Antennas now has its own section in the Forums
There are alternative antenna choices for your UAV when space is a consideration. RHCP antennas are without doubt one of the best choices you can make for your video and telemetry signals. I had wanted to use a RHCP antenna for telemetry for my X-8 but the choices I had come across were pretty space demanding. Surfing the web doing some antenna research I ran across these wheel antennas from Kent Britain WA5VJB, a fellow amateur radio operator. www.wa5vjb.com He's got some really useful designs other than the wheel.
I purchased 2 of the 900 mhz variety for the telemetry, one for in the X8 and one for the ground station . Since the plan is to install all antennas within the X8 these are a great space saving choice over the current offering of cloverleaf antennas. The 900 mhz wheels measure about 4.5 inches in diameter, somewhat smaller than a CD ROM and only slightly thicker. The best part being they are only $ 12.00 each. They can be fed with coax directly or as seen in the photograph you can solder a circuit board SMA connector of your choice to the center in the holes provided.
These wheel antennas are considered to be horizontally polarized at the plane of the horizon however as your UAV gains altitude the wheel antenna emits a cross polarized signal. Here is a quote from hamtv.com “Around 1990 the first launch of a balloon borne payload in the Denver, Colorado Area took place sporting a Little Wheel as the radiator for the on-board ATV transmitter. It was found to work quite well and had some interesting attributes that were previously undisclosed. Those being that the Wheel antenna although being a horizontal radiator on the horizon displayed properties of cross polarization above and below the array - similar to two in phase horizontal dipoles. This worked to the advantage of the ground station by employing a circularly polarized antenna (Helix) - also a fixed horizontal antenna directly below at the launch site would not see the up to -30 dB nulls as the balloon rotated. The horizontally polarized tracking antennas (Yagis) were receiving a signal of constantly changing polarity as the payload rose to an altitude at which it would start to drift away from the launch site and the horizontally polarized tracking antennas would become more effective due to the horizontal polarization emitted from the sides of the Wheel. The pattern of the Wheel is within +/-0.5dB of being a perfect circle in the E-plane. In the H-plane the pattern is +/-35 degrees of the horizontal axis of the antenna. These figures equate to ~ 4.28 dBic gain or 6.38 dBdc. The dBic gain above and below the array is somewhat indeterminable due to the fact that the circular polarization exhibited is quasi-circular or cross polarized; however, it responds to a Helix in a manner consistent with the gain of a pair of like sense Helices in a system looking at one another. ”
This antenna does radiate well (60% efficient) and also radiates well in the second and third harmonics so take precautions to provide filtering for your transmitter.
You’ll want to use a “Wheel” antenna on each side of your link as losses will result if mated to just a dipole when your UAV is overhead. Just mount it in a horizontal position at each end of the link. If it fits inside your UAV that’s great but it could also be mounted flat on the wing as well.
Holes are provided for SMA circuit board jack mount
or direct connection to coax.
This link shows the antenna being tested in a 3D pattern.
https://www.youtube.com/watch?v=vVW9jZG1vI0&feature=player_detailpage
i'll post my actual installation once I get there.
Keep experimenting with radio, get your tech license, it's easy and you'll be contributing not only to this great hobby but ham radio as well.
Comments
Fascinating stuff, to me this is invisible black magic...lol.
If I may ask a question or two, what easily available antennas should I be using on the FOX 700mW FPV system (I'm using 1040MHz) for best range.
What can I do to maintain my Hitec 2.4GHz range using the above system as the last time I tried long distance it went into radio failsafe at about 900m?
@Peppe
@Geoffrey
Hi, Joe from Namibia...I use the following antenna on our Survey UAV ( SurVoyeur -) have a look at my posting on DIYDrones)
1) Simple 1/2wave vertical commercial dipole ( it is simply a coaxial sleeve dipole - costs $18 US), one on ground station, one on A/C, with XBee Pro - 868MHz modems ( legal in our part of the world). With this combination, @ 300mW TX power, we achieve 16km range with solid link.
2) Big wheel -horizontally polarised -( also a design on PCB, but using Rogers low loss materials - normal PCB - FR4 type - has losses to high at UHF) on the Aircraft, DIPOLE ( still vertical!) on ground, range is very much the same - 14 to 16km.
3) DDRR ( Dual ring Radiator) @ 868MHz it is 50mm diameter and 8mm tall, vertically polarised - gain is 1dbD ( ie 1dB over a plain dipole) - perfectly omnidirectional. Again, range with a ground dipole is around 16km to 18km.
4) Super J Pole Collinear antenna on ground station - This is a 'normal J pole @ 868MHz with a phasing coil atop it, and another 1/2wave section added. It has an omni radiation pattern, vertically polarised, and is 500mm tall. Gain is close to 6dBD. Range with this antenna on ground and tested again with each of the antenna in (1) to (3) gave a range of (1)-24km, (2)-22km to 23km, (3)- 26km.
We use this aircraft for land surveying and fly photogrammetry mission - photographing up to 350hectare areas in one mission, which gives around 950photos...we also fly pipelines/road survey missions, with a flight up one way and back down again, taking photos along both ways - up to 50km mission, ie, 25km away and back again, so need reliable datalink coms.
We use the Jpole on ground with the DDRR on the aircraft now - the DDRR is very small and very easy to fit.
I also built a skew planar wheel on 868MHz and measured the radiation pattern - with the typical flight heights we work at - 200meter AGL to 300meter AGL, once the A/C is more than 2km away, we are very nicely within the circular polarisation of the Skew planar on the aircraft. The measured patterns show VERY good circular pol. at elevations of 45deg above and below horizontal from this antenna. At 60deg below horizontal it is strongly elliptical, and directly below a mix of patterns. This is not relevant, as if you are below the aircraft you are also very close. I prefer this antenna above (1) to (3) - helps strongly to eliminate multipath reflections if you use one on the A/C and one on ground, but is is tricky to set it up, and it detunes easily in the fuselage due to proximity of composite materials ( dielectric effects).
Now, before you all show great alarm at my results with cross polarised antenna -
The fact that the polarisation is 90deg 'shifted' has almost no effect in the link range. This is not according to the theory, but therein lies the trap! Theory gives the loss between cross polarised antenna as infinity - not 18dB or 30dB or some similar arbitrary figure.
However, radiation pattern measurements ( in an EMC chamber) show clearly what happens in a typical antenna installation, both in the A/C and on ground. At 868MHz and up, the radiatian pattern is easily distorted by the environment around it. Placing the datalink module on top of the roof of your car gives the antenna a reflection plane that it was not intended to work with ( at least not any of the antenna in 1 to 3 above). This distorts the pattern, normal upwards, but also creates cross polarisation effects, reducing the 'losses' between cross polarised antenna. Placing that uplink antenna on a tripod, but perhaps 2 meters from the metal sides of your car causes large polarisation shifts from multipath reflections. If you use a small metal type camping table to set up your station out in the field, with the datalink module on the table, similar effects occur. So, you never get true single polarisation from the antenna, UNLESS you place it in 'free space' ie, at least many tens of wavelength up above the ground surface, with no reflection planes nearby. All these effects are even more exaggerated when the frequenciy is increase, eg, 2.4GHz or 5.6GHz. The same occurs on the aircraft - long servo leads, battery cable, etc, begin to act as reflectors and radiators, distorting the patterns significantly.
Since the A/C are small in wavelength terms, there is no such thing as true linear or circular polarization from an antenna on the A/C. The antenna cannot be placed far enough from other elements on the A/C.
I have extensive antenna test facilities and design antenna mainly for airborn applications, so it has been an interesting exercise testing the above antenna. I have done similar designs at much lower frequencies ( aircraft coms frequencies and military coms frequencies) on helicopters and fighter jets - at least the comforting factor is that the results are the same, no matter how big the A/C! Of course the bigger A/C are normally of metal construction, so antenna placement is not simple!
Nice Big wheel there Geoffrey, but try use some of the low loss Rogers PCB - you will gain a dB or so...
Regards
The Nampilot...
Joe
More Antenna Alternatives
Found this great product at Sparkfun. 5mm adhesive backed copper foil thats easily soldered and great for making dipoles, vee's or helix antennas for your UAV projects. Mount to fiberglass, plexiglass or directly on your foam plane (avoid the CF spars and any wiring). https://www.sparkfun.com/products/10561 $2.95 for 50 feet.
@ peppe
Thanks for correcting my math for the phasing. Although 3db loss is loosing half the signal. Like antennas, be they quasi circular polarized, like the wheel, or other wise = 0db loss. Its all in the blog.
@ geoffrey
tnx Geoffrey for replying, but I disagree.
180 degrees is the same polarization.... we are talking about vectors 2 dipoles at 90° must to be feed with 90°delay (phisical, electric or a summa of both).... in the 3 elements of wheel I cannot see any delay, so, no plane of vectors change, and no circular pol.
we are speaking of airplanes, so, very different attitude. The best antenna is isotropic antenna, but..... it doesn't exist.... the only way to get closer is a bent-dipole, that is a little worse than a dipole, but no "holes"....(almost)
loss between hor and vert pol 18dB
loss between LH and RH circ pol 30dB
loss between linear pol. and circ. pol. 3dB (signals always present)
I don't like cloverleaf or skew planar (they are not circular polarized, they just have some vectors in different planes, it's not the same)
finally, the best couple is bent-dipole+helix (or circular patch), that is what is used in aeronautical, military and space applications.....
I'm sorry for my poor english..... I hope to talk about more and better
Peppe
@ peppe
If the elements were half wave there would need to be 180 degree phasing lines between the elements to get circular polarization. However these are full wavelength elements and this antenna is a more 2 dimensional representation of the cloverleaf or skew planar wheel which are circularly polarized either right or left depending on the direction of the winding (current). If I had a choice between polarization of strictly dipoles I would choose horizontal over verticle on crowded bands where most everything is transmitted vertically, the 30 db inherent loss over difference in polarization would be of great benefit. This is why it is important to use like antennas. Receiving a vertically polarized signal with a helix versus receiving it with another helix results in loss of signal strength.
@ Veikko Vierola
Here is a photo that thru transparency shows the trace on the reverse side of the board. It is
3 full wave sections.
hi guys
the best "match" is using a vertical bent-dipole (on board) and an helix axial-mode (on ground)
for short distance, a circular-pol patch or a quadrihelix is allowable @ ground station
Big Wheel is essentially a horizontal pol, omnidirectional antenna. Poor antenna
no circular components because no delay lines between arms.
I like L-com Antennas and Amplifiers. Lots of antennas to choose from including LHCP and RHCP in 900, 2.4 and 5.8 bands. Reasonably priced and great technical help and returns. Used them for 11 years and the original equipment is still operational.
link
http://search.l-com.com/search/keywords-Circular_Polarized_
Oh, guess I didn't look close enough... I might try that 5800 patch, but I still wish there were more options out there for 5.8ghz. Anyways, nice find!!
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