I am using a Pixie transceiver, giving 1 W power and 7 MHz frequency. A 40-meter band dipole antenna, providing 5 dBi gain at high angles, has been designed and connected at the output of the circuit to transmit. We are using the concept of reflection from the ionosphere to receive a signal at a distance of 200 km from the transmitter. Our antenna is in the inverted-V configuration. We have designed the same antenna for the transmitter as well as the receiver.

First of all, we have tested it for almost a 15-meter region. We transmitted the signal from the same Pixie transceiver from the transmitter then to receiver and successfully received that signal at the receiver. Now, we tried to perform the same experiment by keeping a distance of 15 km between transmitter and receiver. We received only noise and the original signal was not received.

What modifications are required in my overall system as explained above so that we can receive our signal at a longer range?

Note: Keep in mind I want to receive skywaves, so for that reason, the actual distance is the distance from the earth to the ionosphere and then back to earth.

  • $\begingroup$ So, did you do any rough estimation of the path loss over 15 km? Have you been able to verify the sensitivity of your receiver, or the power of your transmitter? It's important to first narrow down your problem to either transmitter or receiver side. Generally, trying to use a least-complexity transceiver doesn't sound too promising in the extremely-low-SNR regime of low-power ionoscatter communications. I'd recommend to use an SDR receiver instead, and instead of manually keying to use e.g. a serial to produce a long binary sequence, with which it's easy&reliable to correlate at the receiver. $\endgroup$ – Marcus Müller Dec 5 '19 at 16:02
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    $\begingroup$ also, your title mentions that it doesn't work at a range of 2 km, but your text only mentions you've tested it at 15 km. What, by the way, is a "15-meter region"? $\endgroup$ – Marcus Müller Dec 5 '19 at 16:10
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    $\begingroup$ It would be helpful if your transmitter and receiver could be tested. If nothing else, you might compare them to a known-good transmitter and receiver. $\endgroup$ – rclocher3 Dec 5 '19 at 16:19

At 15km you may be too far for line-of-sight or groundwave, and too close for skywave — you're in the "skip zone". You should conduct your test from further away, or else use a lower frequency (80 meters). This is because the highest frequency that will reflect off of the ionosphere depends not only on the ionozation of the ionosphere at that moment, but also on the angle at which the signal reaches the ionosphere. The shallower the angle, the higher the frequency that will work.

Under the current solar conditions it is not possible for a 7 MHz signal to go straight up and come straight down (NVIS) in most times and locations. 200km distance brings the usable frequency up by about a factor of 1.2 - 1.25, which might be enough, at your latitude, for some parts of the day. But 3.5 MHz will usually work during daylight for most places in the world.

  • $\begingroup$ I've worked groundwave contacts on 40m much further than 15 km. However, I was using Morse code, which gets through better than SSB, and maybe my conditions were better and my receiver more sensitive. $\endgroup$ – rclocher3 Dec 5 '19 at 16:15
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    $\begingroup$ @rclocher3 yeah, it should be possible... but with an NVIS-optimized antenna, unfavorable terrain, and not the best equipment, maybe not. I'm just coming up with the best explanation I can :) $\endgroup$ – hobbs - KC2G Dec 5 '19 at 16:18
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    $\begingroup$ I think there's nothing wrong with your answer! Besides, I just remembered that groundwave signals are vertically-polarized, so inverted-vee antennas would essentially be useless for groundwave signals. $\endgroup$ – rclocher3 Dec 5 '19 at 16:22
  • $\begingroup$ @rclocher3 Pretty sure a Pixie is a CW-only transceiver. It'll receive AM, of course, but not SSB. $\endgroup$ – Zeiss Ikon Dec 5 '19 at 16:54
  • $\begingroup$ @rclocher3 Many thanks for your comments. I am using Pixie for experimental purposes, but actually I will be sending the morse code through my antenna. Right now, I am not using any LNA or PA. Can you please provide me the specification and names of the modules you are using to increase the sensitivity of the receiver and power of the transmitter. $\endgroup$ – Ali Hassan Raza Dec 6 '19 at 7:37

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