Watch your image...frequency that is.

[radio8z]

Hello all,

How about some techinical stuff? When you choose your AM TX frequency, consider that you might be on the image frequency of a local station.

AM receivers receive on two frequencies. One, the desired frequency, is what it is tuned to. The other, the image frequency, is 910 KHz. above the desired frequency. If you are broadcasting on this frequency and it is used locally, you may have a problem.

Receivers are designed to reject the image, but do not do so perfectly. I chose 1520 KHz. as my TX frequency, since it was clear here. On the air, it worked great. Though I transmit from an antenna in my basement just so I can cover my yard and do not have a strong signal, I noticed that there was a whistle on 610KHz. right on top of a local blowtorch. Quick math: 1520KHz. - 910KHz. = 610 KHz. Oops!

To calculate what image you might be using, take your TX frequency and subtract 910 KHz. Find out if there is a local station on that frequency, strong or weak, and if so, find another one to use. Try it...subtract 910 KHz. from your frequency, tune your radio to that, and check it out.

Your signal may be perfectly clean and you can still interfere. Though it is a shortcoming in your neighbor's receiver, he won't buy that. Best to head it off.

Regards,

Neil

 

[JasonW]

Hello Neil,

I'm game! Here in Fairbanks, Alaska I have the luxury of only needing to avoid five AM stations on 630, 660, 820, 970, and 1170, *plus* having no distant skywave stations booming in here at night, so avoiding my few local staions' image frequencies is not a problem. Other Part 15 broadcasters aren't so lucky.

I've heard of image frequency, but almost always in reference to FM. (It is mentioned rather often in AM Carrier Current broadcasting literature, though.)

What exactly is the image frequency? Is it an unintentional artifact like a harmonic, or something that AM stations broadcast on purpose (perhaps for the IF sections in some receivers)? Also, why is it always 910 kHz above the station's assigned frequency and not some other value? -- JasonW

> Hello all,
>
> How about some techinical stuff? When you choose your AM TX
> frequency, consider that you might be on the image frequency
> of a local station.
>
> AM receivers receive on two frequencies. One, the desired
> frequency, is what it is tuned to. The other, the image
> frequency, is 910 KHz. above the desired frequency. If you
> are broadcasting on this frequency and it is used locally,
> you may have a problem.
>
> Receivers are designed to reject the image, but do not do so
> perfectly. I chose 1520 KHz. as my TX frequency, since it
> was clear here. On the air, it worked great. Though I
> transmit from an antenna in my basement just so I can cover
> my yard and do not have a strong signal, I noticed that
> there was a whistle on 610KHz. right on top of a local
> blowtorch. Quick math: 1520KHz. - 910KHz. = 610 KHz. Oops!
>
>
> To calculate what image you might be using, take your TX
> frequency and subtract 910 KHz. Find out if there is a
> local station on that frequency, strong or weak, and if so,
> find another one to use. Try it...subtract 910 KHz. from
> your frequency, tune your radio to that, and check it out.
>
> Your signal may be perfectly clean and you can still
> interfere. Though it is a shortcoming in your neighbor's
> receiver, he won't buy that. Best to head it off.
>
> Regards,
>
> Neil
>

 

[radio8z]

> What exactly is the image frequency? Is it an unintentional
> artifact like a harmonic, or something that AM stations
> broadcast on purpose (perhaps for the IF sections in some
> receivers)? Also, why is it always 910 kHz above the
> station's assigned frequency and not some other value? --
> JasonW
>
Hi Jason,

Good to hear from you again. BTW in my post technical was spelled wrong. I sometimes think faster than I can type. And my proofreading is not all that great.

I have given two hour lectures about this. I will try to condense it. No, it is not something that a station transmits. It is a property of any superheterodyne receiver (except double conversion...another time maybe). Superhets mix the antenna signal (Fd) with a local oscillator (Flo) to produce a sum and difference frequency. The difference frequency, in this case, is by design 455 KHz. This signal then goes through what is called and intermediate frequency amplifier (IF) which is designed to pass only 455 KHz. and the tuning here is not variable. This is to allow an optimized design with controlled bandwidth which is best done at a fixed frequency. The local oscillator operates 455 KHz. above the desired frequency, The problem is that ANY signal which mixes with the LO and produces the 455 KHz. which is the only signal that will get through the IF and be treated as the tuned signal. Mixing produces both the sum and difference. Mix 10 with 7 and you get 10, 7, 3, 17.

The receiver has a tuned radio frequency stage (TRF) which tunes to the desired frequency and is supposed to eliminate any other frequencies which may mix to give the 455 KHz. signal.

An example: One tunes to 700 KHz. The LO is 700 KHz. + 455 KHz. = 1155 KHz. The oscillator in your receiver is running at 1155 KHz. So 1155KHz. - 700 KHz. produces 455 KHz. and it gets through the IF. But a signal on 1610 KHz. (the image Fim) will also mix with the LO at 1155 KHz. and produce 455 KHz. which also get through the IF. The only way to separate the 700 KHz. signal from the 1610 KHz. signal is to filter out the 1610 KHz. signal before it is mixed with the LO.

You asked why 910 KHz. This is because the LO is halfway between Fd and Fim. So to get the image Fim you add twice the Fif to the Fd.

Why 455 KHz. for the IF? It was chosen because it was possible to get a good Q factor (needed for selectivity) with available technology in the 30's; A lower frequency compromised the TRF and a higher frequency compromised the IF amplifier. Any frequency nearby is OK. I have a WWII military receiver that uses 562 KHz. I think the industry settled on one frequency (455 KHz.) in order to benefit from mass production for economy. Only one type of coil/capacitor resonant circuits to build by the millions.

This probably made some tuned out, but good answers to good questions get complicated. Check out the ARRL handbook and many others if you want to pursue this. Otherwise, I hope this answered some of your questions.

Neil

 

[JasonW]

Neil, I didn't even notice your typo (I was reading faster than I was thinking, being in a hurry to get ready for work). Yes! That 910 kHz figure kept making my equine senses tickle on the back of my neck all day at work. ("It's related to some other figure I've seen *somewhere*...")

Now that you've jogged my memory, the Part 15 AM Carrier Current literature mentions the 455 kHz IF and the potential for "image noise" a lot. Apparently it's a bigger problem when you're dealing with receivers that are plugged into the same AC line that's carrying the signal.

I've never heard of this problem cropping up with 0.1 Watt Part 15 AM free radiation systems, but it's still a good idea to make sure the image doesn't "fall on top" of another station's channel. Since most of us operate between 1600 kHz and 1700 kHz, it's the hams on 160 meters who would be most likely to hear our image noise. Have you or any hams you know heard this? -- JasonW

 

[radio8z]

> Neil, I didn't even notice your typo (I was reading faster
> than I was thinking, being in a hurry to get ready for
> work). Yes! That 910 kHz figure kept making my equine
> senses tickle on the back of my neck all day at work.
> ("It's related to some other figure I've seen
> *somewhere*...")
>
> Now that you've jogged my memory, the Part 15 AM Carrier
> Current literature mentions the 455 kHz IF and the potential
> for "image noise" a lot. Apparently it's a bigger problem
> when you're dealing with receivers that are plugged into the
> same AC line that's carrying the signal.
>
> I've never heard of this problem cropping up with 0.1 Watt
> Part 15 AM free radiation systems, but it's still a good
> idea to make sure the image doesn't "fall on top" of another
> station's channel. Since most of us operate between 1600
> kHz and 1700 kHz, it's the hams on 160 meters who would be
> most likely to hear our image noise. Have you or any hams
> you know heard this? -- JasonW
>
Hi Jason,

I was surprised that I heard the noise also. I was using a $15 portable and was about 30 feet from the antenna. Guess the field strength was pretty strong there and the cheap receiver didn't have a good TRF filter.

The hams won't hear your interference because the image is above the received frequency, thus the interference will occur to stations operating on a frequency lower than you are using by 910 KHz. On 160 meters they could be on an image of a broadcast station and cause the same problem. I don't know of any hams who have had this happen, but it wouldn't surprise me if it did.

A harmonic from a part15 transmitter might fall into a ham band, but if you are using a resonant antenna it will be extremely weak and not a problem.


Neil

 

[pianoplayer88key]

I have some recent (< 15 years) radios that use a 450kHz filter. (the image is 900kHz below on those radios, not 910.)

Of three frequencies I'm considering, which might be the best option, assuming I disregard everything rfry says about ground systems?

1500kHz
Co-channel: TIS about 8-10 miles west of me, usually only sending out an unmodulated carrier.
Image: local 5kW blowtorch about 10 mi SW of me
1610kHz
Co-Channel: various TIS's in town, but mostly not audible with a Select-A-Tenna and SRF-42 from my location. Has been talking house down the street, then on 1620, now I don't know if it's still there (btw there's a local 25 mi away on 1630)
Image: KSPN has weak, but listenable groundwave signal from Los Angeles.
1680kHz
Co-Channel: at night, KAVT Fresno available with a weak signal by skywave. What's the general rule for interfering with skywave stations? I assume I should stay away from Class A frequencies, but what about the lesser stations, assuming their signal isn't available at my TX's location via groundwave with a Select-A-Tenna?
Image: +20kHz of strong local station, KKOH via skywave.

 

[westlife]

> Why 455 KHz. for the IF? It was chosen because it was
> possible to get a good Q factor (needed for selectivity)
> with available technology in the 30's; A lower frequency
> compromised the TRF and a higher frequency compromised the
> IF amplifier.

Most dial-tuning radios use 455 kHz (the k in kilohertz is lowercase) for the AM IF, but most digital-tuning radios use 450 kHz. In fact I've never seen a digital-tuning radio which uses a 455 kHz IF. Some do use 460 kHz, however, and quite a few car radios use either 260 or 262.5 kHz.

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"This is the New York Emergency Broadcast System satellite channel. They took the crosstown bus."</P>

 

[radio8z]

> I have some recent (< 15 years) radios that use a 450kHz
> filter. (the image is 900kHz below on those radios, not
> 910.)
>
> Of three frequencies I'm considering, which might be the
> best option, assuming I disregard everything rfry says about
> ground systems?
>
> 1500kHz
> Co-channel: TIS about 8-10 miles west of me, usually only
> sending out an unmodulated carrier.
> Image: local 5kW blowtorch about 10 mi SW of me
> 1610kHz
> Co-Channel: various TIS's in town, but mostly not audible
> with a Select-A-Tenna and SRF-42 from my location. Has been
> talking house down the street, then on 1620, now I don't
> know if it's still there (btw there's a local 25 mi away on
> 1630)
> Image: KSPN has weak, but listenable groundwave signal from
> Los Angeles.
> 1680kHz
> Co-Channel: at night, KAVT Fresno available with a weak
> signal by skywave. What's the general rule for interfering
> with skywave stations? I assume I should stay away from
> Class A frequencies, but what about the lesser stations,
> assuming their signal isn't available at my TX's location
> via groundwave with a Select-A-Tenna?
> Image: +20kHz of strong local station, KKOH via skywave.
>
Hi,

That is new information for me about the 450 KHz. IF. You are correct that the difference is 900 KHz. in this case.

As far as choosing your frequency, I would go for the 1680. I try to avoid my local TIS because I don't know who is listening and from where. I used to listen to my suburban city TIS from 12 miles away with a decent signal mobile. They used to give traffic conditions and local event information. Now, for the last week it has been dead air.

Concerning the co-channel skywave, I don't know of an "official" general rule for skywave. What I did was find a frequency that is clear in the daytime, had no useable signals (such as you get from a 5KW 50miles away) +/-20KHz., and at night had two or more audible signals. The only one I found was the one mentioned before in this thread, and it was probably vacant because it was the image of the local station. My thinking is that if at night I hear more than one signal then I am out of their service area and no one around here would be listening to them.

Neil

 

[radio8z]

> > Why 455 KHz. for the IF? It was chosen because it was
> > possible to get a good Q factor (needed for selectivity)
> > with available technology in the 30's; A lower frequency
> > compromised the TRF and a higher frequency compromised the
>
> > IF amplifier.
>
> Most dial-tuning radios use 455 kHz (the k in kilohertz is
> lowercase) for the AM IF, but most digital-tuning radios use
> 450 kHz. In fact I've never seen a digital-tuning radio
> which uses a 455 kHz IF. Some do use 460 kHz, however, and
> quite a few car radios use either 260 or 262.5 kHz.
>
Hello,

Thanks for the update on IF frequencies. Of course there are exceptions to everything, ie. my Yaesu FRG-100 digital radio has IFs of 47.21 MHz. and 455 kHz. (you are right about the capital K). Dual conversion and ceramic filters help.

Indeed, I was using an inexpensive analog tuned radio when I heard the interference. I trust the digital ones have better image rejection, otherwise things can get very complicated.

It seems that since we have advanced beyond the technology of the thirties we can use ceramic resonators to get the right Q at any reasonable frequency.

I appreciate your comments.

Neil

 

[rfry]

> Of three frequencies I'm considering, which might be the
> best option, assuming I disregard everything rfry says about
> ground systems?
_____________

?? Nothing I wrote about ground systems on this board affects the choice of an operating frequency, unless you need shorter effective lengths for the wires in a radial ground system. Is that what you mean?

//

 

[pbolyn]

> I was surprised that I heard the noise also. I was using a
> $15 portable and was about 30 feet from the antenna. Guess
> the field strength was pretty strong there and the cheap
> receiver didn't have a good TRF filter.

I think this is the crux ot the problem. A cheap radio, 30 ft. from a part 15 transmitter is asking for problems. Even a good quality radio close to your antenna may misbehave.

I don't think the IF image phenomenon should be of any concern to part 15 broadcasters. The signal strength will fall off rapidly with distance, so only your closest neighbors MAY experience this if they happen to be listening on a cheap radio. The probabilities are small.

Manufacturers have long ago solved the image problem on the AM broadcast band. Very, very seldom will you hear an IF image on any radio from any but the most powerful, local broadcast stations. It is much more likely that you will hear a very strong local station at many points on the dial due to front end overload.



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Phil B
</P>