VHF RF Choke

Home made VHF rf choke. It has a typical value of 0.3uH made by winding 15 turns of 0.2mm enamel wire over a high value resistor (27K 1/2watt) as a former. Inductor leads are soldered across the resistor leads and the instant glue secure the winding on the former.

Most of my home made vhf rf amplifiers uses this rf choke in its bias and also in the DC supply section. Commercial vhf choke generally comes in a small package suitably for smd but since I have so many enamel wires in my junk box, home brew comes in handy.   ---73 de du1vss

FM Broadcast 55W Push-Pull RF Amplifier

This prototype rf amplifier uses two 2SC2640 VHF power transistor wired in push-pull configuration. The 50 ohm impedance of the input is transformed to lower impedance by the 4:1 broadband coaxial cable transformer  providing a close match to the base of the two transistor.  To operate in class C, the base of the transistor must be at ground potential so the secondary winding of the 4:1 coaxial transformer is grounded at the center position( null).  The impedance of both collectors are transformed by the combination of  inductors L1 , L2 and the 120pF capacitor  and further transformed by the 1:1 coaxial balun.

Both the 4:1 coaxial transformer and the 1:1 balun are made from RG316 teflon coaxial cable and these are cut to a certain length in order to operate at the highest cut-off point which in this case at 110MHz. Actual test of the prototype amplifier, I was able to measure 55 watts output from a 1.5 watts input.   ---73 de du1vss

Quarter Wave Stub Demo

One of the trick that you can perform to your friend (not a Ham) is this quarter wave stub. Looking at the DC (direct current) point of view, the bulb should not light since there is a shorting wire (stub) that will ground any voltage present across the 12V bulb. But strange things happen when a strong rf source is brought near the dipole antenna. The bulb should light brightly! This phenomenon can be easily explained when we understand how the quarter wave stub behaves when excited at its resonant frequency.

In my prototype, I use a 435MHz  3w transceiver to excite the dipole antenna and the 12V pilot bulb was used as an indicator for rf current present across the feed point of the antenna. The antenna itself is approximately 13 inches in total length while the stub length is 6.5 inches. Spacing of the stub was merely approximated to around 1/16 of an inch. Part of the trick is that you can also cut open the far end of the stub and show to your friend how the bulb immediately off at this time.   ---73 de du1vss

Telephone Audio Coupler

This circuit is a simple one, audio signal from the telephone line is sampled by the 600:600 ohm isolation transformer. The combination of 0.1uF/200V capacitor and 500 ohm resistor suppresses the high voltage present in the line during the ring signal.

Signals appearing at the secondary side of the transformer is clamped at 1V peak to peak by the two diodes connected back to back. Audio level at this point can be easily processed by an audio mixer before going to the transmitter. ---73 de du1vss.