How about this to make a cheap 868 Mhz antenna.
- Buy a cheap one off Ebay
- Snap the top off
- strip a bit of insulation off, solder a small piece of single strand wire
- Repeat the following until satisfied - Put top back on, measure with N1201SA and cut until happy (see pics for difference if top is on or off)
Pic 1 shows stock Antenna which resonates at 915Mhz, Pic 4 shows final result - not too shabby (Measurements all at 868 Mhz) 
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Pic 1 - shows stock antenna
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Pic 2 - showing how much extra wire to solder on to tip
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Pic 3 - With top off
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Pic 4 - Final result - with top on
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edit Did some tests with a Heltec.
I sent a regular message within the house (> 10 lambda) and checked the RSSI after alternating between a stock and an altered antenna - did the altered antenna make a difference?
Ignoring spurious results, it did seem that the modified antenna was better - but only by a few dB. The test certainly wasn’t scientific so maybe Loratracker can give a better test?