Radio Broadcast (Nov 1926-Apr 1927)

86 RADIO BROADCAST ADVERTISER Brings ISburSet QToDate The only real advance claimed by the makers of this year's best sets is improvement in tone performance. This improvement can be made in your present set by simply adding the Centralab Modu-Plug. This modernizing device makes your reception equal in tone performance to that of the latest high-priced receivers. Modu-Plug is warranted by Central Radio Laboratories, makers of variable resistances for sixty-nine manufacturers of leading standard sets. Centralab Modu-Plug replaces the loud speaker plug. Gives any degree of tone volume from a whisper to maximum by simply turning the small knob on the plug, without adjustments of other controls. Modu-Plug matches the speaker impedance to the output impedance of the set. Reduces interfering noises. Clarity and faithful reproduction equal the latest developments in perfected performance. $2.50 at your dealer's or mailed direct on receipt of price. CENTRAL RADIO LABORATORIES 22 Keefe Ave. Milwaukee, Wis. Canadian Representative — Irving W. Levine, Montreal Australian Representative — United Distributors, Ltd., Sydney Great Rritain Representative — R. A. Rothcrmel, Ltd., London Centralab Radiohm permits clear, true-tone reception by holding the sensitive regenerative position which immediately precedes the oscillulion point, without distortion or loss of selectivity, A standard unit on leading sets. Retail price, $2.00, ut your dealer's or from us. shown in the accompanying sketches. It consists essentially of a tapped spiderweb coil and a variable condenser. Great care must be observed in the construction of this unit, for, since another piece of apparatus is being added to the receiver proper, it is obvious, if the best results are desired, that the design should be in accordance with the most advanced ideas on low loss efficiency. It is apparent that there would be no use in adding to your receiver an instrument that would effectually block the passage of weak signals, due to the resistance introduced. The variable air condenser employed should preferably be of the straight frequency-line grounded rotor type, and the requirements of the circuit are such that this condenser should have a maximum capacity of o.ooi mfd. The inductance is of the well-known spiderweb type, coupling, as in the commercial type of the Browning-Drake receiver, the Teledyne, and any set in which the antenna is connected directly to some point on the grid inductance of the first tube, it will be necessary to remove the two exterior wires which connect with the antenna and ground posts. In their stead is placed a piece of copper wire no smaller than No. 14 and long enough to reach from the antenna terminal, once around the grid coil, and thence to the ground post. In order that this wire will not cut into the finer wire of the grid coil, it should be covered with a good grade of spaghetti. This single turn should also be made very secure so that it will not come loose. This constitutes the coupling coil through which the received energy is transferred from the loading unit to the receiver proper. The output terminals of the Max. 0 001. mfd.var. / rapatl5th.turn JZ, Spiderweb Coll = g 50turns,4"dia. r Single Turn Coupling to Receiver Proper FIG. 3 FIG. 4 FIG. 5 since such a coil offers a very low resistance to high frequency currents, and is very easy to construct. It consists of fifty turns of No. 22 d.c.c. magnet wire wound in and out of every other slot of the form, and tapped at the fifteenth turn from the beginning. The form may be purchased at a radio dealer's or made at home from heavy cardboard, according to Fig. 1. No shellac, varnish, or other such material is used on this coil. After the coil has been completed, it may be mounted on a baseboard directly behind the condenser by means of a bakelite mounting strip, with a small brass angle. See Fig. 2. It is important that it be placed at right angles to the electrostatic field of the condenser and also that it be out of inductive relation to the first coil of the receiver itself. If any coupling, however slight, exists between the loading unit and the coils of the receiver, the purpose of the one turn coupling coil will be defeated, as no energy should be transferred except at this point. In mounting the switch points on the panel, keep them as far apart as the width of the switch blade will permit. It is also in the interest of efficiency to use points with a very low head, not more than tV inch high. Keep the leads to these points at least an inch apart. Observance of these precautions will result in a very low capacity effect at a point where large losses might otherwise be encountered. All connections should be made with No. 12 or No. 14 copper wire, soldered where necessary, and covered with spaghetti to produce a neat appearance. By connecting the rotary plates of the variable condenser to the ground side of the circuit, all capacity effect from the operator's hands will be entirely eliminated. When the unit has been completed, it may be housed in a suitable cabinet to match that of the receiver. While the panel specified is seven inches high, this may be varied to correspond with the height of the receiver with which it is to be used, thereby presenting a more harmonious appearance. Fig. 3 is a wiring diagram of the unit. Now that we have the unit completed and have erected as large an antenna as circumstances will permit, let us see how we can apply the idea to receivers employing various forms of coupling. Assuming that your receiver employs direct FIG. 6 loading unit are connected to the antenna and ground posts of the receiver, and the antenna and ground are then hooked to their respective posts on the unit. How the connections should be when this has been done is shown in Fig. 4, the dotted lines indicating the former connections. If your set employs aperiodic coupling, as in many modern receivers the changes to be made are very similar. The small coil which is connected between the antenna and ground posts and coupled to the grid inductance is removed and in its place is put the one turn of heavy wire. Fig. 5 illustrates this. In a set in which the antenna circuit is tuned, such as a vario-coupler set, where the outside winding is connected between the antenna and ground and the rotor is used as the main tuning inductance, the primary will have to be unwound and removed. It is not necessary, of course, to dismantle the coupler, but it would not do to leave the unused primary coil in such close relation to the grid coil. The one turn of heavy copper wire is wrapped around the secondary and connected to the antenna and ground posts as before. See Fig. 6. If a tuning condenser has previously been used in the antenna circuit, it may be employed in the loading unit, provided it is of the proper capacity. From the above it is apparent that the oneturn coupling idea may be applied to practically any type of receiver designed for use with an outside antenna. While, in the writer's case, it was used with a long, high antenna, it may also be employed with an ordinary antenna where interference is very marked. If the unit is to be used with a small antenna and a set employing no radio frequency amplification, better results will be secured by the use of a coupling coil composed of two turns instead of one. The operation of the unit is very simple. For frequencies above 857 kc. (wavelengths below 350 meters), the switch lever is set on the first point so that but thirty-five turns of the loading inductance are included in the antenna circuit The dials of the receiver are then adjusted to the settings at which a station is known to come in, after which the antenna condenser is varied until signals are heard. The first dial of the receiver will not read exactly the same as formerly. Examined and approved by Radio Broadcast if