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So, why are usually lower VHF frequencies (around 118MHz to roughly 120MHz) used for towers and higher frequencies (around 130MHz to 136.975MHz) for ACC? What is the advantage of using lower frequencies for towers and higher for ACC since the wave propagates as a line of sight in both bands, that is, the maximum distance of the radio wave should be the same regardless if the frequency is 118MHz or 136MHz?

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    $\begingroup$ What are you basing your question on? I've never noticed such a pattern, have you analyzed a database of frequencies? $\endgroup$ – GdD Jul 26 at 11:57
  • $\begingroup$ I remember a few years ago one Croatian controller said that, I didn't think about it and it didn't bother me, but now I started my ATPL training and the same thing lecturer mentioned when teaching radiotelephony...I started looking at that more deeply and Croatian structure follows that perfectly. Now, I don't know if that's a coincidence or if that's just in our country..Zagreb Tower: 118.3 (backup 119.125), App: 120.7; FSS: 135.050; ACC: 135.8; I mean there will sure be some exceptions,but it seems to me like there is some reason behind it and I don't know which,so I posted the question :/ $\endgroup$ – Darjan Jul 26 at 16:09
  • $\begingroup$ In my opinion, I think there is something behind it, that lower frequencies are reserved for communication near a transmitting station (like towers, ground, etc.) unless all channels are occupied. If all channels are occupied than, of course, higher frequencies will be assigned to towers but I cannot back up that statement with some source or literature, so if someone has any information about it, I would appreciate it. $\endgroup$ – Darjan Jul 26 at 16:19
  • $\begingroup$ This may be an artifact of how your country allocates frequencies; it’s not true everywhere. $\endgroup$ – StephenS Jul 26 at 18:19
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    $\begingroup$ Ground control is very commonly 121.x $\endgroup$ – expeditedescent Jul 28 at 16:45
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There's no specific reason why the lower frequencies are used for towers, but it's not a coincidence that they are grouped together (mostly, there are tower frequencies that are not at the lower end.)

Spectrum management is a complex effort to make maximum use of the fixed amount of frequencies available for use. In the US this falls under the FAA Spectrum Engineering Group AJW-1C.

One method of ensuring reliable safe communication is the intersite analysis. Basically, frequencies have to be used by multiple ground stations based on the airspace volume that the frequency needs to cover. This airspace is referred to as a frequency protected service volume (FPSV).

The goal is that an aircraft in the FPSV will only be able to communicate with the intended ground station. Since towers have a relatively small FPSV, typically 5,000' high (AGL) & 15 nmi radius, the tower frequencies can be reused closer together than a high altitude sector frequency that may cover a 150 mi radius at 45000 feet. A frequency that was used for both high altitude and a tower would be more difficult to de-conflict.

There are also other considerations that can affect the assignment of frequencies. It's all spelled out in the 6050.32B - Spectrum Management Regulations and Procedures Manual. It's rather lengthy at 475 pages. The details of VHF/UHF Comm frequencies is in Appendix 2.

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    $\begingroup$ +1 and ICAO Annex 10, Volume V, §4.1.1 and Doc 9718 define actually a pre-allotment of the VHF band. In this pre-allotment, the aerodrome services are in the lower sub band, and the ACC in the higher sub band. Details are then refined at ITU region level. $\endgroup$ – mins Jul 27 at 14:35
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in that frequency range (118-137MHz) you are correct, the percentage change in frequency from the bottom to the top of the range is small, so there will be no significant change in propagation characteristics, range, data rate, etc. going from 118 to 137. In addition, the same radio antenna can be effectively used across the whole frequency range, simply by trimming it for resonance in the center of the band.

So the real question is frequency allocation i.e., how it's decided who gets to operate where within a frequency band. Those allocations are often arbitrary (for example, there's no reason why one end of a ham radio band would be reserved for morse code as opposed to the other end of the band) but allocating different band segments for different uses reduces the chance of miscommunication.

For example, if tower/unicom was blended into ATC channel assignments, then a pilot could be one channel mistuned and find him- or herself talking to the tower when they were expecting to talk to an en-route controller.

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May not have been an explicit consideration, but the bottom of the band is pretty close to Band II FM broadcast radio.

118MHz could interfere with 96.6MHz transmissions, the difference between them being 21,4 MHz, twice the usual 10.7MHz IF (intermediate frequency) This is known as the "image frequency". It's only problematic with a "high side" local oscillator (LO), 10.7MHz higher than the broadcast station. This problem could arise with any channel up to 108 + 21.4 = 129.4 MHz.

You don't really want interference sources flying over millions of people if you can help it. People living close to ATC transmitters may notice some interference, (which can be dealt with in other ways, e.g. choose a radio with a low side LO) but they might have more pressing noise problems.

So, keep the potential interferers stationary, move the mobile transmitters above 129.5 MHz.

(source : I had a job designing a stereo FM receiver once)

Many years ago I used to hear LF navigation beacons (280 to 530kHz) on an AM radio, via the image frequency. One played "GLG" in very slow Morse code (presumably Glasgow) - these must have been switched off about 40 years ago.

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