Video and Datalink Antenna for airborne and ground tracking systems

Some developments on 2.4GHz video link and 868MHz datalink antenna systems:

          Measuring the 2.4GHz Antenna SWR

                         The Ground Tracking Antenna

This blog shows an integrated 868MHz xBEE datalink antenna and tranceiver and a 2.4GHz Video downlink antenna and transmitter as a single modular unit. Feed it 3.3volts and video as well as the autopilot datastream.

Also shown is a ground tracking antenna system. Two antenna are contained in the head - a multiturn 13dB gain helical antenna for the 2.4GHz video downlink, and a Diamond loop , over the ground plane, giving 8dB gain, for the 868MHz datalink. The antenna base contains a Lipo battery and fits onto a tripod. The base unit is then lined up with North, and the unit tracks the aircraft in flight using the A/C GPS position. Cable wind/unwind software is also implemented to allow the unit to unwind for example when flying circles around the GCS - this process follows a computed geometry to ensure minimum time pointed away from the A/C, and uses the aircraft flightplane to estimate the postion to slew to to start tracking again, if the link were lost.

We have just completed range tests, first with the aircraft mounted on a tall pole, on a high ridge in the desert here in Namibia. The aircraft was able to be rotated 360deg in azimuth and +- 25deg in elevation. Tests were conducted with the ground tracking antenna at 10km from the aircraft position.

Video TX power was 27dBM @ 2.432GHz, into the Skew Planar Wheel. Datalink TX was a 868MHz xBee Pro- range was tested at 25milliwat TX power ( can be set to 1mw, 25mw, 100mw, 200mw and 300mw - 1mw at 10km did not work)

Video was perfect picture with the aircraft at any azimuth or pitch angle ( combined) except for the Aircraft directly facing the ground antenna, at 0deg pitch - the A/C antenna is shielded by the avionics, the on-board IR gimballed IR camera, and the Lipo batteries. Tilting the A/C up or down 5 degrees brought the video back with perfect picture.

Datalink comms was constant with NO dropouts at 25mw bothe ends, regardless of the A/C attitude and azimuth heading. The aircraft was also fitted with 2.4meter carbon fibre covered wings - 315mm chord, and a carbon V tail ( very similar to my mk-I SurVoyeur A/C , only greater wings span and carbon wings.- see the SurVoyeur blogs..)

The Datalink antenna consists of two rings, spaced apart, and is a Slot Antenna, ie, the radiation takes place from the slot formed between the two rings. It is a narrow band antenna- not more than 20MHz bandwith for 1.3:1 SWR - at center frequency it is 1.05:1 SWR.  The first rings were done in copper wire to determine the antenna factors, and then a PCB version made on the router, at various sizes, to determine the size reduction factors due to the PCB dielectric constant.

Wire Rings Versus PCB Versions

An Integrated model was then machined on the router - this module consists of the antena rings and is fitted with the xBee datalink module and serial interface electronics.

                        

This module is later fitted with the Skew Planar Wheel 2.4GHz antenna, and with the video TX module.

The Skew Planar Wheel was constructed thus:

Pre-Cut element wires:                                                                       Wires formed in a 'stretch' jig.

                               Wires Stretched and removed from jig

Wires Formed.

                              Coax End pieces about to be fitted

Coax End Pieces soldered to coax outer and inner

      

                   

                Antenna elements fitted to the mounting Jig and soldered to the coax ends

The 2.4GHz antenna is then fitted to the 868MHz module:

The Video Transmitter is fitted on the underside:

This assembly is then fitted into the antenna bay of the composite fuselage:

SWR measurements were taken in-situ to determine any de-tuning:

SWR AT 2.44GHz:: = 1.13:1

The ground tracking antenna basics follow:

This Consists of a base unit containing the Lipo Battery, the drive electronics, an azimuth servo drive and feedback mechanical system and a yoke carrying the antenna head. The antenna head contains the 2.4GHz helical antenna, the 868MHz datalink Diamond loop antenna, the Video receiver, the xBee datalink tranceiver, and the elevation servo with position feedback.

The Head unit closed:                                                                   The base unit guts:

Azimuth Mechanics and feedback POT

                 The Helical and Diamond antenna in the head unit.

We then flew a max range test to see what a typicall max range might be - Video was lost at 39km, and datalink was still error free with 200milliwats at that range...Video was 'snow free' ( P4 signal) at 32km.

Joe

The Nampilot....

Views: 13667

Comment by Gagarien on May 21, 2013 at 10:47am

Great guys keep it up.

Comment by Paul Feely on May 21, 2013 at 12:26pm
Really impressive work - I love the combined transmitt module, lovely work.

When I looked at the 868Mhz xbees a long time ago they alway had a duty cycle issue - is that gone or do you work around it/ live with it

Cheers Paul
Comment by Gisela & Joe Noci on May 21, 2013 at 12:58pm

Hi Paul,

Since we wrote all the autopilot and GCS code, and since the system is totally autonomous during flight, we have data between A/C and ground only to give A/C position at typically every 5 10 seconds, if in basic flight ( with some other basic info - height, speed, batter, etc, but a very short message. Waypoint acquisition is sent when acquired  etc, so data rates are very low. When we are controlling a camera from ground, such as the IR camera gimbal, the rates increase from ground up, but in a 30minute flight, with 20minutes of camera manipulation, we only use 2.4minutes out of 30 for data transmit time, nicely below the 6minutes/60minute duty cycle. We are able to command a modem reset in an 'emergency' but steer  away from that as we want very much to remain fully within the legal ruling...

Comment by Paul Feely on May 21, 2013 at 2:06pm
Great thanks for the insight - less is more!
Comment by Earl on May 21, 2013 at 4:02pm

How about a 433Mhz telemetry and a 1.2Ghz Video combo. Can it be done?

Earl

Comment by Martin on August 14, 2013 at 4:44am

Could you elaborate on the choice of antennas? I can understand the circularly polarized antennas for video, but the slot and diamond antennas for telemetry. Never seen a diamond loop antenna before. Is it a cousin of the quad antenna?

The setup itself looks neat, but I'm wondering how you have avoided interference issues with the antennas and radios placed so close together.

Comment by Gisela & Joe Noci on August 14, 2013 at 1:13pm

Hi Martin,

The Diamond basically a full wavelength loop spaced in front of a reflector, giving very useful gain. There is no interference since the video is on 2.4GHz and the datalink on 868MHz. They are not harmonically related.

The setup works very well.

If you are interested a useful pdf is in the link:

http://w4rnl.net46.net/download/diamond.pdf

Joe

Comment by Martin on August 15, 2013 at 3:12am

Thanks, that was an interesting read. I can see now why you would use a diamond antenna - it's very compact for it's gain and easy to build. Btw, have you compared linear polarization to circular polarization for video transmission, noise and distance wise?

Comment by Gisela & Joe Noci on August 15, 2013 at 11:18am

Hi Martin, Yes, I have performed a number of tests - first we used a yagi , in vertical and also in horizontal polarization.

Any form of linear polarisation was acceptable at around 4 to 6km, but no more. At 6km we had severe signal loss when the aircraft rolled more than 20 degrees, and in level flight, the signal suffered from severe multipath cancellation with resultant signal drop-out.  Transmitting circular from the aircraft helped a lot, extending the range out to 9km, and eliminated most of the looses in aircraft roll, but multipath was still excessive. Circular at both ends gives us a comfortable 15km with no signal losses at all and no discernable multipath losses. Multipath starts playing a role again at around 28 to 30km.

These tests were done with a linear Yagi of 14dBi gain, and a 3dBi gain 1/2 wave dipole on the aircraft. The circular antenna then fitted to the A/C is the Skew planar wheel in the blog.

At 868MHz the Multipath effect is very much reduced, and can be ignored. However, we are able to operate linear for this, since the data rate is low ( 24kilobaud and a data packet of 120bytes every 120milliseconds), so the signal to noise ratio is high, much higher than the video channel can be.  The video receivers sensitivity are typically between -50 and -60dBm for a clear picture, while the xBee datalink is around -105dBm for a solid link.

Regards

Joe

Comment by Martin on August 15, 2013 at 11:40am

Joe, thank you for the information. I've read mixed opinion on circular polarization. I think you've cleared that up for me.

Am I correct in presuming that the slot antenna was chosen for it's compact geometry?

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