high-tech vehicle you see here was state-of-the-art in 1935 when engineers
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
charger is mentioned. A tungar vacuum tube is a high current rectifier
with a tungsten element and an argon gas filler.
|July 1935 QST|
of Contents]These articles are scanned and OCRed from old editions of the
ARRL's QST magazine. Here is a list of the
QST articles I have already posted. All copyrights are hereby acknowledged. |
See all available
vintage QST articles.
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
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
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
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.
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
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.
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.
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,