BalsaBob

My Feedback: (24)

As I educate myself on the various 2.4 systems ....... I have learned (I think) that line-of-sight between the transmitter and the receiver antenna is a key requirement. Thus I have noticed that Spektrum has options for numerous (satellite) antennas and Futaba uses dual antennas. So, just when I think I am starting to understand the 2.4 systems, I notice that the antenna on the ExtremeLink receiver is just a small nub/protrusion. Are the Spek and Fut antennas setups overkill ? Thanks. Bob

TLH101

My Feedback: (90)

All three systems use different technologies, and as such, different antennas.

dirtybird

My Feedback: (5)

I have flown the xtreme system for several flights with two receivers(not in the same airplane) with no problems. I also have not been too careful about their 2" rule. I have a small airplane where everything is crammed in there.
Futaba and Spectrum are different systems. Maybe they need the other antennas.
BTW all radio waves are LOS. If your 72mhz runs out the tail and the airplane gets comming straight at you with the antenna behind the engine you are likely to get a glitch

bruce88123

All radio waves are NOT line of sight LOS. It depends upon the frequency used. It tends to be that way for the frequencies we use in R/C but lower frequencies can reach around the world with no problem and relatively low power too. No broad brushes please.

heli_Rod

That is not correct. Low frequency waves will and do ben due to tropospheric bending. The higher frequencies, such as VHF, UHF, Microwave, infrared, etc. do require line of sight.

Rod

dirtybird

My Feedback: (5)

Sorry but you are wrong. Lower frequencies cannot penetrate an ioization belt around the earth and are reflected back. That causes them to appear to be going around. There will be skip areas where no reception can be had.

Lomcevak Duck

My Feedback: (14)

Interesting that you say 72 MHz is LOS. At one of my clubs we have a huge valley on the opposite side of the runway from the pits and it is an exciting challenge at our fun-flys to hold a contest to see who can fly the farthest into the valley out of site and come back out. I guess we were all just fortunate to have control of our planes by some strange fluke. And yes, sometimes a hungry bush or tree grabbed a plane.

So what's up with multiple antennas and manufacturers making a big deal about 2.4 GHz being LOS? If this is something we've all been dealing with for years without troubles, is 2.4 LOS rule any different?

dirtybird

My Feedback: (5)

I think you were indeed lucky.
The 72 frequencies are intersperced between TV and FM radio frequencies. Did you ever wonder why these stations are local only? Its not a power problem.
The 2.4 antennas are much smaller and can easily be hidden behind an engine, even a small one.
One of the reasons I bought the xtreme system is a demo I saw Jim Drew make. He put his receiver under a metal sink and run a dowel out thru the drain hole so you could see if the servo moved. He then gave me the transmitter and I got about ten feet away and it still worked! I was impressed. Somehow those radiowaves are sneaking in there and everyone agrees 2.4 is LOS.
I think I will leave this discussion now. If you would like more information about LOS I suggest you look at "Reference data for Radio Engineers" by several authors, or "Fundamentals of RADAR" by Skolnik. These are old books and probably out of print but you could propably find a copy at a library. 2.4GHZ is very close to the popular 10mm RADAR frequency(3.0 GHZ)

Don J

OK My 2 cents,

I believe at UHF Frequencies (450 Mhz) radio waves will travel line of sight PLUS 15 percent (if memory serves me right).
The lower in frequency you go the higher the percentage goes.
Also, Lower frequencies "DO NOT" follow the curvature of the earth.
Granted they will travel a much greater distance line of sight.
They will bounce off the Ionosphere, depending how well it is active that given time.
Just google sunspot cycle and such and there will be lots of reading.

My theory on the dual antennas for the rx'ers is:
Take a transmitter with a vertical antenna and send a signal to a receiver with a vertical antenna.
Now rotate the transmitter antenna to a horizontal position (shifts it 180 degrees out of phase).
Depending on antenna separation, you will have a 10db loss of signal (personally tested myself).
Not a big deal at 72 Mhz, it is a big deal at 2.4 Ghz especially when your transmitter is sending such a weak signal.

Now imagine our R/C planes and how many different positions our receive antennas are in.
One fix would be to add a second antenna and mount it PERPENDICULAR (I believe the manufacturer advises to do this) to the first antenna.
As antenna A shifts out of phase, antenna B comes into phase and receives a stronger signal.

I feel spread spectrum will be great for R/C, but I think there are a few hurdles to overcome yet.
First one being receiver sensitivity and selectivity.
Second one being power issues at the receiver and the transmitter.
It takes a lot more energy to send say, 1/2 watt of power at 2.4 Ghz than it does at 72 Mhz.
Then there are receiver reboot times.
Lots of issues, but I think we will see big improvements to come.

Don J

Rodney

Dirty bird is correct, all RF is line of sight from the source just like light is. It just happens that any matter in the vicinity of the path will deviate and/or scatter the energy in the wave. Direction and degree will vary with the suroundings with the lower freqencies being more easily bent, reflected or scattered. For instance, light from the sun will allow you to see things in the shade here on earth due to refraction and reflection.

d_wheel

The wave length of 2.4ghz requires the use of very short antennas. The small size of these antennas makes it much easier to block the signal with items like battery packs, engines, tuned pipes, etc. Therefore, having dual antennas placed as far apart as possible makes this blocking much less likely. This is not the case with 72mhz because the antenna is so long, some portion of it is more likely to be in view from all angles.

The analogy I like the best regarding dual antennas is this:

Close 1 eye and look at your computer screen. Your one open eye represents the single antenna. Cover the open eye and the signal is lost. Now, leave the first eye covered and open the other eye (second antenna) and wallah, the signal is back.

I realize this very simplistic, but it seems like an acceptable way to explain the importance of using dual antennas.

Later;

D.W.

BalsaBob

My Feedback: (24)

D.W. (and all), thank you all for the replies/information. The single antenna nub/post of the ExtremeLink system is what is puzzling (and intriguing me). It is very compact and appears that it would be the simplest/easiest reciever to install. What I am looking for is a system that will reduce (to the extent possible) a loss of signal to the aircraft. I would gladly be willing to pay the extra $$$ for additional (satellite) recievers/antennas onboard the aircraft .... IF they are needed and do in fact reduce the chance of the signal being interrupted. If additional/satellite recievers/antennas do nothing to reduce the chance of signal loss .... then they may be a waste of time, money and space inside the aircraft. I think I am just simply trying to determine if the use of an ExtremeLink reciever/system (which I am very interested in) will in any way increase the chance that the signal to the aircraft may be lost/blocked. Thanks again.

d_wheel

It might be better to ask how much you are willing to pay for the added reliability and peace of mind. I feel that dual antennas/receivers DO indeed reduce the possibility of signal loss. Xtremelink feels that their 2 way transmitter/receiver communications reduces the chance of lost data because if information does not make it to the receiver, the transmitter sends it again. This appears to work in the majority of cases, but there is no way to convince me personally that this is the best way to handle things. I feel that a system that is designed to reduce the need for retransmission of data is better than one that is designed to retransmit that data because you know that your antenna system is less than optimum.

As for simpler to install, I don't see that as the case with Xtremelink. Since it only has 1 antenna, it is mandatory to find the best place in the aircraft for mounting. Also, you MUST make sure no other wiring or metal is within 2 inches of the antenna. This is all possible and works but it does not make it easier to install or reduce space.

On my Big Stik 40, not only did I have to install the Xtremelink receiver in a special location (on a shelf up higher than the servos), I also had to install a piece of foam rubber to the top of the receiver to make sure the aileron servo extensions did not swing around in flight and violate the 2 inch rule. On 1/4 scale and up aircraft, the manufacturer recommends that you mount the receiver as high as possible to insure minimum shading of the antenna. Some of the installations I have seen have it up in the turtle deck, or canopy area. Not an easy location for mounting as all wiring has to be rerouted. For these reasons, it is not easier to install than a 2 antenna/receiver system.

I replaced the Xtremelink (which worked for many, many flights without any problems) in my Big Stik 40 with a Futaba Fasst system. I simply velcroed it to where I would have mounded any other receiver and then taped the antennas to opposite sides of he fuselage making sure they are 90 degrees to each other. It did not take any longer to install the Futaba than it did the Xtremelink. On my 1/4 scale aircraft, I simply mounted the Spektrum receiver where the Futaba 72mhz receiver had been, and then velcroed the satellite receiver as far away as possible on the opposite side of the fuselage, again making sure the antennas were 90 degrees from each other. Not difficult or time consuming at all.

Hundreds, if not thousands, of Xtremelink systems are in use and most seem to be very happy with them. If you don't mind the added time and work to get it installed correctly then by all means it could be the system for your needs.

Later;

D.W.

BalsaBob

My Feedback: (24)

Thanks again DW ...... I see your point on how the ExtremeLink receiver may not (in some aircraft) be easier to install. As far as how much I would pay for added reliability and piece of mind ..... I would certainly be willing to pay the cost of 1 (or even 2 or 3) additional/satellite recievers. Of course I would only do this for my larger/expensive aircraft (i.e., the ones that take me most of the Winter to build) and not my everyday 32 size fun ship.
That is why I am leaning toward Spektrum ..... as their satellite recievers/antennas will probably give me the 'piece of mind' that I need. My guess is that FUT and EXTR may even be losing some of the market share for not having satellite reciever options ...... Thanks again. Bob

Silent-AV8R

Let me add that there is another significant difference between the Futaba FASST and either of the other two. It is how the signal is transmitted.

Spektrum locks a single pair of channels and uses those. If the noise floor rises too high, the link is lost regardless of how many extra receivers you are using.

The XPS system will hop to a new channel when it sees too much signal loss. In the meantime it can experience a slow down in performance as the noise floor rises.

The FASST system is constantly hopping frequencies. It does so 500 times per second. It is never on a single channel long enough for a problem to affect the quality of the signal going to the RX. The dual diversity antenna arrangement works just fine in this situation.

Spektrum needs the extra remote receivers to overcome the inherent problem with locking single pair of channels. Spektrum is locked onto that pair and cannot hop if there is a problem, therefore it is critical that they do everything they can to avoid loss of signal.

XPS suffers from possible degradation of performance in a situation of increasing noise that is just below the threshold that will make it hop. The single antenna arrangement also makes it installation critical to avoid possible blanking.

The FASST system in my mind is the superior technology.

Don J

It sounds as though the FASST system is true spread spectrum.
The Spektrum and XPS system appear to be more like a trunking system.
A spread spectrum system, in the 2-way radio world, constantly hops frequencies.

Good info here, now I am swayed back to the FASST system.


Don J

d_wheel

Both are considered types of SS. You can see the difference [link=http://www.sss-mag.com/ss.html]here[/link].

Later;

D.W.

pilotpete2

D.W.
I have a hard time imagining a scenario where retransmitting data would be of any help in providing a more robust link between a transmitter and receiver, if data is lost, it's lost, (there is a big difference between "our" data and transmitting a digital file , as in an Ethernet link) I would think that the last thing you would want would be to receive old retransmitted data when link integrity is recovered. If you hold data for retransmission how does it know when to flush the buffer and start sending in real time? Am I the only one out here that sees this as a problem, rather than a fix? Please anyone with comments.
As to antenna diversity, how many have been listening to something of interest on the car radio and just as you ease to a stop for a light you hit a null in the signal that blanks you out right at that spot[:@], I would think dual diversity antenna would help just that scenario.
Pete

d_wheel

According to XPS, the "old" data can be resent many times before the next frame of data is due to be transmitted. If it hasn't been received by then, the next frame is assembled and sent out. It seems good in theory but as I have said before, dual antennas should minimize if not eliminate the need to use this type of system. Yes, I have experienced just what you mention about FM radio blanking. There is one stop sign on the way to work where this is very noticeable!

Later;

D.W.

DavKhy

My Feedback: (2)

XPS just announced that they will be coming out with satilite receivers for their receivers as well as the ability to connect two regular receivers together via wired or wireless methods. They have made it known that these are not necessary but if the customers feel that they would feel better with satilite receivers then they would make them available. Also receiver settings such as fail safe maping, fail safe timing, and other settings will be able to be done wirelessly via computer. Check out XtremePowerSystems.NET then after you finish looking their web site, pick on forums. There is a lot of posts that you will find interesting. Their receiver is bi-directional where the receiver talks to the transmitter and will be able to send telemetry information back to the transmitter to an LCD screen that plugs into the original antenna hole......The future is here. XPS made in America.

pilotpete2

D.W.
Thank you,
That makes more sense, only having retries until the next frame is ready, but I would think the performance of a system that just held last position until valid data was received would be indistinguishable to the person flying the plane.
Pete

d_wheel

All right! Now we're getting somewhere! Guess I will cancel my Futaba backorder and see how this works out.

Later;

D.W.

dirtybird

My Feedback: (5)

One thing you might want to consider is the PPM system we use has always worked in the pulse repetitive basis. In the early sixties I designed an analog proportional system that relied on always receiving a signal. I found there were several conditions under which the signal was lost. Every time the signal was lost I got a glitch. A PPM sysyem repeats the new information every 50 ms and a lost pulse is ignored. In a typical flight lots of pulses are lost on 72mhz. If you don't believe it get a glich counter and try it. Xtreme's technique is not new. The way I see it and extra receiver amounts to redundancy and is not needed.

Mike in DC

So, the "receiver" sends data back to the transmitter? How else would it know if the information was received. Do other systems work this way (the transmitter can hear from the receiver)?

--Mike

d_wheel

Yes, XPS is a 2 way system. The others do not have this feature at this time.

Later;

D.W.