RTR Radio Range Improvement
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Finding your Savage suddenly out of radio range can be disconcerting, to say the least. Increasing the range of your radio gear can help avoid those traumatic moments. This article explains a simple method for maximizing your radio’s range by tuning the antenna matching circuits of the transmitter and receiver.
The range of your radio gear can be further diminished due to external interference, metal fences, etc., so although the range may seem fine, there may be situations where having more range to begin with can help out in closer proximity operation. A Fail-Safe is always an excellent addition to your radio equipment, but the performance enhancements of this article should not be thought of as a replacement for a Fail-Safe.
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Disclaimer
Before performing any of the instructions in this article, check with your country’s communication Regulatory Authority to make sure no laws will be broken. Serious fines and/or possible jail time could occur. Further information for the USA and Canada can be found below in the Legal Issues section, although the accuracy, currency and completeness of the information is not guaranteed. The author, members and owner of Savage-Central shall not accept responsibility for persons breaking the law.
Radio Equipment
The basic radio equipment required for a Savage is comprised of a Transmitter, Receiver, Steering Servo, Throttle/Brake Servo, Fail-Safe, ON/OFF switch, battery and sometimes a third servo if the Reverse Module is installed. This article will focus on the 27 MHz AM TF-4 Transmitter and RF-2 Receiver.
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Figure 1 - TF-4 Transmitter |
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Figure 2 - RF-2 Receiver |
Increases in RF power output have been measured anywhere from approximately 2 dB to 13 dB. For the TF-4, 27 Mhz AM transmitter, a rather small gain of 1.94 dB was measured. Using the Free Space Equation, it was calculated that if the previous range was 200 feet, an increase of 50 feet could be expected. Subsequent to the procedures outlined in this article being implemented, a TF-4 and RF-2 combination were successfully range checked at over 100 yards with the transmitter antenna collapsed. Typically, a range check only requires operation over 100 feet.
A Futaba 2PL 75 Mhz AM transmitter was also trialed and yielded a 13 dB improvement. Out of the box, this radio had a very poor range of approximately 100 feet. According to the equation, the new range would be 447 feet. Once the receiver is aligned, a further increase in range can be expected. A TF-10 27 Mhz FM transmitter also had a high increase available, but in this case, the FM receiver could not be adjusted without expensive test equipment (typical of FM receiver design) and a schematic. Keep in mind that the maximum allowable RF output for a RC transmitter in the USA and Canada is 4 watts for 25 Mhz and 0.75 watts for a 75 Mhz transmitter.
Transmitter
The TF-4 should not be able to exceed the 4 watt allowable limit. As the transmitted RF power out cannot possibly exceed the power consumed by the transmitter, it is important to monitor the multimeter to make sure this current draw does not exceed 333 mAmps which is calculated from a 4 Watt load with a total battery voltage of 12 volts. This provides some safety factor as it does not take into consideration that some of the current is actually being used to power other electronics inside the transmitter. During testing for this article, 160 mAmps could not be exceeded.
Tools Required
Multimeter or DC milliamp meter
Tuning Wand (plastic, wood, or jeweller’s screwdriver
Soldering Iron
Resin Core Solder, 60-40 or equivalent
Philips Screwdriver
Oscilloscope (optional)
Ruler or Felt Marker
Procedure
- Pull off the two channel trim knobs
- Note which position the throttle adjustment slide is in and remove the single short retaining screw from the back of the transmitter. Usually, the slide is located towards the throttle trigger.
- Remove the 6 long screws from the back of the transmitter and remove the back cover.
- Measure the distance from the antenna base to the antenna electrical connector, or mark the antenna at the connector with a felt marker, and then unscrew and remove the antenna (see Figure 3).
- Note the colour coding of the steering control wires where they are attached to the circuit board and un-solder them.
- Remove the two screws that hold the board to the front transmitter case and flip the board over from the trim control end and pivoting on the power switch end. The steering control wires will have to slide back through the slot in the board to do this.
- Unsolder the red power wire from the batter box and connect the multimeter in series with the red wire and the terminal it came from. Set the multimeter to DC milliamps and the range to at least 200 milliamps.
- Extend the antenna and mount it in its electrical connector using the measurement or you mark as a guide.
- If using an oscilloscope, connect the scope lead to the antenna electrical connector and the scope ground to the negative battery terminal on the battery box (see Figure 4).
- Turn the transmitter ON and make a note of the current draw as read by the multimeter and the RF output level if an oscilloscope is used.
- Locate L4 on the circuit board which is the second coil back from the antenna. Using the tuning wand adjust L4 for a maximum on the multimeter or oscilloscope (see Figure 5). Do not adjust any other coil on the board, although it doesn‘t hurt to check L5, which is the last coil before the antenna.
- Turn the transmitter OFF, and disconnect all equipment.
- Lay the antenna aside, re-install the circuit board with the two screws, re-solder the steering control wires and the positive battery wire to the battery box.
- Install the back cover using all 7 screws. Make sure the throttle slide switch is in the same position as before.
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Figure 3 - Dismantling |
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Figure 4 - Adjustment Set-up |
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Figure 5 - RF Waveform |
Receiver
Tools Required
Tuning Wand (plastic, wood, or Jeweller’s screwdriver)
Small Jeweller’s Screwdriver
Helper
Procedure
- Remove the body and then the Radio Box cover from the Savage.
- Temporarily remove the receiver and gently using a Jeweller’s Screwdriver, separate the plastic receiver case into its upper and lower halves.
- With the receiver re-installed and the antenna routed as normal, elevate the Savage so the front wheels are off the ground.
- Turn on the transmitter and the receiver.
- Have a helper walk away with the transmitter until a point is reached where the steering no longer functions or functions erratically. Having the transmitter antenna collapsed can help keep the distance minimized.
- Have the helper continue to operate the steering wheel on the transmitter right and left.
- Locate the input tuning coil (or potentiometer, dependant on receiver) and turn it a little bit one way or the other until steering control is regained (see Figure 6). Make a note where steering is lost in both directions and set adjust the tuning coil for half way in-between. Do not adjust any other component in the receiver or re-alignment with expensive test equipment may be necessary.
- Have the helper walk further away until the steering is again erratic, or lost and repeat the above step.
- Keep repeating this until the adjustment is zeroed in and no further range is obtainable.
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Figure 6 - Receiver Adjustment |
Tips
- Be very careful when adjusting tuning coils. The coil tuning slugs are made from a very delicate material, (ferrite) and can easily be damaged, especially when using a metal screwdriver.
- A plastic or hardwood tuning wand is recommended as a metal screwdriver affects the results and the performance will change once the screwdriver is removed, although this can be accommodated for to some extent by taking multimeter readings while the screwdriver is not in the proximity of the coil.
- When adjusting tuning coils, especially at the receiver, it can be very helpful to note the number of turns being made so the coil can be set back to where it was if necessary. In the case of the receiver, if control seems lost permanently, bring the Savage within a few feet of the transmitter and adjust the coil until operation resumes.
- The receiver antenna length should not be altered and as long an antenna tube as possible should be used. The excess antenna wire must not be cut and should be coiled up inside the receiver box as far away as possible from other radio wiring. Coiling up the excess retains the electrical tuning length, but the gain of the antenna is reduced. Make receiver alignment with the antenna routed as it will be when in normal operation.
- Having the receiver box open is an opportune time to replace the receiver antenna. Solder in a new stranded wire of the correct length (usually 1 meter) and preferably the same gauge. A soldered splice will work for damaged antennas, but once soldered and insulated with heat shrink, it may be impossible to insert the wire in the antenna tube.
- Transmitters that are extremely close to a receiver can cause jittering of the servos. Keep the transmitter a few feet away from the vehicle during testing.
Legal Issues
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Figure 7 - Jail is RC Free ! |
As per the disclaimer above, the legality of making these adjustments will have to be confirmed by the regulating authority of the country in question. That is the FCC in the USA and Industry Canada in Canada.
USA
- Most of the rules regarding RC can be found in FCC Part 95-Personal Radio Services and a link is provided below. Some pertinent “partial” excerpts (which may not be current, or otherwise contain errors) are as follows:
95.210 (R/C Rule 10) How much power may I use?
(a) Your R/C station transmitter power output must not exceed the following value under any conditions:
Channel Transmitter Power(carrier power)(watts)
27.255 MHz ............................................................. 25
26.995–27.195 MHz ................................................ 4
72–76 MHz .............................................................. 0.75
95.221 (R/C Rule 21) How do I have my R/C transmitter serviced?
(a) You may adjust an antenna to your R/C transmitter and you may make radio checks. (A radio check means a one-way transmission for a short time in order to test the transmitter.)
(b) You are responsible for the proper operation of the station at all times and are expected to provide for observations, servicing and maintenance as often as may be necessary to ensure proper operation. Each internal repair and each internal adjustment to an FCC certificated R/C transmitter (see R/C Rule 9) must be made in accord with the Technical Regulations (see subpart E). The internal repairs or internal adjustments should be performed by or under the immediate supervision and responsibility of a person certified as technically qualified to perform transmitter maintenance and repair duties in the private land mobile services and fixed services by an organization or committee representative of users in those services.
(c) Except as provided in paragraph (d) of this section, each internal repair and each internal adjustment of an R/C transmitter in which signals are transmitted must be made using a nonradiating (‘‘dummy’’) antenna.
(d) Brief test signals (signals not longer than one minute during any five minute period) using a radiating antenna may be transmitted in order to:
(1) Adjust a transmitter to an antenna;
(2) Detect or measure radiation of energy other than the intended signal; or
(3) Tune a receiver to your R/C transmitter.
95.649 Power capability.
No CB, R/C, LPRS, FRS, MedRadio, MURS, or WMTS unit shall incorporate provisions for increasing its transmitter power to any level in excess of the limits specified in § 95.639.
- It is further recommended that all of the FCC Part 95-Personal Radio Services be read in its entirety, with particular emphasis on the following sections:
95.209 (R/C Rule 9) What equipment may I use at my R/C station?
95.210 (R/C Rule 10) How much power may I use?
95.218 (R/C Rule 18) What are the penalties for violating these rules?
95.221 (R/C Rule 21) How do I have my R/C transmitter serviced?
95.222 (R/C Rule 22) May I make any changes to my R/C station transmitter?
95.603 Certification required.
95.639 Maximum transmitter power.
95.649 Power capability.
95.651 Crystal control required.
Canada
- Industry Canada is the regulating authority in Canada and most of the rules regarding RC, can be found in Spectrum Management and Telecommunications Radio Standard Specification 210 (RSS-210). A link can be found below. Some pertinent excerpts (which may not be current, or otherwise contain errors) are as follows:
A1.2.1 26.99-27.255 MHz
This band is only for one-way, non-voice usage for remote controls, under the following conditions:
a) Only the following channel carrier frequencies are permitted: 26.995; 27.045; 27.095; 27.145; 27.195 MHz and 27.255 MHz.
b) For single sideband modulation (SSB), the transmitter RF peak envelope power shall not exceed 4 W. For double sideband (amplitude), digital or frequency modulation, the transmitter unmodulated carrier power shall not exceed 4 W.
A1.2.3.1 Carrier Frequencies
(2) 75.4-76 MHz General Remote Control The following frequencies (in MHz) are for general usage remote control of any type other than for control of an aircraft model. Voice modulation is permitted for emergency use if it is of the push-to-talk type. The centre or carrier frequencies (30 frequencies spaced in 20 kHz steps) are as follows: 75.41; 75.43; 75.45; 75.47; 75.49; 75.51; 75.53; 75.55; 75.57; 75.59; 75.61; 75.63; 75.65; 75.67; 75.69; 75.71; 75.73; 75.75; 75.77; 75.79; 75.81; 75.83; 75.85; 75.87; 75.89; 75.91; 75.93; 75.95; 75.97; 75.99.
A1.2.3.2 Standards Specifications
(1) For single side band modulation, the transmitter RF peak envelope power shall not exceed 0.75 W. For double sideband (amplitude), digital or frequency modulation, the transmitter unmodulated carrier power shall not exceed 0.75 W.
