# Do signals have to be more separated on higher frequencies?

Someone told me that 10 meters was the biggest band because you can fit the most channels in it.

He said that in higher frequencies like 2 meter etc, you have to separate the signals more apart for them to work.

Is this true? Can HF frequencies have more signals clustered together, or is he just getting confused because FM takes up more bandwidth?

• It's harder to prevent crosstalk on higher frequencies because parallel wires love to couple capacitatively, but that's about it, I think. – John Dvorak Jan 4 '14 at 22:16

Someone was wrong. The 300 GHz band is the biggest, because it has no upper bound. You can fit an unlimited number of channels in it. See Which band will I be authorized the most bandwidth?

There is no theoretical requirement that signals be more "spread out" at higher frequencies. However, as VHF and up tend to be larger bands, the band plans tend to include wider modulations. FM is much wider (25 kHz) than SSB (4 kHz). Consequently, if the band is primarily FM (such as 2 meters), then signals must be spread further apart to avoid interfering with each other. Nothing to do with the frequency: just the modulation in use.

In principle? No.

Now, on the wider bands, wider transmission modes are often used as well. 28 MHz, for example, is the only HF band that has allocations in the band plan for FM.

Early on, a common trick for going higher in frequency was to employ a frequency multiplier. That works well with FM, but of course also widens the signal. So if you had 25 kHz FM on 144 MHz and frequency-multiplied it by three, you'd end up at 432 MHz but also with a 75 kHz bandwidth. The naïve approach doesn't work nearly as well with amplitude-modulated modes such as AM or SSB though because you'd change the actual modulation as well, so whoever was listening would have to have a receiver built to the same standard that you are using in transmission. (For example, a first intermediate frequency at 144 MHz, obtained by frequency division from the received frequency.) I can't cite sources, but I expect the relative ease of doing frequency multiplication is a major reason why many amateur bands are on harmonically related frequencies.

Consider the SSB portion of 2 meters during an active contest; there's more room to spread out and there are fewer stations within range of any one given station than on HF, but you can easily find stations as cramped together as on HF if it's an active contest, and it's no more or less difficult to work any particular one than under similar conditions on HF.

First of all, I'm going to assume you aren't including microwaves into things. Some of them can hold a truly enormous amount of data, something on the order of thousands of channels. For instance, the highest band is 241-250 GHz, which could hold about 10,000 FM radio quality (200 kHz) stations at the same time.

Comparing 2m to 10m, let's look at the band plans, and see what we can gleam. I'm getting this from ARRL. One big thing to note is that 10M has some FM signals, but is primarily SSB, while 2m has some SSB, but is primarily FM:

10 Meters (28-29.7 MHz)

28.000-28.070   CW
28.070-28.150   RTTY
28.150-28.190   CW
28.200-28.300   Beacons
28.300-29.300   Phone
28.680  SSTV
29.000-29.200   AM
29.520-29.590   Repeater Inputs
29.600  FM Simplex
29.610-29.700   Repeater Outputs


2 Meters (144-148 MHz)

144.00-144.05   EME (CW)
144.05-144.10   General CW and weak signals
144.10-144.20   EME and weak-signal SSB
144.200     National calling frequency
144.200-144.275     General SSB operation
144.275-144.300     Propagation beacons
144.30-144.50   New OSCAR subband
144.50-144.60   Linear translator inputs
144.60-144.90   FM repeater inputs
144.90-145.10   Weak signal and FM simplex (145.01,03,05,07,09 are widely used for packet)
145.10-145.20   Linear translator outputs
145.20-145.50   FM repeater outputs
145.50-145.80   Miscellaneous and experimental modes
145.80-146.00   OSCAR subband
146.01-146.37   Repeater inputs
146.40-146.58   Simplex
146.52  National Simplex Calling Frequency
146.61-146.97   Repeater outputs
147.00-147.39   Repeater outputs
147.42-147.57   Simplex
147.60-147.99   Repeater inputs


Okay, for simplicity I'm going to just block these into total SSB and FM signals, and ignore anything else.

• 10m- Total of 180 kHz FM, 1 MHz SSB.
• 2 100 kHz SSB, 3.5 MHz FM (Or so)

And the channel bandwidths are:

• SSB- 3 Hz (2.8 technically, but...)
• FM- 15 kHz.

Note that SSB takes up a lot less space than FM. Given that, here's the number of channels per band:

• 10m- 334 SSB, 12 FM
• 2m- 33 SSB, 200 FM

Bottom line is, due to the efficiency of SSB over FM, more channels can be used on 2m than 10m at the same time.

Consider you have 10 channels, each of BW=10kHz. Now, if you are allocated 2-3MHz band (i.e., a bandwidth of 1Mhz) then you can have 100 (1MHz/10kHz) channels. But, if you are allocated 200-300MHz band (i.e., a bandwidth of 100MHz) then you can have 10000 (100MHz/10kHz) channels.

So obviously, the higher your frequency you have, the more channels you can fit in.

• Say you are allocated 2-4 MHz, and 2-2.001 GHz. Your higher frequency band has more channels because you defined it to be bigger, not because it's higher in frequency. – Phil Frost - W8II Jan 6 '14 at 15:18