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....
Comments
Hi Daniel,
PM me and I will send you the PDF files.
Regards
Joe
Hi Joe, I'm trying to find some more information on the diamond antenna, unfortunately the PDF file you posted on the previous page of comments is no longer available. Could you point me to an information source?
Thanks,
Daniël
Please post drawings for 868Mhz antenna. It would be great for my RC system
http://diydrones.com/profiles/blogs/rc-system-on-868mhz-based-on-op...
That would be great. I'm also using 869 MHz for telemetry and 2,4GHz for video.
Hi Martin,
How do you mean 'share' ? I can give you PDF drawings with dimensions, etc if that is what you want, for the circular slot antenna - for the skew planar wheel, I can give some drawings that give all the relevant dimensions and then some photos of the antenna assembly in the jigs I made. The pics in the blog do sort of show all that..
Obviously this is for 2.4GHz and for 869.5MHz for the datalink antenna. Other frequencies require the usual development to get the dimensions right...
Joe
The more I look at your design the more I like it. Would you mind sharing the design files for the antennas?
Martin, yes, the two rings so spaced form the dual of a vertical dipole, in other words, the 'slot' radiates , and the slot is horizontal, but the radiation is vertically polarized. It makes for a very compact antenna, easily placed within a composite ( non-carbon..) fuselage and gives a tad over 3dBi gain. This structure also allowed a neat concentric mounting of the skew planar wheel, making the whole unit compact.
In my professional life ( some years ago..) I did a lot of work in the RF field, Spread Spectrum digital comms systems, and designed many antenna systems, mainly for military type craft, so a lot of experience gained then stands me in good stead now!
Regards
Joe
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?
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
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?
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