western Posted June 24, 2009 Share Posted June 24, 2009 OFCOM investigators are out there, you won't spot them until they have cause to knock on your door for a chat or worse consficate the equipment which they have the power to carry out without having a police officer attend. Quote Link to comment Share on other sites More sharing options...
cipx2 Posted June 24, 2009 Share Posted June 24, 2009 I was talking about RECEIVING, not transmitting. Please read my posts. Quote Link to comment Share on other sites More sharing options...
western Posted June 24, 2009 Share Posted June 24, 2009 I was talking about RECEIVING, not transmitting. Please read my post. I did & it's not recommended Quote Link to comment Share on other sites More sharing options...
cipx2 Posted June 24, 2009 Share Posted June 24, 2009 Yeah, thanks for warning me We were not talking about recommended procedures here, I was giving you the scenario that offers the extreme situation were the SWR is as far as it can get from the ideal value. The thread initiator had/has a problem as he couldn't/can't communicate with other stations (this means receiving on one hand). Now we have 2 pages of all sort of replies not relevant to the subject. Sorry, but I don't fell like I want to go along with this so I'll stand by until the thread initiator replies my question or has something else to add. Quote Link to comment Share on other sites More sharing options...
western Posted June 24, 2009 Share Posted June 24, 2009 simple as the rig cannot recieve cleanly it's got to be a antenna or swr problem, unlikely to be a faulty rig as it happened to 2 users. Quote Link to comment Share on other sites More sharing options...
DrT Posted June 24, 2009 Share Posted June 24, 2009 simple as the rig cannot recieve cleanly it's got to be a antenna or swr problem, unlikely to be a faulty rig as it happened to 2 users. Not true! If you read the first post clearly then he says that he and one other could talk to each other and the other three could all talk to each other. If it was an aerial mismatch problem then at the short range of an offroad convoy even a severly mismatched antenna would work. SWR is simply a measure of your antenna's efficiency matched to your CB. It is a measure of the Standing Wave Ratio. Occasionaly he could just detect that the group of three were talking. This is not an aerial mismatch. It is most likely one of two options previously mentioned on this thread 1) The two groups were using diferent modulation ie AM and FM 2) The two groups were using different frequencies despite the rigs all saying Ch10. The close proximity of transmitters could easily overload the front end of a sensitive receiver ona different channel and hence the slight detection of others talking. Without trying the set against another known working example it may be difficult to resolve this one. Quote Link to comment Share on other sites More sharing options...
TheRecklessEngineer Posted June 24, 2009 Share Posted June 24, 2009 Anyone for tea? Quote Link to comment Share on other sites More sharing options...
Bull Bar Cowboy Posted June 24, 2009 Share Posted June 24, 2009 Lets dispel a few urban myths………………….. Firstly MOG ............... Sorry .......... I was one of those men........... There is a strong probability that one group was inadvertently using CTCSS (Continuous Tone Coded Squelch system) which is available on some rigs. Just to be sure that we under stand what ‘overloading’ the receiver means……………. This comes in various guises, A strong signal close by and not necessarily in the received frequency band will cause blocking of the wanted signal due to driving the front end RF amp into a non linear mode. Or, The wanted signal can be at too higher level (usually m/volts as opposed to u/volts) from the antenna which will cause 3rd order intermodulation distortion (from the PLL) and subsequently give very strange results when passed through the first intermediate frequency filter………….this happens when the incoming signal reaches the 3rd order intercept point………….. Every receiver has a dynamic range …………… the cheaper the receiver then generally the worse the dynamic range resulting in mediocre to poor performance. The receivers I normally use are £100K upwards………….. Just to be clear, the receive performance is affected somewhat by the antenna ………. A correctly tuned antenna will present a fully resistive 50 ohm load and act as a filter to unwanted signals due to its resonant frequency, however, in this case the unwanted signal must be outside of the switching bandwidth of the receiver. Christ ………… anybody would think that I did this stuff for a living Ian Quote Link to comment Share on other sites More sharing options...
DrT Posted June 25, 2009 Share Posted June 25, 2009 Lets dispel a few urban myths…………………..Firstly MOG ............... Sorry .......... I was one of those men........... There is a strong probability that one group was inadvertently using CTCSS (Continuous Tone Coded Squelch system) which is available on some rigs. Just to be sure that we under stand what ‘overloading’ the receiver means……………. This comes in various guises, A strong signal close by and not necessarily in the received frequency band will cause blocking of the wanted signal due to driving the front end RF amp into a non linear mode. Or, The wanted signal can be at too higher level (usually m/volts as opposed to u/volts) from the antenna which will cause 3rd order intermodulation distortion (from the PLL) and subsequently give very strange results when passed through the first intermediate frequency filter………….this happens when the incoming signal reaches the 3rd order intercept point………….. Every receiver has a dynamic range …………… the cheaper the receiver then generally the worse the dynamic range resulting in mediocre to poor performance. The receivers I normally use are £100K upwards………….. Just to be clear, the receive performance is affected somewhat by the antenna ………. A correctly tuned antenna will present a fully resistive 50 ohm load and act as a filter to unwanted signals due to its resonant frequency, however, in this case the unwanted signal must be outside of the switching bandwidth of the receiver. Christ ………… anybody would think that I did this stuff for a living Ian Your last point is therefore irrelevant since all the frequencies that were being used were within the frequency range of both the receiver and the antenna. Your two points on overloading are fine but still do not explain the phenomena described, you missed out the fact the the primary receiver stages can act as a receiver in their own right if overloaded by a misstuned signal due to their non linear characteristics and hence provide a signal to the speaker that has been poorly decrypted but is still vaguely audible to the human ear and recogn isable as speech. This is fundamentaly the way that the original "cats whisker" radios worked with a simple diode being the non linear element. If CTCSS was in use then that might explain why two could talk together since that is the purpose of such a system, to exclude third parties from the link despite them being on the same channel. Quote Link to comment Share on other sites More sharing options...
errol209 Posted June 25, 2009 Share Posted June 25, 2009 Edges nervously into thread ... If the the two parts of the other group could communicate, then it can't be because one was on AM (amplitude modulation) and the other on FM (frequency modulation) because even if the two parties picked the same nominal frequency, the AM receiver (RX) of the FM broadcast (TX) would only hear any kind of signal as the FM waveform passed through the centre frequency, which in AM terms is silence. The other way round (AM TX, FM RX), the FM receiver would only pick up a frequency invariant waveform, which in FM terms is also silence. I appreciate a that a variation in FM signal strength would create an AM-like signal ... ... creeps away ... Quote Link to comment Share on other sites More sharing options...
Hybrid_From_Hell Posted June 25, 2009 Share Posted June 25, 2009 3rd order intermodulation distortion Ah yes, the old "3rd order intermodulation distortion" Anyway........ <moderator boots on> I have read this thread, and now that I have had enough asprins feel sort of ok(ish),.... However, reading through, and up to (and not including) BBCs post it seem to me reading some of the comments from some members posting that there is a fair bit 'unpleasantness' / 'attitude' creeping in to the tone(s) and of some replies, this may be either accidental or deliberate - but whichever please re read take note and desist - a polite rerquest </moderator boots off> Nige Quote Link to comment Share on other sites More sharing options...
Bull Bar Cowboy Posted June 25, 2009 Share Posted June 25, 2009 Your two points on overloading are fine but still do not explain the phenomena described, you missed out the fact the the primary receiver stages can act as a receiver in their own right if overloaded by a misstuned signal due to their non linear characteristics and hence provide a signal to the speaker that has been poorly decrypted but is still vaguely audible to the human ear and recogn isable as speech. This is fundamentaly the way that the original "cats whisker" radios worked with a simple diode being the non linear element. Yes, that scenario can occur and indeed has occurred ............. it is normally down to very poor design. However, the devices used in modern front ends are very robust in this respect. Hopefully gone are the days when manufacturers place a couple of back to back diodes across the antenna input to protect the front end............... this type of design error not only provides for a poor receiver performance but also will reradiate in the common modes of FX= (2xF1) - F2, or FX = (3xF1) - (2XF2) or FX = (2xF2) - F1 etc (this is in its simplist form)................... however, where several frequencies are involved then the calculations and possible combinations become vastly more complex. I have programmes here for frequency clearance on multi user hilltop sites that will take about a day of run time to give me all the combinations for a given frequency when compared to the others on site. IMD products do not only get generated by transmitter PA stages ............... RX front ends can also be susceptible. Your understanding of a 'cats whisker' is correct, however, that relied on a high level signal as an implicit design feature.................. device deign has come a long way since then and even mediocre receivers design has a high degree of tolerance to blocking and IMD's..................... Going back the the OP .............. I would suggest that it was caused either by the inadvertent use of CTCSS ( I wouldnt have expected many CB rigs to have the facility of DCS - Digital Coded Squelch) ............. or one group was using AM and the other FM ....................... the FM group would have had the upper hand due to the inherent 'Capture effect' of FM. Edges nervously into thread ...If the the two parts of the other group could communicate, then it can't be because one was on AM (amplitude modulation) and the other on FM (frequency modulation) because even if the two parties picked the same nominal frequency, the AM receiver (RX) of the FM broadcast (TX) would only hear any kind of signal as the FM waveform passed through the centre frequency, which in AM terms is silence. The other way round (AM TX, FM RX), the FM receiver would only pick up a frequency invariant waveform, which in FM terms is also silence. I appreciate a that a variation in FM signal strength would create an AM-like signal ... ... creeps away ... In the perfect world ............yes.............. However, FM transmitters are not true FM ......... they will be phase modulation which will contain a residual AM content. Also the frequency constant part of the recievers will be to be very accuratly controllled and this is not the case with CB rigs .................. FM will be detected in an AM detector as slope modulation if the frequcy is not tightly controlled ................. There are too many variables and factors to take into consideration ........... Ian Quote Link to comment Share on other sites More sharing options...
Eightpot Posted June 25, 2009 Share Posted June 25, 2009 Sounds like fromage in the microphone to me. Have you tried hitting it? Quote Link to comment Share on other sites More sharing options...
cipx2 Posted June 25, 2009 Share Posted June 25, 2009 This for those of you who hate aspirins I uploaded 2 files from which the TQBFox.mp3 is this is source file used for the demonstration. The second audio file is the recording (I have it on video as well) of what an FM receiver picked up when the source file was transmitted 3 times in a row from another transmitter. The second audio has three parts: - the transmission started TX-ing in FM mode (FM transmission received on FM receiver - the ideal match) - after the BEEP the TX was switched to AM mode (AM transmission received on a FM receiver) - after the 2nd BEEP the TX was switched to SSB (USB) mode. (SSB transmission received on an FM receiver) Everything was done in one continuous transmission (i.e I didn't have to release the PTT as the transceiver used for TX is smart enough to let me change the modulation type while TX-ing). Enjoy! TQBFox.mp3 FM_AM_SSB_with_FM_receiver.mp3 Quote Link to comment Share on other sites More sharing options...
martyn668 Posted June 25, 2009 Author Share Posted June 25, 2009 Sorry for starting a post, and then disappearing while the debate went on. There are some very intelligent radio people here (with differing views), and your answers are going way over my head (which is fine, as long as everyone is enjoying themselves ) In reply to cipx2's point, I haven't a clue whether I am on AM or FM. It's a Danita 240FM, so I guess the clue is in the title. However, the group have gone back to France now, so I can't easily ask what the others were using. It sounds like me and one other guy may have had FM transmitters, and the others AM. However, that raises the point - why did CB manufacturers allow that to happen? Two systems, nominally the "same", which can just about communicate, but the quality is so bad that they are not compatible? I guess the next time I see them, I can ask if anyone was using CTCSS (Continuous Tone Coded Squelch system), but by the sounds of it, this is only on newer or more expensive pieces of kit, not the type of stuff you get in 4x4's. Quote Link to comment Share on other sites More sharing options...
cactus Posted June 25, 2009 Share Posted June 25, 2009 I've had my licence 27 years and............................You've pickled the poor lads head............ It is reasonable to assume they were on different frequencies.....................That's all!!! Quote Link to comment Share on other sites More sharing options...
western Posted June 25, 2009 Share Posted June 25, 2009 AM was used in the USA & over here, prior tp the UK Legal FM cb coming into use, AFAIK no CB rigs have CTCSS tones in them & I don't think there is any CTCSS requirement for CB transcievers either UK or Euro/CEPT types. you may have over heard a 10metre Ham users transmiting which is lose to CB 27 Mhz frquencies like this 10 metre rig for example Quote Link to comment Share on other sites More sharing options...
cipx2 Posted June 25, 2009 Share Posted June 25, 2009 .......... It sounds like me and one other guy may have had FM transmitters, and the others AM.However, that raises the point - why did CB manufacturers allow that to happen? Two systems, nominally the "same", which can just about communicate, but the quality is so bad that they are not compatible? I guess the next time I see them, I can ask if anyone was using CTCSS (Continuous Tone Coded Squelch system), but by the sounds of it, this is only on newer or more expensive pieces of kit, not the type of stuff you get in 4x4's. Your rig is one of the most simplest, FM only, CB radios. About all the late AM capable radios have FM also. There's a push button or switch to select whichever mode you wish to. For example http://www.4x4cb.com/public/item.cfm?itemID=1253 - it's the button next to the TX/RX green led. Between your rig and the one I gave the link to, it's not a "compatibility" issue but a difference in "features" and in you case it would have been an "operator error" (theirs). Don't worry about CTCSS, it's not the case here. The main issue is that you couldn't understand what people in the "group of 3" were saying and this excludes CTCSS, DTS, DTMF or any other WERWEGFERFWEXFXEGDWGD systems hypothetically used on CB radios. The wikipedia article is good enough to prove that: http://en.wikipedia.org/wiki/CTCSS Try to find someone who has a UK&CEPT radio and do some tests with you Danita (make sure he uses FM - your CEPT rig doesn't know AM or SSB). Quote Link to comment Share on other sites More sharing options...
A Twig Posted June 25, 2009 Share Posted June 25, 2009 Bloody hell, I'm doing an Electronics and Communications Masters, and I'm lost, the quicker we all get to digital the better... Quote Link to comment Share on other sites More sharing options...
Bull Bar Cowboy Posted June 25, 2009 Share Posted June 25, 2009 Bloody hell, I'm doing an Electronics and Communications Masters, and I'm lost, the quicker we all get to digital the better... Yes, I guess you are right, most of the above is now 'old hat' and there are not many analogue design engineers left............. Shall we carry on the thread and discuss the merits of the ETSI TETRA standard or DMR (Digital Mobile Radio) ............... I'm quite happy to debate quadrature detection and the merits of LSI & DSP's................... Ian Quote Link to comment Share on other sites More sharing options...
Hybrid_From_Hell Posted June 25, 2009 Share Posted June 25, 2009 I'm quite happy to debate quadrature detection and the merits of LSI & DSP's................... Ian Not with a lead pipe smacked hard across your head with the words 'FFS No Stop please' you won't be Bloody show off Nige Quote Link to comment Share on other sites More sharing options...
Bull Bar Cowboy Posted June 25, 2009 Share Posted June 25, 2009 Quote Link to comment Share on other sites More sharing options...
Happyoldgit Posted June 26, 2009 Share Posted June 26, 2009 Is it over yet? Are we done Quote Link to comment Share on other sites More sharing options...
Anderzander Posted June 26, 2009 Share Posted June 26, 2009 Quote Link to comment Share on other sites More sharing options...
SacredDog Posted June 28, 2009 Share Posted June 28, 2009 AM was used in the USA & over here, prior tp the UK Legal FM cb coming into use, AFAIK no CB rigs have CTCSS tones in them & I don't think there is any CTCSS requirement for CB transcievers either UK or Euro/CEPT types. you may have over heard a 10metre Ham users transmiting which is lose to CB 27 Mhz frquencies like this 10 metre rig for example No Miserableolgit, I haven't put my 'TwoPenceWorth' in yet Sorry to disagree with you Western (I do make a habit of doing that sometimes) but a few modern CB's have CTCSS, my Intek M-495 certainly has (it's not that easy to set up & use though). Anyway, my money (if I were a betting man) would be on the two sets of users using different modes (one on AM the other on FM). 73's, Colin M6XSD. Quote Link to comment Share on other sites More sharing options...
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