In-band Tx indicator?

[Skogsgurra]

Good evening,

I have been (very slightly) involved in RC of little air-planes lately. There is a problem with unidentified transmitters that can make these little projectiles go astray. Think of it; a high-speed device with a rotating propeller that cuts into a group of spectators - no nice experience. So it is desirable to locate any transmitter in the frequency band - be it RC transmitters, statics or side-bands from other transmitters.

Question: Is there a device, somewhat like a spectrum analyzer, that can survey and show transmitters in the 35+ MHz band? I was thinking of modifying a SW receiver (adding a sweept capacitance diode to the local oscillator and outputting the tuning indicator voltage to a scope). But, being an old fart, lazy and not knowing much about radio technology, I prefer to find a ready-made thing.

Know about such things?

Gunnar Englund

http://www.gke.org

 

[VEBill]

Maybe a 'VHF {radio} Scanner'. Most cover 30 MHz and up. Most demodulate FM in the VHF-low band (30-50 MHz). I don't know much about R/C, but I assume that they also use NBFM (narrow band FM, you'd have to check).

Due to the scanning speed (maybe up to about 50 channels per second is typical), you'd probably have to manually select (vice 'lock out') the specific channel(s) of interest at the time - manually updating as required throughout the day. As opposed to scanning too many channels. You can preprogram in the channels by number like "25" (but not by Number+Letter like "25A"). Some top end scanners might allow fancy labeling of channels. Cheaper to make a reference list.

A trained human (good radio ears) would have to monitor the channels and listen for signals that shouldn't be there. And distinguish (by ear) the interference from the RC signals that are expected. R/C on R/C interference might be hard to hear except just before the 2nd guy starts up.

Spectrum Analyzers are (generally) not as sensitive and one lump in the RF spectrum looks much like another lump (unless you could make a waterfall plot - got money?). Most humans can't look at the Spec. A. plot and distinguish a valid R/C signal from something else. The human ear (scanner) is a Brazillion times better.

Most VHF is FM and 'capture effect' would limit the ability to hear weaker interference, but then it probably wouldn't interfere anyway (due to the same capture effect). Perhaps the trained operator could hear the interference in-between the R/C transmissions.

Of course, the end result might often be that the VHF scanner operator would be called to testify at the Coroner's Inquiry. What I mean is that if on-channel interference suddenly appears, there's nothing much that can be done except to yell at the hapless R/C operator (who may have already lost control).

Such a monitoring approach might be useful as a preflight check. Most interference is less likely to spring up at exactly the wrong time, but it is playing the odds.

You'd probably want a custom antenna (sleeve dipole in a plastic tube) and use a 10-foor mast to get it up in the air. This would allow much better reception to try to mimic the advantage that the RC airplane has being hundreds of feet in the air.

Just remember that it isn't a 100% solution. Maybe 80%.

Lots of scanners will run on '12 Vdc' (car battery).

US$200 is the ball park price for the scanner, plus some scrounging for a suitable battery and some scrap coax and other junk to make the sleeve dipole. Maybe headphones and a chair.

 

[Skogsgurra]

Thanks! I had no idea where to look. Googling next.

I already got the chair... ;-)

Gunnar Englund

http://www.gke.org

 

[OperaHouse]

I just picked up an old Radio Shack PRO-2020 scanner and in the 30-50MHZ range it scans in 5KHz steps. At 9 steps a second, that is 2.7 MHz a minute. If you can reduce the scan down to 10MHz, that wouldn't be bad. I imagine this is typical performance.

 

[VEBill]

You're refering to the 'search' function where the scanner searches every frequency (by 5kHz channel steps) between two programmed limits. Skogs probably doesn't need to use the search function because he should know the frequencies that he's interested in.

There's only so many standard RC channels. Just program those into the memories.