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Multirotor control antenna positioning for 2.8 GHz

p1ayaone

Junior Member
#1
I'm posting this here because I figure like me, many of you are followers of David's tricopter setup. I started multirotors coming from a fixed-wing background, where we're always told to tilt the transmitter antenna (in a 2G4 system) so it's parallel with the ground and pointing sideways, so it doesn't point at the plane.

Well I use a Hitec 6ch Rx, with a single BODA (i.e. whip) antenna on a mast pointing straight up to the sky on my tricopter. Because it's a multirotor, I can be pretty certain that Rx antenna will always be pointing up - I'm not going to be doing any flips or anything like that.

Should I be re-thinking how I position my Tx antenna?

The Rx antenna being vertical would have the best gain for vertically polarized waves, but of my Tx antenna is horizontal, am I losing a bunch of range needlessly? I'm thinking I should be poining my Tx antenna up to the sky so it's parallel with the Rx antenna.

I never had an issue with the Hitec setup because I've laywas been flying LoS, but I picked up some Fatsharks yesterday to get into FPV. I'm not planning a UHF system for now, but I'd like to get as much as possible out of my Hitec.

Your thoughts are appreciated.

Peter
 

xuzme720

Dedicated foam bender
Mentor
#2
Welcome to the forum!

It seems like we all have been paying more attention to polarization on our video signals but this is really the first time I've heard anyone ask about the control signal. And to be honest, I'm not even sure if the protocol that the radio uses is affected as much as the video link in terms of polarization.

Put it in range check mode and do some experimenting. We all might learn something!
 
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#3
The Tx is more important. You don't really want the Tx antenna pointing directly at the Rx as this is the null zone for transmission. The issue you have with pointing it straight up is if the copter flies directly above you. This could be caused by a gust of wind quite easily at height. If your Tx is horizontal to the ground you will never have this issue.
The null zone for transmission will effectively be to your left or right depending on which way you bend it. If the copter goes to your left or right then you naturally turn to view the copter thus moving the null zone away from the copter. Imagine how difficult it is to move the null zone if both your copter and null zone are above your head and still maintain control.

RC control is far more forgiving than a video signal for polorisation. This is simply due to the bandwidth required on the signal. I would always go with a satellite receiver if possible with the main receiver pointing vertical and the satellite point horizontal. This ensures you get the best signal link at all times. If you can't use a satellite receiver complete a decent range test in various orientations.

And if you are just starting in FPV make sure that your control range is greater than your video range;)
 

Craftydan

Hostage Taker of Quads
Moderator
Mentor
#4
Peter,

I depends on what you're flying, but as long as you're not going for a long range system (LRS), polarization isn't that big of a concern. Most 2.8 systems have plenty of range for small and medium sized planes, because they're too small to see at full range, and the larger planes can afford a satellite to take full advantage.

Your plane also can be at *any* angle at any time, Inlcluding looking down the bore of the antenna. Most newer standard sized receivers have 2 antennas at right angles to each other (I wouldn't buy one without two). If they come out of the case going the same direction make sure they're mounted 90 degrees form each other (taped down coffee stirrers make great antenna guides).

Keep in mind, you're transmitting less than 20 numbers (channel positions + frame headers), *at least* 11,000 times a second. If the rx misses a few updates here and there due to noise you'll never see it. That, and you've got to drop a LOT of packets before the link breaks.

Analog video, on the other hand, is VERY susceptible to interference/noise and has very low tolerance on error rate. Hence, a lot of care is taken with FPV.
 

vk2dxn

Senior Member
#5
I run my control gear horizontally polarized with my tx antenna bent to the side because of how I hold my tx. Polarization does matter greatly with any frequency because if your tx and RX antennas are opposite polarization then the received signal will be cut down something like 3-6db. At close range this doesn't matter because the received signal will be strong LOS but at longer distance the loss will be detrimental to the received signal. It doesn't matter which pozarization you choose as long as both tx and RX are matched. Remember this goes for ALL rf signals regardless of frequency