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
charger is mentioned. A tungar vacuum tube is a high current rectifier
with a tungsten element and an argon gas filler.
Table of Contents
Wax nostalgic about and learn from the history of early electronics. See articles
QST, published December 1915 - present. All copyrights hereby acknowledged.
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 controls.
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
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
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.
The 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.
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.
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.
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.