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Old Sep 27, 2010, 06:46 PM
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Should I choose 2.4Ghz or xxMhz for soaring high over dense urban areas

I'm relatively new to RC soaring and I'm not sure what the pro's choose for radio frequencies. I've read endless discussions and marketing talk about comparisons between 2.4ghz and xxMhz (in my case 35Mhz) but they all seem to focus on static/unchanging RF environments.

One place I plan to soar from is a slope where a densely populated town extends from the bottom. Assuming I'm using 2.4ghz, the receiver will start without almost any competition at all. However if I fly high and far out, the glider's distance to the urbanization could be (much) less than it's distance to me. That means the RC receiver will have to listen for my weak RC transmitter signal amongst the RF noise generated by thousands of wifi networks, household phones, garage doors, security camera's, etc. I have my doubts about how successful it will be in staying connected. That's my 1st doubt about 2.4ghz.
My 2nd doubt is, if my receiver looses it's connection or browns out (*cough* Spektrum AR500 *cough*) in that RF noisy environment, how will it ever be able to find my weak TX signal back again.

So are my doubts valid, and what do the 'pro' RC soarers mostly prefer for frequencies?
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Old Sep 27, 2010, 09:38 PM
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The only reason a sailplane would brown out is just before a blackout.

Think about 2.4 at a large sailplane contest. They seem to work there and that is probably more 2.4 signals at higher levels than all the low power gadgets you are talking about. I switched to 2.4 years ago, even for my low budget models like a Shooter.
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Old Sep 27, 2010, 11:12 PM
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Spektrum quick connect after brown out

When turned on a Spektrum TX looks for two clean frequencies in the 2.4 range and uses those until the power is cycled on the TX, it then looks again.

The RX when powered up, looks at the the last two frequencies the TX was found on, to see if it's still there (quick connect). Should it fail to find the TX on the last two used frequencies, it goes looking for the TX through the 2.4 band.

If the RX browns out (caused by low RX battery) it will once again go for the quick connect and look for the TX at the two last known good frequencies. Assuming this is mid flight and the TX has not been power cycled, the RX will find the TX quickly.

You can prove this and that your RX is quick connect capable. (Early RX firmware was not)

1. Power off TX and RX
2. Power on TX and start moving a stick, keep moving the stick until told not to.
3. Power on the RX and time how long it takes for the servos to respond.
4. Power off the RX
5. Power on the RX and time how long it takes for the servos to respond.
6. Oh you can now stop moving the stick.

If all is well, the time taken in step 5 should be shorter that that in step 3
That is the quick connect function working.

I hope that helps with your brown out concerns.


Barry
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Old Sep 28, 2010, 05:01 AM
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Quote:
Originally Posted by mlachow View Post
The only reason a sailplane would brown out is just before a blackout.

Think about 2.4 at a large sailplane contest. They seem to work there and that is probably more 2.4 signals at higher levels than all the low power gadgets you are talking about. I switched to 2.4 years ago, even for my low budget models like a Shooter.
A 2.4Ghz TX, in Europe at least, is limited to 100mw (and I believe in France even less). Wifi routers also normally output at 100mw. So in terms of power output, all devices are more or less equal. I've never been to a sailplane contest, but how many pilots fly at the same time? I assume max 100. That is still way less 2.4Ghz noise than above a densely populated town. If someone can confirm that they can fly a glider safely, high and far away in a densely populated area, then that'll take my doubt(s) away.
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Old Sep 28, 2010, 07:26 AM
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And most of the wifi routers are inside homes. Add to that things like trees which also reduce the signal strength. There are a lot of things in houses that reduce the signal level so they are not that much of a problem. And if you ever looked at the actual transmissions, most of the time they are not transmitting. It's not like on 35 or 72 where the carrier was on constantly.

So on your 2.4 model TX, the amount of actual data is pretty small vs the available bandwidth. The TX will actually transmit it several times in the time it took your old TX to send the same amount of data. You can't compare 35 to 2.4 based on another transmitter on the same frequency will cause a problem. And all the signals have data in them so your RX can distinguish the data from other data.

Not everything wireless is on 2.4. And if you want speed, you get off 2.4. I don't use 2.4 on my home wifi.

The US transmitters on 2.4 are not that much more powerful than the 100mw limit in europe.

I'm not saying you can't have problems on 2.4 vs 35. Some problems are a little different. But the nice part is a lot of the 2.4 stuff will also log the data on how good your signal was so you have real numbers to verify things were OK compared to the old 1970's technology in the 35/72 mhz stuff.
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Old Sep 28, 2010, 11:07 AM
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Basic rule for ANY wise heavier than air defience enthusiast, DON'T fly over areas whether densely or otherwise populated.

Sorry, NO excuse.

Regards Ian.
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Old Sep 28, 2010, 02:28 PM
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What Ian says: Don't fly over people.

There are just too many things that can go wrong, and loosing the signal is only one of those many things.

Yours, Greg
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Old Sep 28, 2010, 03:11 PM
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Safety topics aside (I agree with no flying over people and houses), I was under the impression that the Spektrum RX only responds to signals if the data packet contains the matching transmitter GUID (from binding) and that it performs checksums on the data packets to ensure the data is not corrupt. Surely this must help with minimising any interference?

Many WIFI routers share the same channel and even in a street full of routers everything still works OK. So the integrity and performance of the data transmission must largely depend on the implementation of a 2.4 GHz system, which includes Spektrum stuff.

Any thoughts on this?
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Old Sep 28, 2010, 03:42 PM
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Quote:
Originally Posted by ulogix View Post
Safety topics aside (I agree with no flying over people and houses), I was under the impression that the Spektrum RX only responds to signals if the data packet contains the matching transmitter GUID (from binding) and that it performs checksums on the data packets to ensure the data is not corrupt. Surely this must help with minimising any interference?

Many WIFI routers share the same channel and even in a street full of routers everything still works OK. So the integrity and performance of the data transmission must largely depend on the implementation of a 2.4 GHz system, which includes Spektrum stuff.

Any thoughts on this?
Based on what mlachow said (that most of the time the TX's aren't actually transmitting), the receiver will only listen for it's designated packets, skipping those with incorrect GUIDs or otherwise mangled by simultaneous packet transmissions (packet collisions).

However, what I am curious to know is, is at what point do these 2.4 Ghz systems actually get overloaded with too many packet collisions that they decide to go into hold mode. If I had to produce such a situation, would it take me 10, 100, 1000, 10000, or more 100mw wifi routers placed next to one another to wreak havoc with a equally powerful 100mw 2.4 ghz RC connection? That would be the point that the silence described by mlachow would be eliminated on all channels.
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Old Sep 28, 2010, 04:08 PM
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This footnote is found in the Frequency plan (for Norway):

The following bands: 13 553-13 567 kHz (centre frequency 13 560 kHz), 26 957-27 283 kHz (centre frequency 27 120 kHz), 40.66-40.70 MHz (centre frequency 40.68 MHz), 902-928 MHz in Region 2 (centre frequency 915 MHz), 2 400-2 500 MHz (centre frequency 2 450 MHz), 5 725-5 875 MHz (centre frequency 5 800 MHz), and 24-24.25 GHz (centre frequency 24.125 GHz) are also designated for industrial, scientific and medical (ISM) applications. Radiocommunication services operating within these bands must accept harmful interference which may be caused by these applications. ISM equipment operating in these bands is subject to the provisions of No. S15.13.

Might be interesting, especially if you fly close to urban areas. A lot of devices share this band, I am not sure what happends in the future

When it comes to airmanship flying over people, not to say urban areas is a no-no in all club-rules I know...

When it comes to Spektrum max 39 could fly at the same time. Not sure when it comes to FASST.
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Old Sep 28, 2010, 05:00 PM
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If you are new to RC, go 2.4 GHz. Absolutely!. The major radio makers all have well established and well tested products.

Are there problems? Sure! There were and are problems on all frequencies. Product defects occur and any radio system can be overwhelmed if the interference is strong enough.

However, when installed properly and supplied by reliable power, the 2.4 GHz systems are at least as reliable as the 72 MHz systems. But the 2.4 GHz systems won't be shot down by someone turning on on your channel.

Go 2.4 GHz!
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Old Sep 28, 2010, 08:19 PM
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Flying over a densly populated area isn't a good idea with any radio. That said, I can't imagine why anybody would buy a new radio that wasn't on 2.4 gHz.
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Old Sep 28, 2010, 08:46 PM
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Only Luddites.
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Old Sep 28, 2010, 08:48 PM
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Hey, I resemble that remark!
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Old Sep 28, 2010, 09:47 PM
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If you want to go GEEK, then get a wifi analyzer and take a look at the 2,4 band in your area. http://www.metageek.net/products/wi-spy.

Otherwise search for all the old 2.4 discussions and all the supposed problems. It works.
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