UBC125XLT Frequency misread

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UBC125XLT Frequency misread

Postby Rob1 » Thu Jun 21, 2018 11:07 am

A puzzler for the forum techies amongst you.....

I am an aero modeller (still using 35meg) and an aviation enthusiast generally. I have just purchased a UBC125XLT scanner, mostly for airband listening, and out of curiosity more than anything else I programmed one of the scanner banks with the full range of 35 meg frequencies (35.000 - 35.250) just to see if there was any action in my vicinity. Nothing going on.....

I then put a crystal (in this case channel 78 - 35.180 MHz) into one of my transmitters and switched it on - lo and behold the scanner burst into life but showing channel 79 (35.190 MHz). Puzzling..??

I then tried the crystal in another transmitter and the same, 78 reads as 79. To cut a long story short I then tried 10 different crystals in 2 different transmitter'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 Transmitter's can rule out faulty R/C gear but can anyone explain why the scanner reads this way....?? or is it a case of I have it set up wrong?
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Re: UBC125XLT Frequency misread

Postby thelad » Thu Jun 21, 2018 12:07 pm

Sounds like you could be over powering the scanner being so close. Hence the rule in 35mhz days of not walking on to the strip to collect a model with the Tx in your hand. Amazing how much 2.4 has changed the hobby! Atleast your radio gear will be cheap :walkietalkie:
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Re: UBC125XLT Frequency misread

Postby Rob1 » Thu Jun 21, 2018 12:49 pm

Thanks for the reply....

Thought about the scanner overload and tried it 'range checking' style with Tx aerial collapsed and 25 metres away from scanner - same result!
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Re: UBC125XLT Frequency misread

Postby m0lsx » Thu Jun 21, 2018 2:25 pm

A scanner is not a calibrated frequency counter & even a proper frequency counter has room for being off frequency, unless it was recently calibrated.
Start by working through some possibilities.
Have you manually checked to see if there is a signal on the required frequency, not just what channel the scanner stops on?
It sounds like you are using programmed channels to do this. If that is the case, what does close call give you?
Did you purchase all of the crystals from the same source? And have you checked the crystals to see that they are what they say they are? It could be that the crystals are all either not what they say they are, or that the signal put out by the TX's is wideband & confusing the Scanner with a stronger signal closer to say, channel 78 than 79.
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Re: UBC125XLT Frequency misread

Postby Rob1 » Thu Jun 21, 2018 4:40 pm

m0lsx

Scanning the band 35.000 - 35.250 does not give me any hits without the Tx switched on. The Tx crystals are all as supplied with branded R/C kit collected over the years and all seem to match the Rx crystals. I have no way of checking them which is why I tried multiple crystals in the trial.

Close call is a good idea - I'll give it a try next time the R/C gear is out.

Thanks for the ideas....
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Re: UBC125XLT Frequency misread

Postby Scan125 » Sat Jul 07, 2018 11:33 am

This may be a STEP issue.

In the 35MHz band the default step values for frequencies is 5kHz. So when the scanner picks up a signal it will display the nearest frequency on the STEP increments. If your channel spacings are less than 5kHz then the scanner will display one channel up or down from what you are expeciting.
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Re: UBC125XLT Frequency misread

Postby Rob1 » Wed Jul 11, 2018 1:07 pm

Nick

This is a possibility. I think we are on 10kHz spacing (Ch78 = 35.180, Ch79 = 35.190)

Will this make a difference...?

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