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AMATEUR RADIO By GARRY CRATT, VK2YBX Build this simple stub filter to eliminate TV inter£erence One of the hazards to radio amateurs operating on the VHF and UHF bands is the possibility of causing television interference (TVI) to neighbours. Fortunately, many TVI problems can be easily cured by building and fitting a simple stub filter to the TV antenna feedline. A recent government proposal to charge a $60 investigation fee to persons who complain of TVI makes this article on the construction of a simple yet effective filter particularly timely. The most suitable type of filter for a particular situation is determined by several factors but the most important in cases of amateur radio interference is ease of fitting. This is because anyone experienc- ing TVI is unlikely to possess much patience - it is important that the filter can be rapidly installed. Secondly, the filter must look acceptable (ie, not look like a can of worms) and thirdly, it must be cheap to make. While there can be cases of TVI caused by IF or audio stage detection (and these situations require specific cures), the primary reason that TVI is experienced in a I I I I A14 OPEN STUB )112 OPEN STUB , 12 SHORTED STUB 290 435 580 870 1015 FREQUENCY (MHz} Fig.1: this diagram shows the effects of both terminated (short circuit) and unterminated (open circuit) quarter wavelength and half wavelength stubs for the 2-metre band. populated area is simply due to overload of the front end of the tuner. After all, a television receiver is expected to deliver a reasonable picture from a signal of only a few hundred microvolts, yet that same receiver needs to remain unaffected by a nearby amateur VHF or UHF transmitter which could have a legal power output of over lkW EIRP. In fact, the transmitting antenna could well be aimed directly at the antenna for the affected TV set. Thus, it is hardly surprising that television receivers do suffer from front end overload in such situations. Theory of operation By far the simplest yet most effective type of filter that can be installed at the antenna terminals of a television set is the coaxial stub. The operation of a stub filter is based on the fact that a cable cut to a quarter wavelength of the offending signal frequency and with one end open circuit (ie, unterminated) presents a very low impedance at the other end (the feed point). This fact can be used to greatly reduce the unwanted signal while letting other signals pass unaffected. A stub also presents this low impedance at odd multiples of a quarter wavelength. When the stub is made to a half wavelength in length, any impedance at the terminated end is reflected at the feed point. Fig.1 shows how the use of either MAY 1989 69 the desired frequency. Two or three such stubs may be cascaded to achieve a notch depth of - 70dB or so, which should be more than adequate for even the most serious overload situation. 0 = '2- 20 z ~ 40 et :::, z ~ ...< 60 TWO STUBS - - SPACED BY "/J4 AT I Construction 80 FREQUENCY 31 Fig.2: a single unterminated quarter wavelength stub is capable of producing 30dB of attenuation at the desired frequency and at odd harmonics of this frequency. Cascading two such stubs will give a notch depth of about - 70dB. This cascaded stub filter was cut to provide around 70dB of attenuation at 146MHz. The input and output leads go to Belling Lee connectors so that the filter can be easily installed in the antenna line. quarter wavelength or half wavelength stubs in either the unterminated (open circuit) or terminated (short circuit) condition can be made to produce filters for the 2-metre band (144 to 148MHz). Such stubs can be used to reduce interference across all TV bands. A quarter wavelength unterminated stub connected across a 75-ohm TV feeder cable is capable of producing 30dB of attenuation at The coaxial stub has the advantage of simple construction and broad bandwidth (single stub only). It can be built using standard TV style coaxial cable and Belling Lee type connectors, so that the entire filter can be made to plug into the antenna socket at the back of the television set. Fig.3 shows the layout of a single stub filter while Fig.4 shows a cascaded stub filter. The length of the stub is quite predictable and will lie between 65 % and 85 % of a quarter wavelength in free space, depending on the "velocity factor" of the ea ble used. That term needs explaining. The "velocity factor" is the ratio of the speed of electromagnetic radiation (ie, radio signals) along a cable to the speed in a vacuum. Since radio signals always propagate more slowly along a ea ble than in a vacuum, the physical wavelength of a radio signal in the cable will always be shorter than for a vacuum. This decrease is given by multiplying the wavelength 'in vacuo' by the velocity factor. Table 2 shows velocity factors of several commonly used coax cables. Fig.5 shows a cascaded stub filter for the 2-metre band. The dimensions shown assume a frequency of 146MHz and the use of cable with a velocity factor of 0.66. Thus, the length of the stubs is: 114A x Vf = 34cm. 4 FEEOLINE ~ _ COAXIAL _ I_ ----~ - - - -7, - -- - -7 r----- - STUB Fig.3: basic scheme for a single stub filter. the velocity factor of the cable must be considered when calculating the stub length. 70 SILICON CHIP _____ - - - - - II II 11 11 11 II II II II II II II 11 II 1 II I II II I, I, _ _ _ _ _ _ _ _ I, II _ _ _ _..... Fig.4: a cascaded stub filter is made by installing two stubs one quarter of a wavelength apart. TABLE 1: TV CHANNEL NUMBERS & FREQUENCY LIMITS Channel 9 195-202MHz Channel 10 208-215MHz Channel 11 215-222MHz BAND I Channel o 45 -52MHz Channel 1 56 -63MHz Channel 2 63-70MHz BAND IV BAND II Channel 3 85-92MHz Channel 4 94-101 MHz Channel 5 101 -108MHz BAND Ill Channel Channel Channel Channel 5A 6 7 8 137-144MHz 174-181 MHz 181-188MHz 188-195MHz Channel ·28 Channel 29 Channel 30 Channel 31 Channel 32 Channel 33 Channel 34 Channel 35 526-533MHz 533-540MHz 540-547MHz 547-5 54MHz 554-561MHz 561 -568MHz 568-575MHz 575-582MHz BAND V Channel 39 603-610MHz TO TV SET Channel Channel Channel Channel Channel Channel Channel Channel Channel Channel Channel Channel Channel Channel Channel 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 610-617MHz 617-624MHz 624-631 MHz 631-638MHz 638-645MHz 645-652MHz 652-659MHz 659-666M_ Hz 666-673MHz 673-680MHz 680-687MHz 687-694MHz 694-701 MHz 701-708MHz 708-715MHz TO ANTENNA BELLING LEE PLU G BELLING LEE SOCKET I. 1-14 X VF FOR 146MHz = 514mm x 0.66 = 339mm Fig.5: a practical cascaded stub filter for the 2-metre band. The stubs should initially be made about 50mm longer than shown to allow for trimming. The stubs should initially be made about 5cm longer than the calculated value to allow precise trimming late r on. They are soldered directly to the feedline (1/4 wavelength apar t) by carefully cutting away part of the outer jacket and braid of the feedline to expose the centre conductor . Connect both the braid and centre conductor of each ~tub to the feedline, then wrap the connection in insulation tape to prevent shorts. The free end of each stub should be left open circuit. Setting up Once the stubs have been soldered to the feedline, the entire assembly should be connected between a 144MHz antenna and a 2-metre receiver . By monitoring the local repeater or beacon frequency (even if this isn 't the precise fr equency to be attenuated), the stubs can be trimmed in small amounts (say 6mm at a time) to provide maximum attenuation a s indicated by the receiver's 'S' meter . Alterna tively, tho se amateurs equipped with a signal genera tor will be able to ti:im the stub to a precise frequency. However, the results will be quite satisfactory using the repeater method, as the filter will have an effective bandwidth of several MHz. Once cut to the length providing ma ximum attenuation, the stubs can be taped to the main coax f eedline and secured with plastic cable ties to make a nea t assembly. When using coaxial stubs one must be careful that the harmonic resonances also attenua ted by the stub do not fall on television chan- Channel Channel Channel Channel Channel Channel Channel Channel Channel Channel Channel Channel Channel Channel Channel 55 56 57 58 59 60 61 62 63 64 65 66 67 68 69 715-722MHz 722 -729MHz 729-736MHz 736-743MHz 743-750MHz 750-757MHz 757 -764MHz 76 4-771MHz 771-778MHz 778-785MHz 785 -7 92MHz 792 -799MHz 799-806MHz 806-813MHz 813-820MHz nels in use. For example, an unterminated quar ter wavelength stub cut to attenuate 146MHz will also attenuate 438MHz and 730MHz. If any of these fre quencies corresponds with a wanted TV channel we would be defeating the purpose of the filter. However, it is possible to shift the ha rmonic resonances. If we cut the stub short er than a quarter wavelength, it can be tuned to resonance by placing a trimmer capacitor across the open end, and the harmonic resonances will move higher in fre quency. It is also possible to cut the stub longer and insert a trimmer in series with the feeder connection. In summary, stub filters for the 2-metre and 6-metre bands are easy to make. They can go a long way towa rds pacifying the neighbours and preserving the image of responsible use of the spectrum by radio amateurs. it: TABLE 2: VELOCITY FACTORS OF COAXIAL CABLES Type RG-6 RG-8 RG- 11 RG-58 RG-59 RG-59(F) RG-17 4 RG-213 Impedance 75 50 75 50 75 75 50 50 Velocity Factor 0. 659 0.659 0.659 0.659 0.659 0.79 0.66 0.66 MAY 1989 71