Frequency / Wavelength Question

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lars128

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So I'm no expert on radio waves and all I remember from high school physics is what frequency and wavelength are. From what I have read, certain bands have operating characteristics, such as the ability to be picked up farther or be better suited for use in tall buildings or urban areas.

So how do various frequencies match up for in transmitting performance? What are the advantages/disadvantages of say 30, 160, 450, and 850 MHz? And most importantly, why does this occur?
 

LEH

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Ouch, short of a doctoral dissertation, this is not an easy set of questions to answer. I'm sure you'll get numerous other responses (and hopefully some good links for more info).

Simply, the lower the frequency, the further it has the possibility of being heard. Now there are numerous factors that will play into this. Output transmitting power, antenna (height and physical length), atmospherics (especially on the VHF-LO [usually the 30-50 MHz band]) and other things.

One reason lower frequencies tend to be able to be heard further is their 'wavelength' is more easily reflected off the ionosphere (called skip. If you ever listened to AM radio in its heyday, it was not uncommon to hear distant stations [there is a thread on that topic in the general forum] fairly clearly at night).

As frequencies climb up, they become more and more line of sight. Now no radio wave can bend with the curve of the earth, but the skip capabilities of the lower frequencies do allow signals to be heard over the horizon.

There is no one ideal frequency range. Depending on what you are trying to do can have a lot to do with what range you transmit on. Again, there are some restrictions due to law and international treaty and as radio spectrum becomes more and more crowded, these ranges or options change. A good example is the 'rebanding' of the 800 MHz range in the US. Read the Wiki here for info on that topic (not to mention the numerous forum entries on the topic).

There are two ways a radio signal can be transmitted (I'll easily stand corrected on this one guys). Either Amplitude Modulation (AM) or Frequency Modulation (FM). Most public service is FM and because it is a bit more stable (less prone to interference) than AM, digital signals work very well over FM. The main areas of AM transmission around today (that I can think of) are AM radio (500 - 1600 KHz), SW radio, aircraft (military and civilian) bands and (though technically it is in the HF area shared with SW) good old CB.

Each band has its own pros and cons. One of the cons to a lot of the 800 or higher systems is they don't work real well inside a building or in very rugged terrain without a nearby repeater. The construction tends to block the signals. Other bands may have problems in rural areas where trees or mountains can block signals.

One of the main reasons (IMHO) for the increased use of higher frequencies has been the crowding of the spectrum. Everyone wants a radio system and the lower spectrum could not support the expansion. Where in the 150 MHz area you might be able to reuse a frequency every couple of hundred miles to avoid interfering with other systems on the frequency, with an 800 MHz system it may drop down to less than 100.
 

manross

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Agree with everything said above. There was something told to me years ago when I was involved in a new Police Operations Building and going to the 800 mhz band. I wondered why 800mhz radios consistently had better communications out of tall buildings and below grade basements. Not only because you could get more channels in the same amount of spectrum, but for comparable power output the higher freqs will work better in tall buildings and basements because the higher freqs (ie 800mhz) will bounce around and out of buildings better than lower freqs. Lower freqs tend to penetrate the buildings interior more and fail to exit as well as the 800 mhz bouncing it self out of the building. I don't know how much scientific basis there is to this, but it always stuck with me and totally made sense to me. Whether it is accurate or not it painted a clearer picture of what happens in buildings when dealing with RF energy.

Marty
 

manross

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The Phoenix, AZ FD did extensive testing of VHF, 700MHz & 800MHz bands of radio communications for firefighting usage. Read the report at http://phoenix.gov/FIRE/radioreport.pdf with conclusions on page 18.
LOL Yep I remember seeing a multitude of different studies (my son is a FF in Colorado Springs) about the advantages and disadvantages of using trunked systems. I worked on the PD side and we had our own set of concerns. That went on for a few years, but the march of technology pushed the powers to be (FCC) more so then I think any actual need. They decided or saw a need to expand capacity and this is the route it went. I think they were pushed this direction by the manufactures too. Eventually everyone will be switched over because there is more money the manufactures can make on a more complex system than the older technology. Man-o-man you should have heard the county SO yell. Their VHF repeater system worked fantastic, and now they had to go and purchase all this new radios and computer stuff just to talk. :D Even though the 911 Authority defrayed some of the cost, it still cost the governments plenty. The almighty dollar won over again. What can be done when the manufactures stop making analog stuff versus digital. If your in charge of a municipalities communications needs, you have to look to the future. It's never ending.......

Marty
 
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zz0468

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Some good comments... I'd like to expound upon a few if I may...

Simply, the lower the frequency, the further it has the possibility of being heard. Now there are numerous factors that will play into this. Output transmitting power, antenna (height and physical length), atmospherics (especially on the VHF-LO [usually the 30-50 MHz band]) and other things.
For one thing, free space loss is directly related to frequency, so all else being equal, lower frequencies have less loss. The difference is significant between, say, low band, and 800.

One reason lower frequencies tend to be able to be heard further is their 'wavelength' is more easily reflected off the ionosphere (called skip. If you ever listened to AM radio in its heyday, it was not uncommon to hear distant stations [there is a thread on that topic in the general forum] fairly clearly at night).
Absolutely true. HOWEVER, there are reflective enhancements that primarily benefit higher frequencies. Meteor scatter comes to mind. Tropospheric ducting is more prevalent at higher frequencies... to a point.

As frequencies climb up, they become more and more line of sight. Now no radio wave can bend with the curve of the earth, but the skip capabilities of the lower frequencies do allow signals to be heard over the horizon.
Well, truth be told, because of the gradient in atmospheric density with altitude, radio waves DO follow a curved path. In radio engineering, this is actually taken into account, and is referred to as 4/3 earth radius. If the earth were one third larger than it is, radio waves WOULD just go around and around. The radio horizon is about 1/3 further than the optical horizon.

There is no one ideal frequency range. Depending on what you are trying to do can have a lot to do with what range you transmit on. Again, there are some restrictions due to law and international treaty and as radio spectrum becomes more and more crowded, these ranges or options change. A good example is the 'rebanding' of the 800 MHz range in the US. Read the Wiki here for info on that topic (not to mention the numerous forum entries on the topic).
Range can vary so wildly that I usually try to avoid even discussing it. It's a complex combination of factors that include transmitter power, frequency, the bandwidth of the modulation, and receiver noise figure (which is a whole book length topic unto itself). Frequency is just a part of it.

There are two ways a radio signal can be transmitted (I'll easily stand corrected on this one guys). Either Amplitude Modulation (AM) or Frequency Modulation (FM). Most public service is FM and because it is a bit more stable (less prone to interference) than AM, digital signals work very well over FM. The main areas of AM transmission around today (that I can think of) are AM radio (500 - 1600 KHz), SW radio, aircraft (military and civilian) bands and (though technically it is in the HF area shared with SW) good old CB.
You can modulate the frequency, amplitude, and the phase of a signal. Every other modulation scheme is a variation, or combination of these three things. Even digital modes.

Each band has its own pros and cons. One of the cons to a lot of the 800 or higher systems is they don't work real well inside a building or in very rugged terrain without a nearby repeater. The construction tends to block the signals. Other bands may have problems in rural areas where trees or mountains can block signals.
One of the most insidious things about this is that 800 MHz is one of the worst pieces of spectrum to use for public safety communications, based on how it behaves. But it's what was available. Trees are a detriment at higher frequencies, generally, and certain species of pine trees are particularly bad because of the size of the needles in relation to the signal.

One of the main reasons (IMHO) for the increased use of higher frequencies has been the crowding of the spectrum. Everyone wants a radio system and the lower spectrum could not support the expansion. Where in the 150 MHz area you might be able to reuse a frequency every couple of hundred miles to avoid interfering with other systems on the frequency, with an 800 MHz system it may drop down to less than 100.
Yep yep! That's exactly what's pushing things higher. There's no upper cap, although I doubt we'll see gamma ray wavelengths used for communications.

All of the above that LEH said is true, but there are some interesting exceptions. For example, I'm active on amateur microwaves, and I have a very high performance 10 GHz ham station that will out talk and out perform VHF and UHF, and performs more like HF when the band is out. 500 mile contacts are a piece of cake, when VHF just doesn't make it.
 

N8DV

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Frequency is the amount of cycles per second that a modulated RF signal covers. A wavelength measured in meters is the length in one second that the radio wave will physically travel in space. Soooooo, the higher the frequency in megahertz (used to be cycles but renamed in honor of Heinrich Hertz) the faster the more cycles in will produce in one second. A UHF frequency say of 800.000 mhz, is 8 million cycles per second. The actual wavelength is less than a one inch ( for argument sake). A 10 meter signal, 28.000 mhz wavelength is approximately 33 feet, give or take a few inches. Hope this helps.
 

zz0468

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Frequency is the amount of cycles per second that a modulated RF signal covers.
The modulation has nothing to do with it.

A wavelength measured in meters is the length in one second that the radio wave will physically travel in space.
No it isn't. A wavelength is the distance traveled while a wave completes one full cycle. The frequency, in hz, is the number of cycles completed in one second. The distance it travels in one second is 186,000 miles, or 300,000,000 meters. That doesn't change, regardless of the frequency or wavelength.

...A UHF frequency say of 800.000 mhz, is 8 million cycles per second. The actual wavelength is less than a one inch ( for argument sake). A 10 meter signal, 28.000 mhz wavelength is approximately 33 feet, give or take a few inches. Hope this helps.
Actually, that wasn't helpful. 800.000 MHz is eight HUNDRED million cycles per second, and the wavelength is 14.8 inches, not less than an inch. 28 MHz is a bit over 35 feet.

These are the kind of posts that make me crazy. *eye roll*
 
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LEH

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Go ahead. Let loose. It can be cathartic.
Hey it wasn't going to be directed at you, but the person you were responding to. You hit the nail on the head. I was going to be snide and sarcastic and really don't need to be.
 
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