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Ham Radio; R/C aircraft control; Video downlink


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I wouldn't use TV band transmitters. My experience with those showed that it's good for fixed links, but as soon as they move the transmission is much less reliable.

It would be a good idea to find a better receiver than the ones you can find in TV's as well, their sensitivity is usually pretty poor.

Thanks for the comments....

I wish you would tell me more about the first point; 70 cm band transmitters being less reliable when moving. I would like to understand this better. In your experience was this a reliability issue with the hardware not working, or a difficulty in signal reception as the plane moved? How much power were you using on the TX, what kind of RX were you using, and what kind of setup did you have for antennas?

And yes... I havn't own a TV for over 20 years [2% of the US population doesn't own a TV by the way], and I don't intend to change that now... I'll be using a box that converts the 70 cm band signal IN, into video and audio OUT (http://www.hamtv.com/ has a few options). I also intend to use an inverted quarter wave on the TX, and a 5 element yagi on the 70 cm RX. With five elements there is significant front to back, but the pattern out the front is rather broad, and so [like horse shoes] close is good enough.

Best Regards,

Chuck

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Just a thought Chuck. A lazy 8 might be a better bet as it has some gain and it's omnidirectional unless you plan on keeping the yagi pointed it the plane. Keerp in mind the yagi should be vertically polarized as you'd lose a few DB with cross polarization. Same with a lazy 8. A 70cm halo might be small enough and a good possibility also for horiz polarization..

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I wish you would tell me more about the first point; 70 cm band transmitters being less reliable when moving.

The 70cm ATV gear is AM. The stuff we use is FM. World of difference in tolerating RF signal issues.

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The 70cm ATV gear is AM. The stuff we use is FM. World of difference in tolerating RF signal issues.

How true... FM is much more efficient, and as long as the receiver can capture the signal the quality will hold up. AM needs more power, and as the signal degrades the picture will snow. Also, random signals (e.g., atmospheric) below a threshold will not show up in the picture (noise immunity) in FM as it will in AM.

The frequencies that we are talking about are different as well; 70cm vs 12cm and 25cm. In the higher frequencies there is more bandwidth available and using FM it is possible to trade bandwidth (modulation index) for noise immunity (a good thing for sure). Also, low noise preamps will help you out. The higher frequencies allow more front to back gain in a smaller physical space but also make the beam narrower requiring a higher degree of tracking (not insurmountable). Higher frequencies have higher free space propagation losses (fspl varies by the square of the distance and the inverse square of the frequency) and shadowing issues.

I could add more, as I'm sure that you could as well...

If I lived in Arizona on the flat, obstruction free, line of sight desert, it would be a no-brainer; FM, as high frequencies as possible to get as big a bandwidth as possible. But, I live in the rolling hills of WPA with it's peaks, valleys, and lots of trees.

It is clear that there are many variables, the pros and cons of which can be debated "till the cows come home." All of this being the case, I'm still interested in the particulars of anyone's experience (band, power, mode, antenna, terrain, observations, and anything else).

Best Regards,

Chuck

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Just a thought Chuck. A lazy 8 might be a better bet as it has some gain and it's omnidirectional unless you plan on keeping the yagi pointed it the plane. Keerp in mind the yagi should be vertically polarized as you'd lose a few DB with cross polarization. Same with a lazy 8. A 70cm halo might be small enough and a good possibility also for horiz polarization..

I'm not familiar with that antenna... and a google search of "lazy 8 antenna" didn't turn up much usefull, probably because "lazy 8" is used a great deal for other things. Can you point me to a specific reference?

Best Regards,

Chuck

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I wish you would tell me more about the first point; 70 cm band transmitters being less reliable when moving. I would like to understand this better. In your experience was this a reliability issue with the hardware not working, or a difficulty in signal reception as the plane moved? How much power were you using on the TX, what kind of RX were you using, and what kind of setup did you have for antennas?

The hardware was fine, worked well when static, but even moving around with it in the hands was enough to cause trouble. I was using a simple wire on the TX (100mW), and a TV with an active antenna to receive. One needed to find the "sweet spot" for the antenna to get a good picture, hence the trouble when moving.

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  • 4 weeks later...
The hardware was fine, worked well when static, but even moving around with it in the hands was enough to cause trouble. I was using a simple wire on the TX (100mW), and a TV with an active antenna to receive. One needed to find the "sweet spot" for the antenna to get a good picture, hence the trouble when moving.

Sorry for the delayed response and thanks for the particulars....

I will be using a receiver that has more sensitivity than a TV. Also I will be using a 5 element Yagi which should get me some "front to back" without being too finiky on the pointing (for those who understand antennas, the front lobe is rather wide). Also, [as I recall] doubling the power is 3db of gain. I will be using 500mW for starters (the TX can do 4 watts and I have a brick amp that can do 25 watts), so that 500 mW represents a +6db gain and the Yagi gives me another +6db. But, hay, this is all theoritical math giberish, what I want to see is how it performs in real life. This I will do over the next few weeks with my daughter's help, and I'll let you all know.

I managed to get the parts for the TV station/reception part of this along with the GPS portion working in the house. Over the next few weeks I'm going to try some experiments (off the plane) to see how the reception is, and as I've stated, I'll fill you in.

To be fare [scientific], I really should do a 2.4 GHz test "side by side" with the 70 cm setup. God save my wallet, but this I would like to do. Any pointers to threads or suggestions on technology in this area would be greatly appreciated (e.g., the most promising technology that I've seen in this area is http://....). I'm basically a scientist at heart, and I would be interested in documenting a comparison of this form. I would also like to know if my "huntches" are correct in these areas. But my primary goal is to "understand" and as such, I would report my findings impartially.

As an asside, I also acquired a "Pandora Pan & Tilt Kit". It was clearly well thought out, and I am looking forward to using it on my Telemaster 40.

The more I read about this topic, the more that I believe that I should be heading down the autopolot route. There is a great deal of information about this on the boards that I have been trying to sort through over the past few weeks. Again any pointers to specific threads or websites that have been found as interesting would be appreciated.

I thank you so much for all of your help!!!! You have all been a great support and have been very helpfull. Manny manny thanks!

Very Best Regards Chuck (73 N3BEZ)

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