Robert Cracknell Posted May 15, 2018 Share Posted May 15, 2018 A puzzler for the forum techies amongst you..... I am an aero modeller (still on 35meg) and an aviation enthusiast generally. I have just purchased a Uniden Bearcat UBC125XLT scanner and out of curiosity more than anything else I programmed one of the scanner banks with the full range of 35 meg frequencies just to see if there was any action in my vicinity. Nothing going on..... I then put a crystal (in this case a 78) into one of my Tx's and switched it on - lo and behold the scanner burst into life but showing channel 79. Puzzling..?? I then tried the crystal in another Tx and the same, 78 reads as 79. To cut a long story short I then tried 10 different crystals in the 2 Tx's and in every case the display showed the next channel up - 71 reads as 72, 80 reads as 81, etc. I thinks using multiple crystals in different Tx's can rule out faulty R/C gear but can anyone explain why the scanner reads this way....?? Edited By Robert Cracknell on 15/05/2018 15:07:13 Quote Link to comment Share on other sites More sharing options...
John Stainforth Posted May 15, 2018 Share Posted May 15, 2018 I suppose the scanner must read the end of the current frequency band (i.e., the start of the next) rather than the beginning. Quote Link to comment Share on other sites More sharing options...
Denis Watkins Posted May 15, 2018 Share Posted May 15, 2018 Your crystal suggests that you are a glider pilot? Try some distance between the Tx and the scanner Robert, then come back Quote Link to comment Share on other sites More sharing options...
john stones 1 - Moderator Posted May 15, 2018 Share Posted May 15, 2018 Mysterious stuff to me lectric, our scanner has txs next to it when we do frequency checks, looks like scanner to me ? Quote Link to comment Share on other sites More sharing options...
Peter Christy Posted May 15, 2018 Share Posted May 15, 2018 I would agree that it sounds like a scanner fault. Most model Txs are pretty good at being on the right frequency - its unusual to find one significantly out. Having said that, most PPM transmitters will read (usually) low on a frequency meter. This is due to the asymmetric nature of the PPM pulse train, which spends more time at the low end of the frequency range than at the high. PCM usually reads correctly as its a symmetrical waveform. I'm surprised that a scanner is reading that far out! -- Pete Quote Link to comment Share on other sites More sharing options...
Martyn K Posted May 15, 2018 Share Posted May 15, 2018 It may be that the bandwidth of the Scanner is incorrect. If its set to WBFM, it could settle anywhere within the slope of the detector. The spacing between 35MHz channels is 10kHz but the transmitted bandwidth will be less than that Martyn Quote Link to comment Share on other sites More sharing options...
Robert Cracknell Posted May 15, 2018 Author Share Posted May 15, 2018 Denis Did as you suggested and put about 30 yds between scanner and Tx - same result. Not a glider guider - just happen to have a few crystals.... Quote Link to comment Share on other sites More sharing options...
Robert Cracknell Posted May 15, 2018 Author Share Posted May 15, 2018 Peter The Tx is PCM.... Quote Link to comment Share on other sites More sharing options...
Robert Cracknell Posted May 15, 2018 Author Share Posted May 15, 2018 Martyn The settings on the 125XLT are... Auto AM NFM FM WFM FMB Setting it on'Auto'it detects the Tx as being AM Quote Link to comment Share on other sites More sharing options...
Kevin Wilson Posted May 15, 2018 Share Posted May 15, 2018 I would expect NFM to give a more accurate reading. The passband on AM will be rather wide. I can only surmise it is the pulse train on the modulation that is tricking your scanner into 'auto' setting as AM. I cannot recall anyone manufacturing a 35mHz AM transmitter, I have only seen 35 FM since the early 80s. Quote Link to comment Share on other sites More sharing options...
Frank Skilbeck Posted May 15, 2018 Share Posted May 15, 2018 Posted by Kevin Wilson on 15/05/2018 19:35:16: I cannot recall anyone manufacturing a 35mHz AM transmitter, I have only seen 35 FM since the early 80s. Acoms did one, but I never saw one at the field, found this one on a sale site https://picclick.co.uk/ACOMS-DIGITAL-PROPORTIONAL-RADIO-CONTROL-SYSTEM-3-CHANNEL-222888367666.html Quote Link to comment Share on other sites More sharing options...
gangster Posted May 16, 2018 Share Posted May 16, 2018 I remember the Acoms 35MHz AM. I recall it didn’t remain available for long. I am racking my brains as I am sure there was an issue about it not working well along side FM. I cannot remember what and cannot for the life of me think why that would be so. Maybe bandwidth issue Quote Link to comment Share on other sites More sharing options...
will -0 Posted May 16, 2018 Share Posted May 16, 2018 Hi, if you know anyone with a 35meg multiplex royal transmitter with synth, it has a built in scanner which you could use to compare results. Alternatively, set up three rxs with e.g. channels 77,78 and 79 and see which one responds. Quote Link to comment Share on other sites More sharing options...
Robert Cracknell Posted May 16, 2018 Author Share Posted May 16, 2018 Will Good idea - gave it a try just now..... The Rx with the paired crystal in is the one that responds i.e. 78/78. The others do not move so I have no reason to suspect anything other than a scanner quirk. Quote Link to comment Share on other sites More sharing options...
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