Sensitive Field Strength Meter
May 1954 Model Airplane News
It was six years prior to this field strength meter construction article being published that Mssrs. Brattain, Shockley, and Bardeen invented the transistor using the element germanium and a point contact "cat's whisker." In 1954, the date of this article, Texas Instruments introduced the first commercially available silicon device - the TI 900 silicon transistor. However, operational frequencies of semiconductors were only in the hundreds of kilohertz, so vacuum tubes were still necessary in higher frequency radio circuits like the field strength meter, which operates in the 30 MHz band. Fortunately, by the mid 1950s there was host of miniature tubes available for use that reduced that physical size and weight and also often worked on lower plate voltages and lower currents so that power requirements were reduced as well. The 1AD4 RF/IF pentode is used in this field strength meter to increase its sensitivity over the passive diode type detector.|
Sensitive Field Strength Meter
By E. J. Lorenz
Your finished field strength meter will look like this. Uses various available meters.
Pot enables voltage adjustment according to the value of the meter used.
Rear view of field strength meter showing placement of component parts. Batteries at the bottom.
Are you really sure your transmitter is putting out? Here's a simple unit for accurate checking.
Range, range, range!
You just don't have it unless your transmitter is properly tuned and the antenna is properly loaded. We have flown excellent flights with our two-tuber using a half-watt transmitter.
In order to obtain the greatest efficiency from your transmitter, we are presenting a simple field strength meter. Perhaps you are using one which consists of a half or full wave diode circuit. This type of meter registers well at close range but has the disadvantage of low sensitivity and the need for a sensitive meter, usually 0-1 ma or less. The described field strength meter employs a sub-miniature 1AD4 RF pentode in a grid leak detector circuit, and uses a 0-3 ma meter, one that every RC modeler has on his bench.
This will not be a full-fledged construction article, for space does not permit, but the schematic diagram plus photographs should be sufficient. In addition, this circuit has been built with a variety of components, such as air trimmers and ceramic trimmers, air coils and slug-tuned coils, and tubes including the 1AD4, IAG4, CK5607, RK-61, XFG-1, CK525, and CK5677. Meters have ranged from 0-1 to 0-5. Results have been similar in all cases. Your unit may be built into almost any size case that will suit the construction; we used a 3006 mini-box.
As far as construction is concerned, keep all leads as short as possible; this is not imperative but always advisable. The B voltage may range from 15 to 45 volts depending upon the value of meter used. When using an 0-1 ma meter, 15 volts are sufficient, and 45 volts will operate a 0-5 meter. Since the unit is on but a short time, we soldered the batteries and A cell in place.
To use the meter, when finished and in its case, switch on the unit and adjust the pot R2 to read about half-scale on the meter. Insert a 12-18 in. antenna (a thin piece of wire soldered to a banana plug) in its socket and rotate the tuning capacitor, or slug, until the maximum dip is obtained on the meter. Remember this: a field strength meter gives only relative readings at a given distance from the transmitter. In other words, if the unit is held by someone, the dip will be greater than if the unit is placed on a box or bench. Once properly tuned, remove it from the vicinity of the transmitter until a .2-.5 ma dip is obtained when a signal is transmitted. Leave the field strength meter in one position and adjust final tank capacitor or antenna loading until the maximum dip is obtained. Be sure the field strength meter antenna and the transmitter antenna are in the same plane; that is, vertical to verrical and horizontal to horizontal. Greatest accuracy is achieved at a distance of at least one wave length away from the transmitter, or about 40 ft.
Posted March 1, 2014