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I’m new to electronics but I'm doing my ATPL pilots license soon and am interested in the EM spectrum and in particular, radio waves. I was wondering what factors affect the maximum distance I could pick up a radio frequency. Say, 127MHz. I know the range depends on the power of the receiver- so let’s say that also stays fixed.

In the world the VHF frequency range used for civil aircraft is 118-136 MHz, I believe. Could I in effect pick up a signal from America from here in the UK if it is reflected off the ionosphere?

I am probably not making myself very clear.

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    \$\begingroup\$ When you said "power of the receiver" I think you mean "power of the transmitter." But it does also depend on the sensitivity of the receiver. \$\endgroup\$
    – JYelton
    Commented Jun 17, 2023 at 17:56
  • \$\begingroup\$ Your altitude will matter a great deal. The 2.5 meter band allocated to air is pretty much entirely AM -- partly because of when it was allocated (WW II, or so) and partly because 'emergency' possibly weaker signals can still be detected even against stronger signals, unlike FM. If your altitude is high enough, it may reach out 100's of km. But there will be no sky wave from it. At the 6 meter band, during some parts of the year, you get sky wave. But not at 2.5 meter (so far as I've experienced.) \$\endgroup\$ Commented Jun 17, 2023 at 23:39

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I know the range depends on the power of the receiver

The range depends on the power of the transmitter and the noise of the receiver. The noise a receiver inherently picks up is proportional to the bandwidth of the receive channel. The tighter the channel bandwidth, the more sensitive you can make your receiver. And, of course, the level of noise picked-up is related to temperature. Higher temperatures mean more noise.

It's also dependent on how directional the transmitter and receiver antennas are. In other words, if all the transmitter's power were focussed in one direction (like a dish antenna) then a regular antenna would certainly pick-up a bigger signal. Then, if you used a very directional antenna at the receiver, the signal received would be bigger again.

So could I in effect pick up a signal from America from here in the UK if it is reflected off ionosphere?

VHF is not noted for ionospheric reflections. I'm sure there will be some but, the main area of the spectrum for these types of reflections is definitely below VHF frequencies. Your target frequency of 127 MHz would not have any reflections of note. So, it's line of sight reception only.

any answer would be greatly appreciated

Noted.

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  • \$\begingroup\$ There are sporadic (and rare at that, I think, at these frequencies?) ionospheric propagation modes, and also reflections from exotic sources like meteor trails. Good luck making a contact over that (but, contacts have indeed been demonstrated this way!)! Atmospheric ducting is also a possibility. \$\endgroup\$ Commented Jun 17, 2023 at 18:25
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    \$\begingroup\$ Aren't the antenna height (at the tower end) and altitude (of the aircraft) more important practical limitations on reception distance for VHF aircraft comms? \$\endgroup\$ Commented Jun 17, 2023 at 19:45
  • \$\begingroup\$ @MarkLeavitt that's why I mentioned line of sight. I have no idea what tower you are referring to; I don't think the question mentioned anything about a tower. \$\endgroup\$
    – Andy aka
    Commented Jun 17, 2023 at 20:19
  • \$\begingroup\$ Maritime VHF range (which is around 156-162MHz) primarily depends on line of sight and far less on transmitter power. The higher the receiver and/or transmitter, the longer the range. \$\endgroup\$
    – StarCat
    Commented Jun 17, 2023 at 20:40
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The range depends on the transmitter power and the receiver sensitivity. More power, better receiver, and better antennas (at both ends) make a longer distance possible.

While it is true that VHF isn't known for "skipping" long distances, it does happen.

  1. Back in the 1990s, the United States Armed Forces Radio station based in Frankfurt, Germany (on 98.7 MHz, if I remember correctly) would occassionally be received in Norway. That's about 1000 kilometers (600 miles) away. One of the DJs actually put a call from a Norwegian on the air one day.
  2. Around 2003/2004 I used to listen to a particular rock station here in Germany. Some evenings in the summer on my way home from work, the German station would fade out and Radio Tataouine would fade in. That's 2200 kilometers (1400 miles.) Both stations transmitted on 87.6 MHz. Radio Tataouine seems to transmit 72000 watts of power.

In both cases, you had to be in the right place at the right time, listening on the right frequency.

In my case, the German station always faded out at a particular spot on the drive home from work. In the winter, it would fade out and that was it - I had to switch to a different station. Sometimes in the summer, it'd fade out and there was Radio Tataouine it its place. Both stations used the same frequency. They should have been far enough apart that interference was impossible - but there it was.

FM broadcast radio is in the VHF range.

You won't be getting that kind of range with your low transmit power aircraft band radio.


Yes, Tataouine. That's probably Luke Skywalker's favorite station. The scenes at the Lars homestead in the first movie were filmed in Tataouine,Tunisia.

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