Any RF antenna design boffins around?

[hendorog]

The clover leaf is already modeled and verified to be a solid antenna, especially for FPV video uses at 5.8 GHz. The only real challenge is to build one that is properly tuned as some pretty detailed precision is needed.

That sounds rather contradictory though: if it's "solid", then why are people apparently having trouble building it to the exact frequency?  Why is the frequency so particular?

Like I said earlier: if the bandwidth is so narrow that it's hard to get it right, it's not a good design.  Good design means good everything: not just "oh well, you didn't build it right" -- a good designer must accommodate the tolerances of the manufacturing process!

Offhand, I don't see actual measurements and discussion of this, so I have no idea how troublesome this is.

Tim

Probably because we are talking about home made antennas, made using sidecutters. High precision is required at that frequency. This ain't HF band where a meter here or there doesn't matter. Materials matter too.
Construction and tuning empirically makes sense for these.

I think modelling means its been simulated in 4NEC.

I think solid means that thousands have been built and flown with good results.

[T3sl4co1l]

Sidecutters are one thing, but there are simple antennas (like conicals) with bandwidth of octaves.  You have to go out of your way to screw that up!

(Obvious problem: wideband antennas center around 150-200 ohms.  You need a matching circuit for 50 ohms.  Which tends to restrict bandwidth...)

Specifically for circular polarization and low directivity, I don't know exactly what would be most appropriate (a helical might be too directional, but is otherwise pretty wide band, and not hard to make).  Maybe a pancake spiral design?

Tim

[hendorog]

Sidecutters are one thing, but there are simple antennas (like conicals) with bandwidth of octaves.  You have to go out of your way to screw that up!

(Obvious problem: wideband antennas center around 150-200 ohms.  You need a matching circuit for 50 ohms.  Which tends to restrict bandwidth...)

Specifically for circular polarization and low directivity, I don't know exactly what would be most appropriate (a helical might be too directional, but is otherwise pretty wide band, and not hard to make).  Maybe a pancake spiral design?

Tim

Yep - I mentioned helicals earlier for exactly that reason. They are often used for the ground station but never on the aircraft.
Anything that goes on the plane needs to be reasonably omni and of course CP.

The other antennas sound interesting, I'll have a search around but do you have any links handy?

[T3sl4co1l]

This is the kind of principle that gets really high bandwidth,
https://en.wikipedia.org/wiki/Self-complementary_antenna
http://www.radioeng.cz/fulltexts/2006/06_03_15_19.pdf

A tapered impedance transformer / balun shouldn't be too bad at these frequencies, for only modest bandwidth.  And probably just one turn's worth of spiral would get the desired polarization, while having a more or less dipole radiation pattern.  It would be easy to cut from foil following a printout, or fabricated on PCB directly.  It could also be made from wires tacked together, but you'd need to make both sides of each element (i.e., trace the outer perimeter), plus cross pieces for stiffness and such.. it would be tedious to fabricate.

Regarding general bandwidth, the dimension of the elements is the first determinant there.  Using very thin wires leads to a high Q (and higher losses, and poorer coupling to space).  This is the typical case with simple wire designs, e.g. monopole, dipole, L, folded, rhomb, etc.  And, I'm guessing, the cloverleaf in this thread.  Wider, especially tapered elements help a lot, even just in one dimension (as the planar construction above).  Conical constructions, and fractal self-similar and self-complementary designs like the log spiral, log periodic, and so on, get very wide bandwidth, which might not be needed here, but shows the ultimate evolution of the concept, and the motivating geometrical principles.

Tim

[Howardlong]

Sidecutters are one thing, but there are simple antennas (like conicals) with bandwidth of octaves.  You have to go out of your way to screw that up!

(Obvious problem: wideband antennas center around 150-200 ohms.  You need a matching circuit for 50 ohms.  Which tends to restrict bandwidth...)

Specifically for circular polarization and low directivity, I don't know exactly what would be most appropriate (a helical might be too directional, but is otherwise pretty wide band, and not hard to make).  Maybe a pancake spiral design?

Tim

Yep - I mentioned helicals earlier for exactly that reason. They are often used for the ground station but never on the aircraft.
Anything that goes on the plane needs to be reasonably omni and of course CP.

The other antennas sound interesting, I'll have a search around but do you have any links handy?

FWIW I spent a good deal of the early 2000s working on circular polarised antennas for space applications. To make a reasonable helical you need to take care on the matching and feed point, and the mechanical fix. Be very careful of plastic standoffs that will have unknown dielectric and loss effects. That will substantially affect both the axial ratio and the directionality. I know that because of work we did on the antenna range with many antennas at the time particularly including helicals and CP patches.

In addition, the off-axis radiation will not be very circular for a patch or axial helical. In fact, the polarisation sense will typically reverse off-axis. That is why Lindenblads are often used in aviation on the ground segment.

One further thing, I don't think CP is used to reduce multipath for FPV, it's to reduce polarisation mismatch fading. I assume you're using linear polarisation on one antenna and CP on the other? That being the case, you already have a 3dB loss. However you are much less likely to see deep fades due to polarisation mismatch.

If both antennas were CP, multipath would still be a problem as reflected CP will be in the opposite polarisation sense, nulling the direct path.

[T3sl4co1l]

Ooh, that's pretty neat:
http://ww2.amsat.org/wordpress/wp-content/uploads/2014/01/70ParaLindy.pdf

This one uses folded dipoles:
http://newt.phys.unsw.edu.au/~map/weather/notes/buildyourown/lindenblad.html
which would seem to help with the matching issue also.

Dipoles are usually pretty modest bandwidth, at least when made with sufficiently sized elements.  I wouldn't think an array would be a problem, as far as bandwidth is concerned (the bandwidth of coupled resonators is dependent on the Q, and the coupling coefficient between them).

Tim

[Howardlong]

Ooh, that's pretty neat:
http://ww2.amsat.org/wordpress/wp-content/uploads/2014/01/70ParaLindy.pdf

Indeed, Tony was a bit of a polymath, he had his fingers in a lot of pies from antennas to embedded DSP, in many respects a man after my own heart. We used to meet up once or twice a year to exchange notes, and imbibed more than is strictly necessary as a result. Very regrettably he was taken from us quite early in life last year, a true gent who distributed knowledge and know-how, and is sadly missed.

[hendorog]

FWIW I spent a good deal of the early 2000s working on circular polarised antennas for space applications. To make a reasonable helical you need to take care on the matching and feed point, and the mechanical fix. Be very careful of plastic standoffs that will have unknown dielectric and loss effects. That will substantially affect both the axial ratio and the directionality. I know that because of work we did on the antenna range with many antennas at the time particularly including helicals and CP patches.

In addition, the off-axis radiation will not be very circular for a patch or axial helical. In fact, the polarisation sense will typically reverse off-axis. That is why Lindenblads are often used in aviation on the ground segment.

Thanks for the info.
The parasitic Lindenblad below looks interesting. I do recall reading about it previously in the context of modifying the stock dipoles which come with the low cost fpv gear.

One further thing, I don't think CP is used to reduce multipath for FPV, it's to reduce polarisation mismatch fading. I assume you're using linear polarisation on one antenna and CP on the other? That being the case, you already have a 3dB loss. However you are much less likely to see deep fades due to polarisation mismatch.

AFAIK most people use CP on both ends for multipath rejection. However, I'm sure some do what you suggest when doing FPV aerobatics.

If both antennas were CP, multipath would still be a problem as reflected CP will be in the opposite polarisation sense, nulling the direct path.

But surely the CP rx antenna and the reflection are cross polarised and so the reflection is attenuated?

I need to blow the dust off some of my gear and do some proper tests. I got a bit sidetracked by this new RF world I discovered, building test gear and experimenting and forgot about actually flying the planes around

Edit: The helical design which became popular for FPV is here:
http://www.rcgroups.com/forums/showthread.php?t=1377791

And the cloverleaf:
http://www.rcgroups.com/forums/showthread.php?t=1388264

[moya034]

Not sure if someone said this already, but in generally, thicker diameter elements in an antenna will make it more broad-banded. (It will cover a wider frequency range at an acceptable VSWR).

(I'm not really sure if this still true in the microwave freqs, but I know it to be true at HF/VHF/UHF)

[Howardlong]

One further thing, I don't think CP is used to reduce multipath for FPV, it's to reduce polarisation mismatch fading. I assume you're using linear polarisation on one antenna and CP on the other? That being the case, you already have a 3dB loss. However you are much less likely to see deep fades due to polarisation mismatch.

AFAIK most people use CP on both ends for multipath rejection. However, I'm sure some do what you suggest when doing FPV aerobatics.

Hmm, I'd have thought polarisation mismatch more than multipath.

If both antennas were CP, multipath would still be a problem as reflected CP will be in the opposite polarisation sense, nulling the direct path.

But surely the CP rx antenna and the reflection are cross polarised and so the reflection is attenuated?

On reflection(!) you are absolutely right. The reflected reverse sense is strongly attenuated due to the polarisation mismatch, it does not destructively interfere. CP at both ends makes sense for mitigating multipath in this case.

One thing to consider is that the further the drone is away, the lower the elevation angle for a given static height. My understanding is that there's a 500' AGL flying limit (at least here in the UK), so realistically having helicals or patches pointing up and down with (say) a 45 degree beamwidths isn't going to be great when the a/c is a little way away: you want greater gain at the lower angles, given inverse square law. Plus there's the complication that off-axis, the polarisation won't be very circular, and in some cases will even be in the opposite sense. Directly overhead, the distance is at its minimum, you don't need much gain overhead.

A Lindenblad is a good idea, but fabricating one for 5GHz will be mechanically challenging.

As a matter of interest are all FPV video links analogue? I can see why of you're trying to minimise latency of you're controlling the a/c, but I thought drones were self flying, or is this RC? If latency isn't a problem, a FEC digital stream would typically provide a better range and be less prone to multipath fading, but it would innevitably introduce latency as part of the temporal diversity scheme introduced to mitigate against burst interference and fading.

Sorry, I have no direct experience of drones and RC. (FWIW, I fly real aeroplanes ;-) )

[hendorog]

One thing to consider is that the further the drone is away, the lower the elevation angle for a given static height. My understanding is that there's a 500' AGL flying limit (at least here in the UK), so realistically having helicals or patches pointing up and down with (say) a 45 degree beamwidths isn't going to be great when the a/c is a little way away: you want greater gain at the lower angles, given inverse square law. Plus there's the complication that off-axis, the polarisation won't be very circular, and in some cases will even be in the opposite sense. Directly overhead, the distance is at its minimum, you don't need much gain overhead.

The altitude limit here is 400' - which gives 100' separation in theory. Not sure about the UK but given the common history its probably the same there.
The horizontal coverage is more important than vertical, as you say the angles become narrow the further away you are and so the area an antenna can cover without moving it is actually quite large. People use antenna trackers or multiple antennas and diversity to increase that area.
I gave up on all that and just mounted the antenna on the googles so I could turn my head when the picture got fuzzy

There has been a big revamp of the rules here in NZ, and flying beyond line of sight is now not permitted. Exemptions can be applied for but are costly and require safety plans etc. 

As a matter of interest are all FPV video links analogue? I can see why of you're trying to minimise latency of you're controlling the a/c, but I thought drones were self flying, or is this RC? If latency isn't a problem, a FEC digital stream would typically provide a better range and be less prone to multipath fading, but it would innevitably introduce latency as part of the temporal diversity scheme introduced to mitigate against burst interference and fading.

Yep, in general all analogue, although digital is becoming more popular as people experiment with HD. Latency is seen as quite important generally.
The quadcopter types all have a flight controller of some sort, usually with an autopilot. Depending upon the goal latency may or may not matter. Dodging in and out of trees is pretty latency sensitive, whereas flying a pre-programmed path for mapping isn't.

Sorry, I have no direct experience of drones and RC. (FWIW, I fly real aeroplanes ;-) )

Yeah I've dabbled with that too Ironically I had a near miss with an RC plane when I was a newbie solo student pilot.

[Richard Head]

I have found QFH antennas to be excellent. There's an excellent chapter on them in "Reflections 3" by Walt Maxwell.

[Psi]

There's new super sensitive receiver preamps you can get that work remarkably well to improve the signal.
Normal receiver = almost complete snow
Receiver with preamp = almost perfect picture.

[Psi]

As a matter of interest are all FPV video links analogue? I can see why of you're trying to minimise latency of you're controlling the a/c, but I thought drones were self flying, or is this RC? If latency isn't a problem, a FEC digital stream would typically provide a better range and be less prone to multipath fading, but it would innevitably introduce latency as part of the temporal diversity scheme introduced to mitigate against burst interference and fading.

Sorry, I have no direct experience of drones and RC. (FWIW, I fly real aeroplanes ;-) )

99.9% is standard definition analog. The few digital systems that exist are for when you absolutely need HD and they all cost far to much.  eg, HD system $2000,  analog SD system $10
The industry is still waiting for a low latency digital system.

Latency really is a serious issue, you cant delay it even by 1 frame when realtime control is needed at the speeds we fly around at. See video below.
PAL 25FPS is 40ms per frame. At 100kph if you delay the video stream by 40ms you are flying more than 1meter behind your actual position.

This is from onboard HD video camera, guy was flying from separate analog video feed.
Set youtube to 1080p 60fps if your PC can handle it, its worth it.

[Howardlong]

I am not sure of the legislation outside the UK, but my understanding is that here the a/c must be within line of sight vision of the operator, so under these regulations what would be the need for low latency video?

I am sure that the cost of digital will reduce, possibly a problem as well as latency is reducing power consumption and therefore battery mass on a light weight design. If extremely low latency is a requirement for remote RC flying, I'm not sure how realistically a digital system could improve over analogue in that scenario: digital systems are dependent on FEC to combat bit loss in both burst and gaussian scenarios, and work by sending redundant data with time separation, hence the latency. In analogue there's no attempt at error correction, which is why you get the fading and fuzziness, and your brain is left to interpolate the errors and see through the noise. In digital, well as we all know, it either works or it doesn't, there's nothing in between for us to interpret.

[hendorog]

There's new super sensitive receiver preamps you can get that work remarkably well to improve the signal.
Normal receiver = almost complete snow
Receiver with preamp = almost perfect picture.

Sounds like an LNA. I have one I got from ebay but never got around to trying it out. Do you have any specs for noise figure to get those good results?

[Psi]

I am not sure of the legislation outside the UK, but my understanding is that here the a/c must be within line of sight vision of the operator, so under these regulations what would be the need for low latency video?

bit of a grey area depending on country.
In NZ you must have an observer who can see the aircraft in order to fly it FPV and be legal.

[Howardlong]

There's new super sensitive receiver preamps you can get that work remarkably well to improve the signal.
Normal receiver = almost complete snow
Receiver with preamp = almost perfect picture.

Sounds like an LNA. I have one I got from ebay but never got around to trying it out. Do you have any specs for noise figure to get those good results?

I'd also say either the receiver is rubbish or there's an awful lot of loss in the cable between the antenna and receiver for the difference to be that much.

[hendorog]

I am not sure of the legislation outside the UK, but my understanding is that here the a/c must be within line of sight vision of the operator, so under these regulations what would be the need for low latency video?

I am sure that the cost of digital will reduce, possibly a problem as well as latency is reducing power consumption and therefore battery mass on a light weight design. If extremely low latency is a requirement for remote RC flying, I'm not sure how realistically a digital system could improve over analogue in that scenario: digital systems are dependent on FEC to combat bit loss in both burst and gaussian scenarios, and work by sending redundant data with time separation, hence the latency. In analogue there's no attempt at error correction, which is why you get the fading and fuzziness, and your brain is left to interpolate the errors and see through the noise. In digital, well as we all know, it either works or it doesn't, there's nothing in between for us to interpret.

FPV flyers wear goggles or stare at the screen and so can't see the model directly, the legislation usually requires an observer.

Therefore the LOS is just a safety thing. The rules are really not that well thought through however, as you should be looking out and listening for full size heading into your area and not observing the model. Full size are much easier to see and hear, its obvious thats what you should look for.
That fixation on the model is what caused the RC guy to fly his model in front of me when I was taking off.

Not many people actually understand the dynamics of what happens when RC and full size are in the same airspace. Things like parallax are a big issue for the observer. 

The power consumption of the video system isn't significant when compared with the consumption of the model motors. The nature of digital dropping out suddenly isn't ideal. A snowy picture is better than no picture at all

[hendorog]

There's new super sensitive receiver preamps you can get that work remarkably well to improve the signal.
Normal receiver = almost complete snow
Receiver with preamp = almost perfect picture.

Sounds like an LNA. I have one I got from ebay but never got around to trying it out. Do you have any specs for noise figure to get those good results?

I'd also say either the receiver is rubbish or there's an awful lot of loss in the cable between the antenna and receiver for the difference to be that much.

Yep I'd say that's fair comment, I suspect they are rubbish, but they are also cheap. The 5.8 ones are based on a single chip from Richwave from memory.
Edit: I am surprised the difference was that much though.

Edit2: This is how cheap they are
http://www.ebay.com/itm/High-Performance-5-8Ghz-Video-AV-Receiver-Module-for-FPV-/111827864924?hash=item1a0975cd5c:g:3vYAAOSwsFpWTZAz

[Howardlong]

That fixation on the model is what caused the RC guy to fly his model in front of me when I was taking off.

Well, nobody died. I was the pilot of a PA28 in an airprox earlier this year, this was roughly the equivalent to having oncoming traffic on the wrong side of the road. We had to take evasive action for fear of a collision as it was not at all clear what the other pilot's intentions were, he was certainly not following pilot 101 procedure inside an airport ATZ: although it doesn't state it in the report, I can only conclude he was lost. Very scary! http://www.airproxboard.org.uk/docs/423/2015039.pdf

The power consumption of the video system isn't significant when compared with the consumption of the model motors. The nature of digital dropping out suddenly isn't ideal. A snowy picture is better than no picture at all

I'd agree with that.

[hendorog]

That fixation on the model is what caused the RC guy to fly his model in front of me when I was taking off.

Well, nobody died.

Heh, yeah thankfully. There are limited options when you are low and slow and something large is whizzing around in front of you. Not sure if many pilots have worked out what they would do.

I was the pilot of a PA28 in an airprox earlier this year, this was roughly the equivalent to having oncoming traffic on the wrong side of the road. We had to take evasive action for fear of a collision as it was not at all clear what the other pilot's intentions were, he was certainly not following pilot 101 procedure inside an airport ATZ: although it doesn't state it in the report, I can only conclude he was lost. Very scary! http://www.airproxboard.org.uk/docs/423/2015039.pdf

Not nice. Steaming in on base like that is just not British