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I am currently working on the ground station for my FPV-project, and need help to decide whether I need a diversity receiver system or not.

About the project:

The transmitter will transmit video from a KX131-camera with overlay from a custom made BOB-4 based OSD on the video channel, and APRS-packets from a TinyTrak3 on the audio channel.

The APRS-packets will be used to calculate azimuth and elevation for the receiver antenna(s). I have built Bob Ball's PIC-based TNC, and is allmost finished with the rotor controller and az/el-calculator. The software is made using basic for PIC microcontrollers (a PIC18-series microcontroller will be used due to advanced trigonometric calculations and lots of float-type variables), and show the calculated angles on a 16x2 LCD-display. The rotor itself uses a 180/180-degree az/el setup with standard RC-servos, which means the antenna will be upside down when accessing azimuth-angles from 180-360 degrees (I will build a more rigid rotor with small geared DC-motors later).

Diversity or not?

Because of the precise antenna rotor, I will use high gain antennas with 30 degree beamwidth or lower. This means I will receive a minimum of reflections from the ground, nearby buildings eg. So, is it really necessary with a diversity system when the antennas have such low beamwidth?

I was also thinking about polarisation-diversity; either LHCP/RCHP, or vertical/horisontal. What do you think about this?

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Polarisation-diversity would be the only advantage. If you are flying distant gentle missions then I say don't bother but if you fly 3D or over head i say go for it.

Terry

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This schematic explains my setup much better...

post-8-1194691667_thumb.jpg

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Polarisation-diversity would be the only advantage. If you are flying distant gentle missions then I say don't bother but if you fly 3D or over head i say go for it.

Terry

Ok, thanks for the reply.

I will fly non-acrobatic and distant.

Another problem just popped into my head:

The rotor will be refreshed a little less than once a second, so when landing near the rotor, it can get problems following the fast motion. This could be solved by a low gain antenna not being controlled by the rotor, fed into the diversity-controller along with the other high gain antenna.

Again, I am wondering if this is neccesary... When the plane is so close to the high gain antenna, I'll guess it will get a good signal strenght no matter where the antenna points.

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When the plane is so close to the high gain antenna, I'll guess it will get a good signal strenght no matter where the antenna points

Hi jonpet,

That's right. Just don't forget about dropouts due to reflections, wherever they may come from.

I strongly recommend to go for a diversity solution. Interference occurs easily and when I watch the leds on my diversity system, it sometimes switches so fast between the two antennas because the wavelength at 2.4GHz is just about 13cm and dropouts are easily produced.

I recommend a combination of a high gain vertically polarized antenna (14-15dB Patch or a 17dB Yagi which matches exactly the frequency of your videoTX because of their narrow bandwidth), and an 8dBi Patch or even an omnidirectional rubber duck for the near field. I had excellent results with this combination, even in urban environments with LOTS of other RF sources around, as there were very strong wireless room to room video transmission systems in the nearby houses, WLAN, Bluetooth, GSM,... Make sure you're the only one on "YOUR" channel, and you'll be happy with this setup :rolleyes:

I'm very interested in your antenna tracking solution and I would love to see how you proceed and to see the results. I was thinking of exactly the same setup with the TinyTrak and so on. Just one little difference, I considered a seperate TX for the APRS to make sure it ALWAYS gets to you, regardless the video link. That's because you need the GPS data to control Az/El of the antenna for the video link, so you must not loose the audio link.

Cheers,

Hartwig

Edited by Hartwig

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I think the question should be 'why not' go diversity. The added cost, weight power used will not be great for your set up.

Terry

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Hartwig, your post allmost completely changed my mind... ;) The question is if I really want to pay 200$ extra to eliminate a few dropouts when landing. When I fly at high altitude (as I will do 95% of the time), the diversity makes no sense.

Another important factor is that I will need to reconstruct the az/el rotor to hold two antennas.

By the way I have just found out I will buy the HeadPlay video googles. Yes, they are expensive, but they are the only option because of my rather wide interpupillary distance, making the cheap video googles with 63mm IOD unuseable to me.

So, the HeadPlay costs 500$, two lawmate receivers plus a 1000mW transmitter cost 215$, the Oracle diversity controller cost 150$ and the antennas cost 100$. That makes 965$ only for the AV-system excluding OSD and antenna-rotator... Way too much... ;)

Today i have taken a few photos of the rotor so far, shifted the oscillator on my BOB4 from 75MHz to 74.25MHz (the soldering was easier than expected :) ), and got my last order from HobbyCity. These parts will be used in my TwinStar II.

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Edited by jonpet55

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When I fly at high altitude (as I will do 95% of the time), the diversity makes no sense.

If you mean you are flying almost overhead then Polarisation-diversity would be an advantage.

Terry

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I can see I expressed myself a little wrong. I meant traditional diversity setup to eliminate problems caused by multipathing. I think I have found a solution to eliminate the polarisation problems. I'll use a circular polarized antenna instead, then I will loose a constant 3dB instead of up to 24dB. Another problem is finding a high gain circular polarized antenna. I think I'll have to make myself a helix (a crossed yagi for 2.4GHz is just too much job)...

Edited by jonpet55

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I have great results with this one (vertical polarization):

http://www.lechner-cctv.de/artikel_einzeln...q7bc8t1plavdjm4

...and this one (circular polarization):

http://www.lechner-cctv.de/artikel_einzeln...q7bc8t1plavdjm4

I can help with translation, in case you need.

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Great! The last one was about exactly what I was looking for! Pretty high gain and circular polarisation. Danke, mein Freund! :D

Edited by jonpet55

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I'll use a circular polarized antenna instead, then I will loose a constant 3dB instead of up to 24dB.

Yes that makes sense if you are happy to loose 3db, I will be interested to hear how circular the patch is.

Terry

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Well, yes, with 3dB loss I will loose half the power, but there are so many other factors I have to consider. Price is a very important one. :rolleyes: Either one antenna with circular polarisation and one receiver, or two antennas with linear polarisation, two receivers and an Oracle...

Instead of using lots of money on the polarisation diversity setup, I could just buy an even more directional antenna (17dB instead of 14dB).

I am considering changing the transmitter antenna too. I'm thinking about an antenna similar to the one I used in EOS1 (search for it here). It's radiation pattern was allmost like a dome (or half a ball), and it had allmost equal components of vertical and horizontal polarisation. The groundplane spaced 1/4 lambda from the dipole made this behavior.

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Look what I found!

Very cheap too; 25$. I think I'll order one of those.

Edit: Seems like the link doesn't work. Go to http://ebay.com and search for "Direction antenna Helicall18 circular polarized wi-fi"

Edited by jonpet55

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but there are so many other factors I have to consider. Price is a very important one.

Sorry :unsure: I miss understood from the start, when I saw the set up you were planning I got the impression that cost was not important, my mistake :(

But I agree if you are going to use a helical then diversity is not needed :)

Terry

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Sorry :unsure: I miss understood from the start, when I saw the set up you were planning I got the impression that cost was not important, my mistake :(

But I agree if you are going to use a helical then diversity is not needed :)

Terry

No prob Terry.

By the way, the helix I found above comes from a page called yagi.pl. They have extremely cheap antennas.

Helical antennas: 10$ to 19$

http://yagi.pl/product_info.php/cPath/50/p...products_id/885

http://yagi.pl/product_info.php/cPath/50/p...products_id/715

http://yagi.pl/product_info.php/cPath/50/p...products_id/787

Parabolic reflector: 8$

http://yagi.pl/product_info.php/cPath/50/p...products_id/658

Parabolic reflector with 2.4GHz antenna: 26$

http://yagi.pl/product_info.php/cPath/50/p...products_id/833

Yagi antenna: 23$

http://yagi.pl/product_info.php/cPath/50/p...products_id/886

To convert from zloty to U.S. dollars, go to google, and type: "xxx PLN in USD"

Now I know where to buy all my antennas... :D

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I would like to try the 18dbi helical as I have never been able to make one with that much gain. Don't understand a word of it though :)

Terry

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Not me either, but the page follows a standard template for thousands of webshops, so you could just go to a similar page in english to find out what the different buttons do. Much of it is self explainig too, like "Anteny kierunkowe 2.4 GHz" (2.4GHz antenna).

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Great! The last one was about exactly what I was looking for! Pretty high gain and circular polarisation. Danke, mein Freund!

You're welcome! :D As I see you discovered a lot of nice alternatives already :rolleyes:

I was very much impressed when I read about your EOS1 a couple of months ago. I'm curious on the performance of the antenna tracking system.

How will you send the APRS data down to the ground station? Will you use the audio channel of the videoTX or an extra TX?

Will you add tracking equipment to your plane as well, as an emergency backup if the link gets interrupted?

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Nice to hear your response about EOS1. The project amazed me when it comes to how far it is possible to reach with the same kind of radio equipment we use here (near 80km with Lawnmate RX & 600mW TX).

The antenna tracking system is working really good so far. I have only tested azimuth though. The APRS data is sent through the audio channel of the video transmitter, decoded by a single PIC TNC, then az/el angles are calculated in another PIC. I am planning on building a simple rudder "return to home" autopilot too.

The RC signals will most probably be transmitted on 433MHz. I will try to transmit the raw modulated PPM signal from the trainer port with a TM-D700, then receive it with a small Alinco handheld radio and decode it with a PIC PPM decoder. If this doesn't work well I will find another more complex solution.

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Very nice! For the RC it might also be possible to use a part of a conventional RC Receiver which gets the PPM data from the handheld radio and does the decoding part, including Failsave and so on. Just an idea, but it will probably be a lot more fun do that programming yourself ;)

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Actually, I will not make the PIC PPM decoder myself. When I started to find a new solution for the RC transmittion, I found out that I was making too much job of everything. My main goal was to design the antenna rotator system and the OSD in the plane, and make a nice FPV-setup that I cound finish in a reasonable time. I didn't had to make every part of it myself. So not to reinvent the wheel, I started to search for someone else who had allready made what i needed: A simple PIC PPM decoder, but with failsafe. I found this one: http://homepages.paradise.net.nz/bhabbott/decoder.html. As you said I could also take the PPM demodulation circuit from a standard RC RX, but I actually don't have any microprocessor decoded receivers (just with simple shift registers), and it would be much less expensive to program one of my PIC's with this firmware, than buing a receiver and ripping it apart...

Edited by jonpet55

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I use these simple decoders from Micron in my UHF R/C radio recievers. Look under "DSP Upgrade Kit".

Terry

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