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Old Aug 18, 2012, 04:17 AM
Registered User
Australia, QLD, Brisbane
Joined Nov 2011
110 Posts
We're looking into cable assemblies !
RPSMA RF extension cables to make installation into airframes and ground installations easier.
I was thinking about two lengths:, 15cm, and 50cm.
15cm would be short enough to get out to the outside of an airframe with minimal loss, and 50cm would give some length to relocate antennas further away.
Longer cables could be made by stacking cables end to end.
Any thoughts and preferences on what you would like?
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Old Aug 19, 2012, 11:03 AM
Rana
Gurgaon - India
Joined Jul 2009
153 Posts
RFD900 tested for 58.5KM @57600 baud on APM1

RFD900 tested for 58.5KM @57600 baud on APM1, done ten times read & write of 50 Waypoints sucessfully, one location was at ground (285mt from sea level) and other location was in hilly area in Kasauli at 1865mt from sea level. 3dBi, single antenna used on both modems.

Amazing performance.

http://diydrones.com/forum/topics/rf...ource=activity
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Old Aug 19, 2012, 01:00 PM
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Joined Feb 2012
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Quote:
Originally Posted by narpat007 View Post
RFD900 tested for 58.5KM @57600 baud on APM1, done ten times read & write of 50 Waypoints sucessfully, one location was at ground (285mt from sea level) and other location was in hilly area in Kasauli at 1865mt from sea level. 3dBi, single antenna used on both modems.

Amazing performance.

http://diydrones.com/forum/topics/rf...ource=activity
Very nice, were you using an RFD900 on both ends?
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Old Aug 20, 2012, 01:34 AM
Rana
Gurgaon - India
Joined Jul 2009
153 Posts
Yes, used RFD900 on both sides.

Quote:
Originally Posted by epyonxero View Post
Very nice, were you using an RFD900 on both ends?
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Old Aug 20, 2012, 07:00 AM
Registered User
Canada, NS, Halifax
Joined Dec 2005
279 Posts
Which is better?

What is the output loss caused by connecting multiple and longer coax cables between the antenna and the module?

Is it a better solution to keep the antenna directly connected to the module and extend the 4 lines that would be required from the UART on the autopilot? i suspect that only +5, GND, Tx and Rx are required. Wouldn't a braided or shielded 4 wire cable to the remotely mounted module and antenna be a better RF solution rather than extending the coax?

As well the module and any RF it generates would be moved farther away from things like GPS and the aircraft receiver and any possible video camera and video Tx caused RF leakage. The only downside is the 800 ma current that the unit draws might require an 18 or 20 gauge wire if any distance is involved.
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Old Aug 20, 2012, 08:02 AM
Registered User
Australia, QLD, Brisbane
Joined Nov 2011
110 Posts
Regarding which is better - the answer is : it depends !

Depending on the antenna and mounting I would generally expect that having an external antenna on the airframe would give the best result.
Reasons being that for a 1/4 wave monopole - a ground plane is required.
The module itself does provide some RF grounding, but, having the antenna external, with a groundplane, is a far better and consistent solution for a monopole. It will also give the best gain and radiation pattern, which will far outweigh the loss in a short cable. Longer cables - it depends on the installation and requirements. As a general rule, shorter RF cables are better, but, depending on the installation, a longer RF cable may make it so much tidier.

Also having a degree of separation between antennas will generally improve the perfomance of all the radios as they typically generate noise in other bands.

Another thing to consider is desensitisation (desense). This is the reduction of sensitivity due to coupling of noise in the RF bands from the electronics on UAV into the antenna. Ideally, all the RF noise producing units (microcontrollers, servos, brushed motors, ESCs etc.) should be shielded and the antennas far enough away not to couple noise into them. Practically, having enough separation between the electronics and the antennas will help enough in most cases. There are some products in development, like servo line RF chokes, GPS band notch filters for transmitters, Antenna groundplanes, etc., which will give another way of reducing noise.

The modems from memory have quite little power drop with voltage drop, giving rated power down to around 4.5V or so. I can check the measurements tomorrow. That said, reasonably thick wire is better.

On the RF cables, I will be able to measure the cables once I get them - the estimated loss for a 15cm cable is around 0.2-0.3dB, for 50dm cable, maybe 0.5dB.

Yes, only 4 lines are required, GND, +5v, TX, RX.

Hope it helps !

Quote:
Originally Posted by small_rcer View Post
What is the output loss caused by connecting multiple and longer coax cables between the antenna and the module?

Is it a better solution to keep the antenna directly connected to the module and extend the 4 lines that would be required from the UART on the autopilot? i suspect that only +5, GND, Tx and Rx are required. Wouldn't a braided or shielded 4 wire cable to the remotely mounted module and antenna be a better RF solution rather than extending the coax?

As well the module and any RF it generates would be moved farther away from things like GPS and the aircraft receiver and any possible video camera and video Tx caused RF leakage. The only downside is the 800 ma current that the unit draws might require an 18 or 20 gauge wire if any distance is involved.
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Old Aug 20, 2012, 08:05 AM
Registered User
Australia, QLD, Brisbane
Joined Nov 2011
110 Posts
Hi narpat007 - Wow, that is an awesome result !
From those figures you should be able to exceed 100km using 2x 6dBi yagis at each end - providing there is clear enough line of sight between them... Hmmmm, wonder who will the first to break 100km
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Old Aug 20, 2012, 07:54 PM
Registered User
Near Austin
Joined Dec 2001
5,370 Posts
What about using some of IBCrazy's quads? Wouldn't that be better than a monopole?
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Old Aug 20, 2012, 08:04 PM
Registered User
Australia, QLD, Brisbane
Joined Nov 2011
110 Posts
The quads sound interesting !, haven't looked into those much at all.
What sort of gain could reasonably be expected?

I guess each case would be different, considering performance, durability, cost, availability, drag etc --
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Old Aug 21, 2012, 11:02 AM
Rana
Gurgaon - India
Joined Jul 2009
153 Posts
Hi Saario !
Infact I had taken 13dBi yagi with for testing this, I was thinking that for such long range, there would be requirement of 13dBi Yagi but I was surprised and it was just unbelievable to see the results without Yagi.

There is clear line of sight between the two tested points. because one point is at 1865mt from sea level.

In 1991, I made a 500 mW TV transmitter made of all hand made coils, and all discrete components Transistors 2N3866. At that time I was a school boy of higher secondary level.

At the TV side, I used high gain UHF booster with high gain Yagi antenna and got clear reception from these two points only from where RFD900 is tested.

Regards

Rana
Quote:
Originally Posted by Seppos View Post
Hi narpat007 - Wow, that is an awesome result !
From those figures you should be able to exceed 100km using 2x 6dBi yagis at each end - providing there is clear enough line of sight between them... Hmmmm, wonder who will the first to break 100km
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Old Aug 23, 2012, 07:49 AM
Registered User
Australia, QLD, Brisbane
Joined Nov 2011
110 Posts
Hi Narpat007,
Would you by chance have the rssi and noise levels for the radios at this range?
It would be interesting to compare it with theory to see how much link margin there was remaining.
If you use the commands
+++ (wait one second till you get OK)
AT&T=RSSI
it will give you local and remote RSSI and noise levels.
Cheers,
Seppo
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Old Aug 24, 2012, 11:34 AM
Rana
Gurgaon - India
Joined Jul 2009
153 Posts
Hi Seppo !
That place is in the country side, far away from my present location.
I have been to that place after many years.

Regards, Rana
Quote:
Originally Posted by Seppos View Post
Hi Narpat007,
Would you by chance have the rssi and noise levels for the radios at this range?
It would be interesting to compare it with theory to see how much link margin there was remaining.
If you use the commands
+++ (wait one second till you get OK)
AT&T=RSSI
it will give you local and remote RSSI and noise levels.
Cheers,
Seppo
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Old Aug 27, 2012, 03:43 PM
Keep calm and carry on.
Oso M's Avatar
United States, VA, Falls Church
Joined Jan 2012
1,352 Posts
Other than the info on this page: http://rfdesign.com.au/index.php/rfd900

Are there any more clear setup and connection instructions?

I'm trying to hook the modem up to a APM2 - and USB/Win7 on the Ground side.
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Old Aug 27, 2012, 04:56 PM
Registered User
Australia, QLD, Brisbane
Joined Nov 2011
110 Posts
Hi Oso M,
The pinout across the serial port is the same order as on the FTDI cables and 3DR radios. Their wiki has some nice pictures of a connection which is the same as the RFD900 across the serial port. http://code.google.com/p/ardupilot-mega/wiki/3DRadio
I will make a few good diagrams showing the connection to APM1/2 shortly.
Cheers,
Seppo
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Old Aug 27, 2012, 07:10 PM
Keep calm and carry on.
Oso M's Avatar
United States, VA, Falls Church
Joined Jan 2012
1,352 Posts
Quote:
Originally Posted by Seppos View Post
Hi Oso M,
The pinout across the serial port is the same order as on the FTDI cables and 3DR radios. Their wiki has some nice pictures of a connection which is the same as the RFD900 across the serial port. http://code.google.com/p/ardupilot-mega/wiki/3DRadio
I will make a few good diagrams showing the connection to APM1/2 shortly.
Cheers,
Seppo
Thanks. I think I've figured it out that far.

I have the two RFD900s powered up and am getting a solid green light. I assume that means the two radios are linked up.

When I jump the UART 0/2 pins from the APM to the RX/TX ins on the RFD (pins 7 & 9) I get a blinking red light, I assume this means I'm sending data.

On the RFD that is connected to my laptop I have a solid green light. and the data/activity light is blinking, but it is very very dim compared to the other modem connected to the APM.
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