Frequency interferance and TV stations

Interesting! Ch 4's audio carrier (which is a well defined peak on a spectrum analyzer) is 71.75. Taking Channel 21's frequency 72.21 and subtracting .455 (typ intermediate frequency) yields 71.755 which is within the BW of the FM audio transmission. I wonder if the rule is because of a precaution or if it is the result of previous problems, and I wonder what the distance to and ERP of the TV station is ...

I am not an expert in this stuff.

However, I was under the understanding that it's the receiver that uses intermediate frequencies. The transmitter shouldn't transmit on anything except the carrier, unless there is some wierd failure.

On the other hand, I wonder if they are thinking that a TV may retransmit on this frequency? The TV must use an internal oscillator that is .455 away from the carrier to mix the signal to 455kHz. If that is the case, then a TV which is tuned to CH 4 just might transmit on that frequency.

This all sounds fairly improbable, though.

I'm going flying!

It shows...;-)

Yes, but the transmitter occupies a large band...typically +- 100Khz for FM audio. And that auidio may be based on a subcarriers that is about

What the OP was saying is that part of that audio subcarrier is a direct image for a model reciever.

No. The TV uses typically a 45MHz IF strip,IIRC - and its a LONG time ago - with a separte sound IF at a ferquency I forget. Possibly a direct take off at 4Mhz.

The IF frequency ius generally selected so as to make shaping teh mandwidh easier. Its about 5-100 times higher than the bandwidth you want.

So for 4.5Khz bandwidth use 455Khz For 450Khz bandwith - FM audio broadcats - 10.7Mhz is teh norm

For 6Mhz (TV video) its usually around 45Mhz IIRC.

In a radar application I have seen IF frequencies of a few hundred Mhz..

If that

I just passed thru and saw this...here's a possible explanation.

If the receiver is tuned for 72.21, and has an IF of .455, then it will have an internal carrier oscillator of 71.755 OR 72.665. The oscillator may be above or below the receive frequency - no rule-of-thumb.

In an area where the channel 4 TV signal is strong it COULD get into the circuit just as the receiver's own internal oscillator does.

TV sets don't use a 455 IF so that possibility can be ruled out.

Another option is plain old overload of signal, even though its offset substantially, if you are close to the TV xmtr site.

Good luck, Bill

Seems like overload would affect more than just one channel.

I like your idea that the internal oscillator in the receiver is syncing with the broadcast audio frequency.

Ah, an expert! Perhaps you can explain it then, rather than just engaging in the noble art of handwaving... ;)

Actually, the OP was asking for a technical explanation of why ch 20 (or

Chris mentioned that it was 455kHz away from the audio frequency of channel 4, and thus that it may be interfering somehow because of this.

I mentioned that the TV station shouldn't have any power on channel 21, but forgot that a large signal might somehow be influencing the internal oscillator of the receiver. This was subsequently pointed out by -ex-.

I also forwarded a theory based on my admittedly trivial knowledge of TV, which you've disproved.

Ok, so that theory is shot.

So, we still don't know why is channel 20 (or 21) prohibited at some fields...

Here is an idea... Could it be that, instead of CH 4 affecting the receiver, the RC radio is somehow making annoying noise on channel 4? How about the idea that the RC receiver is actually broadcasting a buzz on the audio frequency?

If somebody has a channel 21 crystal and a TV that isn't on cable, they can try this out fairly easily.

I'm out of place in that I'm not an RC'er. I'm a TV guy so I look at it from a different perspective.

Your RC transmitter isn't going to bother a TV set unless you are REALLY close. The RC receiver local oscillator fer darn shure won't bother a TV unless its sitting right on top of it and then its unlikely.

I'm not really sure why Ch 20/21 would be specifically prohibited anywhere but remember that 72-76 MHz is a shared band. There are other services using the same freqs. Forestry Service, garage door openers, etc. tend to use this band in some areas so maybe the restrictions are local to the extent that there have been interference issues in the past.

Just to blab a bit...some of you may remember an airliner that went down near Harrisburg PA in the 70s. This was officially blamed on leakage from the local cable TV system that uses the same frequencies and caused havoc with the airliner's navigational systems. Ever since then the FCC has been VERY STRICT on cable tv leakage to the extent where that kind of thing just simply shouldn't ever happen again. Trust me on this...part of my job is running these tests and maintaining the monitoring systems (and droning quarterly paperwork).

The point I'm making is that if a commercial airliner's systems can be messed up by something like a weenie-power cable tv system signal at

Dunno if the original issue really considers sources interfering with RC or if RC interferes with other parties when certain channels are said to be 'prohibited'. The problems are essentially of the same ilk.

-Bill

| >> I am not an expert in this stuff. | > | > It shows...;-) | | Ah, an expert! Perhaps you can explain it then, rather than just | engaging in the noble art of handwaving... ;)

You want TNP to explain why somebody has banned something (but we're not sure exactly what) at some unnamed fields in some unnamed state?

In any event, you don't always have to be an expert to tell that somebody else isn't one.

| Actually, the OP was asking for a technical explanation of why ch 20 (or | 21, he can't remember) is prohibited at some fields, and thought that | perhaps it was because of TV ch 4.

| >> On the other hand, I wonder if they are thinking that a TV may | >> retransmit on this frequency? The TV must use an internal oscillator | >> that is .455 away from the carrier to mix the signal to 455kHz. | > | > No. The TV uses typically a 45MHz IF strip,IIRC - and its a LONG time | > ago - with a separte sound IF at a ferquency I forget. Possibly a direct | > take off at 4Mhz. | | Ok, so that theory is shot.

And any signal that a TV receiver emits on the 72 mHz band should be _very_ weak anyways, and shouldn't cause problems unless the TV is defective or really really close to the plane.

| So, we still don't know why is channel 20 (or 21) prohibited at some | fields...

Ultimately, even if we get an `expert' here, we still won't know.

You'll need to ask the people at the fields who actually banned it. There's a lot of `voodoo' claimed when people talk about interference, and it may be that the channels are banned for no good reason beyond that Joe crashed two planes on those frequencies six years ago because he forgot to charge the batteries (but it was blamed on interference, of course.) Or maybe not.

If it's really channels 20 and 21, it could very well be that there's a pager tower nearby that broadcasts on 72.200 mHz -- right in between the two channels. Here in Austin, TX I've detected strong digital transmissions on 72.660, 72.860 and 72.960 mHz. None of the local clubs have banned the channels adjacent to those signals, but maybe none of the clubs have a pager tower that close to the field. Or maybe people just haven't put 2 and 2 together yet and figured out why they're crashing. Or maybe it's an unwritten rule that you avoid those frequencies ...

| Here is an idea... Could it be that, instead of CH 4 affecting the | receiver, the RC radio is somehow making annoying noise on channel 4? | How about the idea that the RC receiver is actually broadcasting a buzz | on the audio frequency?

Seems unlikely. Any signal emitted by the receiver would be very weak, and you're not likely to be flying that close to a TV anyways.

However, I've heard that (some?) scanners emit a weak signal 10.7 mHz above or below what they're listening to -- and our dual conversion receivers may very well do the same. But it would be a very weak signal, and I wouldn't expect it to interfere with TV unless the TV station was very weak and/or the TV set was very close.

Do we even know if the place where these channels are banned has a TV station on channel 4?

since at close range you can put lines of interference onto a TV tune

to channel 4 with a TX on 72 mhz... I can see where it would be eas for a TV station broadcasting on channel 4 to interfere with R operations. The wat the numbers add up for the frequencies o channel21 and 22 (RC) It would just seem prudent to not use them in a area that has a TV station broadcating on channel 4

-- fhhuber50677

----------------------------------------------------------------------- fhhuber506771's Profile:

on a single conversion RX, with low side LO injection the Image Freq is within the Ch4 video bandwidth.

Image freq is a freq, that will mix with the 2nd harmonic of the Local Osc., and produce a signal in the IF Band.

TV ch4 visual = 66 +1.25 67.25 audio = 67.25 +4.5 = 71.75, FM energy from 71.675 to 71.825 color = 67.25 +3.58 = 70.83

ch 20 RC Freq: 72.19Mhz ch 21 RC Freq: 72.210Mhz with .455Mhz IF

Image frequency, single conversion, .455Mhz IF =

Also, the FCC does not want interference FROM RC:

enjoy :)

And some of the other part of it is:

at low RF to IF seperation, i.e. 455Khz, the RF Front end filter will not/cannot be narrow enough to provide much attenuation to the image freq. not without loss and cost!!

if the IF is 10.7Mhz, the Image freq. is 21.4Mhz removed, and the RF Front end filter might well be able to greatly attenuate the image frequency. Unless it gets into the RX as a result of un-shielded enclosure :)

a bit contrary too what was previously posted, IF freq. are not selected based on a bandwidth in particular. some basis, but not as important as :

*ability for the RF section to reject the LO freq from appearing at the RF input. i.e. LO -re-radiation.

• the in-band intermod or spurious frequencies produced by the selection of the freq., based upon mixer output of Mrf=/-Nlo, where M,N are integers.

• 1/2 IF Response

• lastly, the availability, complexity, Cost, etc., of using a certain freq. for a required bandwith. As per Nat. Philo.

just some fyi.

Ok, just to make sure I have this:

If the IF is 455kHz, then mixing the input from the antenna into the internal osc = target fre - IF gives a signal around the IF. However if target freq - 2 * IF is also there in the input from the antenna, then that will be mixed into the output at IF, because the mixing is symmetric around the oscillator frequency.

Thus, you have to filter out the 'image = target - 2 * IF', or it'll get into the signal. However, in order to filter the image, and also considering that the signal is huge near a TV station, you need a filter with a very high Q, which is expensive.

Do production receivers actually use .455 as the IF? I've seen at least one receiver design that uses 10.7 MHz, due to the cheap ceramic filters available for FM, and possibly because of the relative simplicity and economy of building a filter with a Q of 10 as opposed to 100.

Bob Monsen wrote in news:wqWdnZOh3 snipped-for-privacy@comcast.com:

Okay, I'm *really* not an expert, so I'm working strictly from memory, but I remember that we had concerns (never proved it was a problem) with this back in the early/mid 80's, and as I recall, it's not that channel 20 had a problem - it was that combining a Tx on Ch 20 with TV channel 4 would cause problems for other receivers.

Not sure it's still a problem for modern FM dual-conversion Rx, but since my understanding is so fuzzy in the first place . . .

I think this is as good as any answer I could give. Local oscillator re-transmission is fairly low - and statutorally low in many domestic appliances.

However the mere mechanics of radio design mean that all sorts of remixing, rectification and re-transmission of signals can occur in the presence of nothing more complex than a rusty fence...

In short, if your field has problems on band X, avoid band X...unless you have a few thousand dollars of frequency counters, communication receivers and spectrum analysers that can make a case for why...

Its not.

Thats called something else. Usually spouruious response.

Image frequency is the frequency that ths 2xIF freqency away from wanted.

Dual conversion recievers of quality use input RF filtering to get rid of everything except a braod band in e.g. the 60-80 Mhz band, followd by a first mixer to knock down to 10.7Mhz where the image can be removed by a sharper ceramic tuned circuit, followed by a final mix down to 455Khz, where ceramic filters shape the response to eliminate adajacent channels...also they should have good quality AGC to prevent input overloading, and harmonic generation, a quality local oscillator with low harmonic content and a smooth 2nd order mixer that itself does not introduce harmonc content beyind what is necessary to achieve mixing.

Needless to say you won't get all of that in a commercial design, or any of it in a simple park fly single conversion set.

The fact is a cheap parkfly reciever has almost no AGC, no input RF tuning whatsoever, input signal going straight into a chip base oslcilator and mixer, and maybe obnly a siungle 455kHz ceramic filter. It will be likley to pick up adajcent channels and many other frequencies inside and outside the band, but in general it relies on the fact that the wanted signal should be many times stronger.

Does this mean that using one of these cheap receivers (a HItec HAS-3MB, for example, ;) I could be subject to failures due to nearby flyers?

I've actually been quite happy with my current tx/rx setup. I routinely fly next to high tension power lines, near large shipping containers, and get more than 800m, even with other folks flying nearby. I never get any spurious control movements, even when the transmitter is off.

Admittedly, I haven't flown near any TV transmitters.

(btw, thanks for the patient explanations. I suspect the prohibition is due to the image frequency issue.)

Yes.

Lucky you.

"If it works, use it"

Probably.