Highpass Transmission Line Filters

The arrangement at (A) will stop the passing of all signals below about 45 Mc. from the antenna transmission line into the TV set. Coils Li ore eoefi 7.2 micro/ienrys (17 turns no. 24 enam. closewound on %-inch dia. polystyrene rod) with the center tap grounded. It will be found best to scrape, twist, and solder the center tap before winding the coil. The number of turns each side of the tap may then be varied until the tap is in the exact center of the winding. Coil L2 is 0.6 microhenry (12 turns no. 24 enam. closewound on %-inch dia. polystyrene rod). The capacitors should be about 16.5 imfdbut either 15 or 20 ceramic capacitors will give satis factory results. A similar filter for coaxial antenna transmission line is shown at (B). Both coils should be 0.12 microhenry (7 turns no. 18 enam. spaced to !6 inch on %-inch dia. polystyrene rod). Capacitors C2 should be 75 mifd. midget ceramics, while C3 should be a 40-mifd. ceramic.

nated as a source of interference to reception, work may then be begun on or in the vicinity of the transmitter toward eliminating the other two types of interference.

Taking Out More or less standard BCI-

the Fundamental type practice is most commonly used in taking out fundamental interference. Wavetraps and filters are installed, and the antenna system may or may not be modified so as to offer less response to the signal from the amateur transmitter. In regard to a comparison between wavetraps and filters, the same considerations apply as have been effective in regard to BCI for many years; wavetraps are quite effective when properly installed and adjusted, but they must be readjusted whenever the band of operation is changed, or even when moving from one extreme end of a band to the other. Hence, wavetraps are not recommended except when operation will be confined to a relatively narrow portion of one amateur band. However, figure 1 shows two of the most common signal trapping arrangements.

High-Pass Filters High-pass filters in the antenna lead of the TV set have proven to be quite satisfactory as a means of eliminating TVI of the overloading type. In many cases when the interfering transmitter is operated only on the bands below 7.3 Mc., the use of a high-pass filter in the antenna lead has completely eliminated all

TVI. In some cases the installation of a high-pass filter in the antenna transmission line and an a-c line filter of a standard variety has proven to be completely effective in eliminating the interference from a transmitter operating in one of the lower frequency amateur bands.

In general, it is suggested that commercially manufactured high-pass filters be purchased. Such units are available from a number of manufacturers at a relatively moderate cost. However, such units may be home constructed; suggested designs are given in figures 2 and 3. Types for use both with coaxial and with balanced transmission lines have been shown. In most cases the filters may be constructed in one of the small shield boxes which are now on the market. Input and output terminals may be standard connectors, or the inexpensive type of terminal strips usually used on BC and TV sets may be employed. Coaxial terminals should of course be employed when a coaxial feed line is used to the antenna. In any event the leads from the filter box to the TV set should be very short, including both the antenna lead and the ground lead to the box itself. If the leads from the box to the set have much length, they may pick up enough signal to nullify the effects of the high-pass filter.

Blocking from Operation on the 50-Mc. ama-50-Mc. Signals teur band in an area where channel 2 is in use for TV imposes a special problem in the matter of blocking. The input circuits of most TV sets are sufficiently broad so that an amateur signal on the 50-Mc. band will ride through with little attenuation. Also, the normal TV antenna will have a quite large response to a signal in the 50-Mc. band since the lower limit of channel 2 is 54 Mc.

High-pass filters of the normal type simply are not capable of giving sufficient attenuation to a signal whose frequency is so close to the necessary pass bandof the filter. Hence, a resonant circuit element, as illustrated in figure 1, must be used to trap out the amateur field at the input of the TV set. The trap must be tuned or the section of transmission line cut, if a section of line is to be used for a particular frequency in the 50-Mc. band. This frequency will have to be near the lower frequency limit of the 50-Mc. band to obtain adequate rejection of the amateur signal while still not materially affecting the response of the receiver to channel 2.

Elimination of All spurious emissions

Spurious Emissions from amateur transmitters (ignoring harmonic signals for the time being) must be eliminated to corn-

Figure 3

Figure 3

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