INSTRUMENTS FOR ANTENNA DEVELOPMENT AND MAINTENANCE R. P. Haviland, W4MB The work of developing and testing a new antenna is much less if proper tools are available. This includes the tools for making measurements of the performance of the antenna at each stage of its development. Even if you are just assembling a store-bought antenna, its best to do some testing before it's at the top of the tower. And for that used bargain beam from the hamfest, measurements can tell if its really a bargain, or just a source of aluminum. The following is partly a review of antenna measuring techniques and instruments. The rest is a compilation of methods and tricks which have been found useful. The methods include some suggestions for modification of standard devices for better use in antenna work. Let's start with some very old-fashioned items that I havn't seen used in years. SIMPLE VOLTAGE AND CURRENT MEASUREMENT Two useful tools for the antenna developer are a light bulb and a neon tube. Or more properly, a handful of bulbs of various ratings. The filament ones will be used as current indicators, and each rating is good for only a 10-1 range of current or so. Two types of neon bulbs are useful, the small 1/10 watt ones with rod electrodes, and the larger 2 watt ones with two D-shaped electrodes. Low-voltage dial-lite bulbs are readily available in current ratings from 60 ma to several amperes. Some power is needed to operate these: in a 50 ohm line one ampere is 50 watts, and 100 ma is one-half watt. These levels are transmitter levels rather than signal generator levels, but as discussed later, the transmitter is a common test instrument. Use the filament bulbs whenever a current indicator is needed. A useful item is a pair of small boxes, each with two RF connectors joined by a lamp socket as in Fig. 1a. One unit makes a useful telltale indicator that current is flowing. With two connected in series by a quarter-wave length of transmission line, as in Fig. 1b, they become a visual SWR indicator: when the bulbs are equally bright, the SWR is 1:1. This is especially handy in adjusting the base tuning elements of a low frequency vertical. Of course a SWR meter is more accurate, but, for example, do you want to expose your Bird (tm) to the rain. Or keep a spare in your emergency kit. Later a better lamp type of SWR indicator is shown. A neon bulb is useful as an indicator of voltage. As for the filament bulb, it is a wide-band device. A quick check of antenna performance is to bring a neon bulb close to the ends of antenna elements, or touching if low power is being used. The effectiveness of Yagi elements is shown by their differing brightness. Unbalance between the two ends of elements can be detected the same way. And don't forget the old test of a transmitter- If the bulb glow is rose rather than orange, suspect the presence of a VHF parasitic. Be careful: mount the bulb at the end of a dowel or rod of insulation to keep hands away from high DC and RF potentials. The sensitivity of a neon bulb can be increased by a passing a small DC current through the bulb, just enough to cause a tiny glow. A circuit is shown in Fig. 2a. If the glow starts when the switch is closed and continues with the switch open, the RF voltage is between the striking and extinction voltages, typically between 50 and 67 volts. Adding a calibrated potentiometer as in Fig. 2b converts the indicator to a measuring instrument. The RF voltage is equal to the the striking voltage of the neon bulb minus the dc voltage from the potentiometer. Adding a short dipole at the bulb-choke terminals makes a crude field voltage indicator. A four foot fluorescent bulb makes a good indicator of antenna performance during the final full-power check of a new antenna. It also impresses the neighbors. BETTER VOLTAGE AND CURRENT MEASUREMENTS. While the above are useful indicators, for good work measurements should be possible. This means, at least, an indicator with a calibrated scale. Professionals now use digital instruments, but most are too expensive for amateur use. But don't forget to watch the swap and surplus sales. RF voltmeters are invariably a rectifier-DC voltmeter combination. A solid-state type is shown in Fig. 3a. A germanium diode is shown, for good response at low voltages. Such a unit is good to about 50 watts at 50 ohms. For higher voltages, silicon rectifiers can be used, or the vacuum tube rectifier of Fig. 3b. Units can be calibrated by comparison to an already calibrated unit. Other methods are to use a thermocouple ammeter as a standard, measuring the voltage across a known load, or to shunt the coupling capacitor with a larger one, using a low frequency AC voltmeter as the standard. These RF voltmeters are so useful and inexpensive that they are usually built into other items of equipment, as in the transformer ammeter and other devices described later. One or two separate units are often useful, for example, used instead of the bulbs in Fig. 1b. The oldest method of RF current measurement called for a hot-wire ammeter. As sketched in Fig. 4a, a piece of resistance wire expanded from passed current. The expansion was converted to rotary motion by a drum, with a needle on a calibrated scale indicator. You will probably have to visit a museum to see one. The next method, Fig. 4b, used a thermocouple connected to the hot wire, this feeding a standard DC meter. These were a common part of military equipment through WWII, and can still be found in junk-boxes, on swap tables and in surplus. The small antenna connector box of the "Command Sets" used a separate thermocouple-meter system, good for the power range of 10-100 watts. The dc meter is not calibrated in amperes, but an arbitrary linear scale. Its poles are shaped to give approximately a linear reading with power. Meters with internal thermocouples can be found, with a non-linear scale calibrated in amperes. A major advantage of these units is that they are nearly insensitive to frequency. They can be calibrated on DC. For greater accuracy, they should be checked on several frequencies, using a known load and an RF voltmeter to determine the current. The major disadvantage of these units is fragility, to mechanical shock damage and to burnout on overload. Today, it is almost certain that current measurements are made with an RF transformer, RF voltmeter combination, as in Fig. 4c. The voltage across the resistor is proportional to current. The scale will be non-linear for small currents, due to the square-law action of the diode rectifier. The suggested technique is to use a thermocouple type for calibration, plus one or two transfomer types for routine work. RF ammeters can be used much more than they are. The natural area is in base-fed verticals, on the lower bands. They are also useful where balanced feed is used, as in rhombics, the Zepp, and even quad loops fed by parallel coax or twin-line. The ARRL Antenna Handbook gives construction details of a specialized form especially useful for measuring current on guy wires, towers and other conductors. To be complete, another RF voltmeter should be mentioned. This is a receiver with a meter type of S-Meter. The LED bar- graph type is too coarse for most work. The best procedure is to calibrate the S-Meter with a steady signal fed through an attenuator box, as described in the ARRL and other handbooks. The meter reading on noise can be used as the reference, or the level of the generator if it is so calibrated. One of the common uses is to measure the antenna pattern of a friend's antenna. (Most hams can't get enough separation between the their own pair of antennas to do proper measurements. See later.) SIGNAL GENERATORS The basic elements of a signal generator are shown in Fig. 5. They are really simple. An oscillator for the desired frequency, some method of indicating the signal level, and an attenuator for setting the level are the key elements. Most station transmitters have at least two of these requirements, if not all, and have the great advantage of being available. And it is likely to be the continued terminal of the antenna system. Thus, as mentioned, the bulk of amateur antenna work uses the station transmitter. Despite the convenience, a transmitter has drawbacks. Many won't cover an adequate frequency range. It's often difficult to set the output power to a low level, needed for many measurements and required to prevent unnecessary interference. Solid state transmitters don't like the widely varying loads of much antenna work. And it's often inconvenient to move the transmitter to the work area. Some of the problems can be avoided by using a power attenuator between the transmitter and the antenna. Fig. 6 shows a design, with two values of components being shown. These are intended to reduce a level of 10 watts to either 1 watt or 0.1 watt. The 10 watts is chosen as a level found on most power meters. Such a unit will reduce interference, keep solid state transmitters happy, and give a source of known impedance. It's rarely worthwhile to build a signal generator. Low cost used units can be found at hamfests. Don't forget to look at the older ones using tubes. They may need some replacement capacitors, and a good cleaning. In compensation, they are rugged. There are some wide range new units at reasonable cost. One thing to look for is a constant level output terminal at about 1 Volt, to connect to a digital frequency meter. While signal generators have calibrated dials, the resolution is usually not sufficient for good antenna work. Also, the output of many generators is low, so a simple instrument such as a SWR meter may not work. The ARRL handbook describes amplifiers which can bring the power up to necessary levels. Don't omit the output filtering of these, since harmonics affect the accuracy of many measurements. If you do need to home-brew a generator, Fig. 7 shows a simple design of fair performance. It is intended for use over a single band, providing several fixed xtal frequencies and variable frequency. Because the transistor output is a square wave or nearly so, the output filter is necessary. The above types are basically single frequency generators. Spread frequency generators are also useful. The two commmon types are the noise generator, and the swept frequency generator. While the single frequency system can use broadband detectors, such as a simple RF voltmeter (except for the harmonic error problem), the broadband type must use a frequency selective detector/indicator. The most common type is the station receiver. The circuit for a simple noise generator is shown in Fig. 8. The Microwave 1N21 diode shown isn't the only type which can be used, but it has high output. Low voltage Zener diodes are the other common type. The low frequency coverage is set by the size of the coupling capacitor and the isolating choke: the values shown are good for all HF bands. The upper limit is determined by the stray inductance of the coupling capacitor and stray capacitance to ground. With normal components and open construction, usefulness extends through the UHF. In use, the noise generator replaces the oscillator and the selective voltmeter (receiver) the detector in Fig. 5. Early swept frequency oscillators used a mechanically varied capacitor to cover the swept range. WWII versions are sometimes seen at hamfests. Current designs use a voltage variable capacitor, a diode designed for high interelement capacitance. Generators usually provide an output voltage synchronized to the instantaneous frequency, but a few have supplied only a trigger. Either is used to control the detector indicator, a CRT being common. The detector itself must be broadband, for example, a simple RFvoltmeter, or a SWR meter. The generators are often found at hamfests, disguised as TV sweep generators. SWR METERS For too many hams, the only antenna measurement made is of SWR. Many tales of poor antenna performance have been traced to concentration on this, and neglect of other measurements. Use the SWR measurement for what it should be: the measurement of conditions between a properly working antenna and a properly working transmitter or receiver. In this connection, don't forget that the SWR of an ideal dummy load is 1:1. But it doesn't radiate very well. To say this again for emphasis, the job of an antenna is to radiate, not to have low SWR. The matching unit is the SWR control, whether at the antenna or at the shack. Of course, SWR is of some importance. Coax attenuation does increase as SWR goes up, but this is usually important only on VHF and up. More important is the possibility of puncture at high power, or of a short due to heating and softening of the dielectric at a high current point. Pay attention to line ratings when setting SWR goals. The other problems, an unhappy solid state transmiter, RF on the mike cable, and narrow operating range, disappear if an antenna tuner is used. It is easier to use this antenna tuner if a SWR meter is permanently in the line. Just after WWII, a simple SWR indicator appeared, as shown in FIG. 9a. It works because there is simultaneous magnetic and electric coupling to the transmision line. These combine to separate the forward and reflected waves. When the two lamps are of equal brilliancy, the SWR is infinite, when one is out, it is unity . Fig. 9b shows an unbalanced line version. One of these makes a useful indicator of antenna change if left permanently in the line. One in the emergency or field-day kit can be useful. The modern type of SWR meters use variations of this principle. The one of Fig. 10a uses the same parallel line structure as in Fig. 9b, but with separate lines for forward and reflected components. In Fig. 10b, a RF transformer is used for the current component, and a capacitor for the voltage. Designs are available for the power range from about 1 watt to many KW, and for freqencies through UHF. Separate units or measuring heads for HF and VHF are best, but most types will correctly indicate 1:1 SWR over a wider frequency range than shown on specification sheets. Check using a matched dummy load. For example, a typical line type intended for CB use gives good results on 144 MHz, and is usable on 220. Virtually all of the units on the market use a single meter, switched from forward to reflected. The two meter type is much easier to use. They are more expensive though, so you might want to add the second meter in a bolt-on box. You can replace the origional calibration pot with a dual unit, or add the pot externally. The meter doesn't need to be large: use the built-in meter for the important quantity. Usually, this is reflected power during antenna adjustment, and forward power in station operation. One of these modified units between the match-box and the transmitter, plus calibrated dials on the match-box makes for fast tune-up/operation on multiple bands, over the entire band extent. These SWR meters work on the basis of resistance comparison. If the resistor at the end of the pickoff line in Fig. 10a is replaced by a capacitor, the comparison is of reactance components. Automatic antenna tuners use a form of this device to control the reactance cancellation components of the tuner, plus a normal SWR type to control the feed resistance setting components. A combination R-X indicator can be built, but there are better techniques available. PRECISION MEASUREMENTS TO REPLACE SWR While it is possible to design an exact matching system from SWR information by "cut and try", the process is easier if the impedance can be measured. Also, the terminal impedance of the antenna tells a lot about what is going on. Any serious antenna work requires impedance measurement. The single item of SWR does give a measure of the magnitude of the impedance. A second measurement is necessary to get the angle of the impedance. This can be the position of voltage minimum (see later), but getting this may not be convenient. An easy measurment is to add series resistance to the line, Fig. 11a, and measure the new SWR. The intersection of the the two SWR circles on a rectangular or curved (Smith) impedance chart gives two resistance and reactance magnitudes: a third measurement is needed to select the correct value of the two possible solutions. This can be either the series capacitor or inductor as in Fig. 11b or 11c. The third circle on the chart now identifies the correct point. A computer solution of this resistance plot geometry is easy. The equivalent shunt elements are also usable. These series and shunt elements are useful in extending the calibration range of the measurement bridges discussed later. Use the series element for very low impedances, and the shunt for very high ones. Calculate the unknown from series ans parallel impedance relations. For greater accuracy and ease of use, an impedance bridge or similar device is needed. The basic principle is shown in Fig. 12a, and a typical series arm bridge in Fig. 12b. Low cost versions are made by combining a noise generator with a simple bridge, as "The Noise Bridge". They are available commercially, or can be built to the designs in the ARRL, RSGB and Radio handbooks. Most are only designed for 50 ohm lines and for low reactances. Series elements as in Fig. 12b, or equivalent shunts, can be used to extend the measuring range. Some designs indicate only resistance, depending on the depth of the null to indicate when reactance has been eliminated. In the precision field, a General Radio RF bridge for the 0.4-60 MHz range occasionally appears at hamfests. The price tends to be high, since their usefulness is well known. More common, and much lower in cost is the HP VHF Bridge, excellent for scale model work in the 55-500 MHz range, and usable down to about 5 MHz with some problems. The HP RX meter for 0.5 to 500 MXz occasionally appears, as does the GR UHF Admittance Meter for 20-1000 MHz. Newer equipment such as network analyzers, vector impedance meters and so on may appear, but these take more than loose change to buy. All of these units have the same basic test set-up, shown in Fig. 12a. Source and generator can be as discussed above, or can be of the many special types recommended by the manufacturer. Old HP and GR catalogs are often found at hamfests and used bookstores, and are the best guide to identification of possible items. There are many useful hints for use in these, also. Instruction and technique books may be found. Fair Radio can supply copies for some equipment, and it may be possible to get microfiche copies from HP. Call their nearest office for order info. The ARRL Antenna Handbook, the RSGB handbook and the Radio Handbook have descriptions of RF bridges or equivalent measuring equipment for home construction. See Reference Data for Radio Engineers and Terman's Radio Engineers Handbook for comprehensive discussions of theory plus some practical use material. These devices are not difficult to use. Setup can be a little tedious. Evaluation of the measuring results used to be more so, but there are now computer programs to get evaluated results by punching a few keys. The actual measurements are more fun than a chore. THE SLOTTED LINE For some reason amateurs have not paid much attention to the slotted line, Fig. 13 a/b, or to its open wire analog, Lecher wires, Fig 13c. A 3 or 6 foot length of either makes a excellent impedance measuring element on UHF and VHF, and a 20 or 40 foot temporary Lecher wire in the back yard is good for 6-10 and 6-20 meter work. The travelling detectors are the most problem, but they can be a simple RF voltmeters of Fig. 3a. The terminals should contact the wire for very low power, but a high enough level to allow capacity probe coupling to the conductors is better. The measurements needed are the ratio of the highest and lowest voltages, and the position of the lowest. Commercial slotted lines, both coax and waveguide, generators and detector/indicator elements for the VHF through SHF range are not uncommon at hamfests. These precision devices are really nice for the upper bands. Older editions of the ARRL VHF Manual had a lot of data on this family of devices. The RSGB VHF-UHF Manual is very good, giving complete construction data on a sloted line. The Radio Handbook has some information. CALIBRATED LINES Impedance bridges of any form give the impedance at their terminals. But in antenna work, the item of interest is the impedance at the antenna. The impedance transforming effect of the transmission line must be accounted for. The very old proceedure involved plugging numbers into equations. The Smith chart was invented to get the same answer by fast graphical plotting. Today a computer program solves the equations, and gives both number values, and Smith Chart plots. Any of these solutions take some time, and are not convenient when an antenna is being developed by cut-and-try. Its easier to have the measurements give the antenna values directly, which is possible by using the fact that the impedance values repeat each half wave of line. The trick is to get the right line length. This can be done by measurement, cutting the line to the value PK*LAM/2, LAM being the wavelength at the test frequency and PK the propagation constant stated by the manufacturer, usually 0.67 for coax. Since the propagation constant does vary some from batch to batch, and since the end connectors change the effective length, it is better to measure the effective length. For this, short one end of the line. A small disk with a center hole is good if there is no connector: one of these can also be used to short a cable connector receptacle. A small coil is connected to the other end, and the resonant frequency meaured with a grid-dip meter (see later). For greater accuracy, measure with coils of 1,2 and 3 turns, and plot the frequency against the number of turns. Project the curve to the zero-turn axis to get the true resonant frequency. The line length is 4 times the wavelength,i.e., 4*299.8/f. The transformation is exact at only one frequency. Assuming that the antenna is near resonance, a small change in frequency has little effect on the resistance measurement. The reactance value change is approximately the impedance of the line multiplied by the fractional wavelength change. If the frequency change is appreciable, use the Smith Chart or computer. Another use of line sections of known effective length is to move a SWR minimum point into the range of the measuring device. This is the way to use a short slotted line below its normal lower limit. For a 1 meter line, extensions of 1, 2, 3 etc, meters length are needed. Another line use is as a shunt for the antenna terminals, to bring the terminal impedance to a value suited to the measuring instrument. If a T connector is used, its length must be compensated for. If you take time to make up any of these lines, label them carefully, and save them for future work. But don't store them in sunlight, in a damp area, or with too tight a coil. Treat them like precision tools. FIELD STRENGTH METERS Most amateurs are surprised to learn that they have a field strength indicator they are not using. It's the SWR meter. With a telescoping antenna at one connector and the sensitivity control turned to high, it can be used in a lot of situations. Some of the small indicators sold for CB use have a special jack for such an antenna. A better design uses a tuning coil, as in Fig. 14. This increases the sensitivity, and reduces the effect of interfering signals. If you live near a broadcast station, the tuned type is a necessity. See the handbooks for variations in design, including design for field intensity measurement. Commercial designs are fairly common at hamfests. A unit can be calibrated by measuring the strength of a high-end broadcast station at one or more known locations , and using the strength curves the station filed with its FCC application. With care, a calibration can be obtained with a short vertical, with known power fed to it. The feature of the Signal Strength meter is that it indicates the RF field, which is the entire purpose of the antenna. One is mandatory for proper adjustment of a low frequency phased vertical array. In development of a new directional antenna, the best indicator of gain performance is the pattern. The field strength meter is one way to measure this, but see the later comments. THE GRID DIP OSCILLATOR One of the most neglected antenna measuring devices is the grid dip meter. It is used to get the resonant frequency of an antenna element, of a trap, a guy or boom. It gives an indication of the usable bandwidth and even of the loss. Adjust the sensitivity control on the dipper for a good dip indication. A narrow, deep dip occurs if the element being checked is high Q, which also means low loss. A narrow shallow dip usually means low coupling. A wide band reduction of reading with no pronounced dip indicates loss. To be really useful, three additions or even modifications of the dipper are needed, as shown in Fig. 15. The first at (a) is to allow use of a digital frequency meter, making the dipper a precision resonance indicator. At the dip, the frequency of the dip oscillator is determined by the coupled circuit, if it is high Q, and is the frequency of the coupled circuit at the exact dip, even for lower Q circuits. For occasional work, a temporary loop or a pickup antenna on the frequency meter is adequate. But for regular use a connector should be added to the dipper, with the loop permanently mounted. Alternatively, connection to some internal pickup point may be used. In the tube type, a capacitor to the cathode has been used. In tunnel diode and transistor types, it may be necessary to add a FET isolation stage, coupled to the tuning circuit. These should really be built in by the designer/manufacturer. The second addition, Fig. 15b, is provision for capacity coupling to the measured element. In antennas this is to the end of an element, or to the inside end of a trap. The capacitor can be a 2 to 5 PF ceramic, one end lead wrapped around one coil-plug pin, the other ending in a small alligator or battery clip. This is the connection to the circuit being measured. The frequency scale calibration is affected, especially at the high frequncy end of the dial. For repeat work, this capacitor should be built in, connecting to a separate jack. Again, these should be provided by the designer of the dipper. The third change, Fig. 15c, is construction of a set of special coils, shaped to give good magnetic coupling to a linear element, the antenna wire or tube. The easiest description is that these are shaped like a wire coat hanger. You could try to make the inductance match that of the regular coils, but the frquency meter connection makes this unnecessary. In use, the straight section is brought close to the antenna. The dipper now allows direct measurement of the resonant frequency of the element. In the Yagi, the reflector should be resonant a few percent below a design frequency, and the director a few percent above. The design frequency depends on the goal of the antenna, maximum gain or maximum F/B ratio. The percentage depends on the bandwidth goal. Usually the radiator is made resonant, but it does not need to be. Specification sheets sometimes give the resonant frequencies. An advantage of the dipper measurement is that it includes the effect of element taper, clamps and the shortening by the boom. Its a valuable check on the design details. Use it to check the calculated resonant frequency. OTHER INSTRUMENTS All of the above have been electrical measurements. Several mechanical devices are useful. The most basic is a steel tape calibrated in meters and centimeters. It saves no end of conversions. With just a little practice, it's easy to think in meters rather than in feet. Another mechanical device is a spring scale. One use is simply to check the weight of elements. A more important one is for proof testing of element and clamping strength. The element projected area times the wind loading (50 lbs per square foot typically) gives the element load. The center clamp must stand this as a pull along the boom. The distributed load can be converted to an equivalent end load, and the scale used to place this at the element end. The boom to mast fitting must withstand the entire load of the antenna. A torque wrench is useful if working on the larger beams, and on towers. Most amateur mechanics pride themselves on the ability to tighten fittings correctly. Perhaps so: a torque wrench is better. THE ANTENNA RANGE An antenna range can be a place for repeated development of antennas, or simply the place used to check out "that new beam". Results depend on several fundamentals, common to all ranges. The most important is the factor of distance between the antenna and the measuring device. This must be great enough to allow the emitted signal to be essentially a plane wave at the measuring point. For a point source (or detector pickup) Kraus gives this as d=k*a*a/lambda, where lambda is the wavelength, and a is the maximum antenna dimension, in the same units as wavelength. The k is determined by the accuracy needed. A value of 2 is satisfactory for work on the main lobe for gain. For adequate resolution of the sidelobes, a value of 4 can be used, except that for very low-sidelobe designs, a value around 9 is needed. For a typical small triband yagi, with a turning radius of 23 feet, a is equal to 14 meters. The minimum separation is very nearly 20 meters, or 66 feet, or 130 feet for sidelobe checking. But on 10 meters it would approach twice as much. Also, this is for a point source or pickup. If this were a half wave long, about another 33 or 66 feet would be required on 20 meters, or 16/32 feet more on 10. Try the values for a large moonbounce antenna on 2. Not too many amateurs have space for an adequate range. For individual stations an interested, friendly neighbor is important to a personal antenna range. Even with room, there are points to watch. A major one is ground reflection. The effect of this decreases with height of the antennas. It also decreases if the antennas at both ends of the path are directional, but this also means that greater separation is needed. Pattern distortion occurs from reflections, as by a roof, or the side of the building. And moving objects are a delaying nuisance. These factors are the reason pattern measurements are often neglected. If made, they are usually by cooperation of another Ham, sometimes a local some miles away, sometimes DX. Neither is really good, the first because of stray reflections, the second because of fading. There are several possibilities for Ham Club activity here. Some clubs maintain an antenna trailer for field day and emergency use. Addition of a good field strength meter or a low powere remotely controlled transmitter makes this into a portable measuring unit. Temporary parking in a low traffic area well away from the antenna under test shouldn't be any problem, but warning cones and lights are indicated, and review of plans with the police may be in order. Two well separated flat roof buildings make a good range if there is no reflecting traffic between them. An individual, but better a club, might be able to make arranagements for their use as a range. Schools, high school or above, may be more cooperative if there is an arrangement for student participation- good publicity and a source for new hams, as well. Many of the computerized repeaters have a signal strength measuring/ reporting routine. This doesn't seem to attract the attention it deserves, either from the station manager or repeater users. With a little work on equipment stability and on software revision for easy use, a real antenna range can be developed that is available to all users. Control can be over the air, or by a special telephone line. The idea can be extended. There are many relatively low cost transmitters , receivers and transceivers which cover all amateur bands (and more) under computer control. One of these at the repeater site or even a special site would make a remote antenna range a multi-band activity. The largest problem would be antennas, but high efficiency isn't needed, so small loaded crossed loops or dipoles would serve. A really good installation would have choice of antennas for both polarizations. There is a place for creative club activity here.