RADIO FREQUENTLY ASKED QUESTIONS
(The following information is for radios manufactured to operate within the USA under FCC ruling)
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Can_I_change_crystals_on_my_radio?
What_is_Hi-_band_Low-band?
What transmitters and
receivers work together?
Will_servos_interchange_between_radios
Why_no_channel_20_and_21?
Is_my_transmitter_legal?
How long is receiver antenna?
What is a Mode 1
vs. Mode 2 transmitter?
How do you reverse a servo?
Can I change crystals on my radio?
The changing of a transmitter crystal by the user is illegal under FCC rules. All transmitters should be sent to the manufacturer or a certified maintainer for altering the frequency of the transmitter. The transmitter will be retuned after changing the crystal to ensure that it is within specifications on the new channel. If your transmitter uses a plug-in module then you can purchase additional modules on different frequencies and legally swap modules in the transmitter.
Most receiver crystals can be changed without retuning. Futaba does specify a high-band and a low-band receiver and crystals should only be changed within the band on these receivers. (See Hi-band Lo-band below)
Most receivers are tuned to the center of the band and can use any crystal within the band. Early JR and current Futaba receivers have their front ends tuned to the center of either the high-band or the low-band. The low band is channel 11 thru 35 and high-band is channel 36 thru 60. On these unit only a crystal within the sub-band should be used.
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What transmitters and receivers
work together?
Pulse sequencing - Although I know of no valid reason, manufactures have chosen to differ in the position for which channels are placed in the encoded serial output. The stick pot you connect to the encoder channel 1 input defines the function of the first channel. Any stick pot could be connected to any input.
Futaba and Hitec use a sequence of aileron, elevator, throttle, rudder. JR selected throttle, aileron, elevator, rudder. While Airtronics chose to send elevator, aileron, throttle, rudder.
On AM modulation, all transmitters will operate all receivers, and if you wanted to use a Futaba receiver with a JR transmitter you would just have to remember that you plug the servo’s into the receiver based on the transmitter sequence and not the receiver definition. The receiver does not care which control is on what output.
On FM PPM we have a different story. Manufactures have chosen not only to rearrange the pulse sequence but to also select opposite high/low frequency shifts.
Positive vs. Negative Shift: When working in FM PPM mode, Futaba and Hitec are compatible, and JR and Airtronics are compatible. This is because of the way that each has chosen to implement the frequency shift to represent a data bit. One has selected to send the lower frequency normally and shift to the higher frequency to represent a data pulse, while the other sends the higher frequency normally and shifts to the lower frequency to represent a data bit.
JR and Airtronics are compatible xmitter to receiver, and Futaba and Hitec are compatible. What this means is that a JR receiver will work with an Airtronics transmitter and vise versa if you remember to plug the receiver according to the transmitter sequence. Hitec also make a positive shift receiver compatible with JR and Airtronics. They also make a shift selectable receiver that can be used with all transmitters.
Futaba and Hitec are also compatible using the same rules.
Airtronics and Hitec now offer a transmitter where you can select the shift mode that means that they can be set to work with either brand receiver.
Note: To my knowledge, radios designed for
countries other than the USA do not have shift incompatibilities.
PCM encoding/decoding is proprietary to each manufacturer. To my knowledge, no manufactures PCM receivers will operate with another manufactures transmitter.
If you do choose to operate one manufactures receiver on another manufacturers transmitter, be sure to range test the combination before using it.
NOTE: I recently received an E-Mail from an individual stating:
I am a RC
pilot flying airplane and helicopter. Recently, I faced funny problem. I have
JR 9303 and RD 6000. When I switch models in JR, my Airtronics receiver started
not responding correctly. However, when I did the same with JR receiver,
everything worked fine. I though it is a bad receiver. However, When I came
back home and started checking again, I found funny thing as follow.
Case 1.
JR 9303, JR
receiver, and Airtronics receiver (7ch DC, CH 48)
è è when model changed from on to another, Airtronics receiver
start not responding. However, JR receiver showed no problem.
Case 2.
RD6000, JR
receiver, and Airtronics receiver (7ch DC, CH 48)
è è both worked good
Case 3.
JR 9303, JR
receiver, and Airtronics receiver (7ch DC, CH 48)
è è back to previous model, everything worked fine.
Observation
è è 1. Airtronics receiver with JR sometimes not working right
è è 2. Airtronics radio and JR receiver work OK always
è è 3. with Airtronics, workd not good when throttle is low on
JR radio. However, no problem was found when the throttle was high.
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Will servos interchange
between radios
To my knowledge, all of today’s servos are interchangeable between manufacturers if you alter the plug. If you alter the plug does not mean just changing it to fit in the hole, but also ensuring that the three wires are correctly oriented. Specifically, old Airtronics (non "Z" connector) puts the + power lead (red) on the outside, while JR, Futaba, Hitec and Airtronics "Z" place the + power lead in the center. To use an old Airtronics servo on JR, Futaba, Hitec or Airtronics "Z” you must alter the position of the power leads in the plug. Likewise to use JR, Futaba, Airtronics "Z" or Hitec on an old Airtronics they must be altered to the old Airtronics configuration. The new Airtronics "Z" connector is the same as the JR and Hitec connector.
Although there is a slight design difference in the connectors (See Figure below), all of them are now using the .1 inch pin spacing, and with a little rework the Futaba plug can be made to cross fit between JR, Hitec, Airtronics "Z" and Futaba radios. Looking at the Futaba plug from the end you will find a tab coming out the side that is used to “key” it into the socket. If the tab is removed, and you bevel the edges, then the plug will fit into JR, Artronics "Z" or Hitec. Watch out because you no longer have the keying tab to prevent you from plugging this in backwards in the Futaba. The JR, Airtronics "Z" or Hitec plug, with the above precaution, will fit into a Futaba without change.
Note: A standard JR, Hitec, Airtronics "Z" and Futaba servo
will not be damaged if plugged in backwards. They just won't work.
If a JR, Hitec, Airtronics "Z" or Futaba is plugged into an old
Airtronics, or vice versa, without correcting the power lead configuration, it
will burn out the servo.
In 1988 when narrow band RC channels were first introduced, RC channel 20 was found to create a problem when operating close to a channel 4 TV station. The problem did not affect the channel 20 users, but potentially affected all other channels. The wide-band single conversion receiver was the most popular receiver in use during this time period and was the receiver affected by the channel 20 problem. To understand how this problem occurs we must understand the internal workings of our receivers.
The single conversion receiver has a front end (RF section) which is tuned to broad carrier frequency, this is followed by an IF (intermediate frequency) section, which was tuned to the industry standard 455 KHz. An oscillator is required in the receiver to beat against the incoming carrier frequency and produce the 455 KHz IF frequency. This beating of the frequencies, called “heterodyne”, is the basic operation that we call conversion today.
The advantage of this conversion to 455 KHz is that it is easier to narrow-tune the bandwidth of this lower frequency and that one receiver, with a fine tuned IF bandwidth, can be made to operate across a wide spectrum of channels by changing the beat oscillators frequency. Applying this to our environment we see that the front end (RF section) of our receivers are tuned to a bandwidth, which covers multiple channels. An internal crystal controlled oscillator, whose frequency is controlled by the crystal plugged into the receiver, generates the beat frequency necessary to convert the incoming signal to 455 KHz. The 455 KHz IF section of a wide-band receiver is tuned to pass frequencies from 435 to 475 KHz (40 KHz).
With this understanding, we can explain the problems and the reasons for excluding RC channel 20 (72.190 MHz). Television channel 4 broadcasts is audio on 71.750 MHz. This channel 4 audio signal is received by all receivers but is normally disregarded by the receiver as it falls outside of the band pass filters. However, if RC channel 20 is in use, its frequency (72.190 MHz) will also be received by all of the RC receivers. These two frequencies when mixed produce a difference frequency of 440 kHz. This fell into the old wide band 455 kHz IF amplifier frequencies, so the IF would amplify the 440 kHz mix of the channel 20 transmitter data with the channel 4 TV audio, in addition to the wanted data from our transmitter. This produces a noise injection into the receiver, which made it impossible for the receiver to distinguish the wanted data. The solution was not to use RC channel 20 at RC flying fields where there was a strong TV channel 4 signal.
In 1991 the odd numbered channels were implemented. Now there was channel 21 (72.210 MHz). It mixed with TV channel 4 audio and produced a 460 kHz difference frequency. So now, even narrow band receivers, which have their IF, tuned to 445 to 465 were susceptible to interference. Clubs, which had channel 20 problems, now banned channel 21.
Evolution to Dual conversion receivers, with narrow tuned 10.7 MHz first conversion stages, and even tighter tuned (10 KHz intermediate sections) has virtually eliminated the problem. But....... There are still old single conversion 20 and 40 KHZ bandwidth receivers in use today and the channel 20/21 problem will remain until these units are phase out.
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Is my transmitter legal?
If your transmitter is narrow-band then it is legal to operate. The problem is that the AMA gold sticker program, which identified narrow band equipment, is no longer used.
Narrow vs. Wide Band Operation:
Wide band: A term used in R/C to describe a system that utilizes a 40
KHz maximum bandwidth. This was termed Silver Stickered.
Narrow Band: A term used in R/C to describe a system that utilizes a 20
KHz maximum bandwidth. This was termed Gold Stickered.
These terms were created to identify the evolution of bandwidth restrictions in
getting from our initial 72MHz allocation to our current 50 channel operation.
Initial operation in the 72MHz band used equipment that operated at an 80 KHz
bandwidth. Newer equipment operating in the 40 KHz bandwidth was introduced
around 1985 and is what is known today as wideband. This equipment operated on
channel 38 thru 56 (even numbered)
A two part evolution to 50 channel operation began in 1988. First the use of
the old 80 KHz bandwidth equipment became illegal on Dec 20, 1987. Twelve new
channels 12 thru 34 (even numbered) were allocated to replace the old 80 KHz
bandwidth channels. These channels were defined as narrow band and were
required to operate in a 20KHz bandwidth.
Wide band equipment continued in use on the upper channels until 1991 when the
remaining channels were implemented. Most clubs banned the use of wide band
transmitters at this time.
The use of wide band transmitters became illegal under FCC rule on March 1,
1998.
Note: Current FCC regulations require that an RC transmitter operate within an 8KHz bandwidth.
Sec. 95.663 Emission bandwidth. "(b) The authorized bandwidth for
any emission type transmitted by an R/C transmitter is 8 kHz."
Now we can derive from this that no 72 MHz transmitter using a bandwidth
greater that 20 KHz is legal today. Although this is a good statement, how do
we determine that?
1) Any transmitter using the old 80 KHZ bandwidth frequencies is illegal. (If
it's on 72.080, 72.160, 72.240, 72,320, 72.400, 72.960 or 75.640 it is an old
80 KHz unit)
2) If it was manufactured on a odd numbered channel, or a channel below 38,
then it is narrow band and legal.
3) If it was manufactured after 1991 it is legal.
4) If it has an AMA Gold Sticker on it it's legal.
5) Futaba also states that "ALL of their FM transmitters" meet the
20KHz requirement. Not true with their AM units. So if it is a Futaba FM unit
it is legal.
This leaves us with the units that were manufactured from early 80's through 1991
on channel 38 through 56 (even numbers) that may not have met the 20 KHz
bandwidth. Although the AMA Gold Sticker program was in effect, the program was
dropped and current unit do not carry them.
Bottom line is if the unit was manufactured before 1991 on channel 38 through
56 it should have a gold sticker, but how do you know when it was manufactured.
Caution, Following the 1988 through 1991 years when the new frequencies were allocated, many people replaced the original crystals in their transmitter with one of the new frequencies to get to an uncrowded band. All of these units were not retuned and may not meet the narrow-band requirements even though they are now on an odd channel or a channel below 38. The odd numbered channel or a channel below 38 only applies to radios manufactured on those frequencies.
How long is a receiver antenna?
Normally 39 inches for any 72 MHz frequency.
What is a Mode 1 vs. Mode 2 transmitter?
Mode I - Right stick horizontal is aileron, right stick vertical is throttle. Left stick vertical is elevator, left stick horizontal is rudder.
Mode II - Right stick horizontal is aileron, right stick vertical is elevator. Left stick vertical is throttle, left stick horizontal is rudder.
To understand why we have Mode 1, you must go back to the days of reed radios. In a reed radio you did not have a 2 axis stick or fully proportional channels. Each control had its own self centering switch that you manually pulsed to operate the control. Your two primary switches were rudder (or aileron) on the right side of the transmitter (right hand thumb operated), and elevator on the left side of the transmitter (left hand thumb operated). Looking at the right switch it moved horizontally. When you moved the switch to the left, the servo went to full left and stayed there. When you released the switch the servo returned to neutral (These were true self centering servo’s). To turn left at less than full throw you would pulse the switch. The same applied to elevator on the left switch except it moved up and down.
When the proportional radios came out, the sticks were connected so that the right horizontal was aileron, and the left vertical was elevator. This was because reed equipment flyers were use to this mode of operation.
I won’t get into the argument of which mode is the best, but most new flyers, with no previous preferences, start with mode II.
To reverse a servo you must reverse both the two leads going to the motor, and the two outside leads of the feedback pot. On early servo's this was easily accomplished as the circuit board was attached to the motor and pot using wires. On newer servo's, the board is usually soldered directly to the back of the motor and the pot has long legs on it which also directly attach to the PC board. Reversing one of these newer integrated board servo's is much more difficult.