PPM-fed dipole for long-distance flights...

[AnthonyRC]

Yeah, the title sounds a little strange, but here's the idea:

In order to have a little fun with long-distance FPV flights I am more limited by 'uplink' range than 'downlink' (thanks to 17dBi yagi-style 2.4GHz antenna at the Rx end of the downlink).

So first thought was to modify the Futaba 9C transmitter to install a BNC connector (or similar), but I didn't really want to go drilling holes, or modifying a perfectly good transmitter.

Second thought was quite elegant, or at least it seems that way for the moment. Build a simple 72MHz dipole (using 2 x ~1m alu poles), mount the 72MHz RF module in the center along with suitable LiPo, and run a thin coaxial cable from the PPM input to the buddy-box connector of the Tx.

Advantages...
- No coaxial cable loss (RF feed cable could be an inch or so long)
- No need to modify the transmitter
- Very simple to build

Any opinions/suggestions/recommendations/interest?

[Mr.RC-CAM]

In the USA, these sort of things have been done to 6-meter ham R/C Tx's (we aren't allowed that on 72/75Mhz). A high gain antenna, sufficiently mounted off the ground, will certainly improve the range. But even so, there is so much room for improvement on the Rx side, it might be best to start there before hacking the Tx.

[vintage1]

For really long distance, consider running it at a few megawatts, and just feed the PPM in with no RF at all.

otherwise study propagation versus frequency charts, and go down to the AM broadcast bands. Should be good for about 600 miles with only a few kilowatts.

Or take advantage of the modern life, and run it off a satellite cellphone..;)

[AnthonyRC]

In the USA, these sort of things have been done to 6-meter ham R/C Tx's (we aren't allowed that on 72/75Mhz). A high gain antenna, sufficiently mounted off the ground, will certainly improve the range. But even so, there is so much room for improvement on the Rx side, it might be best to start there before hacking the Tx.

How would you improve the Rx side?

[vintage1]

In the USA, these sort of things have been done to 6-meter ham R/C Tx's (we aren't allowed that on 72/75Mhz). A high gain antenna, sufficiently mounted off the ground, will certainly improve the range. But even so, there is so much room for improvement on the Rx side, it might be best to start there before hacking the Tx.

My guess is 3-10 times range with a very carefully constructed PPM type reciever using 'best technology' front ends and a bandwith of about 4Khz.
That equates to a noise figure (thermal) of about -140dBm Say -135dBm with a sensible FET front end.

At 50 ohms drive impedabce that imnplies a noise level of around 40nV..good digital decodes should be able to operate with an S/N of around 10dB, ten times better than anythng on the market probably - about three times the range.

Should be able to get to ten miles or so if the model is above the horizon, and there are no other transmitters on the aame frequency in the area. Transmit on two or more simulatenous frequencies and it gets better..

[vintage1]

How would you improve the Rx side?

Low noise front end. Best FET. Liquid nitrogen cooled if necessary

Double or treble conversion to split the gain at any one frequency to something sane and stable.

Ultra careful IF filter design to reduce out of band noise.

Digital encode/decode to remove jitters and glitches. Plus delta style modulation to reduce bandwidth needed

Double or treble frequencies used to overcome multipath problems and specific problems associated with a single frequency.

[AnthonyRC]

Low noise front end. Best FET. Liquid nitrogen cooled if necessary

Double or treble conversion to split the gain at any one frequency to something sane and stable.

Ultra careful IF filter design to reduce out of band noise.

Digital encode/decode to remove jitters and glitches. Plus delta style modulation to reduce bandwidth needed

Double or treble frequencies used to overcome multipath problems and specific problems associated with a single frequency.

Ah... I see, clearly much simpler than slapping a couple of bits of alu tube, a lipo, and a RF module on a pole :confused:

[Mr.RC-CAM]

How would you improve the Rx side?Just adding a counterpoise Rx antenna will gain you 2 to 3dB. Mounting the Rx antennas away from all the installation noise sources, ensuring they are full 1/4 length, and tweaking the front end inductor for best matching, AFTER you install everything, would probably gain you 1 to 6 dB more.

Most importantly, carefully choosing your R/C Rx will ensure you start with one that has the best sensitivity and selectivity ( http://webx.dk/rc/RX-measurements/komplet-listen.htm has some practical sensitivity numbers). BTW, if cost is no object, then there is a new diversity R/C Rx from Germany that is offered on all the popular bands, including 72Mhz.

Once the Rx end is cleaned up, changes to the Tx end, if any, began to make sense.

[vintage1]

Ah... I see, clearly much simpler than slapping a couple of bits of alu tube, a lipo, and a RF module on a pole :confused:

No, but simply switching to a DC set with a low noise front end and more gain will net you around 10dB..see Mr RC-CAM's measures for another ten.. that gives you 3x more range already..the rest is the other 10dB...:D

[AnthonyRC]

One of the challenges here on the Rx side is the choice of plane :-)... I'm using a slowstick, for several reasons, one being of course the safety aspects of out-of-view flight (weighs in at about 500g). Another important reason is the cost (or lack of)... if I loose one (or two) while experimenting I wont loose too much sleep.
(the lack of GPS and all other navigational aids is intentional, its much more of a challenge to fly purely by recognising the landscape)

So... given that choice of plane, I'm not sure where I'd put the counterpoise in order to remain omnidirectional. The antenna is already 1/4 wavelength long, but of course its longer than the plane itself.

I already use a DC Rx (Electron 6), mounted as far away from the 2.4GHz Tx as possible (Tx emits 500mW).

Rc-CAM, thanks for the receiver site, very interesting, looks like the Electron does 'ok'. The diversity receivers are interesting, but given the wavelengths in question it wouldn't seem that spacial diversity is a real plus, what do these add?

[Mr.RC-CAM]

So... given that choice of plane, I'm not sure where I'd put the counterpoise in order to remain omnidirectional. There would be some benefit by just having the 1/4 wave counterpoise "mirror" the existing antenna installation as much as you can. Certainly an improvement over relying on the bunched up servo cables, which is the default counterpoise.

For sure, tweaking the antenna matching coil, after everything is installed, will help. It would be like winning the lottery if the factory tweak was perfect once everything was in place.

I already use a DC Rx (Electron 6), mounted as far away from the 2.4GHz Tx as possible (Tx emits 500mW). Everything electrical on the model is a noise source, including the ESC and the servos. Wiring an all. Distance is your friend when it comes to Rx and antenna mounting.

looks like the Electron does 'ok'.Wow, it does surprisingly well, considering its low cost. But, there are other choices on his list that have bit more gain and smiley face'd ratings. Just remember, that for every 3dB of Rx improvement, a doubling of the Tx power would be required to do the same. Every 6dB (4X increase in Tx power) earns you a doubling of range, regardless of how you achieve the dB's (Tx or Rx end, does not matter).

The diversity receivers are interesting, but given the wavelengths in question it wouldn't seem that spacial diversity is a real plus, what do these add?On your small slowstick, probably not much range benefit at all. Would earn you some cool bragging rights and a much lighter wallet.

[AnthonyRC]

RC-Cam, Thanks for the feedback, interesting stuff to consider.

So... thats for the Rx, what would you estimate would be the gain improvement at the Tx end of the dipole that I described. I'm sure that I don't make a great counterpoise when using the standard antenna.

[Mr.RC-CAM]

I predict that your custom Tx dipole will add +2dB to your gain budget.

[AnthonyRC]

Time to do some range tests I think.

[Mr.RC-CAM]

Why not go for something with a bit more gain than a dipole? Perhaps something with a useful beam spread that has 6dBi of gain. Get it up on a portable mast, well off the ground, and I suspect you would be happy with the net gain (especially if you optimize the rx end too). If the stock Tx antenna is horribly mismatched, then the practical range increase could be much more.

Don't forget to consider the effects of cross-polarization. That can create a -20dB penality. So, make sure the Tx and Rx antenna polarizations match and always fly level to maintain it.

[JimDrew]

A little birdy told me that in about 45 days the R/C world will have access to a reliable bi-directional link with a 5 mile range, and it drops into your existing system...

[AnthonyRC]

A little birdy told me that in about 45 days the R/C world will have access to a reliable bi-directional link with a 5 mile range, and it drops into your existing system...

Very interesting... an evolution of the spectrum product?

[lazy-b]

guys, I guess, cellphone is more reliable, and it offer two-way communication, Just need to tap on the Speaker and Microphone.....with a bandwidth of 3 Khz, I guess its more than enough to control just 4 servo.......and cellphone is cheap.......can use a single chip FSK modem, just need a Modem chip and a Pic-controller on Transmitter and receiver side.

[JimDrew]

LOL! I don't know about your cell phone service in your area, but there is no way that I would trust it in my plane! A dropped call would be a REALLY bad thing!

[sodman12]

A little birdy told me that in about 45 days the R/C world will have access to a reliable bi-directional link with a 5 mile range, and it drops into your existing system...


Price? Can you give us any more info?

[kubing]

guys, I guess, cellphone is more reliable, and it offer two-way communication, Just need to tap on the Speaker and Microphone.....with a bandwidth of 3 Khz, I guess its more than enough to control just 4 servo.......and cellphone is cheap.......can use a single chip FSK modem, just need a Modem chip and a Pic-controller on Transmitter and receiver side.

Hi, Can you give me a little detail. Good project to built.