Radio Broadcast (May 1928-Apr 1929)

Practical 5-Meter Hints By ROBERT S. KRUSE JT HIS short article contains some oper "\ ating data which Mr. Kruse did not cover in his article on 5-meter transmission and reception in the September issue of Radio Broadcast. Several "freak" transmission effects on the 5-meter hand are also explained. — The Editor. JT THE 5-METER WAVEMETER IN THE preceding stories the very attractive possibilities of the 5-meter band have been discussed. There will be added here a few time-saving hints. Very early it became evident that the average experimenter did not make a satisfactory job of building and calibrating a 5-meter wavemeter. At the writer's suggestion the General Radio Co. developed the very rugged little meter shown on this page, which remains the only one on the market, although the band-covering DISTANCE.MULS FIG. I. "WEST PEAKS" meter of the same firm also has a 5-meter coil. The meter here shown is most convenient when mounted on a short wooden or bakelite panel, although the hand-capacity effects are small because of a good C/L proportion. In the previous articles the plate and filament supply were not mentioned. It is best to use alternating filament supply, but one should not use the center-tap of a transformer secondary. It is far more satisfactory to use a double 100ohm resistor or a 200-ohm potentiometer. 1 he latter has the advantage of permitting one to find exactly the right point. Either method permits one to use any toy transformer or even a bell-ringing transformer. The plate supply may be raw a.c. as a. starter, as was suggested, but had better be rectified. The filter should not have a condenser next to the rectifier tube. First should come a choke of 1-5 henrys inductance, then a 2.0-mfd. condenser, then a choke of 10-100 henrys and finally another condenser, as large as convenient, shunted by a resistance of 25,000-100,000 ohms. The reasons for this type of filter are good but toe lengthy to mention here. Although the ux-852 tube is excellent for the 5-meter band, no proper rectifier for it is lable. One is comlled to use electrolytic rectification, or else a too-laige kenotron, mercury arc or d.c. generator. A new gas tube may be available soon. When the set is taken into the field one must of course have some sort of automatic key at the station to keep making signals. A suitable one is shown herewith. It consists of the motor of a 16" electric fan, a standard Boston Gear Works worm gear and a cam in the edge of which are milled code letters as desired. A bakelite cam worked out with a file is just as good. The edge of the cam bears on a brass strip screwed under the knob of an ordinary telegraph key which controls the transmitter. Parenthetically, it is best to key the set by the method shown in Fig2 — for reasons good but lengthy. Having the key going one gets into the field with the receiver and is then uncertain whether things are proceeding well. The appearance of the following effects is normal and reassuring. AUTOMOTIVE EFFECTS DEFORE one is long afield the automobile injects itself into the picture. Its ignition noises are rather troublesome, although this is lessening as the Model T Fords and their spark coils diminish in number. After a little practice one can distinguish the type of motor and its condition to a considerable degree by listening with a 5-meter receiver as a car passes. Cars with monthly service programs have vastly the best average performance as re ards the steadiness of their 5-meter signals. If one is near the road another effect will be observed: each passing car detunes the received signal a trifle. This statement is made general, though we must admit lack of evidence against ihe Rolls Royce, which did not come along that road. It was possible to tune the station out, and then bring it back by interposing a suitable automobile. "Suitable" means any type of about the right size and body structure; a Buick and a Nash are interchangeable for the purpose, whereas a Franklin and a Stutz are not! The gist of the difference lies in the difference between the wood-and-aluminum body of the " Benjamin" as compared with the fabric-covered OSCILLATOR FIG. 2. THE OSCILLATOR AMPLIFIER KEYING DIAGRAM THE AUTOMATIC KEY Stutz body, and the only possible value of the stunt is to suggest the importance of small absorption effects at 5 meters. WEST S PEAKS TTO C. H. West of 2csm we are indebted for 1 first calling attention to the curious effect shown in Figure 1. He noticed that in driving along a road there appeared to be "humps" in ihe signals at 12-foot intervals when one was some 3 or J mile from the station. Subsequent observations produced the same effect at various stations, the spacing between H peaks remaining reasonably constant, (pi though sometimes the peaks are entirely absent at a particular station. The same thing has since been observed at f-meter wavelength with all the dimensions of the picture about one seventh as large. It looks like an interference pattern, but in most cases appears to be stationary and of much the same proportions regardless of the things alongside the road at that place. 371