Appendix 2

Toroidal chokes

The "braid-breaker"

The braid breaker is a toroidal choke for the HF bands, and is made by winding several turns of co-axial cable through a ferrite ring as shown.

It is effective because interfering currents flowing down the braid create an external magnetic field, and the device acts like a ferrite cored choke in the path of these currents. For wanted signal frequencies. howeven equal and opposite currents are flowing in the inner and outer of the co-ax, and their fields cancel; the ferrite ring, therefore, does not impede signal flow.

The choke should be made using small diameter (standard) co-ax and the ring should not be wound more than 2/3 full, to reduce capacity between the ends of the coil. The assembly should then be taped to secure the turns, and a co-axial plug fitted at each end. The coil can then be readily interposed between the co-axial downlead and the TV aerial socket.

Mains filter choke

This can be made in the same way by threading the mains cable through a ferrite ring. Again only wind 2/3 full.

Ferrite rings of 'split' construction are available (see Appendix 6). They can be installed on existing cables without rewiring the plug.

Appendix 3

Braid breaker and high pass filter

This combined braid-breaker and high-pass filter prevents HF signal currents from flowing down the inner or outer of the co-axial cable. L 1 has a low reactance to HF or VHF signals. thereby by- passing the interfering current. whereas C 1 and C2 have high reactance to HF/VHF. effectively blocking signals to the receiver. L2 provides a futher 'short circuit' across the connections on the Rx side of the filter.

At UHF the conditions are reversed. C 1 [4p7]and C2 [4p7] presenting a low impedance path to signals to the receiver and the impedance of L 1 and L2 is so high as to be ignored. R 1 [1M5] is for static discharge of the outer screen.

Appendix 4

Fitting of ferrites and RF bypass capacitors

Capacitors should be disc ceramic, and fitted with the shortest leads possible. to keep inductance low.

The ferrite bead should preferably be fitted on the base lead of the transiston but where this is not possible it should be fitted in the PC copper as shown.