Table of Contents
Wax nostalgic about and learn from the history of early electronics. See articles
QST, published December 1915 - present (visit ARRL
for info). All copyrights hereby acknowledged.
The 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.
The story appeared in QST magazine. The mission of the mobile unit
is to enable laboratory equipment to be carried into the field to make
observations on various radio phenomena. 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.
An Experimental Station on Wheels
By H. Selvidge, * W9BOE
Clad in copper and with chromium plated fittings. The station
looks the part
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
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.
Looking through the rear door of the mobile laboratory.
Ample space is provided for convenient manipulation of all controls.
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.
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.
Posted January 18, 2021
(updated from original post on 4/11/2013)