UHF: 2 radios transmitting at the same time ?

Submitted: Wednesday, Jan 23, 2008 at 15:39
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Hi all,
Something that comes to mind periodically, when driving in convoy.
Sometimes when I transmit a message to the convoy, upon releasing the transmit button, one of the other guys is broadcasting a message also.
Just wondering who hears what...eg :

Car A and Car B coincidentally transmit at the same time, what does Car C hear ?

Cheers...Lionel.
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Reply By: MrBitchi (QLD) - Wednesday, Jan 23, 2008 at 15:48

Wednesday, Jan 23, 2008 at 15:48
Usually garbage....
AnswerID: 283347

Reply By: Member -Signman - Wednesday, Jan 23, 2008 at 16:04

Wednesday, Jan 23, 2008 at 16:04
Whichever is closest to the receiving car- or who has the best signal- or the more power..
So if you (car X)..is talking to Car Z...and I (in Car Y)..have a stronger signal- My signal will over ride your signal...and Car Z will not hear you !!
AnswerID: 283349

Reply By: Member - Geoff C (QLD) - Wednesday, Jan 23, 2008 at 16:06

Wednesday, Jan 23, 2008 at 16:06
With FM the receiver always hears the strongest signal at its antenna. In most cases, unless only one signal is heard.
Geoff
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Follow Up By: Member - Geoff C (QLD) - Wednesday, Jan 23, 2008 at 18:06

Wednesday, Jan 23, 2008 at 18:06
Sorry, senior moment, take out the unless. Granddaughters are lovely but I'm easily distracted.
Geoff
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Reply By: Member - Phil G (SA) - Wednesday, Jan 23, 2008 at 18:16

Wednesday, Jan 23, 2008 at 18:16
"Someone's coming over the top of you" usually means the rest of the convoy only heard garbage.
The other guy who was transmitting has heard nothing. And you're thinking Oh S**t
AnswerID: 283375

Reply By: Bonz (Vic) - Wednesday, Jan 23, 2008 at 19:03

Wednesday, Jan 23, 2008 at 19:03
If the vehicles are equidistant and have similar radios then you are likely to hear "white" noise, sort of like a ccccccccccchhhhhhhhhhhhhssssssssssttttttttttttttttt noise.

It is caused by the two signals cancelling each other and attenuating each others signal. If the signals are out of synch then you will get a "doubling" of the signal which can damage your set an your eardrums, that is why they have in-line fuses in the power leads to the set and a failsafe disconnect at the base of more expensive antennas
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Follow Up By: disco1942 - Wednesday, Jan 23, 2008 at 22:26

Wednesday, Jan 23, 2008 at 22:26
Bonz

Where did you get that theory from? Never heard it myself.
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Follow Up By: Bonz (Vic) - Wednesday, Jan 23, 2008 at 22:27

Wednesday, Jan 23, 2008 at 22:27
Its on the internet, it MUST be true
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Follow Up By: Member - SNAKE (RAOUL) QLD - Wednesday, Jan 23, 2008 at 23:06

Wednesday, Jan 23, 2008 at 23:06
Bonz
I have never read such a load of crap in all my life.It is this sort of misinformation that only confuses people new to radio and does nothing to enhance their enjoyment.Inline fuses and failsafe disconnect??.I dont know what failsafe disconnect means but fuses in line protect the radio from dills blowing radios to bits. Do yourself a favour and read up on the subject.Cheers and with your permission may I publish your reply in our local Amateur Radio Newsletter. Snake and Josie
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Follow Up By: Bonz (Vic) - Wednesday, Jan 23, 2008 at 23:13

Wednesday, Jan 23, 2008 at 23:13
hahahahaha of course you can publish it in your local radio newsletter if you think that your members will learn a thing or two. You have my express permission to republish my intentioned waffle.

If you don't know what a failsafe disconnect is then how can you refute its accuracy or otherwise? I have read up on the subject for many years and have come to the opinion that if someone cant see a tongue in cheek reply amongst all the others that profess to speak the truth then they will probably sus it out when the aysk for a failsafe disconnect at their local Dick Smith's.
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Follow Up By: Steve from Top End Explorer Tours - Wednesday, Jan 23, 2008 at 23:17

Wednesday, Jan 23, 2008 at 23:17
Hey Bonz

I have been waiting for someone to spell the sound of white noise.

Thanks for clearing that up.

Regards to the family.

Cheers Steve.
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Follow Up By: disco1942 - Wednesday, Jan 23, 2008 at 23:41

Wednesday, Jan 23, 2008 at 23:41
Bonz

"and have come to the opinion that if someone cant see a tongue in cheek reply amongst all the others that profess to speak the truth then they will probably sus it out when the aysk for a failsafe disconnect at their local Dick Smith's."

A lot of people asking these questions do not have enough knowledge to sort the smart comments from the the accurate ones. Members like you with your verbal diarrhoea who insert your smart comments in as many threads end up not being particularly funny and disturb these threads. There are too many threads with the first few replies posted by your type just hanging around who think you are funny.

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Follow Up By: Bonz (Vic) - Friday, Jan 25, 2008 at 22:12

Friday, Jan 25, 2008 at 22:12
Thanx PeterD, I do take your comments on board but I very much doubt that people can't sort the wheat from the chaff. If mine was the only comment you may have a point, but how is my comment any different that someone saying you should run 50PSI on the gibber roads around Innamincka (which I have seen before) in a group of replies to the obvious contrary. Come on mate, I know it was a smart comment, it was constructed in that manner. And if they couldn't sort it then what, it wasn't like I was professing that they do something in a life threatening manner or something that was going to break the bank, like everything on here its all part of the research into things, some of which is waffle, some of which is gold (like my reply to the PDA issue with navigators which it appears does not fall into the verbal diarrhoea basket). Surely you don't take everything here as gospel mate, you'd have five different tyres on your car at 5 different pressures, four different fridges, six different cars, eight different swags etc etc etc. All that waffle said I do take your point as I said and will be very careful in future when I crap on.

Steve, you can thank Crazie for that spelling, he elucidated me in that vagary.
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Follow Up By: Gone Bush (WA) - Saturday, Jan 26, 2008 at 09:25

Saturday, Jan 26, 2008 at 09:25
Bonz,

don't take that criticism to heart. If those of us whose sense of humour is at a completely different level suppressed it where would the world be?

We wouldn't have Monty Python, The Simpsons, Daffy Duck, or several cracker campfire conversations I can remember.

On a convoy many years ago a call came over the radio asking for an explanation about some distinct lines through the overcast cloud above us. I didn't have a clue what caused it but I said they were isobars, lines joining areas of equal barometric pressure. You see them on the TV weather every night.

A laod of BS for sure but it sucked most of the convoy in and kept everyone entertained.

I assume that PeterD is a Discovery owner and was born in 1942.

The Defence rests.


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Reply By: Red Frog - Vic - Wednesday, Jan 23, 2008 at 20:05

Wednesday, Jan 23, 2008 at 20:05
In theory FM exhibits "capture effect" which says that the strongest signal should completly overide the weaker signal so the weaker signal is not heard at all but with the narrow band FM used for CB the capture effect is rather weak resulting in audio not much better than AM when received signals mix.
AnswerID: 283399

Follow Up By: disco1942 - Wednesday, Jan 23, 2008 at 23:13

Wednesday, Jan 23, 2008 at 23:13
The capture ratio is generally considered to be 13dB. If one signal is greater the other by 13dB you will only hear that one. If the signals are closerin amplitude you are likely to hear a heterodyne caused by the two signals (whistle) and bits of both audio signals as the result of incomplete capture.

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Follow Up By: Red Frog - Vic - Thursday, Jan 24, 2008 at 17:50

Thursday, Jan 24, 2008 at 17:50
13db eh! can go far better than that, 13db would be a poor radio.
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Reply By: AndrewX - Wednesday, Jan 23, 2008 at 21:22

Wednesday, Jan 23, 2008 at 21:22
It's commonly referred to as "doubling" in radio circles. Usually results from poor radio technique. Listen before you push the transmit button and say "over" when finished.
AnswerID: 283422

Follow Up By: disco1942 - Wednesday, Jan 23, 2008 at 23:21

Wednesday, Jan 23, 2008 at 23:21
In convoy situations people at one end of the convoy often can not hear those at the other end particularly when there are 10 or more vehicles involved. Making a bald statement like that is nearly as unhelpful ar Bonz's smart comments above.

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