A receiver antenna length is not critical by any means , it can be to short.
All you had to do is stop by any model airplane hobby shop and measure an antenna.
I walked over and measured mine. "36" inches. any thing 36 or more of a number 22 copper wire will be fine.
If you do not have room for 36 inches. You can coil it up but you will lose range. The bigger and further apart the windings the better best not to over lap the windings.
You can see several mostly P.R. diesel engines or steam tenders with a antenna wraped around and insulated off the roof. Then coil the rest of the length inside.
Length is very critical for transmitters. There you have to worry about (SWR) standing wave ratio or reflected back power a real killer.
I can't claim this to be absolutely true, but I believe the UAVs are probably controlled via "Spread Spectrum"/"Frequency Hopping" radio and the data itself is encrypted such that there would be no way to break into the signal and take over the craft.
"Spread Spectrum"\"Frequency Hopping" is where the transmitter and the receiver both know a particular sequence of frequencies/channels to tune to for just a few milliseconds each before moving on to the next frequency/channel. Unless you KNOW the sequence of frequencies you cannot transmit on the correct one to get data to the receiver. The actual frequencies can range all over the place, including TV and commercial radio, not just frequencies reserved for military use. The length of time on any one frequency is so short that even if you were watching a TV channel that the control signal used in the series, you would be hard pressed to see or hear a slight blip.
Also, although the "set" of frequencies might be somewhat limited for any one "mission", the series is not a repeatable series. The selection of the next frequency is determined by a pseudorandom number generator, so, say, frequency X might be followed by frequency R one time, but the next time frequency X is selected it might be followed by frequency E, or Q or any frequency from the "set" for the mission. The "set" might contain many dozens of frequencies. Nobody can just listen with a broadband receiver and determine which frequencies are in use and what the sequence is. Besides, with all the other military devices in use doing anything remotely similar (voice, data, etc.) there is so much "noise" on the same frequencies, the listener wouldn't know which signals are destined to which device.
Spread Spectrum/Frequency Hopping also keeps anyone from building an extremely high power transmitter to overpower the receiver and render the device uncontrollable. It would require so much power over so large a frequency range as to make it too expensive to do and would eliminate their own communications.
The frequency series and the encryption keys would be selected at launch time so it would require considerable "insider knowledge" to gain control, and no one person knows both pieces of information. It is conceivable that one or both pieces of information are not known by anybody on the ground at the lauch site. They are probably not even be known by the person doing the actual flight control.
There is a commercial R/C control for toy airplanes, cars and trains now that does something "similar". See http://www.spektrumrc.com/ It doesn't necessarily "encrypt" the data, but each transmitter and receiver know an "address" that is sent with each piece of control information such that many Transmitter/Receiver pairs can all be on the same frequency and not interfere with each other. The two devices "train" with each other at the moment they are turned on and "learn" the address to use. It is supposed to be completely "glitch free" (the radio interference caused by the locomotive wheels on the rail is ignored by the receiver since it would not contain the 'address' data). It also eliminates the problem I had with the kid down the street wrecking my servo!
Semper Vaporo
Pkgs.
I'm not too sure that it is legal to alter a transmitter in any way, except within parameters set by the manufacturer with approved parts for THAT particular transmitter. There are specific limitations on Effective Radiated Power and the antenna will most definitely have an effect. (That is the point of altering the antenna, right?)
If you replace the antenna with one designed for that transmitter then I think it would be legal (but I do NOT know that for sure!). You can change the crystal to alter the transmitter frequency, but only within the frequency range specified for that type of transmitter and its usage... The crystals in my R/C all look pretty much like many others I have seen, same diameter pins and distance between them, but I know I am not permitted to put one in that would make my transmitter use a television channel frequency (not that the rest of the circuitry would be "tuned" well enough at a frequency that far away from the designed frequency).
In the old "Citizen Band" system (I'm not sure if that even exists like it used to be 40 years ago... the last time I knew anything about it) transmitters were not "supposed" to be connected to antennas of over a certain height or of a certain "gain"... but lots of people seemed to think that CB was their gateway to being famous "Disk Jockeys" to be heard all over the world and bought power amps and special antennas and mounted them on towers or buildings that put them over the "legal" height and Effective Radiated Power and thus inteferred with other users that should not have had to worry about interference from someone in the next city over.
Also, if you "soup up" your transmitter, you could damage somebody else's equipment if your signals overpower their receiver.
This has happened to me, when the kid down the street got a new R/C car and his transmitter was on the same frequency as mine and his "steering" was set to drive the servo beyond what my receiver servo was capable of moving and it stripped the gears in MY servo (approx $20.00 to replace). It was kind of my fault, because I turned on my receiver before I turned on my transmitter, but he was a block away and I didn't even know he was around until I saw my servo move to the stop and then whine as the motor spun and the gears didn't... then I saw him walking down the street steering his car from one side to the other.
All very interesting everybody, I'm using Loco-Linc RC and have been told about changing the factory antenna with another manufacturers antenna for better reception and distance, LEGAL??? or not.....
Byron
cokeoven wrote:By the way, what would the penalty be for getting caught with a plane radio in a model railroad locomotive? A small fine or worse?
The penalty would probably depend on what your transmitter interferes with and the damage caused by that interference.
A local radio control model airplane club was asked to move their airfield several miles away from Fort Huachuca, one of the sites where Unattended Aerial Vehicle (UAV) pilots are trained. The reason given was interference with military communications and that some club members were losing control of their planes when they flew too close to the post.
Some of the frequencies used can allegedly interfere with commercial aviation, too.
Oops! You're right! I hadn't thought about that. And I should have remembered it... it cost me a bundle when I got R/C for my trains. I had to buy a "Car" controller and wanted to control 3 things (Throttle, Reverser, Blower) and "Car" controllers that have more than two channels are much more expensive than "Airplane" controllers with multiple channels for some dumb reason (maybe it is just that they saw a sucker come in!).
You should be able to send the R/C to the mfg to have it retuned for ground frequencies. You might also be able to simply replace the transmitter and receiver crystals (which the law allows the consumer to do without license if the mfg has provided a simple way to do so... i.e.: no soldering or "tweaking" of coils or 'tuning' of parts). Check with your R/C hobby shop for the availability of crystals for "Ground frequencies" for you model of R/C.
I don't think there is anything special about antenna wire. I believe any old copper or steel wire will do. I do know there are restrictions (laws?) about using frequencies for air RC on ground RC applications though.
-Brian
Any piece of wire can be used. Copper is probably best, but brass, steel and aluminium will also work. It should probably be electrically insulated, but can be bare wire, as long as it doesn't short itself out someplace if wound in a coil or loop, or to a ground somewhere along its length. Don't use coaxial wire (a coax has one conductor inside an insulation sleeve that is covered by another pipe of wire that is usually a bunch of very thin wires woven or braided around the insulated inner wire).
There is some need to make it a particular length. I assume it "was" just a wire sticking out the side of the receiver module. That makes it a "mono-pole" type and it should be 1/4 the length of the signal frequency it is to receive. You need to know what frequency the R/C operates on. Probably something in the 27MHz range? Look for the number on the label of the R/C (transmitter or receiver module) or on the crystal that plugs into it. Take that number (in MegaHertz, "MHz") and divide it into the number 234. The answer is the length "in feet" that the antenna should be.
Example: 27.045 Mhz is what it says on the crystal in my R/C.
234/27.045 = 8.652246256 feet.
Now, can you cut a wire to that "exact" length? Probably, but is it necessay to be that EXACT length? And, do you have room for that many feet of wire hanging off the side of your train? The answers are: NO!
The longer the wire, the better (up to a bit over 8 and a half feet), but you don't have room for that much wire. Besides, the wire you cut off was probably much shorter. The reason it was shorter is because the receiver has a coil of wire inside it that makes up for the original antenna length to make it 1/4 wavelength. I bet the wire that you cut off was between 1 and 2 ft long. Maybe more, maybe less. It would be nice if you knew what that length "was". Take a guess and cut your new wire close to that length. The closer the length of the antenna 'system' is to 1/4 wavelength, the greater the distance you can cover with the R/C system. But it is NOT necessary to be all that accurate.
I guess you could experiment by starting with something you know is longer than the original wire and see what distance you can still control things. Then cut it a bit shorter and see what distance you get. If it improves, then cut some more off. Keep it up until the distance gets worse. Get a new wire and cut it to the length of the where you got the best distance of control (maybe some average or middl'in value in the range of best distances).
Another thing you could do is go to a hobby shop that sells R/C systems to see if you can find the same model of receiver and measure the length of the antenna.
One more thing... unless you have an FCC license, do not mess with the transmitter.
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