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
@ Peppe,
Peppe, I believe one needs to define the surrounding parameters when quoting 'arbitry' numbers in order to make them quantitative and non-arbitrary. Two CIRCULAR polarised antenna that are of opposite polarisation sense, and having PERFECT axial ratio, ie, ellipticity of 0db, will have infinite loss when facing each other in the far field. This is the correct theory, as presented by Kraus and Johnson ( Antenna Engineering Handbook, 3rd edition, page23-7) It is VERY clear. I said a rejection of infinity for a perfect antenna, you said 30dB. After I indicated this is 'arbitrary' you changed your figures to >30dB which is still not true....For example, if one circular (clockwise) antenna has a 4dB axial ratio, and another (anti-clockwise) one has a 8dB axial ratio, then the minimum loss between them will be 4dB and the maximum loss 16dB, as one of the antenna is rotated 360degrees in its direction of predominant circular radiation. see the graph on page 23-9 of Johnson - it is clear. As the axial ratio of an antenna worsens, ie, becomes more and more linear, so the minimum loss will decrease, and the maximum loss will increase. This is shown when the two antenna are purely linearly polarised. When the RADIATED WAVE FRONT of each antenna is in the same polarization plane as the other one, the loss is minimum. When at 90degrees, the loss approaches infinity. This is NOT the same as merely having the physical antenna in the same plane, ie, both 'vertical' for a dipole, for example. There radiated wave fronts will ONLY be in the same plane if they are not affected by their surroundings, which can be approximated by raising them high and far from earthly objects. When the axial ratio of two antenna approaches infinity ( truly linear) the electromagnetic induction of like energy in a cross polarised condition approaches ZERO, ie, loss approaches infinity. This is fundamental Antenna Theory.
And it is the very fact that antenna radiation suffers cross polarization due to proximity to other object that allows the use of arbitrary cross polarised antenna on mobile platforms - there will be losses, but never infinite...
The' big wheel', Skew planar, DDRR, etc are all antenna that perform well enough for commercial use in the environment that all hobbyists play. I have been a Radio ham for 40 years, and have designed antenna for many commercial systems and military programs, for NATO, etc. I have a lot of test equipment at my disposal, but one can hardly expect the average hobbyist to try and develop a Quadifilar Helix and get the phasing correct using a multimeter....The big wheel and the DDRR work very well on my aircraft, and will perform admirably for amateur use.
We have a version of KIWIT ( also an A/C I developed and presented the prototype on this website) which sold to an Asian country in quantity for surveillence, using an on-board video camera and DDRR antenna, with a 1 watt video TX on 2.1GHz ( one of there military frequencies) . They achieve 15km range with a 5turn helical on ground station..
But if we wish to debate this further maybe we should resort to regular email - it is not really for this forum...
Take care
Joe Noci
73
De V51JN/ZS6JGN
very good, Geoffrey
I will do, as I will finish my X8. now it's only a flying machine.....I have already all components, but the FreeTime.......
anyway, the plan is: RC radio RangeLink 433MHz, videolink 5.8GHz, XBee for APM2.5 900MHz, another RC radio 2.4GHz
all antennas on board are vertical, bent-dipoles
on ground station, antennas will be true circolar, helix and patch.
I experienced helix very good decoding telemetry of OUR amateur satellite OSCAR-40, on 2.4GHz downlink, ranging more than 50.000Km (not a typo, 50 thousands of Km)
my professor said "everything works", meaning that, without any comparisions, every piece of metal could act as antenna.... so, no adjectives, but numbers....
airborne antennas have some limitations: air drag, weight, and good or better lobe at any attitude...
I prefer to have simplest antenna, lite, and really omnidirection, on board (a dipole have >95% efficiency)
commercial doesn't mean the best...... ;-)
I'm I8UZA since 1975, and KJ4UMJ (extra) ex Ka1nok, etc.) some world and continental records in HF, EME, satellite, up to SHF, etc.
chat soon again, keep u posted
best
Peppe
@ Joe
Can your share your source for the DDRR antenna? What size ground plane are you using under it?
@ peppe
Please share some photos and test results from your installation. I am always interested in what works best.
I'm sure there are a lot of interested community members who would benefit from your photos and actual test results.
There seems to be a good deal of everyday in field testing of the cloverleaf and skew planar wheel antennas by RC hobbists and from what i've read they seem to be out performing other set ups for a vareity of reasons.
I spoke with Kent Britain (creator of the Big Wheel) on the phone and his antennas are being used by News agencies to relay video from helicopters to ground stations, so the cloverleaf or at least his variation of it are being used commercially.
I too am an active amatuer radio operator since 1986, KB6WSQ.
Looking forward to seeing photos of your application.
just to reply to Joe
@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.
in clear band, data link is different respect to video 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.
how you do measure circularity?
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.
WRONG in theory and in real measures
This is not according to the theory, but therein lies the trap! Theory gives the loss between cross polarised antenna as infinity (false)- not 18dB or 30dB or some similar arbitrary figure.
ARBRITARY????? take a look to "Antennas" Kraus, and "Antenna Engineering" Jasik
pol mismatch:
hor vs vert ( > - 20 dB )
lin vs circ ( - 3 dB )
RHCP vs LHCP ( > 30 dB )
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.
for that, you should use true circular pol, and NOT cross pol, that is another thing, as you know.....
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!
I had extensive test, in last 35 years, in anecoich room, free space, satellite link, and daily use, and theories are always proved and confirmed by measures and observations. Having a true circ pol antenna at one side of link is the best way to avoid fading and nulls. My helicopters use quadrihelix antenna to have a stable datalink at any banks, and a bent-dipole or bent-vertical to avoid "holes" in COM. never seen poor antennas as wheels, cloverleaf, etc, in pro use. I have seen, as very simple and useful, in amateur radio (i'm licensed and very active, since 1975 in Italy and USA)
Nice Big wheel there Geoffrey, but try use some of the low loss Rogers PCB - you will gain a dB or so...
good advice, use always good material
Regards
The Nampilot...
Joe
anyway, Joe, thanks for sharing info and experiences, it's the only way to get better
hoping we could continue
best
Peppe
No single antenna is perfect for all situations. Until recently diversity on the AC side wasn't readily available, however the RFD900 1 watt data radio has change this. The diversity reception in this unit is on a packet by packet basis with transmit using the most recent receive antenna selection. No longer stuck to one antenna we can now deploy a combination of antennas and polarizations suitable for what ever condition the flight calls for.
@ Gisela & Joe
Thanks for the great comments and sharing your knowledge.
Thanks Joe, I looked through the ICASA website for regulations regarding 1-1.3GHz band and drew a blank, in the documents I looked through I could find no mention of these bands.
Have been using an inverted vee on the vTx and dipole on vRx and have had range out to about 2.8km and on that same flight the standard Hitec radio was also still fine but on a subsequent flight the Hitec's range as mentioned was only 900m so something has changed, I must go through my system taking what you've posted into account.
Thanks again
@Graham
Hi. I do not know of any commercially available antenna types of the above for 1040MHz - I have a feeling that band is not a 'license free' band either? The various license free bands cover 433MHz, 868MHz, 900MHz, 2.4GHz, 5.6 or 5.8GHz, etc, some valid in some countries, some not...Antenna availability as with all else depends on market and sales, and the markets are with the popular frequency bands..
Re the Hitec 2.4GHz -
There are two issues here. Antenna wise I recommend a GOOD quality patch antenna on the radio - not one made from standard FR4 printed circuit material, but from the low loss Rogers PCB laminates. Standard FR4 at 2.4GHz can cut the antenna efficiency by half... Fit the antenna on the top of your radio so that when your radio is slug in front of your in the working position, you can easily aim the antenna at the aircraft direction.
Then in the aircraft fit either a big wheel or a skew planar for best results. This should get you 2km easily..
BUT - a problem on all electric aircraft is the electrical noise generated by the motor ESC. The 3 esc phase wires to the motor carry high currents - 10 of amps, switched at high frequencies, up to 44KHz in some instances. That in itself makes a very efficient very broad frequency RF generator, with frequency harmonics radiated up into the GHz region. Although these harmonics are not at the precise RCS receiver frequencies, the RX input can be swamped by the energy in the radiated noise and the result is that the receiver is desensitised, ie, you received signal has to be stronger than the noise for the receiver to see it. So your 'poor' range could be also as a result of these effects. Try to keep the RX antenna FAR as possible from the ESC - try to keep the wires between the motor and ESC as short as possible - say a few cm at worst. Do that and then try better antenna if still unhappy....
Regards
Joe
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