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I am working on a system that should transfer some integer data on a distance of couple of kilometers in a mountainous area and should be as cheap as possible. I've read about LoRa modules and the like but they are quite expensive and also tend to be much less effective when surrounded by obstacles, much like any other transceiver that uses UHF frequencies.

However lower frequencies have much easier time reaching long distances. So I thought if it is feasible to make a transceiver with frequency range of let's say 10 Mhz (The number is arbitrary, the point is lowering the frequency), and transmitting data at extremely low bit rate with ASK modulation. If the antenna size would be 1/20 of the wavelength would such a device be feasible or are there some considerations that make the idea impossible?

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  • \$\begingroup\$ How much energy do you have to send and receive? \$\endgroup\$ – Andy Aug 16 '17 at 11:42
  • \$\begingroup\$ By energy you mean how much power I can pump into my transmitter? Let's say it's 2-5 watt transmitter. I don't really have a design ready for it just interested if the idea is even worth thinking about. \$\endgroup\$ – Anthropomorphous Dodecahedron Aug 16 '17 at 11:44
  • \$\begingroup\$ en.wikipedia.org/wiki/Longwave - "Because of their long wavelength, radio waves in this frequency range can diffract over obstacles like mountain ranges and travel beyond the horizon" \$\endgroup\$ – JimmyB Aug 16 '17 at 11:44
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    \$\begingroup\$ You could have a look here: en.wikipedia.org/wiki/ISM_band to see what bands are (possibly) available in your region. Note that even if it is available, data communication might not be allowed (only voice). \$\endgroup\$ – Bimpelrekkie Aug 16 '17 at 11:52
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    \$\begingroup\$ Most likely you'll only be able to use 27 MHz citizen band frequency (legally). Also, I think, SSB-FSK modulation will be much better than ASK \$\endgroup\$ – Archimedes Aug 16 '17 at 11:57
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If the antenna size would be 1/20 of the wavelength would such a device be feasible or are there some considerations that make the idea impossible?

A monopole that is 1/20 \$\lambda\$ will have a radiation resistance of a few ohms and a capacitive reactance of about j1000 ohms: -

enter image description here

So your first fight is radiation resistance (equivalent load-resistance of what is transmitted as an EM wave) versus antenna loss-resistance and this latter may be of the same order as the radiation-resistance so immediately, you are wasting significant power in order to transmit power.

With a quarter \$\lambda\$ antenna the radiation-resistance is about 37 ohms so it's easier to deliver power to and has less percentage loss due to it being much higher than the loss-resistance.

If you use a resistor of (say) 40 ohms to produce a 50 ohm load on your transmitter (antenna is 5 ohms radiative and 5 ohm loss) you can see that 1 watt into 50 ohm (7 VRMS) becomes about 10 mW transmitted as an EM wave. Do the math!

This is a power loss of 40 dB so you have to think about using a transformer to effectively lower the output voltage from the transmitter in order to get a decent "match" to the 5 ohm radiation resistance.

At this point I haven't calculated radiation resistance - I've just used the graph above and made an estimate. So you should grab the formula from the internet and get a more likely number.

The next problem is tuning-out the capacitance of j1000 ohms. You would need a series inductor of equivalent reactance and this needs to be carefully chosen to make the circuit electrically resonate so that you can deliver power effectively.

So you'll need a transformer of about 3:1 turns ratio (50 ohm to 5 ohm) and an inductor of 1000 ohms to get a decent efficiency figure. It's doable at 10 MHz and should be OK but the antenna will be easily detuned by obstacles so you need to take care of moving objects around the antennas at both ends of the link.

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  • \$\begingroup\$ In regards to the impedance matching: I have -j1000 capacitive load which I tune with j1000 inductor. I also have a 2 ohm radiation resistance. I now have 2 ohm real component + 0 ohm reactive load. Am I right in concluding so? Can I not use let's say LC L matching network to make it look like it is 50 ohm load? \$\endgroup\$ – Anthropomorphous Dodecahedron Aug 24 '17 at 13:35
  • \$\begingroup\$ Yes you can but don't forget that the low radiation resistance is matched or exceeded by the loss resistance. \$\endgroup\$ – Andy aka Aug 24 '17 at 14:57

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