A couple technical questions

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i-7ashe

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  1. Does it take more electrical energy to transmit HF and lower frequencies than it takes to transmit VHF and above?
  2. Are there any wavelengths beyond radio waves and gamma rays?
  3. Is it true that the lower the frequency, the lower the amount of data that's being sent? I read on this wiki that military submarines can only send around 300 bit/s – or about 35 eight-bit ASCII characters per second when sending in ELF, and a few characters per minute when sent in ELF. Is there a chart or an article somewhere showing if there's any relation?
  4. If I have an antenna that's ~1cm, will it still be able to pick up and send HF and lower signals?
  5. Isotropic antennas can't really be achieved, but what types of antennas are most like isotropic anyway?
Thanks a lot for reading. It's ok if you can only answer a few, I just need some help with this.
 

majoco

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Well, nobody seems interested so here's my 10cents.
1. No - but if you are looking to actually contact a particular receiver rather than just a random distance then HF power is more likely to make the contact rather than VHF. VHF works over long distances if you have a straight line between you and him - visual contact in fact. This is why you can hear aircraft on VHF for 300miles or more as long as they are at 25,000ft or more - and 25watts will do it just fine. But HF will run out long before that - the ground wave may not go more than 25miles before it fades out - and the first return of the sky wave may not come down until maybe 100miles - unless you make your antenna and the right frequency to make what they call NVIS - near vertical incident skywave - it goes up and it comes straight down. But then again there are "QRP" contacts - very low power, say 5 watts or less, and particular forms of modulation that will go around the world. So the short answer to your question is - maybe!
2. Yes - DC to daylight - and past daylight!
3. There are round-the-world communications on extremely low frequencies - see - List of VLF-transmitters - Wikipedia - some of those transmissions are in 'Hertz' - but the reception is of very low signal strength so they have to resort to very complicated methods of digging the signal out of the mud - look up 'coherent detection'. Your GPS receiver uses these techniques - the signal is actually much lower than the noise - but the noise is random and the signal is not - so if you keep sampling time and time again you will find that the noise is not
re-occuring but the signal is, then you can cancel the noise and what is left is signal - but as you say the data rate is very slow.
4. A 1cm antenna will receiver signals best where 1cm is a quarter-wavelength - so the frequency will be 300/4cm - which if I have pressed the right buttons is about 12,000MHz - hardly HF. The best antennas are based on a quarter wavelength so if you want to receive, say 10MHz you need a quarter of 30metres, 7.5 metres, 25 feet.
5. None, but a simple dipole is about 2.1dB down on an isotropic radiator.
Now lets wait for the flame-throwers who have been lurking in the back room - just going to pounce..... :)
 

RFI-EMI-GUY

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These are some of the very questions I have had over many years in the hobby/biz.

1) I recall learning from a TV station engineer that the most coveted channels were the low VHF ones like Channel 2 (we have no channel 1 here in the US long/short story, guy invents FM, he is a genius, guy invents TV but he's a schmuck. TV guy outsmarts FM guy, FM guy jumps from hotel window while impoverished, sad story)

The UHF TV stations require a humongous amount of power to generate the same signal as the low VHF one, so operating a Channel 2 station is very economical.

2) Beyond my payscale, but I would venture "yes" everything is an electromagnetic wave.

3) It is true that you generally need a higher frequency band to transmit greater bandwidth. However from what I have read, this is more of a practical limitation than a hard and fast "laws of physics" limitation. If you can carve out 20 MHz of bandwidth between 1MHz and 21 MHz you can impose the same amount of data as between 1001 and 1021 MHz. The submarine communications using VLF is a method of providing global coverage thus they are saddled with painfully slow communications that is more on the order of: "If you can read this signal, no need to bomb Moscow."

https://www.seas.upenn.edu/~kassam/tcom370/n99_4.pdf

4) Yes, but not very well

5) The sun is an isotropic radiator.
 

spongella

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I have a confession to make.... I was very interested in his excellent questions but did not know the answers. Thank you majoco and RFI-EMI-guy for your responses.
 
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Hi 7ashe

I'll take a stab at one of your questions-- #3
.
The reason that those frequencies carry such limited data rates is not because of the physics of the low frequencies, but becuz of the limitations on the antennas the transmitters use.

Those wavelengths are tremendously long- and even though the stations using them have antenna systems that cover acres, they are still tiny fractions of these wavelengths.
In order to radiate anything effectively they have to be highly tuned- a very high "Q" factor. As such, the bandwidth that they resonate at is extremely narrow- in the <100 Hz (!) range.
As data rates increase, so does the bandwidth occupied by the signal. It is very easy to exceed that narrow tuning range of these antenna with what we'd consider very slow baud rates.
Its not the frequency used, but the limitations of the antenna system.

Some of the earliest single sideband transmissions played upon this high Q antenna phenomenon. A transmitter shifting its frequency sufficiently could null out a sideband- these antennas were that narrow and sharp.

Lauri :sneaky:
 

SteveSimpkin

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These are some of the very questions I have had over many years in the hobby/biz.

1) I recall learning from a TV station engineer that the most coveted channels were the low VHF ones like Channel 2 (we have no channel 1 here in the US long/short story, guy invents FM, he is a genius, guy invents TV but he's a schmuck. TV guy outsmarts FM guy, FM guy jumps from hotel window while impoverished, sad story)
Very sad story. Edwin Armstrong deserved better.
Edwin Howard Armstrong - Wikipedia
 

Boombox

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As the others have said, a one centimeter antenna will pick up HF, if the signals are strong enough. It just won't pick it up very well. I have a portable that has picked up China with just 15-20 cm of whip antenna. But those signals were broadcast at 100-200 KW, and prop conditions were really good.
 
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