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I'm trying to implement a point-to-point communication system in a mountainous environment where the base station and the client are at least 15 km apart, without any line of sight (LOS). The base station could have more transmission and receiving power than the client, which should be portable (ideally, it should be light and battery powered). The data rate is very low with a 30 character string being transmitted every 10 minutes.

I saw that XBee-PRO® 868 modules claim a range of 80 km in LOS, but in a severe environment like a wild mountain place, how well would it perform? What are the alternatives?

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    \$\begingroup\$ Are we talking about bare rock mountains, or vegetation-covered mountains? \$\endgroup\$ – Dave Tweed Sep 26 '13 at 16:07
  • \$\begingroup\$ Have you considered using satellite phone type technology? This should overcome the terrain problem and can operate with low power and physically small aerials. \$\endgroup\$ – JIm Dearden Sep 26 '13 at 17:20
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    \$\begingroup\$ @DaveTweed we are talking about rock mountains starting from 2500m tall, so there isn't any type of vegetation \$\endgroup\$ – phingage Sep 26 '13 at 18:46
  • \$\begingroup\$ @JImDearden Satellite is a solution, but you must be under a carrier and typically fees are very high, I would like to investigate a more cheap and operator free solution \$\endgroup\$ – phingage Sep 26 '13 at 18:48
  • \$\begingroup\$ I think in that kind of terrain, you're going to find that there is no free lunch. LEO satellite systems are ideal for that environment, but if you can't use them, then I think the next best thing would be to create a mesh network of solar-powered VHF/UHF repeater nodes that give you the coverage you need. \$\endgroup\$ – Dave Tweed Sep 26 '13 at 19:10
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As with any radio system the bottom line is what can be achieved without the hindrance of obstacles. In free-space the baseline is dependent on what frequency you transmit at and how sensitive your receiver can be. The power path loss equation is this:

Loss (dB) = 32.45 + 20\$log_{10}\$(f) + 20\$log_{10}\$(d)

Where f is in MHz and d is in kilometres. This equation tells you how many dB of power loss you can expect at a given distance with a given carrier frequency.

It's formulated from work by Harald T. Friis and a decent reference is here. It doesn't assume any gain in the antennas, so it is a real baseline. Antenna gain brings lower path loss but greater directionality, and for a dipole the gain is about 1.76 dB.

If you are transmitting 10 dBm (10 mW) at 100 MHz over a distance of 10 km, you can expect to receive a power of:

10 dBm - (32.45 + 40 + 20) dBm = -82.45 dBm (5.7 nW)

How much power does your receiver need? A useful equation is this:

Power (dBm) needed by a receiver is -154 dBm + 10\$log_{10}\$(data rate) and as you have a data rate that is quite low you can expect better performance than Wi-Fi!

The question states 30 characters every ten minutes. I'm going to assume that this is transmitted as a burst of 30 x 10 bits in 10 seconds plus a preamble of 100 bits to get the receiver locked in - that's 400 bits in 10 seconds or 40 bits per second.

The receiver power required is therefore -154 dBm + 10\$log_{10}\$(40) dBm = -138 dBm

This assumes that the transmitter and receiver are bespoke items designed to run with this low data rate. It's not easy to get a receiver sensitivity below -120 dBm, so whatever radio system you use read the small print and investigate. Off-the-shelf items are probably incable of transmitting at very low data rates, so they should be avoided.

Anyway, you need to receive -138 dBm and, over 10 km with a 100 MHz carrier, you can expect to receive -82.45 dBm. This sounds good enough considering that you may get a few more dBs from antenna gain.

But on earth, no matter what the terrain appears to be, there will be added attenuations that are really difficult to account for and describe here. There's a thing called fade margin and this, as a rule of thumb basically says - try to ensure your received power is at least 20 dB greater than its baseline sensitivity - this means that if you designed a receiver requiring -120 dBm you should expect to receive -100 dBm on a good day.

Given the terrain I'd hazard a wild guess that you need to have 20 dB more up your sleeve and this just about brings you to being able to get 10 km from a 10 mW transmission.

Hopefully you now have the formulae to work out what power you need to get 15 km. Another helpful fact is that the target may not be moving very fast and can be tracked from one end using a high gain antenna such as a Yagi-Uda antenna. You might have seen wild-life films where a bear or a puma has a radio transmitter attached to them and some guy in a field is pointing the Yagi-Uda antenna this way and that in order to locate the direction the animal is in. This might work to improve things.

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  • \$\begingroup\$ I'm curious where you got the receiver required power equation from; I'd suspect that this number is dependent on the receiver and isn't universal. \$\endgroup\$ – akohlsmith Sep 27 '13 at 23:51
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    \$\begingroup\$ @akohlsmith The formula is derived from noise power per Hz due to the receiver not being at ambient rather than absolute zero in temperature. The noise power increases with bandwidth requirements (bigger data rates) and assumes that the energy per bit required is 100 times bigger (can't remember which right now) than that of the noise. It's from a book by Christopher Haslet entitiled "the essentials of radio wave propagation" link: books.google.co.uk/… try page 15 on the link. \$\endgroup\$ – Andy aka Sep 28 '13 at 10:50
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This link will require more than the standard 900mhz ISM band. In my experience the only way to get 900mhz to work would be to use the Xtend modules, and use about 2 or more repeaters on the high points between the two location. Otherwise, this will require something under 150mhz . See this view enter image description here

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    \$\begingroup\$ "m" = milli; "M" = mega. "P" stands for pedantic LOL \$\endgroup\$ – Andy aka Sep 27 '13 at 12:17
  • \$\begingroup\$ I'll look to xtend modules thank you, unfortunately put repeater is not an option due to restriction, we are inside a national park \$\endgroup\$ – phingage Sep 27 '13 at 17:58
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Could be done with APRS (transmitting requires Ham licence) in 10m or 20m band. The 2m band can be used for non line of sight communication if digipeaters are available in the region.

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  • \$\begingroup\$ Problem with 20m band is antenna size, base camp can have as much as power and size as want, but client should be portable, infact that should be bring by hikers so size and weight are very strict \$\endgroup\$ – phingage Sep 26 '13 at 18:53
  • \$\begingroup\$ A matched antenna may not be needed for receiving, but could be deployed to reply (easier with trees though) - wire dipoles and string are pretty light to carry. End fed / random wires can be used with an adaptive matching network, or possibly just by eating the inefficiency and assuming duty cycle is low enough to manage the amplifier heating. But true independence from terrain only happens when ionspheric conditions are such that the frequency of interest will be reflected even on a near perpendicular incidence (as needed to reach the local area). \$\endgroup\$ – Chris Stratton Sep 26 '13 at 19:10
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    \$\begingroup\$ also, be advised Amateur Radio bands cannot be used for any type of commercial, or quasi commercial communication system. they are to be used only by amateurs, conversing, or otherwise doing amateur tv transmission or other hobby type projects and builds. Amateur serving under the command of a commercial enterprise, forwarding their traffic, are breaking the rules. \$\endgroup\$ – Andyz Smith Sep 26 '13 at 19:17
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    \$\begingroup\$ Check if your country has an HF band allocation for ISM, and what emission power is allowed. APRS protocols can be used on ISM bands for commercial purposes, with private repeaters, but not with any connection to the Amateur service. Separate the technology from the regulatory domain. \$\endgroup\$ – James Cameron Sep 28 '13 at 2:46

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