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My goal is to build a battery powered RF transmitter that will reach its accompanied receiver through trees, tall grass, brush, and some outside terrain at a reasonable distance (up to a couple miles would be great).

The main thing I want to focus on is the transmitter for now but for completeness, think of the receiver as a handheld device. The transmitter needs to be small and run on a small battery. The antenna can be probably a foot long. Basically think of those GPS dog collars that emit their GPS location via RF, as this is essentially what I'm hoping to make.

I am expecting that I will want/need to use a transmitter in the 900MHz range as that seems to be what would work best for this application based on what my research has shown. Unfortunately I'm having a difficult time figuring out what a good chip or module would be to use in this case. I will likely need to pair it with a GPS chip and a simple microcontroller(?).

I expect the data rate needs for this will be as follows:

  • 6 bytes for a fairly precise GPS coordinates
  • Between 4 and 32 bytes for a unique identifier
  • Some packet overhead
  • I think transmitting this message once per minute would be plenty sufficient and it would still be usable at higher periods (5 minutes?)

I'm not sure what the packet rate limitations are at the 900MHz range or the max power output (2 Watts maybe)? Are companies like Garmin able to achieve this by via licensing? If I used license hardware would that make a difference?

I have a good amount of software experience and some assembly/C/C++/C# for the programming aspect of the microcontroller but hardly any RF or signal processing experience.

Can anyone recommend some components that would be useful for this application? Explanations for why these components would work best would also be helpful.

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  • \$\begingroup\$ This is not necessarily achievable. Extremely efficient technologies like LoRa or Sigfox can do it for very low data rates using license-free provisions. To move more data, you're probably talking about licensing users or else buying spectrum and putting up a large network of towers - or more likely renting capacity from a mobile carrier who already has. Without your data rate requirement, this is "too broad" and will probably be closed - even with it, that's a likely outcome for this sort of question. \$\endgroup\$ – Chris Stratton Jul 22 '17 at 4:12
  • \$\begingroup\$ maybe with a couple thousand watts @1GHz , leaf loss and Friis loss for a couple miles. Try CB band \$\endgroup\$ – Tony Stewart Sunnyskyguy EE75 Jul 22 '17 at 4:16
  • \$\begingroup\$ A GPS coordinate and an identifier seems like it should be a very small amount of data to me \$\endgroup\$ – Joe Phillips Jul 22 '17 at 4:17
  • \$\begingroup\$ @JoePhillips - depends on how often. The really narrowband/long-range-on-low-power systems and their networks are limited by regulation and node QOS capping by packet rate as much as actual data, often to only a handful per day. So once you send a packet at all you might as well send the (rather modest) MTU - what you can't do is make frequent updates every few seconds/minutes. \$\endgroup\$ – Chris Stratton Jul 22 '17 at 4:20
  • \$\begingroup\$ It's questionable if once per minute is achievable in a non-licensed category. \$\endgroup\$ – Chris Stratton Jul 22 '17 at 4:26
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Leafy trees make great microwave reflectors. Not a good choice of frequency unless you just need to reach a neighboring house.

Try a Long wire antenna or high gain antenna and using the LF band not UHF and choose 80dB max loss for reliable communication .

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Assume line-of-sight. Assume 10,000 meters (6 miles). Assume 900MHz (1/3 meter). The path loss is 22dB + 10*log10[ (distance/wavelength)^2 ],

or 22dB + 10 * log10[ (10,000 / 0.33)^2 ]

or 22dB + 10* log10 (900,000,000) or 22dB + 89dB = 111dB. PathLoss. Lineofsight.

Assume 1,000 bits per second. Really low datarates are exciting, but carrier tracking becomes necessary, and phasenoise is a bother.

The required energy from the receiver antenna is

-174dBm/rtHz

+30db for 1,000 bits/second

+20dB, for excellent SNR and no need for error correction

+10dB, for antenna matching, Noise Figure, etc


-174 + 60 = -114dBm

Just add -114 to +111, and find the isotropic ERP is -3dBm or 0.5 milliWatts

Line of sight.

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Tony Stewart suggests the LF band (0.3 to 3MHz) and long-wire antennas. Reason? tree leaves reflect small-wavelengths, similar to leaf size.

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    \$\begingroup\$ Then add 10dB per leafy tree \$\endgroup\$ – Tony Stewart Sunnyskyguy EE75 Jul 22 '17 at 6:14
  • \$\begingroup\$ So you're saying the reflections are a bad thing? Due to noise I assume? \$\endgroup\$ – Joe Phillips Jul 22 '17 at 14:16

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