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An Experimental Station on Wheels
July 1935 QST Article

July 1935 QST
July 1935 QST Cover - RF Cafe

Table 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.

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. 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.

An Experimental Station on Wheels

By H. Selvidge, * W9BOE

An Experimental Station on Wheels, July 1935 QST - RF Cafe
Clad in copper and with chromium plated fittings. The station looks the part

Looking through the rear door of the mobile laboratory - RF Cafe
Looking through the rear door of the mobile laboratory. Ample space is provided for convenient manipulation of all controls.
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.

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.

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  4/11/2013

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