Two Radio Puzzles

Radio Record, Volume II, Issue 6, 24 August 1928, Page 23

Two Radio Puzzles

HAV®H had two experiences that appear somewhat. unintelligible to me, and an explanation might be of interest to your readers. Firstly, I had a super-heterodoyne set and a neutrodyne set alongside of each other. The super-het. was receiving well off the loop only and not grounded, but of course being immediately under the aerial and only a few feet from the lead-in, but in no way connected. The neutrodyne was connected to the aerial, and its own batteries, but no loudspeaker. I found on tuning the neutrodyne (which had no speaker) to the same station that the super-het. was receiving, that the volume of the super-het was considerably increased. Of course I was not satisfied with one trial, but got the same result time after time. I have been at it long enough to know that I was not being deceived by a chance synchronising. This experience is perhaps more easily explained than my second one, which was as follows:-I had a superhet. set working on aerial and loop but no ground. Suspecting the wet B battery (which was an alkali one) to be at fault, I substituted dry batteries and the set operated faultlessly. I then had the alkali battery standing on the floor at the foot of the console superhet., and got to work with a voltmeter to test it. The battery was connected to nothing. On putting the second terminal of the voltmeter on to a cell in advance of where the first terminal was, the reception of the set was considerably upset-there was a distinct variation, and this happened again and again, as I tested the battery a few cells at a time. Can you elucidate please?-Mostyn Jones (Te Kuiti). he Phenomenon Explained. Our contributor M.I.RE.., to whom we submitted this letter, writes :- Mr. Mostyn Jones’s first problem is very easily explained. The neutrodyne was connected to the aerial, and the super-heterodyne was not, but the latter was connected to the loudspeaker. Presuming the super-heterodyne' to have six valves and the neutrodyne five, the net number of valves in operation between aerial and loudspeaker would be eight, because the first two valves in the neutrodyne would act as radio frequency amplifiers, and this amplified energy would excite the super-heterodyne by virtue of the inductive coupling existing between the two sets, and thus in effect add two radio frequency stages to the superheterodyne. As a matter of fact, the third valve in the neutrodyne, which under normal circumstances is a detector valve, would act to a certain extent as an amplifier and assist the before-described action somewhat. Had Mr. Jones shifted either of the two sets, an angle would have been found where this coupling arrangement would have had a maximum effect. With respect to the second set of circumstances, as far as can be judged, this is a freak effect dependent on the fact that a receiver using a loop as a pickup, becomes very much more sensitive if an aerial is attached to the receiver, and the earth left disconnected. The aerial under these circumstances, will impulse the receiver, due to the aerial becoming ‘an aperiodic pickup. If the earth is connected, the aerial takes up a definite time period, and is no longer aperiodic, and the received signals are consequently

materially reduced ‘unless the natural period of the aerial approximates the waye length being received. Presumably, the wet "B" battery described by Mr. Jones as having been located close to the receiver, had a material selfcapacity with respect to the receiver and to earth, and increased the coupling between receiver and ground just sufficiently when the self-capacity of his body was also connected to the battery sufficiently to disturb the efficiency of aperiodic action of the aerial. This latter explanation is purely hypothetical, but as a loop-driven re-. ceiver attached to an aerial and not earth, is particularly sensitive to néatby capacity effects, it is not at all an improbable explanation.