Show due to the widespread increase in power of many of our broadcasting stations the need tor some sort of ap capable of cutting out strong ly interfering stations Is becoming felt more and more there Is not much fun in sitting through the program ot one particular station night after night especially when a better pro gram Is being offered by some other station which 13 drowned out by the nearer or more powerful transmitter interfering signals writes howard E rhodes in the radio broadcast magazine sometimes are caused by driece pickup by the colls of the set so that its natural selectivity Is of no avail in eliminating them this oc curs when the set is being operated in proximity to a broadcasting station I 1 with the result that the field strength in the vicinity la great enough to in duce currents directly in the various colls of the receiver under these conditions it will be found possible to hear the program with the antenna disconnected the only practical methods to be aed in eliminating this tape of inter farence are either shielding of the re beiver or using some form of closed field coll such as a horold it the set Is to be shielded it Is necessary to line the entire inside of the cabinet the cover and the panel with thin copper any joints in the copper sheet are soldered together and con Is finally mide between the shield and the minus A terminal which in this instance Is connected to ground this puts the entire shield at ground potential thereby excluding from the colls any external electrical fields it should also be pointed out that athla shielding will also eliminate any interference caused by power lines motors etc provided they are causing trouble by setting up currents directly in the colls however it the interference Is reaching the set by way of the antenna system shielding will be no aid the use of horold colls Is coming into very common uie in in where there la need of an in ductor tint Is unaffected by external fields why horold coll Is favored the gist of mr rhodes explanation why the horold type of coll Is becom mended for use where ordinary colla will cause broad tuning by their pick up qualities Is as follows colls will as Is the case with other colls have induced in their turns cur rents which ordinarily would nullify the selective qualities of a receiver however the voltages induced in op halves of the coll oppose each other and the resultant effective pick up voltage Is zero this nullifying feature Is apparent in several other types of colls but Is probably more so in the horold the article in radio broadcast continues as follows there Is a second type of interfering signal that causes considerable trouble and against athla type the two cures so far described are useless it the signals being induced in the an tenna are sufficiently strong it is pos elble diat a receiver may not have a selectivity sharp enough to eliminate them completely although its selectivity under ordinary conditions kiy be perfectly satisfactory for instance the receiver might be operated at a location quite close to a powerful broadcasting station so that the signal strength from this station in combar ison with the strength of the that it Is desired to receive from other stations la so greit as to produce con interference even though the receiver Is considerably from the interfering signal it Is apparent then that in some way this powerful signal must be impeded so as to decrease its strength the roost mw piece corn of apparatus for use in this connection Is the so called which Is wave trap actually a filter circuit tuned to absorb the interfering signals these wave arapi trapi are strutted ted very easily con and cost little they merely of consist an ordinary coll and a denser the con traps are connected in the antenna circuit and are adjusted so that nt the frequency of the inter bagnal they have impedance a very aleh the exact value on whether the unit Is connected in or parallel with the antenna this high impedance prevents the rial from passing down through primary of the antenna so into the receiver at the sai time these wave traps offer impedance to all other frequencies methods of operation there are several methods which these traps might be operate fig 1 shows the most common in the li oi constitutes trap circuit and L C the antem coupler and tuning condenser ct fe receiving set it will be seen trap Is connected between the ajtai post of the receiver and the antena lead in fig 2 represents a slight different method of connecting ft trap in the circuit in this late method the wave trap a coupled to the antenna ahls tote alve coupling Is obtained by few turns of wire about one endotti coll li one end of this new ilni connects to the antenna and the otic to alie antenna post of the recelle this circuit Is practically to that of fig 1 with the that somewhat sharper tuning Is A talked whether the trap circuit of W or eliat of fig 2 Is used depend ste a considerable extent upon the chan ast the antenna system 1 well insulated antenna with a bior lead in and a short ground good ground such as the cold vat mains gives best results with the coupled circuit every ss should be made to have as ghori ground and antenna as Is possible b if it Is necessary to use a long lead to a rather poor ground dijt the direct coupled aavo trap ably give better results hen using the latter direct coupling as illustrated la ft 1 some small change in the tuning the antenna condenser will be noti whereas inductive coupling p no change in tuning tills represents a blight advantage fior of inductive coupling stow will be possible to continue to use fr fiame old dial readings after the K has been installed if the trip Is poorly made aa sir rhodes in conclusion it layf only eliminate the interfering segni but will also decrease the strength those signals we desire to receive most common causo of poor reil with wave traps as due to the we high lesl stance colls and lowge condensers tonden sers it Is therefore eb that the trap operate efficiently in order to obtain such operation t Is generally best to use a fairly ass coll and a large condenser large coil the elimination 13 kusiw more complete but the neutral band Is larger so that the trap feres with reception on wave le adjacent to that wave on interfering station Is operating satisfactory results arc obtained v low loss coils are used such as spa wound solenoids while a horold be used in the inductively couple et cult |