Antennas/Antennas_CD-ROM_Walnut_Creek_September_1996.iso/articles/instrums/antmeas.txt

Text File | 1996-06-30 | 33KB | 658 lines

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.