Currently my location is Nepal. We use the ICOM f-7000 radios without the antenna tuner. We use the frequencies round about 5-9 MHz, for communication over distances between 150 and 400 km.

During the summer time during the day the radio works fine in all frequencies. But during the winter time there is hardly any communication possible on any frequency. The best chance is the lowest frequency within the range. What could be the problem?

  • $\begingroup$ Tell me, what's the weather like in the winter around where you are? Precipitation? $\endgroup$ Nov 29 '18 at 12:01
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    $\begingroup$ What time of day are attempting to use the radio? How far are you trying to reach (or are you just listening)? $\endgroup$
    – mike65535
    Nov 29 '18 at 13:54
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    $\begingroup$ Could you define "works" vs not works? $\endgroup$
    – mike65535
    Nov 29 '18 at 16:44
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    $\begingroup$ mike65535 works = basically can do voice comms and not works = no voice comms. We are just using it for simple voice communication. no any hi-tech medium. no ale modes. Phil Frost we are doing simple radio voice communication from locations varying from 150km to 400kms. We are using the LSB and the USB mode with the highest power. Note: we haven't got tuners or power amp. Lastly I forgot to mention in my earlier comment we have the Himalayan ranges on the north just 132 km direct distance. (does that affects) $\endgroup$ Nov 30 '18 at 0:33
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    $\begingroup$ While this answer gives a way of finding out which is the best frequency to use between two points, it does not attempt to answer the question $\endgroup$
    – Scott Earle
    Dec 1 '18 at 10:14

At distances of 150 to 400 kilometers, you will largely be relying on reflection of your signal by the ionosphere. In the winter the atmosphere receives less solar radiation and the ionosphere's index of refraction is less. This increases the critical frequency in the winter, putting the receiving station in the skip zone unless you use a lower frequency.

To gain an intuitive understanding of how this works, look at a glass of water. Looking through the side of the glass at a shallow angle it appears reflective like a mirror. But at a steeper angle it becomes transparent like a window:

enter image description here

The ionosphere interacts with radio waves in much the same way: different layers have different indices of refraction, and at some angles this can result in the radio waves being reflected back towards earth. For the purposes of terrestrial radio communication, we need the ionosphere to be like a mirror, not a window.

This is what creates the skip zone. For short distances, the angle to the ionosphere is too steep, and thus the ionosphere looks like a window. In this example, communication with anything closer than 1000 km is not possible by ionospheric reflection, because the signal instead escapes to space:

enter image description here

This refractive behavior is frequency dependent. A prism works because the index of refraction of glass is different for the different frequencies of visible light. It is the same for the ionosphere. As the radio frequency decreases, the waves are bent more. At some point, even waves straight up are reflected back towards earth, and the skip zone disappears:

enter image description here

The frequency at which this occurs is called the critical frequency.

The critical frequency also changes with the seasons, time of day, and solar conditions. Generally, more solar radiation increases the critical frequency. So the critical frequency is higher during the day than the night, higher in summer than winter (because the days are longer), and higher when the solar cycle is at its maximum.

So what's happening to you in the winter is the critical frequency is lower due to the shorter days and less radiation of the atmosphere, and so at higher frequencies the receiving station is too close because it's in the skip zone. If you get just the right frequency you might even find you can communicate with farther stations, but not closer ones. So, you must decrease frequency to get below the critical frequency where your signal will be reflected at nearly vertical angles.

  • $\begingroup$ Phil does it means i need even lower than 6 Mhz ? because we have licence from round about 6-8.5 Mhz at the moment. sometime with luck on side the lowest among the range of frequencies i have mentioned works. $\endgroup$ Dec 1 '18 at 14:11
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    $\begingroup$ @AngryHooman Sometimes yes, depends on the space weather. I'd consult a real-time critical frequency map. You can also use VOACAP to get probabilistic estimates of reliability in the long term. $\endgroup$ Dec 1 '18 at 16:51
  • $\begingroup$ Some hams say sites like this one are better than VOACAP, as it shows real-time propagation with actual QSOs. This one is a better real-time site, but it's down for now. $\endgroup$
    – Mike Waters
    Dec 1 '18 at 20:59
  • $\begingroup$ That seems like an apples-to-oranges comparison: VOACAP is about prediction, not current conditions. $\endgroup$ Dec 2 '18 at 0:26
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    $\begingroup$ @Mike Waters There's also the website of the Australian Bureau of Meteorology. They have relatively good HF prediction tools, and have actual measurements from ionosondes as well. Unfortunately, the ionosonde measurements don't cover central-south Asia, from what I can see. Unlike most ham tools, these also cover non-amateur bands as well. $\endgroup$
    – AndrejaKo
    Dec 2 '18 at 10:55

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