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Another VHF problem


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Reading Andy's thread about 1 way VHF reminded me about asking the following:-

While at Oulton Broad last week, waiting for my call to enter the lock one of the other boats (I think it was Col) shouted over to me that they were calling us. I have a brand new fixed VHF and a portable - we didn't hear a thing.

When in the lock I appoligised to Keith (lockkeeper) saying that we didn't hear anything. He said not to worry and also said that sometimes when you are near other higher powered radios your signal would get knocked out. He was talking about the receiving signal. I don't see how this is possible - surely you would be able to 'receive' regardless of what anyone else had on board... I can understand the 'transmit' signal getting lost, but not receiving ?

Now I know we have some very very clever radio guys on here and I was wondering if they had ever heard of this before, or do you think it was a case of me getting confused with what he was saying ? But then, why didn't we hear the call on either of our radios ? Squelch was turned down, and volume was up, INT setting etc..

I didn't want to hi-jack Andy's thread, but perhaps there is a common problem ?

One last question too:- While using my handheld on high power, when I try to transmit the radio double beeps - which means the channel is busy, if I turn it onto low power its fine. This is on a lot of channels including 16 which obviously you would need to transmit on full power if there was an emergency .. This happens all the time, not just every now and again.

I've read the manual a couple of times and there is no explanation for it.

My I-phone works perfectly !! lol !!!

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I'm not sure Luke and I can't profess to be much of an expert on Marine Radio even though I work with PMR and Airband kit

I have a feeling (though I'm not certain) that channel 16 is 156.8 simplex which if so, could possibly cause an issue if two or more people simultaneously transmit and you are within range of the other transmitting parties. I'd reckon Dave S needs to come to your rescue on this one :wave

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Jim is correct, in that Ch16 is a 'simplex' channel, whereas Ch80 for example, which is a Marina Channel, is Duplex, and you would receive on Ch80 (from the marina), but transmit (to the marina) on a different frequency,

With FM radios, we talk in terms of the received signal being 'fully quieting', if its a good good signal. That is to say, no background noise. Background noise (discriminator noise) is what you would hear if you opened the squelch without out any received signal present.

As the recieved signal gets stronger, so this noise diminishes, until it gets strong enough to 'fully quieten' the discriminator of the receiver (discriminator = the bit that recovers the voice info from the frequency modulated carrier wave).

If a very local/much stronger transmitting station had his PTT (press to talk) button pressed, but with without out speaking, the carrier (radio signal) would 'block' any weaker stations on that channel. As you wouldn't be hearing a voice on the signal, you might assume that it isn't there.

However, the fact that Col was, I assume close by, and could hear the lock keeper calling you, would indicate that the channel was clear. This leads me to think that you had the 'squelch threshold' set too high, and as such the squelch on your receiver wasn't opening, hence you not hearing the call.

The squelch should be set to a level that just quietens the receiver, or just enough to prevent very weak signals (if you don't want to hear them) from opening the squelch. Albeit, probably not a good idea if monitoring Ch16, as a distress call might be missed.

Bear in mind that marinas, and I assume locks and bridges will be the same, are only allowed to transmit on low power (1watt). So, if you were calling them from distance on "high power" (25 Watts), they would likely hear you, but you wouldn't hear their reply.

As for your 'beeping' noise, I don't know of the 'top of my head', and you can demonstrate the next time I see you.

Hope this helps

Dave

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Hi Luke

It's called capture effect when the strongest signal in the area overides the smaller transmitted signal. This is the reason why one should first put out your signal on low power.

Many other things could have happened with respect to frequency harmonics especially with poorly serviced equipment. The analogy I use is the old taxi radios coming over the TV or as was reported many years ago, someone's cooker receiving radio signals !

Maybe you should retune your cooker as a more efficient means of comms? LOL !!

Regards

Jim Lad

AKA

Lieutenant Bogey

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However, the fact that Col was, I assume close by, and could hear the lock keeper calling you, would indicate that the channel was clear. This leads me to think that you had the 'squelch threshold' set too high, and as such the squelch on your receiver wasn't opening, hence you not hearing the call.

I always set the squelch just above the interferance noice level (I used to be a cb'r in my younger days)

The squelch should be set to a level that just quietens the receiver, or just enough to prevent very weak signals (if you don't want to hear them) from opening the squelch. Albeit, probably not a good idea if monitoring Ch16, as a distress call might be missed.

There is no way I could have all that noise going on, it'll drive me mad !!

Bear in mind that marinas, and I assume locks and bridges will be the same, are only allowed to transmit on low power (1watt). So, if you were calling them from distance on "high power" (25 Watts), they would likely hear you, but you wouldn't hear their reply.

As for your 'beeping' noise, I don't know of the 'top of my head', and you can demonstrate the next time I see you.

This is what we will do Dave !!! Then I can show you exactly what I mean !

Hi Luke

Maybe you should retune your cooker as a more efficient means of comms? LOL !!

Its a microwave Jim ! lol !

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All good stuff, just a point to put the domination or capture effect into an understandable context. If you listen to AM signals on medium or short wave on a normal domestic radio you will hear all sorts of beats and hetrodynes (whistles) however it is rare to hear the same effect on FM. This is due to the fact that with AM the interfering signal needs to be at least 50 times stronger to dominate and completely wipe out the other signal. With FM it only needs to be twice as strong.

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