the procedure and I noticed something funny going on. On the set I would only hear one half of the conversation - From the lock.

So I got out my Silva S12 handheld VHF and tuned it into the same channel. I was suprised when the handheld would only receive one side of the conversation as well - only it was the other half, from the incoming boats only.

I would have thought that the set should pick up both transmissions?

Steve

 

[Solitaire]

Re: VHF half\'n\'half

Ch 18 is a duplex channel. Most sets are class D and will not pick up duplex transmissions. Exactly the same as Ch 80 for marinas. Duplex channels have to different frequencies.

 

[stevebrassett]

Re: VHF half\'n\'half

Why would the two radios pick up different sides of the transmission? Could the fixed set be set up as a land station?

 

[Dave99]

Re: VHF half\'n\'half

I'm gussing u might not know what that means... duplex channels transmit and receive on different frequencies. If u press the PTT 9push to transmit) your set will act in semi duplex mode. It will transmit on the other frequency and when u release it will switch to the receiving frequency. Simple channels use the same frequencies for transmitting as receiving.

 

[muckypup]

Re: VHF half\'n\'half

I see.. I can understand then why I can only pick up the lock on my fixed set VHF.. But why then is my handheld picking up the wrong side of the conversation?

Steve

 

[Solitaire]

Re: VHF half\'n\'half

Very strange - Ch 18's frequencies are A - 156.900 MHz and B - 161.500 and is a port operations channel. Mind you, strange things can happen with Silva kit /forums/images/graemlins/wink.gif. But it could also be that one of the sets is set to a US channel. 18A, which is a US channel is a simplex channel on 156.900 MHz. This coincides with one of the channels used on UK ch 18.

Edit - judging by Muckypup's post Mobo forum this is what has happened. He has set his handheld to US ch and it's picking up the "A" frequency.

 

[Solitaire]

Re: VHF half\'n\'half

Have you got an operators licence?? You should have been taught what simplex and duplex channels are /forums/images/graemlins/grin.gif

 

[IanR]

Re: VHF half\'n\'half

Been there, got the T shirt

If you call up ch80 and accidentally set the set to US Ch80 rather than International you transmit on the marina side of the duplex.

I have a handheld where unfortunately this is a rather easy to trigger function

 

[brianrunyard]

Re: VHF half\'n\'half

You've probably forgotten that when you did the course for your VHF certificate, you covered Simplex, Full-Duplex and Semi-Duplex operation.
Simplex = Both stations receive and transmit on the same frequency (eg Ship to Ship)
Full-Duplex = Ship has separate transmitter and Receiver. Ship transmits and receives on different frequencies (eg. Ship transmits on frequency A and receives on frequency B. Shore station Receives on frequency A and transmits on frequency B) therefore both can talk at the same time.
Semi-Duplex = Uses one radio. Works 2 frequencies like Full Duplex, but can't transmit and receive at the same time. The set changes frequency when the PTT is pressed.
Could be that your hand held is operating on USA mode and therefore tx and rx on same freq, could be that you only heard the boat because the lock was out of range.

 

[VicMallows]

Re: VHF half\'n\'half

I find this a very useful 'feature'. When listening to a marina or lock on Ch80 I often set the H/H to 'US' so I can hear the boat side along with the lock on the fixed set. You get a better impression of what's going on and it also enables you to check the boat-transmit channel is clear before transmitting.

It would be far more sensible if Ch80 was a simplex channel, but it's a re-used one from the days of ship-to-shore telephone connection and is designated as a duplex outside the US.

Vic

 

[muckypup]

Re: VHF half\'n\'half

I understand now. I just need to work out how to change the set back to international (and as you say back to US when required).

Thanks!
Steve

 

[steverow]

Re: VHF half\'n\'half

Steve,

This is called semi duplex or half duplex working or more correctly in marine radio mode as "Two Frequency Simplex"
That is: Radio A receives channel A but transmits Channel B
Radio B Transmits Channel A but Receives Channel B
This way both can hear each other, but other stations can only hear one side, unless they either switch between the two frequencies or listen to both channels concurrently.
Half duplex working is used extensively in PMR, and it's beauty is that a Base station can set up what is known as "talkthrough" which makes the base station a repeater..ie couples the RX to the TX modulator, therefore doubling the range mobile to mobile.
However, the way it is used in marine radio, ie both stations press to talk..it is more correctly known as "two frequency simplex."
The RYA radio course actually teaches it as Duplex..but in fact that is incorrect.

The term Simplex and Duplex refer to the "mode" of operation..not how many frequencies are involved.

It's called half duplex because one channel is not left open constantly, ie only one station transmits at a time...or....the base station TX is left open constantly..only.

Full duplex working is like a telephone call where both channels are constantly transmitting for the duration of the QSO.

Simplex is the mode you are most familiar with, ie all radios on the same channel
for both receive and transmit.

There are more variations on this theme depending on the complexity of the radio network, like reverse repeater working, but these are the only three modes you as a user are likely to come across.


Steve.

 

[steverow]

Re: VHF half\'n\'half

Dave,

Sorry to be picky, but this is not Duplex.
I do wish the RYA would get it right and modify their courses to teach this correctly.
Where both radios are Press to Transmit (PTT) the mode is SIMPLEX.
So what you call duplex is in fact correctly known as "Two Frequency simplex"

Semi or Half Duplex is where one channel..usually the base transmitter is left open constantly..

Full Duplex is where both transmitters are left open constantly...ie a telephone call.

The terms Simplex and Duplex refer to the network mode, not how many radio channels are in use.

They are still called Duplex from a hang over from Ship to Shore R/T working, when at least in the early days it was proper duplex working..but they havent been used in this mode for years and it really is about time it was corrected.

Steve.

 

[damo]

Re: VHF half\'n\'half

Ch80 also labelled "P1 Reverse 80" on the marina transmitter

 

[Solitaire]

Re: VHF half\'n\'half

Thanks for raising those points - I've noted them!

 

[graham]

Re: VHF half\'n\'half

I have heard of people here in Cardiff having the same problem with Silva hand helds(The barrage locks use channel 18) There nasty bits of kit anyway ,personally I wouldnt have a Silva radio.

"Harlequin "is an impressive boat,where do you moor her?

 

[muckypup]

Re: VHF half\'n\'half

She is in Swansea marina. She looks fairly impressive in the photo until you see her in the water moored in between an aquastar 48 and a Sealine something-or-other... still, we are very happy with her (and the bible does say "Thou shalt not covert thy neighbours aquastar".

Steve