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TX16/ R88 Low RF - Am I doing something wrong?


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Morning all,

 

I had my first flights with my new TX16Smk2 yesterday. An issue came to light....

 

Iam paired with Radiomaster R88 receivers, and the installation is identical to the Futaba clone receivers in both the models I flew yesterday, so antenna placement is known to be at least OK.

 

The R88s are set up in the TX16 as Frsky D8. I have fine tuned them, as per the instructions, but they needed minimal offset (-4, to those that know!).

 

I range tested both models. Here is the first (small) issue; what is the actual range test criteria? All I can find on tinternet advises >30 paces. I had a controllable connection to 60 paces, but got RF low warnings from 35. However, this only occurred when I was facing directly away from the model. If I turned at all, the RF restored.

 

The RF critical setting is as factory; 42 dB.

 

So this appeared OK, if not as solid as I would hope. Bear in mind I've come from a background where no such information is provided (a FF9), so don't know whether this is good, bad or indifferent.

 

So, in the sky, first model gave many RF low warnings. I flew very close, but still they came, and then an RF critical. So I landed. Went through the range test protocol again, and it seemed a bit better. The only thing that had changed is both TX and RX had been power cycled.

 

Second model, exactly the same thing.

 

Second flight on the first model seemed OK with fewer warnings, and just as I was thinking it was a warning threshold issue, it briefly went into failsafe. This was actually at quite close range, and relatively low so distance does not seem to be a factor. On this, I landed (safely), and called it a day.

 

Have I missed something during set up? Or is something broken?

 

Much as I love the radio, it's first task is to provide a solid control link. Fundamentals first, I need to get to the bottom of this.

 

I don't have any compatible receivers other than R88s to try, so diagnosis is limited to those at the moment.

 

Not sure I enabled logging (that seemed a step too far for me at the moment!), but will look and see.

 

What should I check gents?

 

Thank you

 

Graham

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I was about to say that what you are experiencing is not that abnormal, until I read that you had suffered a fail-safe. (Well done for setting it BTW! I'm amazed by how many seem to not bother!)

 

On my old and trusty Taranis X9D, I often get the odd RSSI warning in flight, usually when the model is turning at the edge of the "box", and aerial orientation is less than ideal. I've also noticed that the "Pro" receivers seem to give more warnings, though I've not noticed any signal losses. I suspect the "Pro" receivers output a lower RSSI level, and the threshold needs adjusting for them, though its never bothered me enough to do it.

 

Ground range: Like yourself, 30 paces is my "yardstick", and I usually get around double that or more, given reasonable aerial orientation.

 

First things I would check would be aerial orientation. In the model, make sure the aerials are at approximately 90 degrees to each other, and that they are clear of any metal work or carbon fibre. (Carbon fibre makes nearly as good a shield as metal!)

 

On the Tx, my preferred orientation is with the tip of the aerial pointing into the ground about six feet in front of me. This places the two "dead spots" (one off each end of the aerial) in the ground in front of you and behind your head. You shouldn't be flying in either of those places ? ! It also gives the maximum signal strength in a doughnut shaped arc in front of you, which is where the model spends most of its time.

 

Having said all that, you really should not be suffering a failsafe at relatively close range, no matter how bad the aerial orientation. I would also check the switch harness for volt drops.

 

You should be able to log the RSSI, Rx voltage and maybe packet losses on to your sd card in the transmitter. It might be worth doing that next time you take it out. You can view the logs as graphs in OpenTx Companion, and these may give you a clue as to what is going on.

 

Best of luck!

 

--

Pete

 

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Thanks Pete,

 

Some useful nuggets here. 

 

I did suspect antenna orientation, but it looked fine. I have one antenna pointing directly downwards, and another along the fuselage. To be sure, I repositioned this one so it was inside a clear acetate window in case servo leads were creating a shield. Didn't make any noticeable difference. To be honest, if the system is so sensitive to this or the TX antenna position, it sounds a bit too marginal for my liking!

 

I need to explore the failsafe thing and replay what happened in my head. I'm starting to doubt that this is what happened. The reason for my doubt is I would have expected to hear an 'RF lost' report and 'Telemetry re-stablished' (or somesuch) announcement, and I don't recall that I did. So something else may have happened. I may have logs, in which case I should be able to diagnose more accurately. 

 

I'll keep thinking...

 

Graham

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As Peter, I would also say antenna orientation. I have flown an R88 with TX16s and no in flight warnings at all.

Range checked O.K but did give RSSI warnings depending on which way I pointed the TX aerial.

The aerial on my TX16S will not physically point at the ground without me going inside and altering its fitting. (maybe next time I have to have the back off - but can't imagine why.) Currently I can only have it pointing vertical (directly at my face ) or sideways.

Any chance a low receiver battery voltage affecting it but unlikely ??

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Hi John,

TX antenna was pointing sideways. I can't rule out low receiver pack voltages, but I agree that it's unlikely.

 

Trying to access logs at the minute, but struggling to get the companion to work. The joys of owning a mac; you can download it, extract it and get the icon in the apps folder, but nothing happens when you click it. Without the companion, the logs are unavailable, and so no help.

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I do get a lot of RF warnings on my X10S (see my latest Mustang vid to hear them!) but that is down to having an ACCESS Rx and not having reduced the warning levels to 38 and 32 (critical). I would also suggest using VFR for warnings as well (discover first in telemetry, I've then got a logical switch to operate if less than 50%).

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Could the battery pack in your model have blocked the signal to the antennae?

This would be particularly acute in smaller models.

 

I try and place the antennae not only at 90 degrees to each other but also not parallel to any other metallic items in the plane.

This means servo wires / pull pull wires / wing joiners and also I try to keep them away from servos and flight batteries which can also blank the signal.

Were the warnings coming when the model was in the same direction the Tx antenna points?

 

When you get to see your logs, look at signal strength on the graph. You are looking for episodes where the signal dropped below the 42 threshold.

You could always plot the data via something like Excel or just look at the data below 42. It may be that your logging might not pick anything up at all especially if it only logs at 1 second intervals.

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Given this is a brand new TX my priority would be to discount that as a first port of call. Try borrowing a “known good” Frsky D8 RX that you can slide into the model without changing anything else. Range test and (if it passes) fly it see if you get the same behaviour; if it’s fine you can discount the TX, if not then some more detailed ground tests might be in order, or returning the TX for replacement if you don’t want to do that. 

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Update:

 

Mike at HobbyRC advised that whilst the R88 is OK, it is not built to the same standards as FrSky receivers and is sensitive to antenna positioning. With that in mind, I returned to the field this evening to try again.

 

I also went through the fine tune procedure again. Initial range checks very nearly sent me home as I couldn't get to 30 paces before RF was lost. However, on checking the antenna one of them was very close to the feed from the main batteries. I missed this because they are only in this position when the battery is fitted, I checked without...

 

Once I moved one antenna, range check went up to >80 paces, and I got no RF warnings during flight. 3 flights without any issue at all.

 

So it seems Mike is spot on; the antenna positioning is far more critical than it was, but once correct, all seems well.

 

Graham

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