Method and apparatus for testing survival radios

A test apparatus is disclosed which is particularly adapted for testing survival-type radio transmitters in the field. The apparatus includes an enclosed metallic test chamber into which the antenna of the radio transmitter is placed. A movable load carriage within the chamber applies a load to a predetermined point on the antenna which results in simulating in the test chamber the radio frequency absorption/reflection characteristics of the antenna in free space. A direct measurement of the radio frequency energy absorbed by the load is thus indicative of the performance of the radio transmitter and its antenna in free space.

GENERAL DESCRIPTION 
This application relates to radio test equipment and more particularly to a 
method and apparatus for testing emergency/survival radio transceivers and 
beacons in the field. 
Portable emergency radios serve the crucial function of allowing victims of 
aircraft or boat accidents to transmit signals over assigned emergency 
frequencies to enable rescuers to locate them. It can be of particular 
importance for pilots of military aircraft to have the operation of their 
survival radio transmitters tested before each mission. Such testing 
requires test equipment which is suitable for use at front echelons and 
which is low in cost and complexity, easily operated by relatively 
untrained field personnel, and capable of verifying the operation of the 
equipment tested with a high degree of confidence. 
Laboratory-type instruments for testing survival radios are usually not 
available in areas where such radios are deployed. Furthermore, the skills 
of typical field personnel are usually not sufficient for the operation of 
laboratory-type test instruments. Moreover, the testing of survival radios 
with such instruments usually need to be performed within screen rooms to 
prevent radio frequency interference from being generated on the assigned 
emergency channels during the testing operation. Such screen rooms are 
expensive to construct and are not typically available in the field. 
Currently available test equipment which is suitable for field use suffers 
the disadvantage of not providing a complete test of the radio. For 
example, equipment exists for testing the RF power output where the 
antenna for the radio under test is replaced by a simulated antenna load. 
Unfortunately, such equipment does not test the operation of the antenna 
itself. Thus, a radio which is completely inoperable by reason of a faulty 
antenna could pass a test in which the antenna was replaced by a dummy 
load. 
In accordance with the present invention, a survival radio test apparatus 
is provided which overcomes all of the above-discussed disadvantages. The 
apparatus is relatively inexpensive and simple to operate and simulates 
the actual operating conditions of the ratio under test. 
The chief component of the apparatus is a metallic test chamber of novel 
design into which the rod-type antenna of the radio to be tested may be 
inserted. The chamber has dimensions such that it acts as a wave guide 
having a lower cutoff frequency well above the operating frequency of any 
of the radios to be tested. Such a waveguide will not support axial 
propagation of radio frequency energy having a frequency below the lower 
cutoff frequency. Thus, such radio frequency energy would not be radiated 
from the ends of the waveguide even if these ends were open. Because of 
the fact that one end of the test chamber of the present invention must 
have an aperture for allowing the antennas to be inserted into the 
chamber, the lower cutoff frequency characteristic of the chamber is 
important to ensure that substantially all of the radio frequency energy 
radiated by the antenna of the radio under test is absorbed within the 
chamber. Since survival-type radios operate at frequencies between 100 and 
300 megaherx (MHz), an illustrative embodiment of the test chamber 
described herein is dimensioned to have a lower cutoff frequency of 600 
MHz. 
The interior of the chamber is provided with a movable load carriage which 
may be positioned to connect a resistive load to any desired position of 
the antenna. A calibration procedure, described herein, determines, for 
each type or model of radio to be tested, the position at which the load 
carriage should be placed in order that the energy absorbed by the 
resistive load shall equal the energy which would have been radiated by 
the antenna of the particular radio type being tested had that radio been 
positioned "free space". The test chamber also includes means for 
connecting the resistive load to a power meter which is calibrated to a 
particular reading (e.g. center scale) for the minimum acceptable power 
output of the particular type of radio being tested. 
To test a particular type of survival radio, the load carriage is moved to 
the predetermined position corresponding to that radio type. The antenna 
of the radio is appropriately positioned within the test chamber and the 
lid of the test chamber is closed. Upon activation of the radio 
transmitter power meter will read center scale if the radio is operating 
properly. Any reading substantially below center scale reading indicates 
that the radio (or the antenna thereof) is defective. A reading above 
center scale indicates that the performance of the radio exceeds minimum 
specifications. 
It is an object of the present invention, therefore, to provide a survival 
radio test apparatus which is portable, simple in construction, and 
requires little technical skill to operate. 
It is a further object of the invention that the apparatus be adapted to 
test many types of existing survival radios and be easily adapted to test 
radios of new design. 
It is a still further object of the present invention that the test 
apparatus be capable of verifying the operation of both the radio under 
test and the antenna thereof by simulating actual operating conditions. 
It is yet a further object of the invention that the testing of the radio 
not result in significant radiation which may create radio frequency 
interference with other radio facilities. 
It is yet a further object of the invention that calibration of the test 
apparatus be relatively easy to perform.

TEST CHAMBER STRUCTURE 
FIG. 1 is a generally pictorial view of the test chamber 10 of the present 
invention. Basically, the test chamber 10 is a metallic box which acts as 
a wave guide. Such a wave guide exhibits the properties of a high-pass 
filter having a lower cutoff frequency which is determined by the 
dimensions of the waveguide. In the preferred embodiment illustrated by 
FIG. 1, test chamber 10 has nominal dimensions of 
30".times.5".times.31/2". These dimensions result in a lower cutoff 
frequency of 600 MHz. With these dimensions, the test chamber 10 will not 
support the propagation of RF wave energy travelling along its 
longitudinal axis below 600 MHz. Radiation of such energy from the ends of 
the chamber 10 is thereby prevented even if an end of the chamber has an 
aperture such as the aperture 18 described below. The survival radios for 
which the present apparatus is intended operate at frequencies between 100 
and 300 MHz. Thus radio transmissions within the test chamber 10 in the 
100-300 MHz frequency range are substantially contained within the 
chamber. This feature of the invention prevents undesirable radiation of 
RF interference on the assigned emergency channels which must be kept 
clear for actual emergency transmissions. 
Referring to FIGS. 1, 2 and 3, test chamber 10 includes a pair of side 
walls 12 and 12a, and end wall 13 and a bottom 32. A top cover 14 is 
attached to the side wall 12 by a hinge 15. The top cover 14 may be 
secured to the other side wall 12a by means of one or more conventional 
latches 16. A power output terminal 17, which is adapted preferably to 
accept a conventional BNC-type cable connector, is mounted on the top 
cover 14. The test chamber 10 is completed by a cradle assembly 20 which 
is removably secured outside the end of the chamber 10 opposite end wall 
13. 
Cradle assembly 20 comprises a cradle 20a which is supported by one or more 
legs 24 and connected to a butt plate 21. Butt plate 21 has an aperture 18 
through which the antenna 39 of a radio 22 to be tested may be inserted 
into the test chamber 10. Preferably, a plurality of cradle assemblies 20 
are provided, each one of which is dimensioned to support a corresponding 
radio type. The butt plate 21 of each of these cradle assemblies should be 
identical, however, so that the antenna 39 will always be guided into the 
same position in the test chamber 10 irrespective of the radio type being 
tested and of the position of the antenna 39 relative to the body of the 
radio in each such radio type. 
In the presently preferred embodiment of the invention, butt plate 21 
includes a pair of holes 29. A snap slide fastener assembly 23 comprising 
a clip 28 slidably mounted on a stud 27 which is mounted on the butt plate 
21 is positioned adjacent each of the holes 29. A pair of angle flanges 25 
are affixed to corresponding side walls 12 and 12a of the test chamber 10 
at the end of the chamber to which cradle assembly 20 is to be secured. 
Each of the flanges 25 includes an outwardly protruding guide pin 26 which 
is dimensioned to be received by the corresponding hole 29 in the butt 
plate 21 thereby guiding the butt plate 21 into the proper position. With 
the butt plate thus in position each clip 28 is slid downward to engage 
its corresponding guide pin 26, thereby removably securing the butt plate 
21 and cradle assembly 20 in place. 
It will be appreciated that the means for securing cradle assembly 20 to 
the test chamber 10 just described is meant to be illustrative only, and 
numerous alternative means for removably securing the cradle assembly 20 
to the test chamber 10 will be readily apparent to practitioners of the 
art. 
Referring to FIG. 2, two guide rail supports 30 are mounted on the bottom 
32 of the chamber 10. Two parallel guide rails 31 are mounted between the 
guide rail supports 30 and pass through corresponding holes 101 in the 
body 120 of a load carriage 100 shown in FIG. 4. Guide rail supports 30, 
guide rails 31 and the body 120 of the load carriage 100 are formed of 
non-conductive plastic material such as polystyrene. 
Also mounted on the bottom 32 of the test chamber 10 is an indicia strip 37 
which is also formed of a non-conductive material. The indicia strip 37 
includes a plurality of equally spaced indicia marks 38 and, preferably, a 
numbered scale for the indicia marks (not shown). 
FIGS. 4 and 5 show further details of the load carriage 100. As previously 
noted, the body 120 of the load carriage 100 includes a pair of holes 101 
through which the guide rails 31 pass. The body 120 of the load carriage 
100 also has a cut-out 108 for accepting the indicia strip 37. The body 
120 also includes a V-shaped notch 102 for receiving the antenna 39 of the 
radio 22 under test. A resilient metallic (e.g. beryllium-copper) spring 
contact strip 103 is affixed within the V-shaped notch 102, as by screws 
104. A non-conductive latch 109 having a notch 110 is pivotably mounted on 
the body 120 alongside the V-shaped notch 102. A pair of metallic angled 
flanges 105 are mounted on the body 120 alongside each of its edges facing 
the side walls 12 and 12a of the test chamber 10. A resilient metallic 
spring contact strip 106 is formed over each of the outside edges of 
flanges 105 so that the spring portions of strips 106 maintain sliding 
electrical contact with the side walls 12 and 12a of the test chamber 10. 
Strips 106 are secured to the flanges 105 and maintained in electrical 
contact with it by means of ground straps 107. A ground strap 112 is also 
connected between the flanges 105 to ensure that equal (ground) potential 
is maintained at the side walls 12 and 12a of the test chamber 10. 
A load resistor 111 is connected between the resilient strip 103 and one 
end of the center conductor 34 of a coaxial cable 33. The shield or outer 
conductor of cable 33 is connected to ground at one of the ground strap 
assemblies 107. 
Referring again to FIG. 2, it will be noted that cable 33 is removably 
secured to the top cover 14 of the test chamber of plastic clips 36 
leaving sufficient play in the cable to accommodate movement of the load 
carriage 100 over the length of chamber 10. The other end of center 
conductor 34 of cable 33 is connected to the center conductor of the power 
output terminal 17 in the top 14 of chamber 10 and the corresponding end 
of the cable shield is connected to the top cover 14 by a lug 35 so as to 
be in electrical contact therewith. A metallic spring contact strip 19 is 
soldered or otherwise electrically and physically connected to the inside 
surface of cover 14 opposite sidewall 12a so that when the cover 14 is 
closed, strip 19 makes electrical contact with side wall 12a. 
A wire 17a is removably connected between cable 14 and the corner of the 
test chamber 10 defined by the intersection of end wall 13 and sidewall 
12a to insure that cover 14 is not opened too far. 
THEORY OF OPERATION 
It is believed that a discussion of some of the theoretical principles 
underlying the present invention will be helpful in understanding the 
calibration procedure for the test chamber and the test procedure for 
survival radios utilizing the test chamber with are set forth below. 
It is known that if RF energy is applied to the feed point of an antenna 
which is located in free space (i.e., removed from any electrically 
conductive obstacles by a substantial distance), a portion of this energy 
is absorbed by the antenna and radiated therefrom and the remainder of the 
energy is reflected back to the feed source. It is also known that the 
ratio between the amount of applied energy of the applied energy which is 
absorbed by the antenna to the amount which is reflected is related to the 
ratio between the magnitude of the resistive component of the antenna 
impedance to the magnitude of the reactive component of the impedance. 
Thus, proper opertion of a radio system may be determined by measuring the 
RF energy which is absorbed by the antenna. If the energy applied to the 
feed point of the antenna (forward power), and the energy reflected back 
from the feed point (reverse power) is measured, the power absorbed by the 
antenna may be determined by subtracting the magnitude of the reverse 
power from the magnitude of the forward power. 
If a radio antenna such as the one discussed above is placed within the 
confines of a metal box its impedance will appear as almost a pure 
reactance (no resistive component) to RF energy applied at the feed point. 
An antenna so placed would, therefore, reflect back essentially all of the 
power applied to it. It has been found, however, that if the antenna is 
loaded somewhere along its length with a resistive load connected to 
ground, a portion of the energy applied at the feed point will be absorbed 
by the load and the remaining energy will be reflected back from the feed 
point. For a given value of resistive load, the ratio between the energy 
absorbed by the load and the energy reflected back from the feed point is 
determined by the position at which the load is connected to the antenna. 
Thus, by connecting a resistive load of known value to an appropriate 
point on the antenna, the reflection absorption characteristics of an 
antenna in free space can be simulated by an antenna enclosed in a metal 
box. 
If the power absorbed by such an appropriately positioned load attached to 
the antenna of a radio under test is directly measured (e.g., by a 
wattmeter), this measured value is a direct indication of the power which 
would have been radiated by the antenna had it been positioned in free 
space. Such a method for measuring the RF power output of a radio under 
test serves to verify that both the electronic circuitry and the antenna 
of the radio under test are in proper working condition without requiring 
actual radiation of RF power of the antenna beyond the confines of the 
box. This consideration is of extreme importance in the testing of 
survival radios since the assigned emergency channels on which they 
operate must be kept free from any radio transmissions other than those 
which are initiated in actual emergency situations. 
CALIBRATION PROCEDURE 
Utilizing the above-discussed principles, the test chamber is calibrated 
for each type of radio to be tested as follows. Reference to FIG. 6 will 
be helpful in understanding the following discussion. 
Prior to beginning the calibration procedure, a calibration antenna 
assembly 204 is prepared. This calibration antenna assembly comprises an 
empty radio case and antenna simulating closely the radio type for which 
the calibration procedure is to be performed. The calibration antenna 
assembly 204 is placed on a wooden tripod or post at shoulder height in a 
relatively clear area (e.g., at least 100 feet from any significant 
electrically conductive obtructions). The feed point 205 of calibrator 
antenna assembly 204 is connected to the input terminal of a directional 
watt meter 202 (e.g., a Bird Wattmeter, Model 43, or equivalent) by means 
of a coaxial cable 206 having a length equal to one-half wavelength at the 
frequency at which the calibration is to be performed. For example, if the 
test chamber is to be calibrated for radios operating at 243 MHz coaxial 
cable 206 may be a length of RG 58/U coaxial cable 16 inches long. 
Utilization of such a cable causes the impedance of the antenna at its 
feed point 205 to appear at the point at which the cable 206 is connected 
to the directional wattmeter 202. 
Wattmeter 202 is connected to a power amplifier 201 (such as a Boonton 
Model 230A or equivalent), which in turn is connected to a signal 
generator 200 (such as a Hewlett-Packard Model HP-608E or equivalent). 
Preferably, signal generator 200 is tuned to a frequency which is close to 
but not exactly equal to the assigned emergency frequency in question, to 
prevent radio frequency interference on the emergency channel. For 
example, if the calibration is to be performed for survival radios which 
operate on the 243 MHz emergency channel, signal generator 200 may be 
tuned to 241 MHz. It has been found that the errors resulting from 
performing the calibration procedure at frequencies so slightly removed 
from the actual emergency frequency are negligible provided that all 
calibration measurements are performed at the same frequency. However, 
where feasible, calibration should be done at exactly the frequency to be 
used. 
The output level of signal generator 200 is now adjusted so as to obtain a 
predetermined forward power reading (e.g., 200 milliwatts) on the 
directional wattmeter 202. The directional wattmeter 202 is then reversed 
and the value of the reverse power at antenna feed point 205 is recorded. 
Next the calibrator antenna assembly 204 is removed from tripod 203 and 
installed within the test chamber 10. To do so, the test chamber assembly 
10 is first placed in a horizontal position with the latches 16 facing the 
operator, with the top cover 14 open. A cradle assembly 20 corresponding 
to the radio type for which the calibration is to be performed is selected 
and secured to the remainder test chamber assembly with clips 28. The 
calibration antenna assembly 204 is then placed on the cradle 20 with its 
antenna 39 extending through the aperture 18 in butt plate 21. The antenna 
39 is extended full length and pushed into the V-shaped slot 102 of load 
carriage 100 so as to engage and make electrical contact with the 
resilient strip 103. The antenna 39 is then secured in place by pivoting 
the latch 109 so that the end of its notch 110 engages the antenna 23. Top 
cover 14 is then closed and latched and the test chamber is ready to be 
utilized in the calibration procedure. 
The power output terminal 17 of the test chamber should have a load 207 
connected to it which duplicates the input impedance of the indicating 
instrument to be connected to terminal 17 during testing. Preferably load 
207 is provided by allowing this indicating instrument 210, 212 (FIG. 7) 
to remain connected to power output terminal 17 during the calibration 
procedure. 
Forward and reverse power readings are then again recorded from the 
directional wattmeter 202. Assuming that these readings are not identical 
to the readings previously obtained when calibrator antenna assembly 204 
was in free space, the load carriage 200 is then repositioned so that its 
load resistor 111 is connected to a different position of the antenna. 
This procedure is repeated until a position of the load carriage 200 is 
found at which the forward and reverse power readings of the directional 
wattmeter 202 are identical to the readings obtained when the calibrator 
antenna assembly 204 was in free space. This position of the load carriage 
200 is then either recorded directly on the indicia strip 37 for the radio 
type involved or, alternatively, the numerical value of the indicia mark 
38 at the position of the load carriage is recorded in a table. The table 
may be taped to the inside of the top cover 14 of the test chamber so as 
to be available to the operator of the test apparatus during subsequent 
testing operations for any of the radio types to be tested. 
The described calibration procedure is repeated for each type of radio to 
be tested, utilizing appropriately prepared calibration antenna assemblies 
204. The procedure is also repeated for each frequency at which a 
particular type of radio may be equipped to operate, utilizing a cable 206 
which is cut to the appropriate length for each such frequency. 
TESTING PROCEDURES 
FIG. 7 illustrates the apparatus of the present invention set up for 
testing survival radios. An output meter 210 preferably having the 
characteristics set forth below is connected to the power output terminal 
17 of the test chamber 10, either directly or through an optional 
attenuator 212. 
It has been found that as a result of the calibration procedure described 
above, the load resistor 111 will be positioned at a point along the 
length of the antenna 39 at which the impedance of the antenna 39 equals 
that of the load applied to it at that point. With the load resistor 11 so 
positioned, the feed point of the antenna 39 assumes the impedance of the 
feed point in free space and the voltage/current distribution on the 
antenna 39 within the confines of the test chamber 10 simulates the 
voltage/current distribution of the antenna 39 in free space. 
The inpedance of an antenna of typical survival radio transmitter varies 
between 73 and 350 ohms along its length. The impedance of the load 
applied to such an antenna should, therefore, be selected to fall within 
this range. 
In a preferred embodiment of the invention the load impedance has been 
chosen to be 123 ohms. This load includes a 73 ohm load resistor 111 which 
is mounted on the load carriage 100 and connected between the strip 103 
and the center conductor 34 of coaxial cable 33. The remaining 50 ohms of 
the load is supplied by the input impedance of the output meter 210 and 
the characteristic impedance of the optional attenuator 212. This 
selection of the value of the load resistor 111 above makes possible the 
use of a wide variety of commercially available output meters having input 
impedences of 50 ohms and RF attenuators having characteristic impedances 
of 50 ohms. 
A further desirable characteristic for the output meter 210 utilized in 
conjunction with the present invention is the inclusion in it of a 
low-pass filter 212 which rolls off its response beginning at 300 MHz. It 
will be noted that typical survival radio transmitter radios having a 
nominal operating frequency of 243 MHz may actually radiate energy having 
a high harmonic content at 486 MHz. If the total radio frequency energy 
emitted by the antenna of such a radio is measured without first 
eliminating the energy emitted at the harmonic frequency, an output power 
reading may be obtained which indicates that the power output of the radio 
is acceptable, when in fact only a portion of the total power output is 
being radiated at the correct operating frequency. The inclusion of the 
300 MHz filter 211 prior to the meter circit and movement stage 212 of the 
output meter 210 insures that only radio frequency energy radiated at the 
correct operating frequency of the radio under test will be taken into 
account in determining whether the power output of that radio is 
acceptable. It is preferable that the output meter 210 and optional 
attenuator 212 to be used during the testing of each radio type should be 
connected to the power output terminal 17 as illustrated by FIG. 7. During 
the calibration procedure the output meter 210 should be calibrated to 
read center-scale for correct operation of the lowest-powered radio type 
to be tested. For example, an AN/PRC-90 radio has a specification of 100 
milliwatts average power output per channel. The RT-10 radio, on the other 
hand, has an output power specification of 200 milliwatts. If both of 
these radio types are to be tested, the output power meter should be 
calibrated to read 100 milliwatts center-scale without an attenuator 
inserted between the output terminal and the meter. During the calibration 
procedure and testing of the 200 milliwatt RT-10 radio, a 3 db 50-ohm 
attenuator is inserted between the input terminal of the meter 210 and the 
power output terminal 17, thereby effectively calibrating the meter to 
read center-scale at 200 milliwatts. For other output power values, 
corresponding attenuation is included to provide center reading at rated 
power. 
OUTPUT POWER TESTING 
Testing of the power output of a survival type radio utilizing an 
appropriately calibrated test chamber 10 of the present invention is 
performed as follows. 
The load carriage 100 of the test chamber is first moved to the position 
which has been determined during the calibration procedure to be 
appropriate for the radio type being tested. A cradle assembly 20 
corresponding to the radio type under test is then assembled with the test 
chamber 10 and secured thereto with the slide latches 23. The radio 22 to 
be tested (with fresh batteries) is then placed on the adaptor cradle with 
its antenna 39 extending through the aperture 18. The antenna 39 is 
extended full length so as to be received by the notch 102 in the load 
carriage 100 so as to make electrical contact with the resilient strip 
103. The latch 109 is then pivoted over the antenna 39 so as to secure the 
antenna 39 within the V-notch 102 of the load carriage. The top cover 14 
of the test chamber is then closed and secured, and the output meter 210 
and appropriate attenuator 212 (if any) are connected to the output 
terminal 17. The radio 22 is then activated and its relative output power 
read from the output meter 210. If the output meter shows that the RF 
output of the radio is substantially below center scale the radio is 
defective. 
RECEIVER TESTING 
Although the test chamber 10 of the present invention has heretofore been 
discussed only in relation to its utilization in performing tests upon 
radio transmitters, it may also be used to perform field tests upon radio 
receivers. 
If the antenna of a radio transmitter is installed in the test chamber, and 
the load carriage of the chamber is positioned at the appropriate location 
for that transmitter, it has been found that the magnitude of the radio 
frequency energy which will escape from the chamber will be 35 DBM below 
the magnitude of the radio frequency energy dissipated by the antenna 
within the chamber. Thus, a transmitter dissipating 200 milliwatts within 
the chamber is analogous to a transmitter dissipating 0.05 milliwatts in 
free space. Under such conditions the signal level is quite low within a 
short distance from the shielded transmitter and undesirable radiation of 
RFI on assigned emergency frequencies is minimized. Moreover, a receiver 
responding to a test transmitter within the chamber at a location of 150 
feet from the chamber or more may be regarded as a serviceable receiver. 
This may be used as a test for operation of receivers intended to operate 
with the survival radio. 
Although specific embodiments of the invention have been disclosed for 
illustrative purposes, it will be appreciated by those skilled in the art 
that many additions, modifications and substitutions are possible without 
departing from the scope and spirit of the invention as defined in the 
accompanying claims.