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Boneheaded questions that I should know the answer to, but don't

B

Basnya

Okay we know that the reason we power down at night on AM is to minimize skywave propagation or as I always called it 'skip'

Two questions:

1) Why is the AM BC band subject to skip at night, but amateur HF is subject to it during the day?

And the really dumb one:

2) If you could transmit an FM signal on the 550- 1700 BC band, would it still be subject to the skywave?

Again, apologies for this, but I've always been curious about it.
 
As to #2, of course it would. AM, FM, CW or phase modulated (the "quadrature" information in AM stereo), anything in the MW wave band will skip at night.

As to #1, I'll make a feeble attempt to explain--MW signals are fairly long wave lengths--ranging from around 1800 feet at 540 to about 580 feet at 1690. It takes a while after sunset for ionized layers to form that are of a size that will reflect rather than scatter these long wavelengths. Amateur signals, especially 20 meter band and above, are much shorter, i.e. 20 meters as opposed to 300 meters at 1,000 khz. Duh!

During the day, small reflective layers high up in the atmosphere will form briefly, reflecting these short-wavelength signals, but not the longer wavelength signals. AM skip is relatively consistent, it is better during the long nights in winter, and not as good in summer, but it will usually develop most every night. HF skip is fickle, affected by sun spots or the lack of them, other factors. So some days 20 meters is open around the world, other days reception is poor. All depends on how the reflecting layers form, how high they are, how strongly reflective they are.
 
Ionization of various ionosphere layers favors different wavelength at different hours upons different paths.
The short answer is that the AM broadcast wavelengths are so long they shoot through instead of making the bounce.
At some point in higher frequencies (shorter wave length) they CAN bounce and make the hop. And it changes when they daytime ionization
and ionization background noise fades away. It get very complicated by time of day and frequency.

There is no reason that an FM signal would not "work" on the 550-1700 range.
There is not really enough bandwidth in that band to make it workable.
Each AM station gets 10khz. Each FM get 150khz.
You'd get very few stations to fit on the dial, and there may be some problems with designing tuning circuits for that kind of deviation at such low frequencies. The deviation would have to be narrower.
But it probably would skip just fine.
 
I have a question which is a bit off point here and also moot now that TV is gong digital nest year. Why was AM used for modulating the video information on analog TV?

When I worked in cable TV we used "reverse stream' modulators to send programming back to the head end from our studios. These modulators using frequencies below channel 2 used FM for the video. Was this because of frequency, that is that AM worked better for video on the higher (up to UHF) ones?

I know, another duh, probably. :-[
 
nmoore6676 said:
Why was AM used for modulating the video information on analog TV?
Click to expand...

Analog broadcast television uses AM to conserve r-f bandwidth, so as to allow more stations in the same spectrum space.

Your cable system was closed circuit, and didn't need to consider the bandwidth limitations needed for over-the-air broadcasting.
//
 
NTSC Analog television is single sideband, AM with full visual carrier. The upper sideband is removed and replaced with the FM aural signal.

As for transmitting FM on the 540 to 1700 medium wave band. It works. I have done it (with an FCC STA). I transmitted the normal AM signal AND a narrow band (1 kHz deviation) FM signal on the same carrier. With such little deviation, I was able to transmit FM audio up to 15kHz and stay within the NRSC mask. I did this through a 50KW Continental 317C transmitter.
 
frankberry said:
NTSC Analog television is single sideband, AM with full visual carrier. The upper sideband is removed and replaced with the FM aural signal.
Click to expand...

Sorry, but NTSC (analog) TV is broadcast with the full-amplitude carrier and the upper AM sideband up to about Fcv + 4.2 MHz. This allows room at Fcv + 4.75 MHz for the FM sound carrier at about 10% of the power of the unmodulated visual carrier.

The lower AM sideband of the visual r-f waveform is attenuated so that at Fcv - 1.25 MHz and below it is at least -20 dB w.r.t the upper visual sideband amplitude (-42 dB at Fcv - 3.58 MHz).

RF (RCA Broadcast Field Engineering, 1965-1980)
//
 
R. Fry said:
nmoore6676 said:
Why was AM used for modulating the video information on analog TV?
Click to expand...

Analog broadcast television uses AM to conserve r-f bandwidth, so as to allow more stations in the same spectrum space.

Your cable system was closed circuit, and didn't need to consider the bandwidth limitations needed for over-the-air broadcasting.
//
Click to expand...

Like I said, that was a duh. I just was thinking that FM would have been better because when I got TV from an antenna it was subject to ignition interference, vacuum cleaners and other appliances as well as thunderstorms. All of that goes away with digital though.
 
frankberry said:
NTSC Analog television is single sideband, AM with full visual carrier. The upper sideband is removed and replaced with the FM aural signal.

As for transmitting FM on the 540 to 1700 medium wave band. It works. I have done it (with an FCC STA). I transmitted the normal AM signal AND a narrow band (1 kHz deviation) FM signal on the same carrier. With such little deviation, I was able to transmit FM audio up to 15kHz and stay within the NRSC mask. I did this through a 50KW Continental 317C transmitter.
Click to expand...

I'd like to hear more about this. How did you phase modulate the carrier? Did you have to make any adjustments to your antenna system to pass the phase modulation?
 
I used an FM Exciter which operated at 101.0 MHz. I adjusted the deviation to 100 kHz and then devided the output frequency by 100 for an output frequency of 1010 kHz.
I fed the output of the divider to an RF amplifier (manufactured by Delta. The output module from their C-QUAM Exciter) and fed this into the transmitter to replace the crystal.
Since the station was C-QUAM stereo at that time, I didn't need to retune anything.
I'll be happy to e-mail you some of the FCC data if you will send me an e-mail. My e-mail address is [email protected]
 
frankberry said:
I used an FM Exciter which operated at 101.0 MHz. I adjusted the deviation to 100 kHz and then devided the output frequency by 100 for an output frequency of 1010 kHz.
I fed the output of the divider to an RF amplifier (manufactured by Delta. The output module from their C-QUAM Exciter) and fed this into the transmitter to replace the crystal.
Since the station was C-QUAM stereo at that time, I didn't need to retune anything.
I'll be happy to e-mail you some of the FCC data if you will send me an e-mail. My e-mail address is [email protected]
Click to expand...

What did you use for a receiver, how wide was the IF passband, and what was the FM signal-to-noise ratio at, say, the 2 mV/m contour?

I see no problem making this work under "laboratory conditions", but if you transmit FM with an upper modulating frequency limit of 15 kHz, sidebands extending up to +/- 15 kHz from the carrier will be generated. At a deviation of only 1 kHz, the first 15 kHz sideband pairs would be quite low in amplitude (because the modulation index is less than 0.07) so it's possible that they would indeed fall within the NRSC mask limits. However, at such a low modulation index, the FM signal-to-noise ratio and ability to overcome AM crosstalk would suffer greatly. In the noise-filled medium-wave band, I would expect disappointing performance unless the RF signal was very strong.
 
I have the information in my FCC filing. I'll be happy to scan my paperwork and e-mail it to you if you will send me your e-mail address.

I used a standard AM receiver. IF bandwidth was not a problem. I multiplied the IF by 75 and then heterodyned the output to produce a 10.7 MHz signal which I injected into the IF strip of an FM radio.

The result was wideband FM at the receiver.

There were issues caused by the amplitude modulation.
 
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