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REMOTE CONTROL By BOB YOUNG Operating an R/C transmitter There are many considerations that go into operating a modern radio control transmitter and we will examine some of these in a practical way. In particular, we will have a look at how the antenna radiates the signal and how this can affect the control of your model. To begin with, the prime factor is transmitter output. The considerations involved in this one aspect of R/C design are numerous and cause most of the spurious service calls and practical problems encountered in operating on a busy club field. This entire column will be devoted to this one aspect, although the solo operator does not encounter many of these difficulties. The circuit designer is well aware that, as the popularity of the sport grows, the number of people using flying fields and boating/car sites is increasing. Because each site can only support a given number of users, it is important ·that the minimum output power needed for operational safety and reliability be used. This allows more fields to be operated within any given city. Two hundred watt linears are definitely out in the model business. Unwanted interference is not merely annoying - it can have lethal results. A model travelling at 160km/h can kill. Besides that, government regulations on the use of R/C equipment limit the power to 2 watts total DC input with a maximum EIRP (effective isotropic radiated power) of + 30dBm (1 watt). Most modern systems use less than this, with 500-600 milliwatts being typical. This will give out of sight range on a model aircraft. At this power, fields can be as close as 5km apart. In Australia, we are allowed to operate on the frequencies 29.72 to 30.00MHz, 36.00 to 36.60MHz, and 40.66 to 40.70MHz. The 36MHz band is limited to competition type aircraft and watercraft only (Dept. of Transport and Communications Fig.1: the radiation pattern from an antenna is not symmetrical but has a number of lobes instead. The signal is weakest at the antenna tip so don't point the antenna at the model - it could crash! 12 SILICON CHIP DOC 60 and DOC 303, 1988). All other frequencies are illegal for radio control of models. Reduced range One very interesting and important point to keep in mind when operating models, particularly aircraft, is that the range is reduced as the Rx (receiver) antenna is brought closer to the ground and this effect is even more noticeable as the frequency is increased. The effect is worsened by the "lobes" of the transmitter antenna whereby, when the antenna is vertical, the signal strength radiated from it varies quite dramatically according to direction. Fig.1 shows the transmission lobes for a typical antenna and as you can see from this, the signal is weak at points A and B, and directly along the ground. Thus, flying low a long way out is not recommended. When range checking your model, be sure that the Rx antenna is the same height above the ground each day. When the Rx antenna is within 1 metre of the ground, the range is very badly affected and great changes in range will occur for every 15cm change in height. The moisture content of the ground also plays some part and results can differ from day to day as a result. Changing receivers from model to model will often also give differing results when range checking. The reason is often simply that the Rx antenna in one model is closer to the ground than in the other. Don't point the antenna Another important consideration that designers and flyers must take This Christmas at David Reid Electronics 127 York Street SYDNEY 2000 Ph: (02) 267 1385 MAIL ORDERS P.O. BOX 0103 SYDNEY 2000 FAX: (02) 261 8905 David Reid Electronics POST & PACK $5-$25 .... $4 $26-$50 ... $5 $51+ . . . .. $8 BUSINESS HOURS Mon-Fri B.30-5.30 Thur 8.30-7 .00 Sat 9.00-1.00 avoid pointing the Tx antenna at the model, placing the Rx antenna in close proximity to battery and servo leads, or flying low a long way out. Car enthusiasts beware Unless you take precautions, the antennas on adjacent transmitters can absorb or reflect your signal, leading to momentary loss of control. There's over 50 cars on the starting grid here so there's plenty of potential interference problems. If an unwanted signal is very close, it can swamp the front end tuned circuits in your model. into account is that the transmission lobe from the Tx antenna is theoretically donut shaped. This means that the signal is the weakest at the tip of the antenna. Thus, flying low and pointing the antenna at the model is definitely taboo. It can lead to a momentary loss of signal with serious consequences for the model. This condition is aggravated by 14 SILICON CHIP the fact that the transmission pattern is not a perfect donut. It is instead quite jagged in shape, resulting in weak signal areas in the most unexpected places. All of this must be taken into account by the system designer when setting the parameters for Rx sensitivity and Tx output power. So the flyer should keep in mind the possible dangers. He should Still on the problems associated with transmission patterns and levels, there is a special problem for the R/C car enthusiast. The average model car meeting always looks to me like an old Errol Flynn movie. There are 10 to 20 drivers, all waving their transmitters around excitedly, with frequent clashes of antennas. Add to this the fact that the track is usually in close proximity to the drivers and you have a recipe for trouble. What model car enthusiasts must realise is that other transmitter antennas act like reflectors and directors. This means that other antennas near your transmitter's antenna can absorb or reflect your signal. Thus, each adjacent antenna reflects some power away from the line of radiation. As well, the tuned circuit at the base of the other antennas will absorb some power, as is illustrated readily by placing another Tx, fitted with an output power meter, in close proximity. Its meter will show a reading, even though the Tx is switched off. The end result of all of this is that when a car comes into the straight and the driver of that car is at the far end of the queue, with many transmitters between him and his car, the signal is probably at its weakest point. Add to this a sudden savage increase in radiated power from much closer transmitters falling on the Rx antenna, servo leads, battery leads and circuit board lands, and the stage is again set for glitches (momentary loss of control). Great care must be exercised if you want to win races consistently. It is surprising how many complaints I receive from drivers who feel their receivers are out of tune or worse still, that other transmitters are off frequency. Once I point out these simple facts their problems often go away. There are of course always genuine Developed by the author, this 16-channel transmitter can control a range of functions on a model truck, including steering, brakes and gear change. cases of interference but they are rare. Another common problem here is the positioning of the Rx antenna in the car and this will be dealt with later. Designers of radio control systems have their special problems and the foregoing is one of them. If an unwanted signal is very close and very strong, it can swamp the front end tuned circuits in the Rx, The effect is similar to that which you experience when your hifi system is subjected to a CB'er driv- ing past your front door. The music is drowned out in a burst of voice transmission. This problem is not to be confused with an Rx bandwidth problem (ie, the ability of an Rx to reject signals close to the Tx frequency), although this does play some part in the problem. The bandwidth problem will be discussed in a later column. It is primarily an artificial problem, caused by the close proximity of many transmitters. Most of the problem probably comes from the base of the audio or IF (intermediate frequency) transistors rectifying the RF signal present on the circuit board tracks and demodulating it. I noticed that the problem got much worse after the change from metal Rx cases to plastic. I have even encountered situations where, after removing the Rx crystal from lts socket, the Rx could still be operated at close range. Some particularly bad cases will operate with power applied to the audio stage only. When you are on the end of the Radio-Controlled Motorbike from Dick Smith Electronics / Ii Currently available from Dick Smith Electronics, this radio controlled motorbike is typical of the diversity of models that are available. The transmitter (above) is a 2-channel proportional unit giving control of steering and motor speed. It is priced at $199 (Cat. Y-2541) from all Dick Smith Electronics stores. DECEMBER 1989 15 ...-------------------'l,,,,,,.---4,________,__.,..__..... ~-+9.6V .,.. r ---, TEST I I INPUT n--➔-----41._,__,..__ _ __.,..__--4..,___,..___..,_____...,__..,__ __._---4.,___---4~--~--+--,.__---0METER- fig.2: this circuit is typical of transmitters in use today. Note how the meter and associated diode is fed via a capacitor from the antenna. That explains how you can read the power absorbed by your antenna from other transmitters when your transmitter is switched off. queue therefore, you are really up against it, so please be careful. The same applies to aircraft taxiing in the pit area or landing or taking off in close proximity to other transmitters. The answer to this problem is really simple. Do not set your field up in such a way that models come within 7 metres of any Tx. Avoiding a crash Finally, the most dangerous situation of all is when two transmitters operating on the one flying field are separated by several hundred metres. Even though they may be on separate frequencies, if one model flies close by the other Tx, the disparity of signal levels can be so great that the Rx in the model cannot function properly. The most likely outcome is a crash. This is a situation which should never be allowed to exist on any model flying field. It is most often encountered on fields mixing power models and gliders using bungee and winch launches. Thus we can see that quite apart from battery consumption and government regulations, there are many considerations that go into just how much power should be fed into a radio control transmitter. Excess power is just a nuisance. Before leaving the Tx output stage, let me give a few practical tips. Do not run your Tx for extended pertods of time with the Tx antenna collapsed as this will result in a large increase in current 16 SILICON CHIP Club meetings like this are becoming increasingly popular. If you want to win, keep your antenna clean and the rest of the gear in tip-top shape. in the output stage with subsequent overheating of the output transistor and possible failure. This is fairly common problem for the poor old serviceman. Antennas The antenna is also a very important item. Most telescopic antennas on modern R/C transmitters are just barely adequate for the job. Ideally, we should be using ¼-wave antennas but on the frequencies we are · using these would be up to 2.5 metres long and are just not practical. The circuit designer therefore uses base loading coils to artificially extend the length. Unfortunately, telescopic anten- nas are quite flimsy and it is easy to loose one or more sections of the antenna. If this happens, the Tx will be very badly detuned and a noticeable loss of range can result. Do not fly with a damaged antenna. Likewise, always keep the joints clean and lubricated. CRC 2.26 is ideal for this job. I am always amazed at how much dirt comes out of those joints. I don't know how some transmitters that I see keep going. As you can see, the Tx RF section is quite a vital section of your R/C system. Learn how to use it, look after it and you will be rewarded with models that live a long and comfortable life. See you next month. ~