FASST receiver antenna placement

[DrJustice]

How do you folks handle the FASST receiver antennas?

I used the supplied antenna tube, cut to length, to make a rigid support. I might make a proper bracket that mounts between the tail boom supports and holds the tubes in place. The only issue is that vibration might make the tubes chew on the leads at the receiver end, but I don't really think that will be a problem.

43742

DJ
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[steffgiguere]

You don't need those tubes. I tie wraped a piece of nitro gas tube to the tail boom supports and passed the antennas though.

[DrJustice]

It would be tempting to run them along the supports, but that would compromise the antenna diversity, and from the front they will mostly be obscured. If anything I'd want a wider spread than the tubes have in the picture, getting the prescribed 90 degrees - easily fixed by making a bracket.

DJ
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[wlfk]

Worse than that, if the supports are carbon fiber, they will have a very bad effect on the tuning of the antenna.

K

[DrJustice]

Just for fun I measured the resistance in the tail boom supports and frames of my Trex 450SEV2. The boom supports measure ca. 20 ohm from end to end. Any two points on a frame section represents a resistance of 10-20 ohm. These parts are indeed conductive enough to potentially affect antennas.

DJ
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[steffgiguere]

Well that is how I have it on my 450 and everything is great. I have had about 50 flights with my 450 on 2.4 with this setup.

[DrJustice]

I'm sure it works fine steffgiguere, but it isn't optimal.

Part of the hobby for me is to build and set up things to perfection, not necessarily because it makes an immediate difference, but because it is good fun :)

BTW: I changed my setup slightly from the picture. The tubes now rest on top of the boom supports, next to the end fittings. That gives a 90 degree spread and a good distance from the heli. Both antennas are visible from almost all angles.

DJ
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[dvelez3]

Here is mine, I've been flying it like that for 3 months now, no issues.

Doug.

[DrJustice]

Aha... I was debating with myself which way to place the receiver. Hmm... I could turn it like you have and still use the tubes for rigidity and spreading by fixing them to the landing gear. Anyway, your setup looks sound enough.

DJ
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[steffgiguere]

How do I vue a picture of how you rooted your antenna? Sorry I just saw it again higher in the post.

[Tomas Ahl]

The only issue is that vibration might make the tubes chew on the leads at the receiver end, but I don't really think that will be a problem.
If you put 0.5 - 1 inch of silicon nitro fuel tubing at the transition from the tube to the receiver you may further reduce the potential for this problem as then you avoid getting sharp bends of the coaxial cable when it exits the tube and enters the receiver.

[istandalone]

fuel tubing has a thousand uses! i put shrink wrap on the coax part of the wire, thinking that if somehow it touches metal or carbon it won't cause interference. so far, so good.

[medflight]

Do you have to have a 90 degree split or could you go more? On my 600N's I'm thinking of running one out each side of the plastic electronics tray on the front. This would make them 180 degrees out. One of them should be in line with the xmitter at all times.

[centuryman]

I cut a v shaped peice of plastic and attach the receiver in the center a zip tie works well then run the antenna through fuel tubing up to the last 30mm at the end. and then i attach the tubes to the v shaped plastic with packing tape and one zip tie for good measure. this holds the antenna in the perfect 90 degree oriontation for optimal signal reception. I have 6 helis set up this way with the fasst receivers and no problems. It also holds them off any cabon that may be close.

[istandalone]

i think they say 90 degrees as a baseline, so that the wires are sticking out at two different directions and likely one on each side of the model. 180 should be fine, but i'd definitley range check. maybe call futaba support to find out for sure?

[wlfk]

180 degrees is the worst possible relative orientation.

If you have only one antenna, and you oriented it until it got the best possible signal then turned it 90 degrees in any direction, you would in theory get no signal at all. If you turned it another 90 degrees so it was pointing at 180 degrees to the original orientation, you would get the best possible signal again.

So, ideally you should have 3 aerials, all pointing at 90 degrees from each other. 2 antennae at 180 degrees are no better than 1.

K

[medflight]

As per the instructions, "If possible, please make sure the two antennas are placed at 90 degrees to each other. Please note: This is not a critical figure, however, the most important thing is to keep the antennas away from each other as much as possible."

Today, I called Hobby Services and asked if the antennas had to be at 90 degrees or if it was o.k. to go more or less, say 180 degrees. All the guy would say is they HAVE to be at 90. Who do you believe, the instructions or a tech that obviously hasn't read the instruction?

I don't see how 180 is any worse than 90. In both positions a heli could still be oriented so that only one antenna has a direct path to the transmitter. I thought the reason for two antennas was so if one wasn't getting the signal the other would, hence the reason for the split. With two at 180 degrees one will get the best possible reception and the other will get the worst. With only one antenna there is nothing to fall back on.

[DrJustice]

The antennas should be placed so that at any angle, you will have a side on view of at least one antenna. With the antennas at 180 degrees you will see the antennas as a single point from two diametrically opposed angles. This is the one zone where a whip antenna has very little sensitivity, whereas side on an antenna has the best sensitivity. As you may know, you should never point the TX antenna directly at the model for the very same reason. So the 90 degree spread is the optimal configuration for the two antennas. The radio will work practically just as fine if your antenna spread is a bit less, but anywhere near 180 degrees is an invitation for Murphy. So no need to believe either Futaba or Tower on this, just believe general radio theory (old ham here...). You have paid for the radio diversity that two antennas gives you, so why not use it and reduce the chance of a radio 'black out'.

DJ
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[wlfk]

don't see how 180 is any worse than 90. In both positions a heli could still be oriented so that only one antenna has a direct path to the transmitter. I thought the reason for two antennas was so if one wasn't getting the signal the other would, hence the reason for the split. With two at 180 degrees one will get the best possible reception and the other will get the worst. With only one antenna there is nothing to fall back on.

OK - you're half right. But this is one of those things that you have to get 100% right.

There are two issues at play here. One is shielding, and as you say it's good to have a direct path from the TX to the antenna.

The other issue is orientation. They always used to say you should never point the aerial at the model, because this gives you the smallest possible signal. Same applies here. Ideally, the transmitter/receiver aerials should be parallel to each other, like this:

| |

Worst possible is:

_ |

By putting the antennae on the helicopter like this: _ | or \ /, you're minimising the chances of the worst-case scenario affecting both antennae.

When they say 'about 90', there is a lot of latitude. If you put the antennae at 45 degrees rather than 90, you still get a pretty strong signal. If you do the maths, even 30 degrees would probably be OK. But 180 - if you get it exact - is a recipe for disaster.

K

[medflight]

I knew about not pointing the tx antenna directly at the acft but wasn't applying that to the rx antenna. I was only trying to keep the "line of sight" clear. Thanks for the help.

[DavidH]

As for the 180 deg spacing. Most of the models I see using the Futaba FASST receivers have the antennas at 180 degs. They didn't seem to be having any problems.
For example if the receiver is laying flat on a table. They have placed a 90 deg bend in each antenna away from each other. So the tips are facing 180 degs opposite direction.

David

[wlfk]

Well, more fool them. Why do something the wrong way when it's so easy to do it the correct way.

I once managed to provoke a lockout on a FAAST system. I turned the TX power down to 10mW and put a 2.4ghz TX on the helicopter. It locked out when the helicopter's horizontal aerial was facing towards me, and the vertical antenna was shielded by the c/f frames. When you turned the helicopter 180 degrees so the vertical antenna was no longer shielded, then it started working again and worked over a much longer range too.

So, you can get away with a lot, but antenna orientation does make a big difference. It may be that you can get away with something that's suboptimal for a long time - perhaps forever. But on the other hand, you might not. Helicopters are mostly operated over a relatively short range, so perhaps this is why people are having fewer problems than if they were running large gliders, or the like.

K

[ErichF]

I do wish Futaba had something like Spektrum's datalog, so we could see if our antenna's are being used effectively. You can spend half a day setting up both your antennas, and have one not ever get used by the system.

I don't have a pic yet, but on my Caliber 90 I have one antenna pointing upward from the RX tray, and the other hangin down below the radio tray, resting on the bottom of the canopy pointing aft. So, these antennas are not only 90° apart in direction, they are on different planes (one above, the other below the fuse and electronics pack.)
Range check was double book spec of 50 paces, and flying FAI patterns takes the machine a lot farther out than my 3D counterparts. I must have done something right.