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I have a Lora radio device that runs at 868mhz, it came with a tiny stubby antenna.

I want to use the device out and about so wanted to know if it will be OK connected to a magnetic/car antenna I got for 2m use?

As antenna seem to be listed as 2m/70cm -- and 868 is 34.5cm (about half of 70cm) -- I was thinking it would be fine.

The actual antenna is around 45cm long.

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    $\begingroup$ Most antennas aren't made to work on second harmonics. You'll note that 70cm is (approximately) the third harmonic of 2m. $\endgroup$ Nov 26, 2021 at 3:05
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    $\begingroup$ I think the correct monopole for 868MHz would be 8cm long? Maybe your "stubby" is actually perfect. A correct much longer antenna might be a colinear. $\endgroup$
    – user10489
    Nov 26, 2021 at 12:41
  • $\begingroup$ I guess that's the question here: why do you not want to use the antenna that came with it? Experimentation? Or there is some problem you are trying to solve? Maybe edit the question and tell us what problem you are solving, if any. $\endgroup$
    – user21789
    May 3, 2022 at 17:39

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LoRa is only 25–100 mW. Given that you mention 868 MHz you are probably using the SDR860 European standard, which means a maximum of 25 mW ERP.

Even a grossly mismatched antenna system wouldn't be a big deal in terms of safety (for people or equipment), at least for testing. Would it be wasteful using the energy to warm the antenna system slightly? Probably, but you won't care unless you are planning on sipping battery power for weeks or months. I suppose it depends on the device doing the sending, and whether it can handle the mismatch and potential higher voltages. But most LoRa equipment I've seen is made for more rugged IoT situations so I imagine the antenna connection will not be so delicate. (But you might want to investigate that anyway.)

So, to answer your question, you are probably ok to just experiment with the antennas you have, though the stubby will probably be the most efficient -- in free space with a decent ground of some kind.

By my back-of-the-envelope calculations a $1/4\lambda$ for 868MHz would be approximately 8.6cm. This might mean you are feeding the antenna at location with a higher voltage node. Well, maybe. We don't really know anything about this antenna, other than its length. And who knows what the ground plane looks like to the monopole.

But who knows? Many commercial LoRa external antennas are $1/2\lambda$ in length, so basically a vertical dipole with whatever counterpoise/ground it can find.

You may have stumbled upon a reasonable configuration because the always imperfect ground offered by the car body and the extra length of the monopole combined to give you an accidental lower feedpoint impedance and a lower takeoff angle resulting in decent results.

Honestly, there is no way to really know without analyzing the system in place. Otherwise, experimental results speak for themselves.

Not to mention that the whole point of LoRa is to provide bursty, low-bandwidth, low-power "connected" networking comms in marginal situations. So the mode is rather forgiving. If you can get a signal out enough to "connect" then that's what matters.

Since you are experimenting anyway, why not simply hook up piece of large gauge single-core copper wire cut to the approximate correct electrical length and then trim until you get the result you want from 2-3 listening nodes?

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We tried it out- in parallel with devices with stubby antenna and with a device with a proper high quality antenna.

It is quite a hilly area around here and it seemed clear that even low power stubbies were plenty to cover 'line of sight' - the other antenna didn't seem to add much (if anything). We'll be doing more trials and I'll update if there is some significant difference.

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    $\begingroup$ Hello and welcome to ham.stackexchange.com! Please do explain how you managed to transmit on 868 MHz with an antenna designed for 144–148 MHz and 420–450 MHz. I can't imagine that the transmitter was very happy with such a mismatched load. $\endgroup$
    – rclocher3
    Dec 3, 2021 at 0:38
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    $\begingroup$ I just plugged it in... a Lora Heltek (v2.1) and the magnetic antenna on top of the car. And sent messages over bluetooth to the Heltek and it sucessfully Lora'd them to another board back home. The landscape (hilly) meant we didn't get more than 1.5Km away before hills blocked the signal, but it was OK upto there -- I don't know how reliable the reading is - but testing with a second lora device in the car reported a signal strength of 98% from the antenna on top of the car; $\endgroup$
    – pperrin
    Dec 4, 2021 at 12:26
  • $\begingroup$ +1 for "just trying it out" which is the poor ham's VNA. $\endgroup$
    – user21417
    May 4, 2022 at 22:06
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I would agree that if you're just experimenting your probably fine as long as your power stays low. But why? The stubby antenna will most certainly be most efficient as it is matched to your application. I also like experimenting with such things, but for routine operation I'd stick to what you know works (from the manufacturer or your own design!).

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