An Experimental Station on Wheels
July 1935 QST Article
high-tech vehicle you see here was state-of-the-art in 1935 when engineers at the
Cruft Laboratory at Harvard University outfitted it to do radio research. Clad with copper and chromium fittings,
the vehicle contained transmitting and receiving equipment along with various test equipment that included a high
stability frequency reference. In the article a 'tungar'
charger is mentioned. A tungar vacuum tube is a high current rectifier with a tungsten element and an
argon gas filler.
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An Experimental Station on Wheels
By H. Selvidge, * W9BOE
Mobile radio laboratory which might well be called the answer to a ham's prayer has been constructed at the Cruft
Laboratory, Harvard University, in Cambridge, Massachusetts. The object of the mobile unit is to enable laboratory
apparatus to be carried into the field to make observations on various radio phenomena. Two main objectives will be
pursued with the present apparatus installed in the truck. First, observations on the propagation characteristics
of ultra-short waves will be undertaken. It is hoped that observations on several different frequencies say 60, 120
and 240 mc., will lead to a better understanding of the processes of their propagation. Second, ionosphere measurements
will be made.
Clad in copper and with chromium plated fittings. The station looks the part
Looking through the rear door of the mobile laboratory. Ample space is provided for convenient manipulation of all
The ultra-high-frequency apparatus consists of a three-band receiver operating on 50, 120 and
240 mc. and is of the super-regenerative type, using self-quenching Hartley circuit detectors. There are three separate
detectors, one for each band, and they feed a single audio stage. In this way a very rapid change from one band to
the other can be made. The detectors for 60 and 120 mc. are 76's, while the 240-mc. detector uses a 955. A radio frequency
stage will soon be added to each, using the new 954's. The audio stage consists of a 42 feeding a small dynamic speaker.
There is a 50-mc. transmitter using three of the 6A6 type tubes; one as a speech amplifier with the elements
in parallel, one as a Class-B modulator, and the third as the oscillator in a t.p.t.g. circuit. A novel gadget is
the r.f. indicator used with this transmitter. It consists of a flashlight bulb in the antenna lead, the filament
being focused by means of a lens on a jeweled insert in the front panel. Contrary to all predictions, it works, giving
a very nice indicating light when everything is working properly. This transmitter is primarily for communication
purposes, as most of the ultra-high frequency observations will be made on signals sent out from the laboratory, at
a fixed location. The antenna consists of a quarter-wave aluminum rod pivoted at the bottom and mounted on the side
of the truck, and is fed by a concentric feeder with the outside conductor grounded. The antenna is arranged so that
it can be raised or lowered from the inside, and if it strikes an obstruction while in motion, it folds down, and
then springs back up into the vertical position.
For reception on the lower frequencies, a National HRO is
carried, with coils covering from 10 to 600 meters. This receiver works very well using the mobile antenna.
A relatively high-power transmitter is carried for the ionosphere measurements. It operates in the 3.5-, 7- and
14-mc. bands, as well as five experimental frequencies in that range. It consists of crystal-controlled oscillator
using an RK-20. Crystal switching and plug-in coils provide the means for convenient frequency shifting. The use of
AT-cut crystals insures freedom from frequency drift caused by temperature changes. The single-phase r.f. output of
the oscillator unit is usually fed through a phase-splitting network which gives two-phase r.f. which is then fed
to two amplifiers, each using two RK-20's in push-pull. Normally a single 59 would supply the necessary excitation
for the amplifiers, but there is a loss in the phase-splitting network because of the necessity of providing a good
wave form, so plenty of oscillator output is required.
The two amplifiers feed through two antenna matching
networks, and if these properly phased outputs are led to two half-wave Hertz antennas crossed at right angles to
each other, a circularly polarized wave will be transmitted which will be used in some of the ionosphere measurements.
The amplifiers can he modulated, in their suppressor grid circuits, with pulses for ionosphere measurement purposes,
or voice, using the modulator from the 60-mc. transmitter. Provision is also made for c.w. operation.
auxiliary equipment includes a cathode-ray oscilloscope, a pulse circuit for forming the pulses used in the ionosphere
measurements, and a standard signal generator operating from 2.4 to 1000 meters. A portable 5 meter transceiver will
also be added for possible emergency use. Recorders of various kinds will also be installed as the occasion demands.
One of the outstanding features of the outfit is that all of the apparatus can be operated from self-contained
power supplies, or on external a.c. when it is available. This is accomplished by using two batteries carried on one
side of the truck under the running board. One is a six volt size for lighting filaments. The other is a large twelve
volt battery used for lighting the RK-20 filaments, through a dropping resistor, and running three dynamotors. One
of these dynamotors gives 500 volts d.c. at 200 ma. for running the ultra-short wave equipment. A second gives 1000
volts d.c, at 400 ma. for plate voltage on the large transmitter. A third gives no a.c. at 150 watts for running the
HRO, a soldering iron, or other small a.c. apparatus. The external a.c, is taken in through a long cable, and feeds
through a watt-hour meter, so the amount used can be paid for. A 15 ampere tungar charger is used to charge the two
power batteries, as well as the car battery, whenever a.c, is available. An a.c. operated power pack supplies the
high voltage for the large transmitter when the dynamotor is not used, Two G.R. Variacs are used in this unit to compensate
for changes in line voltage which are likely to be found in small communities.
All the units have aluminum
panels and dust-covers, and are made for relay rack mounting, thus making it easy to change or replace them. The relay
racks are built into the steel framework of the body, which is mounted on a Packard roadster chassis. The steel frame
is covered with a layer of linoleum for the inside finish, a half-inch layer of celotex for heat and sound insulation,
and an outer skin of copper for shielding purposes, The roof is reinforced and covered with a rubber mat so that it
can be used as a platform for setting up various antennas. A socket in the roof is provided so that a collapsible
mast can be erected for operating from a fixed location on the longer waves. A loop can also be mounted on the top
for direction finding and field strength measurements. Onlookers are always thrilled by the chromium plated ladder
giving access to the top. A blanket roll is provided and there is space on the floor in the rear for sleeping. Two
trailers are available if there is more to be carried than the truck will hold. The truck and one trailer recently
returned from a trip to Washington, D. C., where they were exhibited at the conventions of the American Physical Society
and the I.R.E., as well as at the Washington hamfest of April 27. The truck and apparatus behaved in fine style and
many interesting contacts were made on the 5-meter band on the way down and back. As a result of this "shakedown cruise"
several additional spring leaves were added to the already-reinforced springs in order to carry the load properly.
When operating in the amateur bands the truck signs W1FQV. On the general experimental frequencies it uses
W1XAJ portable. For special experimental frequencies in the ultra-short wave region W1XDJ portable is used.
The amateurs associated with the project are Paul B. King, Jr., W2BWF, who is cooperating on the ionosphere measurements
and the author, who designed and built the radio apparatus. Both are graduate students at the Cruft Laboratory. Jack
Pierce, ex-1EB, was responsible for much of the body work on the truck, and John De Young, W1HHW, assisted in some
of the construction and testing of the apparatus. This project is being carried out as part of the radio research
work in progress at the Cruft Communications and Physics Laboratory of Harvard University, and is under the general
supervision of Professor Harry Rowe Mimno of that laboratory.
* W1FQV. Cruft Laboratory,
Harvard University, Cambridge, Mass.