How reliable are harmonics?

[ErikB]

I would like to use my sacnner (a 246t) to scan for R/C air frequencies that are in use before I go out and fly. The R/C air frequencies are all in the 72 mhz range. My 246t does not cover that band, but I have noticed that my R/C Tx puts out a harmonic at 145.9400, exactally twice the origional 72.97 freq. It is quite strong and consistant. My question is how reliable are these harmonics? I am guessing that scanning the 2X harmonics of the origional freqs might work but is probabally not a reliable method to find what freqs are in use. I would think all radios are a little different and all might not spit out this harmonic?
All input is appreciated.

 

[eorange]

I wouldn't count on radiated harmonics always being there. Harmonics can be caused by poor circuit design, inappropriate antennas, and many other factors. High quality transmitters shouldn't produce harmonics.

I used to fly R/C many years ago...isn't there a frequency board with visible tags to show who is currently flying with what frequency?

Erik
KA3FYU

 

[ErikB]

I like to fly slope gliders out in the middle of no where, I think there are less than 10 RC guys in the county, there are no clubs, etc. I would like to be able to scan to find out what freqs are in use since there is no organized freq board, club, or the like.
If you are fimiliar, my Tx is a Futaba 9c, not exactally cheap, works fine for flying planes, do you think it might have something wrong with it?

 

[StarkFlyer]

Take a look at this. http://www.hobbico.com/fieldequip/hcap0340.html

I fly electric RC at local parks and I have one. It does the job.

I doubt there is anything wrong with your 9C. Having the transmitter close to the receiver can show up as harmonics, birdies, etc. The receiver's RF amplifier can easily get overloaded.

Dick

Dick

 

[N_Jay]

I like to fly slope gliders out in the middle of no where, I think there are less than 10 RC guys in the county, there are no clubs, etc. I would like to be able to scan to find out what freqs are in use since there is no organized freq board, club, or the like.
If you are fimiliar, my Tx is a Futaba 9c, not exactally cheap, works fine for flying planes, do you think it might have something wrong with it?

You can not tell if you have a problem without knowing the level of both the primary frequency and the harmonic.

 

[ErikB]

I have seen that, and have heard it works pretty good. I was almost considering getting one, but then I thought maybe I should just put that 50 bucks towards another scanner. It is one more thing I can use to justify a new purchase to the wife.
"you know honey, not only do i need this new 396D digital scanner to listen to the new digital LE signals, but I can also use it while out flying to prevent crashing planes, you know, a saftey thing".........

 

[ErikB]

You can not tell if you have a problem without knowing the level of both the primary frequency and the harmonic.

I am not sure what you mean. Are you talking about how many watts each freq is putting out?

 

[N_Jay]

I am not sure what you mean. Are you talking about how many watts each freq is putting out?

Yes.

Lets say you have a 4 watt transmitter. +6 dBW which equals = +106 dBm

Lets say you 2nd harmonic is 10 mW (A very receivabel signal) is +10dBm

So even if the harmonic is 96 dB down it is still a decent strength.

 

[eorange]

Do the old reliable test: take your glider to the airfield and power on the receiver and see if any servos start to fritz - that will be a sure sign stray signals are around. Even give it a gentle hand launch and ensure you have control.

And...once your glider is way up there, there isn't much you'll be able to do about it once your scanner starts squawking about intruders.

Also...isn't your Futaba a digital radio? Those were long after my time, but I thought digital radios allowed you limit the control to just your planes, even if others share the same freq. Wow, that 9C looks sweet, after having been out of the hobby for over 15 years. I remember getting excited about dual rates as the hot new feature!

Erik
KA3FYU

 

[ErikB]

Do the old reliable test: take your glider to the airfield and power on the receiver and see if any servos start to fritz - that will be a sure sign stray signals are around. Even give it a gentle hand launch and ensure you have control.

And...once your glider is way up there, there isn't much you'll be able to do about it once your scanner starts squawking about intruders.

Also...isn't your Futaba a digital radio? Those were long after my time, but I thought digital radios allowed you limit the control to just your planes, even if others share the same freq. Wow, that 9C looks sweet, after having been out of the hobby for over 15 years. I remember getting excited about dual rates as the hot new feature!

Erik
KA3FYU

Erik- I usually do the "old reliable" but I figured the scanner might be more sensitive to weak signals that might hit me when I was way up there.
The 9C is both PCM (digital) and PPM (FM analog), you can select which you want. Some new recievers have whats called TSR (something like transmitter signiture recognition) which supposedly will lock on to your transmitters unique signal and block all others on the same freq.
I did a test while I was home for lunch. I fired up another Tx I have, this one is a different brand (Hitec Flash 5) on the same freq as my Futaba. I put it in one end of the house with the antenna down and turned the scanner on in the other end of the house. Again, 145.9400 came through strong as before. I programmed all 50 2nd harmonics for the RC band into my scanner and will try to figure out in the coming weeks how effective this is.
Erik

 

[ErikB]

I did a little more testing. 2 more R/C transmitters, 2 new freqs, one was a third brand I had not tried yet (GWS).
I found all radios tested spit out a predictible 2nd harmonic at 2x the origional freq. The only this I really did not test is range. I did all of these indors with the Tx antenna down and got as far away from the Tx as i could and still be indoors. I will try to find out how far these signals can go outside when I get more time and it warms up outside.
I know R/C air radios broadcast at about .75 watts, what kind of power would the harmonic be putting out?

 

[N_Jay]

I did a little more testing. 2 more R/C transmitters, 2 new freqs, one was a third brand I had not tried yet (GWS).
I found all radios tested spit out a predictible 2nd harmonic at 2x the origional freq. The only this I really did not test is range. I did all of these indors with the Tx antenna down and got as far away from the Tx as i could and still be indoors. I will try to find out how far these signals can go outside when I get more time and it warms up outside.
I know R/C air radios broadcast at about .75 watts, what kind of power would the harmonic be putting out?

A bad TX is 30dB down and a good one is 70+ dB down

You are putting out +28 dBm (about)
and a good receiver is sensitive to -115 and -130
A short path will be between 20 dB and 60 dB of loss

So a bad TX you will hear loud and clear and very good one will be marginal.

Edit;
These are all off-the-top-of-my-head numbers so don't start nit-picking!

 

[ErikB]

A bad TX is 30dB down and a good one is 70+ dB down

You are putting out +28 dBm (about)
and a good receiver is sensitive to -115 and -130
A short path will be between 20 dB and 60 dB of loss

Thanks for the info. Here is a stupid, newbieish question. I am assuming dB is decible, and as I understand it, more decibles=stronger signal. So what does dBm stand for?
Erik
_______________________________
Non-Communications Professional

 

[ErikB]

whille I am asking, I saw you referenced dBW earlier, is this decible watts? And how is that differnent than watts.
Thanks.

 

[N_Jay]

dB is a comparative measure. It tells you only the difference between two values.
dBm is the level compared to 1 milliwatt
dBW is the level compared to one watt.
0 dBw is +30 dBm

More dB can be stronger or weaker depending on the context.

 

[ErikB]

dB is a comparative measure. It tells you only the difference between two values.
dBm is the level compared to 1 milliwatt
dBW is the level compared to one watt.
0 dBw is +30 dBm

More dB can be stronger or weaker depending on the context.

OK, so when you say my TX puts out +28 dB, this is in relation to what?
Thanks for the info!!

 

[Al42]

OK, so when you say my TX puts out +28 dB, this is in relation to what?
Thanks for the info!!

30dbm (he didn't say db) is 1,000 miiwatts (log 30 above .001 watt), or 1 watt. 1/2 watt would be 3db below that or 27 dbm. Your 3/4 watt is about 28dbm.

BTW, putting the antenna down could actually increase the second harmonic. First it's a mismatch. Second, if it's resonant at 145 MHz when it's down it'll transmit the 145 MHz signal better than the 72.5 MHz signal. (And any hams in the area are going to be very upset with you - that's right in the middle of the 2 meter ham band. I don't know if there's any activity around there on 145.94 (there is here on 145.495, even though that entire area - 145.5-146 isn't used tas much as other parts of the band) but if there is, they'll let you know.)

If you could hear the third harmonic, that might be even stronger.

 

[N_Jay]

OK, so when you say my TX puts out +28 dB, this is in relation to what?
Thanks for the info!!

That is why I said +28 dBm

 

[ErikB]

Ok, thanks guys, it is making a little more sense now. Thanks for all the replies.
FYI, the only Ham repeater in the county is set up on 147.1200 and has next to no traffic (I usually monitor it just for fun).
Lots of bandwidth up here. Nobody wants lots of Hams upset with them.