Independent tests prove lack of frequency hopping with XPS

[KrisW]

	Quote: Originally Posted by rdgood
	Well I am going to jump in here a bit. So this is kinda pointed at Kiwi - as a question. To me this whole 2.4 GHZ issue is at it's weakest point a problem of wave length. Granted I know a little about heterodyning and such getting info off of a carrier freq. Which is the main issue at stake!! Now - from what I can tell 2.4 GHZ has a wave length of .125 meter which is approximately 4.9 inches. The antennae are actually dipoles two short half wavelength stubs ~ 2.5" in a dipole arrangement. Now since we are dealing with microwave frequency behavior - microwaves like antenna's that deal with E and H waves to get good reception. Thus the dipoles. Now to me IMHO - Futaba has the best way/method that I have seen so far. But to me it would seem that if Futaba or JR for that matter, wantedto make a completely bullet prooof system they would put alot more dipoles on the airplane. It wouldn't be that hard heck you could probably put them as a tape on afair all over the fuse exterior on the rudder on the bottom of the wings. Know the reason I say this - I saw a NASA study where a group of engineers powered a remote glider with microwave RF beamed up at the plane using only the RF energy as the source of power. The bottom of the wings was a dipole array sort taped on the covering. I also think we are depending alot on 2.4 GHZ penetrating alot of solid and metallic material. Which will decrease its db energy also. I do beleive that 2.4 GHZ is just fine for smaller say 25% or below. But for larger 30% and above I beleive that there is too much metallic material that could stand in the way between the receiver and the transmitter. I also beleive that the problem gets worse with tuned pipes - which will block/bounce off of 2.4 wavelength. It just sure seems to me that we are getting closer and closer all the time to perfecting this. Big - Big Caveat here - I know enough about RF micro wave propagation to be dangerous. So If i have a few items backwards/ or just plain wrong please excuse my interpretation of info I have gathered over the years. I have a hard time beleiving that 2.4 will penetrate a DA150 and tuned pipes without diminishing the signal significantly. Especially if the 1/2 inch dipoles are between you and the transmitter. On another note - 900 MHZ has a wavelength of about .33 meter which ~ 1 foot. Now wouldn't it be better to transmit on 900 MHZ since it would only take an antenna of 6 inch dipole which is bigger than most engines/jugs and metallics on the plane. I could two see two 2 dipoles which would be about 12 inches being able to catch more wave info than an two 1/2 inch dipoles. My two cents - I hope I dont get ripped to hard here!! Ronster

Nice post Ron, and it illustrates the strength of satellite receivers. . like Spektrum. . that are placed in strategic locations to avoid ever losing signal due to engine/exhaust mass getting in the way. i was actually going to use extensions and put a couple in the wings of my 40%, well clear of the engine/prop/pipes and CF wing tube, but that seemed a bit ludicrous so I just spaced them well apart, and high/low in the fuselage at all different angles to get the best spread of dipolation (is that a word???) throughout the area. But, as with XPS. . single antennae are going to find RF shadow areas, especialy behind big chunks of metal or CF, which IMHO makes any system utilizing only a single receiver and antenna very questionable over the long term.

As far as I am concerned. . the more the merrier for antennae/satellite receivers with 2.4g, which is why I appreciate the ability to do this with the Spektrum setup.

Concerning 900 mhz .. yes it does have better range, a higher maximum output power limit, and a longer wavelength . all of which make it better than 2.4 at the limit of receptivity or in questionable circumstances (like behind a big 150 cc engine). But at the same time 900mhz is not legal in as many countries as 2.4, and the constraints on power output on that frequency band are is stricter in many countries. Add in that 900 mhz is, like 2.4, a wide open frequency band, and there are lots of higher powered units already there, plus the bandwidth being narrower (or less deep, using Kiwi's analogy) and you have to wonder if 900 mhz is actually a better place to fly rc than is 2.4g.

JD seems to think so, or so he has said, sinc ehe is supposedly coming out with a 900 mhz unit soon. But i've had enough jumping around frequency bands for now. . 72 and 2.4 are plenty for me.

[XJet]

	Quote: Originally Posted by Kiwi
	Panzlflyer, You touched on something I cannot do so far and that is jam the entire 2.4Ghz spectrum. Short of hauling the door of a microwave oven and bypassing the lockout I canr find a way to do it. Its actually very hard, much harder than jamming 72Mhz for example, Correct me if I['m wrong here XJET as I can be with this stuff.

Nope, you're pretty much right Kiwi.

The 2.4GHz band is 80-times bigger than the 72MHz one so to entirely saturate it would require (all else being equal) 80-times the amount of power required to saturate the entire 72MHz band.

	Quote:
	2.4 Ghz is like a square box. That is the channels maybe 13 wide but they can also be 10 deep. 72 Mhz on the other hand is 50 wide for example but one deep. So if you put out a blast of RF on 72 that covers all 50 channels then 72Mhz is down for the count.

Yep, spread-spectrum not only encompasses the spread of frequencies but also something known as the timeline.

Think of it as a party-line for RF.

Back in the old days (am I showing my age) we used to have as many as five or six households on a single phone line. We were able to share a line because our individual use of that line was relatively brief and intermittent. If you analyzed it you'd probably find that any individual user was probably "on the phone" for no more than 1 percent of the day. That left plenty of time for others to use the phone when you weren't.

Of course there was also the issue of how to know which house an incoming call was for so we gave each number a distinctive ring sequence. We lived in a house where the phone would ring two shorts and one long if the call was for us. Our neighbors had different ring sequences (three short or one short and two long for example). This is pretty much the same way that the GUID system works on spread spectrum. Since multiple systems may be sharing the same frequency(s), the call to each receiver is identified by a unique number that is specific to an individual transmitter.

So in the world of spread spectrum, several users can share the same channel(s) without any interaction being noticed.

This can't happen on 72MHz because a 72MHz set is effectively on the phone 24/7. Each line (frequency) is entirely monopolized by any radio that's operating on it -- there are no breaks in the transmission to allow another set to send its own information and even if there were, there's no provision for unique ring-sequences to let the receivers work out which data is for which plane.

(oops, rambling again).

[XJet]

	Quote: Originally Posted by adjonym
	I know that XPS claims that, and I know that it doesn't I have no idea on how JD implemented it though, since he refuses to say(and how to make it jump). I just thought about it, and honestly don't see how it can be so hard to implement(make it work as advertised).

Actually it's damned hard and most of that difficulty is caused by the thing JD claims is so good about XPS -- its reliable-transport layer.

	Quote:
	The part that changed channel should be able to send "i changed channel" on the "old" channel in a sequence where it changes between the "old" and "new" channel until it recives an "ack" packet saying that the channel switch is complete.

The problem with that is that if the "old" channel is hit by interference, the two ends can't talk to each other to exchange information about where they're going to hop to. If a predetermined alternate channel is used there's no guarantee that the interference won't be there too.

If you go back through this thread you'll find a discussion about the issues that surround trying to implement the system that XPS claims to use -- which probably explains why it doesn't work (or at least why it doesn't work well enough to be useful).

[Panzlflyer]

Thanks, Kiwi and Xjet, was curious.
If someone was that determined to get us there will be other issues to worry about.
I have been on Spectrum since it came out in 35%s down to Outlaw style and have yet to find fault with it.
Beats me how it stays linked in an Outlaw as I have it crammed in and the roll rate is something else.
About half the club is on Spectrum..no Fasst as we all deserted to the dark side and I have yet to see a problem.
Only thing I see is that it has taken away the cries of being hit! So there has to be another issue for folks to belly ache about.

[Flyfast1]

I have been using XPS in 100cc planes for over six months and have never had an issue with the signal being blocked by the engine. Even when standing directly in front of the plane with the antenna removed, the transmitter operating on the lowest power setting and the end of the antenna coax pointing directly at the plane, I have full control and cannot not make the receiver go into failsafe. The only time I have ever been able to block the signal with something in the plane is when the receiver was directly behind a very large aluminum air tank in the plane. I could make the receiver go into failsafe during range testing (on the ground) when I was standing so that the receiver was directly behind the aluminum tank. It was only at a very specific spot: if I moved even two feet left or right, control was re-established. I addressed this issue by moving the air tank into the tail and moving the receiver higher in the fuselage. Over 50 flights with this configuration and this plane without any issues.

That being said, I am all for additional safety and if XPS offers satellite receivers or alternative antenna configurations, I will give serious consideration to using them.

-Ed B.

[BaldEagel]

Conversly on my interferance prone site in one of the areas of know interferance the Rx goes into lock out and no matter the orientation or height of the Tx, the Rx remains in lock out, Spektrum & FASST do not have any problems on this site.

Mike

[Flyfast1]

	Quote: Originally Posted by BaldEagel
	Conversly on my interferance prone site in one of the areas of know interferance the Rx goes into lock out and no matter the orientation or height of the Tx, the Rx remains in lock out, Spektrum & FASST do not have any problems on this site. Mike

Mike,

Are you saying that with your XPS system you cannot even connect to your plane on the ground at your field? You have no response and the system does not go into failsafe?

-Ed B.

[Kiwi]

Guys,

Apologies for not getting the video up. I cannot render this new Sony format properly and its going to take a day or so to find a fix.

Today I leave on a business trip and wont be back home until Thursday next week. I think I will shoot the vid again using a camera with a memory stick as they take seconds to edit and post. I forgot the KISS principal here and its bit me in the butt as usual.

Sorry again guys. Keep an eye here for it and I will also out it on You Tube this time to make it even more available.

[XJet]

	Quote: Originally Posted by Kiwi
	Guys, Today I leave on a business trip and wont be back home until Thursday next week. I think I will shoot the vid again using a camera with a memory stick as they take seconds to edit and post. I forgot the KISS principal here and its bit me in the butt as usual. Sorry again guys. Keep an eye here for it and I will also out it on You Tube this time to make it even more available.

No problems Kiwi, as my wife keeps reminding me -- this isn't your job it's just a hobby :-)

[gregw]

	Quote: Originally Posted by Kiwi
	Guys, Apologies for not getting the video up. I cannot render this new Sony format properly and its going to take a day or so to find a fix. Today I leave on a business trip and wont be back home until Thursday next week. I think I will shoot the vid again using a camera with a memory stick as they take seconds to edit and post. I forgot the KISS principal here and its bit me in the butt as usual. Sorry again guys. Keep an eye here for it and I will also out it on You Tube this time to make it even more available.

Kiwi, thanks for all the effort. I look forward to the definitive test comparing XPS, Assan, Spectrum and FASST in how they handle interference in a controlled environment.

I would also suggest that you upload the video to http://www.vimeo.com instead of Youtube since it allows everyone to download the original video without the compression, and vimeo videos are much clearer than Youtube even when compressed. Just remember to ensure that the source video is 500MB or below, since that is the upload limit on vimeo.

[XJet]

	Quote: Originally Posted by gregw
	I would also suggest that you upload the video to http://www.vimeo.com instead of Youtube since it allows everyone to download the original video without the compression, and vimeo videos are much clearer than Youtube even when compressed.

Unfortunately it looks as if you have to sign up (just what I need, another id/password) before you can access.

The advantage of YT is that *anyone* can watch its content without the signup hassle and they can also be easily embedded in other webpages.

YT also now has a High(er) Definition option which makes much of the newer stuff look very good indeed.

[Daemon]

You only have to sign up to post your video, or to download the original file.
Otherwise it's identical to YouTube for most other people (don't need to sign up
to view compressed version), but does offer higher quality compression that favors
image quality over high motion rates, and I suspect your video will lend itself to that well.
If people want the highest quality version available they can sign in and download it.

ian