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

Text File | 1996-06-30 | 9KB | 191 lines

ANTENNA MEASUREMENTS
R.P.Haviland, W4MB

LEGENDS FOR FIGURES



Fig. 1. Use of Lamp Bulbs as Ammeters
    a) Use as wide band RF ammeter. Estimate current by
brightness. Or place a second bulb with known current from a
battery, rheostat and ammeter as close as possible, adjusting
current for equal brightness. The RMS RF current is the same as
the ammeter reading.
    b) Two bulbs separated by 1/4 wavelength make a SWR
indicator. Equal brightness is 1:1 SWR unless line length is
accidently such as to give a false equality. If the relative
brightness changes on moving the 1/8 wave section to the
transmitter end, the new relative brightness is the correct
indication.

Fig. 2. Neon Bulb Indicators
    A small neon bulb is a useful indicator of the presence of
RF. Sensitivity can be increased by forming the leads into dipole
form. Or:
    a) Bias the bulb with a small DC current from a battery, to
just above the striking voltage.
    b) Use a higher adjustable voltage to measure the difference
between the striking voltage with RF present and absent. This is
equal to the peak RF voltage across the terminals.

Fig. 3. Diode Voltmeters
    a) Germanium diode RF voltmeter for low voltage
measurements. A 1N914 silicon diode will withstand higher
voltages, with a small loss in sensitivity.
    b) A vacuum tube diode voltmeter for high voltage RF
measurements. Also useful in transmitter development. Use
appropriate safety precautions.

Fig. 4. RF Ammeters
    a) Hot Wire ammeter, measuring the current by amount of
elongation of a wire heated by the current. Liable to damage by
shock, or by too high current.
    b) Thermocouple ammeter, which produces a small DC current
by measuring the temperature of a wire heated by RF with a
minature two-metal thermocouple. Liable to damage by excessive
current. Very good as a calibration standard.
    c) Transformer ammeter, measuring the voltage induced in the
secondary by the current in the one-turn primary winding. Now the
most popular RF ammeter.

Fig. 5. Elements of aa Signal Generator
    Essential elements of a RF signal generator, producing a
voltage of known level at a known frequency. Frequency accuracy
depends on the oscillator calibration accuracy, and on drift. An
external digital meter is necessary for precise work. Amplitude
accuracy depends on the accuracy of the RF voltmeter, and on the
attenuator. A good design provides both the pre- and post-
attenuator outputs, typically 1.0 and zero to 0.1 volt. Frequency
swept generators use a low frequency form of FM modulation,
linear vs time, or sine wave, both typically syncronized to the
power line frequency. Power line filtering or the double
shielding of the oscillator as in the best designs is not shown.

Fig. 6. Transmitter Power Reducer
    Values for a power attenuator, to reduce transmitter levels
for antenna measurements. Designed for 10 watts input, this can
be constructed for 1 or 0.1 watt output to a matched load. The
unit can be mounted with the resistors in oil, or they can be in
a performated metal case for good air circulation. Intended for
use with in input RF wattmeter: a voltmeter as in Fig. 3a can be
included.

Fig. 7. Simple Signal Generator
    Single IC signal generator for rf work, values for 14 MHz
being shown. The coil must have good Q, and an all xtal design is
easier. The output may be increased by replacing the 3 parallel
open collector inverters with a high voltage type, the 50 ohm
resistor being fed from 12-25 volts.

Fig. 8. Diode Noise Generator
    Elements of a noise generator using the noise produced by a
reverse biased diode. The circuit assumes a DC current path
through the external circuit. If not present, a resistor equal to
the the line impedance may be shunted across the output
terminals. Intended for relative measurement, but the noise
output can be calibrated by comparison to the signal from a good
RF signal generator. Precision diode types are occasionally found
at hamfest tables.

Fig. 9. Balanced Line SWR Indicator
    a) This post WWII open wire line indicator shows low SWR
when the bulb towards the transmitter is bright as compared to
the one towards the load. Dimension shown and bulb types suitable
for a transmittter of 50-100 watts output. Sensitivity is
adjusted by the spacing from the main line.
    b) Coax version of a).

Fig. 10. SWR measuring Instruments
    a) Transmission line directional coupler, an extension of
Fig. 9. Sensitivity is much higher, due to use of the diode
detector. A typical unit made for the CB band is usable for the
low power range from about 0.1 to 10 watts.
    b) Coupler/ indicator which separates the magnetic and
electric field coupling to the line. Careful construction is
neded to eliminate the effects of stray coupling, see the ARRL or
other handbooks for design details.


Fig. 11. Series Elements for SWR to R,X Measurement
    Series or parallel resistance and/or reactance can be used
to give R and X measurement from 2 or 3 SWR values. Use a
rectangular or Smith chart, or computer program, for calculation.
Typical values for 14 MHz are shown at a) resistor, b) capacitor,
c) inductor. For the parallel equivalents, use the relation Xp *
Xs=Zo * Zo.

Fig. 12. Elements of Impedance Measurements
    a) Basic measuring setup, of signal generator, bridge and
detector. A wide band detector is often used with a single-
frequency generator, but a narrow band detector (receiver) must
be used if the signal source is wide band, such as a noise
generator or a swept frequency oscillator.
    b) Basic four arm  bridge, usually designed for equality of
the upper and lower arms. The standard arms may be a center
tapped transformer, or two resistors, or may be more complex. The
series adjustable arm type is good for dioles below resonance, as
shown for the series RC unknown element. Parallel arms may also
be used, as in the GR RF bridge and some noise bridges.
    c) Transmission line bridge. the resistive component of the
unknown is determined by adjusting the relative capacitive and
inductive coupling to the line ends, and the reactive component
by the fractional wavelength departure from line center at
balance. This is the principle of the HP RF bridge. Other design
are found in the literature.

Fig. 13. Transmission Lines for Measurement
    a) Top view of a slotted or trough transmission line,
usually designed for a Zo of 50 ohms. A probe moves along the
open top of the line to give the voltage variation along the
line, which gives the SWR. The position of the voltage minimum
gives the second measurement needed to calculate R and X.
    b) Cross section of a trough line. The probe end is usually
smaller than the conductor diameter. Such lines can be built with
1 by 3 inch extruded aluminum, or can be sheet metal folded
around a 2 by 4 for forming. The equation gives the conductor
diameter and position for the design impedance. A 6 foot length
is good for 144 MHz and above.
    c) Elements of a Lecher wire system, which may be  designed
for 270, 300, 450 and 600 ohm impedance. If room is available,
useful at HF, for measurement, or for check of the SWR accuracy
of another instrument.

Fig. 14. Basic Field Strength Meter
    The essential elements of a field strength meter measuring
the electric field. A magnetic field measurement type omits the
pickup whip, and enlarges the coil into a small loop, or a
ferrite "loop-stick". Low sensitivity types omit the tuned
circuit, but become sensitive to stray fields. An amplifier may
be added to increase sensitivity. See handbooks for design
details.

Fig. 15. Dipper Modifications
    a) Addition of a pickup loop for magnetic coupling to a
digital frequency meter. The loop should be permanently mounted,
together with its associated cable, after determining that the
pickup level is adequate for the meter in use. The dipper is now
a precision resonance indicator.
    b) Addition of a coupling capacitor to permit capacitive
coupling to a resonant circuit, for example an antenna. The coil
connection can be one or two turns of the lead around the coil
pin, but an internal connection to a tip jack is better. Should
be connected to a high voltage point. In an antenna this is the
element end. In a trap antenna, it is the inside end of the trap
resonant on the band being investigated. Traps are best checked
when separated from the rest of the element. A shallow dip is
reason to suspect a poor trap. This is an alternate to a) for the
frequency meter.
    c) Coil shape to increase magnetic coupling to a linear
circuit, such as an antenna element. Several turns are needed for
the lower frquencies. Large coils tend to damage the coil
receptacle, so mechanical support should be added for these. A
pair of hooks can be built into this, to hold the dipper at a
constant position to the element. Coupling should be to a high
current point. In dipoles and trap dipoles, this is the element
center. Using this with a) allows determination of the exact
resonant frequency of the element, especially important in
parasitic beams. The assembly is very useful in checking for
tower, guy and boom resonances. It is also useful in TVI
elimination, for searching for unwanted resonances in the
transmitter and TV antenna, and as an interfering signal source.