FrSky S6R receiver has low RSSI

[gorc]

Hi all,

I have a low RSSI problem with my plane using Taranis plus and Rx S6R. I know my Tx is OK since I have this only with this plane.
The plane works well but I receive the telemetry message of low RSSI even if the plane is near the Tx and over a nice clear grass field. No trees around, no other flyers.
The distance between the plane (Rx) and Tx is pretty small, around 100 feet distance, so pretty close.
I'm sending 2 pictures of the antenna placement, one on each side of the bottom fuselage.
The plane is a flat EDGE-540 EPP.

Here are the details of the antennas placement:
- Of course they are at 90 degrees from each other.
- They are spaced as far as possible
- No metallic objects near the tips of antennas (> 10 cm).
- The closest conductive material are very small carbon rods but they are at least 10 cm from tips of antennas. Carbon rods are the black segments in pictures.
- The antennas are wrapped and protected by small heat shrink tubing and tubing are glued with hot glue. So I can slide in and out the antennas in the tubes.

What is interesting is that the low RSSI is always (and only) detected at the same area in the sky, at about 45 degrees up and at 2HPM o'clock of Tx are approx 100 feet so to my right.

I've checked if the antenna wires are loose in the receiver and their not. They seem well snapped in.
I have not made an electrical ohm conductivity test from tip to inside the Rx.

Finally when the Tx is near the plane (Rx) at about 6 feet, the RSSI is around 90 dB.

Does anyone has some suggestions for me to check?

Thanks,
Gorc

 

[Wayne]

You may have 2 issues here..

1- The receiver may be a little weak. It happens. Sometimes it is just a bum antenna. Take a close look at the crimp near the plug, or look for any hard folds on the coax.

2- RF interference at your field. Since you report it always has an issue in the same spot, you may well have some localized RF issues at that site. Not uncommon these days. I have run across it at various fields. Something to be aware of. Your Spektrum buddies may have issues flying through that zone.

The combination of these 2 issues may be what you are seeing.

 

[gorc]

Thanks Wayne,

In my opinion, your suggestion # 1 is more likely than # 2.

For # 2, there are no other flyers around, no trees, nothing, only grass. This field is far from possible emissions.
The only thing I can imagine would be a small underground river nearby that acts as a shunt, but I really doubt it. It is a big sod field.

For #1: For sure I will need to open the receiver and check the 2 antennas. There might be something wrong with one or two antennas.
If one is damaged that would explain the directional failure (plane direction and attitude) when the plane is located at the failure position

What would be really interesting would be a way to check the antennas without opening the Rx, maybe by using the Tx with some sort of test.
If someone has a good test, please let me know.

Thanks,
Gorc

 

[gorc]

Wayne,

I've tried to make an electrical test but the tip of the antennas are way too small to probe.
Also on the receiver side, there is some sort of glue that makes it difficult to probe.
So I'm not sure if the antennas are electrically OK.
I took 3 pictures, 2 of the joint between the antenna and coax wire and one on the receive side.
One antenna seems OK but the other one may have a broken antenna wire (the 708 at the end), but I'm really not sure.
It's the best pictures I could take with my cell phone.
See pictures and let me know what you think.

Thanks,
Gorc

 

[jure]

.. the other one may have a broken antenna wire (the 708 at the end), but I'm really not sure...
Thanks,
Gorc

it looks like there is no continuity in the center conductor in that antenna.
Maybe an optical illusion ?

can you replace the antenna and test again ?

experiment / test suggestion:
assuming that the antenna in pic ...708 is bad, we may also consider that the other antenna is good.

In that case , you could completely enclose the supposedly good antenna in aluminum foil , and test again.
Then , if the assumptions are correct, the signal loss would be enormous ( like -40dB for example ) ! ,
( as none of the two Rx antennas would be fully operational )
So , the conclusion would be that the ...708 antenna is bad.

 

[HENNY]

it looks like there is no continuity in the center conductor in that antenna.
Maybe an optical illusion ?

can you replace the antenna and test again ?

View attachment 20773


experiment / test suggestion:
assuming that the antenna in pic ...708 is bad, we may also consider that the other antenna is good.

In that case , you could completely enclose the supposedly good antenna in aluminum foil , and test again.
Then , if the assumptions are correct, the signal loss would be enormous ( like -40dB for example ) ! ,
( as none of the two Rx antennas would be fully operational )
So , the conclusion would be that the ...708 antenna is bad.

The actual antenna is the bit at the end without black covering. It's normally 32mm in length, the black covered part is the antenna feeder. So if that 32 mm is damaged or shorter you essentially have lost the use of that antenna and would need to be replaced.

 

[alm_nl]

I'd also do a range-test without the heat-shrink tubing to see if it makes a difference. Sometimes heat-shrink can interfere with the signal because of what it is made of. If it does, use something else.

 

[gorc]

Thanks guys for your responses and suggestions.
It's not the heat shrink the problem because I made a quick test and in my other planes I have the same heat shrink and my RSSI is very strong.

I'm more and more sure the problem is a bad antenna.
I will make some tests with a faraday metal case and do a test similar to what jure suggested.
I will come back to you guys with results.
Thanks
Gorc

 

[HENNY]

Thanks guys for your responses and suggestions.
It's not the heat shrink the problem because I made a quick test and in my other planes I have the same heat shrink and my RSSI is very strong.

I'm more and more sure the problem is a bad antenna.
I will make some tests with a faraday metal case and do a test similar to what jure suggested.
I will come back to you guys with results.
Thanks
Gorc

One last suggestion. In Ethos it is possible to reduce the Telemetry RF power to 25mw. Not sure if this option is available in OTX. Go to the telemetry page and it hides in option in Ethos.

 

[gorc]

Hi all,

OK, I made a few tests and the only conclusion I can make is that one antenna is worse than the other one.
But I'm still perplexed about some dB numbers obtained.

I put the two antennas in a metal box with half of the black part of both antennas inside de metal cage/box and still I could read 77 dB.
77 dB is much higher than what I was expecting. I don't have an explanation for that. I was expecting something near the 40's dB.
My metal box had no big holes in it. Very small in order to pass an antenna. Grounding the box or not did not change anything.
Then getting one antenna out of the box at the time, I could clearly see a reduced dB (not more than 5 dB) for one of the antennas.
Not enough in my opinion to get a real concrete conclusion. The RSSI monitoring is not precise enough.

Then I made the "telemetry lost" by placing the antennas very close to the Tx (even if I have UNI-RX V83) and only one of the two antennas gave me the telemetry lost message. Interesting.
This is why I came to the conclusion that the antenna NOT giving me telemetry lost when super close was the bad antenna.

However, I don't understand how two antennas in a metal box (Faraday cage) would still give me 77 dB.
I have to mention that I was not very far (Tx to Rx), about 4 feet.

I just bought a few new antennas and will try them and see if there is a difference. I did not have any spares.

Any other thoughts would be welcome.

Thanks,

Gorc

 

[alm_nl]

Grounding the box or not did not change anything.

It was connected to the minus pole of the battery/bec/receiver as well?

 

[gorc]

Yes, this is where I connected to the ground. No effect if connected or not. dB readings were the same.

Does a floating metal box will stop 2.45Ghz? The metal box sides are thick, about 3 mm of solid metal.

My big mystery here is how can two antennas in a metal box still can get 77 dB and I get low RSSI in the field?

Gorc

 

[alm_nl]

Very strange, do other wires go into the box and if yes, did you shield these wires as well? If not, it might be that the signal is being transfered over the wires into the box reaching the receiver that way.

 

[gorc]

Yes, excellent question.

This is my explanation of the 77 dB.
The 2 standard antennas and receiver were inside the metal box but not the ESC servo 3 wires, they were 50% outside. These are not sheilded.
So, since the Tx is not far, these wires pick-up the Tx signal and they enter inside the metal box.
Once they are inside, they bounce like in a microwave oven and the real antennas will pick them up. In my opinion, this is the culprit.
For my next test, I will try adding aluminum foil on these wires to prevent the pickup of the Tx signals.
This might be cause of getting the 77 dB, these 3 wires act as antennas and propagate the Tx signal inside.
I will try this. But I'm not sure thin aluminum foil will be enough.

Does this make any sense?

Gorc

 

[HENNY]

Just to be aware only one antenna is in use at any given moment. The receiver will switch antennas based on the signal strength received. How far away was your TX from the metal box. At close range some of the RF energy will be absorbed by the box and re radiated, so put some distance between the box and the TX, I suspect the signal strength will fall off quickly.

 

[gorc]

Hi Henny,

OK I made a few tests, here are the results.

I put ann Rx components ESC + Rx + battery inside the metal box and now I got an RSSI around 58 dB which is much lower than 77 dB.
So some wires of ESC were radiating Tx signal to the antennas.
The distance between Tx and Rx for this test is approx. 4 feet and 30 feet.

However, by putting out one antenna at the time from the metal box, I was unable to get a clear picture of which antenna is good or bad.
They seem to be both the same. The RSSI is not precise enough to get a clear conclusion. With the two antennas out I had around 85 dB which is still a bit low.

When I receive my new antennas, I will make more tests but I'm afraid it might the Rx itself the S6R which is weak.
And these S6R are not easy to find.

So I have no clear conclusion, unfortunately.

Gorc

 

[HENNY]

Hi Henny,

OK I made a few tests, here are the results.

I put ann Rx components ESC + Rx + battery inside the metal box and now I got an RSSI around 58 dB which is much lower than 77 dB.
So some wires of ESC were radiating Tx signal to the antennas.
The distance between Tx and Rx for this test is approx. 4 feet and 30 feet.

However, by putting out one antenna at the time from the metal box, I was unable to get a clear picture of which antenna is good or bad.
They seem to be both the same. The RSSI is not precise enough to get a clear conclusion. With the two antennas out I had around 85 dB which is still a bit low.

When I receive my new antennas, I will make more tests but I'm afraid it might the Rx itself the S6R which is weak.
And these S6R are not easy to find.

So I have no clear conclusion, unfortunately.

Gorc

There is always the possibility that the receiver is fine and other factors are at play

 

[alm_nl]

Yes, excellent question.

This is my explanation of the 77 dB.
The 2 standard antennas and receiver were inside the metal box but not the ESC servo 3 wires, they were 50% outside. These are not sheilded.
So, since the Tx is not far, these wires pick-up the Tx signal and they enter inside the metal box.
Once they are inside, they bounce like in a microwave oven and the real antennas will pick them up. In my opinion, this is the culprit.
For my next test, I will try adding aluminum foil on these wires to prevent the pickup of the Tx signals.
This might be cause of getting the 77 dB, these 3 wires act as antennas and propagate the Tx signal inside.
I will try this. But I'm not sure thin aluminum foil will be enough.

Does this make any sense?

Gorc

Yes, that's why I asked.

Curious what will happen when you replace the antenna's.

 

[gorc]

There is always the possibility that the receiver is fine and other factors are at play

Hi Henny,

What other factors besides the receiver? The antennas were well placed on the plane and there was no interference on the field.
All electronics were well secured as well.

Thanks,
Gorc

 

[jure]

in the end, maybe there is a defect in the receiver .
The signal path from the antenna connector and the the RF chip itself may have some damage compromising the system performance.