I’m working on long-range (60km+ one direction, 200km+ total range) FPV plane. Almost entire plane is built from scratch (batteries, airframe, autopilot, antennas) and i’m trying to choose right components.

For 60km+ range i probably can’t go with single antenna – i need array of antennas (other options – LNA (good LNA are very expensive!), more power to VTX (and i need electric power for motor, also – interference), trackers on both sides (too complex and error prone)). Now just need to choose correct polarisation option…

So, my options at this moment – circular polarized array (CP BiCircles – i like circles :P ) or linear polarized array (again, bicircles). Both have high gain (~16dBi for CP array, ~17dBi for LP array), both can be built to be flat (another reason why yagi antenna is out of scope – too big to transport).

Lots of resources on internet about CP/LP choice, in general all agree about:

• LP is worse in banked turs, CP works well in any direction (agree 100%, that’s why satellites use CP – but question “how much banking is too much for LP?”)
• For the same volume/weight LP offers higher gain (serious reason why i’m reverting to LP – huge CP antennas, easy damaged in transportation, higher gain antennas (such as QFH) are draggy a lot)
• CP is a “must have” for higher frequencies (again, agree – LP on 5.8GHz sucks ballz due to multipathing issues, but for long range i’m going on 1.3GHz setup)
• LP antennas are easy to make (and this is great – i killed three cloverleafs while flying in forest in one day, repairing them takes lots of time. Long range plane keeps antennas safe, but sometimes they break anyway and…)

So, question remains – how much banking is too much for LP, and how much image quality i will loose for using LP? Knowing this, i can choose right antenna array. To answer this question, i did simple test. Conditions:

• 18dB attenuator on plane (cut range by 4 – i.e. 10km with attenuator = 40km without. Well, at least in theory)
• my old CP antennas – verified, 15km without attenuator is “easy mode”, VSWR < 1.5 when new vs brand new, just “cooked” Vee’s + BiCircle (BiCircle gain = BiQuad gain and almost the same radiation pattern)
• The same frequency (1280MHz), the same plane with the same setup (and the same battery), identical weather conditions (well, if possible – liftoff during sunset without wind, after second test landed in almost dark environment, with strong wind)
• If possible, very bad conditions (RF noise – cell towers nearby, willage, mobile phones; clouds, moisture)

What i got (distance is top-middle):

OK, my old CP antennas lived trough hell (lots of crashes, repairs, re-soldering, tunning) and probably do not have < 1.5 SWR anymore. But i didn’t expected them to perform so bad, compared to LP…

< 5km range with CP (OK, bit better signal quality compared to LP – less noise, no sudden signal loss), that’s approx 20km total range. 10km range with LP (still good signal when banking – check video) – that’s approx 40km total range without attenuator and in better weather/RF conditions (and really simple antenna construction, Vee’s won’t be destroyed by trees anymore) – i think, we got clear winner here.

Next in work queue – BiCircle antenna array for 60km+ range (and i guess 80km range should be easy too).

Notes:

• results are valid for low freq (0.9-1.3GHz) video transmission. On higher frequencies (2.4GHz and esp. 5.8GHz) it’s a different story
• Yes, i will perform “max range” tests without attenuator, when i finish my long-range plane. Currently i can’t go past 30km to one direction (temperature issues, also some issues with current plane setup) so i can’t test single antenna to it’s limits, not talking about array…
• BiCircle and BiQuad have identical gain and (probably) just minor differences to gain pattern. I choose BiCircle for it’s look and simplicity. Also, tested on different frequencies – works great, so why bother bending corners?
• In good weather (no forests around, no rain/moisture, no clouds layer, no RF noise) range should be better. Take off attenuator, mount antennas and… =)