QRP RF Voltmeter from your Junk Box

A useful tool that can be easily crafted out from your junk box is this rf voltmeter. I discovered this circuit online and created a  prototype out of it. I found this project very useful in reading RMS voltage in my HF /VHF projects. I use it to determine the output power of my rf amplifier, set the null in my balanced modulator, determining the peak voltage output of my oscillator, peaking a band pass filter, etc... 

It takes only 3 components to start with. I use 1N60 germanium diode to rectify the high-frequency AC signal into a DC voltage compatible with any digital multimeter. My prototype rf voltmeter is mounted in a small PCB assembled in surface mount fashion. Two connectors soldered on the edge of the pc board to accommodate the positive and negative probe of my digital multimeter.

Connect the alligator clip to the common ground of the circuit you are measuring. Minimize the length of the alligator clip wire to avoid introduction of errors in your measurements, particularly on higher hf bands. In my prototype the lead wire for the alligator clip is just over 6 inches.

When reading the Vrms, take the measurement across the 50 ohm dummy load. For instance, if you measure 13V using your digital tester, the computed power is around 3.38watts, calculated using the formula P = (Vmeasured)² / 50.    ---73 de du1vss

40M DSB Transceiver

The circuit described here is actually a variation of the  MDT40 project by VK2DOB,  A simple double side band suppressed carrier (DSBSC) transceiver which operates on 7MHz. I find this project simple and a good start if you are interested into home brewing your own rig so I decided to make one.

In the original circuit, it uses a ceramic resonator as a common oscillator for both receive and transmit stage but I have difficulty in finding the parts here in Manila so I decided to use a vfo whose circuit was copied from KD7REM, a rocksteady vfo. Stability is considerable during start up which usually takes less than 30minutes of wam-up and can be used with a couple of QSO without correcting the vfo for a drift. T50-6 toroid was chosen as a core of my inductor where 25 turns of enamel wire was wound. RF feedback was taken at 5 turns from the cold end. I found by experiment that by keeping the capacitance large in the L/C ratio, the oscillator is more stable than the other way around. MPF102 is also used since JFET is more stable than its BJT counterpart.

The balanced modulator and the receiver pre-amplifier stage are constructed together in one board. There are four 1N4148 used in the balance modulator and please note that these diodes are carefully matched by their forward bias voltage. A 500 ohm potentiometer is used to find the null in the balance modulator. Carrier suppression is easily done when you already have completed the transmitter stage since this can be just adjusted by the power output of the rf amplifier stage.

The rf transistors found  in the transmitter stage are uncommon so I decided to redesign completely the entire stage using common transistors that are locally available here. 2SC2078 was chosen as the final stage since it is cheap and offers a 4W to 5W ouput at the HF frequencies.

Above is the prototype of my DSBSC transceiver that I made. It is housed in a plastic casing (black) and on the top (grey plastic casing) is the additional 15watts linear amplifier constructed using a push-pull IRF510 mosfet transistor.  ---73 de du1vss

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.

4:1 Unbalance to Unbalance Coaxial Transformer

This is my first time to implement the 4:1 unbalance to unbalance coaxial transformer to match the input and output impedance of an rf power transistor. My prototype rf amplifier originally had a lumped LC network in both input and output sections and only have +/- 3MHz of bandwidth but after the addition of 4:1 coaxial transformer, the bandwidth has now increased to +/- 6MHz from the center tuning frequency of the amplifier.
The coaxial transformer is about 1/16th wavelength long from the highest frequency in which the amplifier will work and should have 25 ohm of characteristic impedance. In my prototype amp it uses a pair of RG-178 cable paralleled to arrive at the 25 ohm requirement.

This is the simplified diagram when looking at the transformer however, the characteristic impedance of the coaxial cable also plays an important role in the impedance ratio. For a 50 ohm to 12.5 ohm transformation, it calls for a 25 ohm cable but since I don't have this kind, I simply paralleled two RG178, a 50 ohm cable.

The schematic diagram of my prototype amp tells that aside from the 4:1 coaxial transformer, LC network was still employed to further matched the impedance of the base and collector of the power rf transistor. The only limiting factor that prevents the amplifier for a broadband operation is the LC network!

My prototype amp after the inclusion of the 4:1 coaxial transformer. I might replace the old PCB to facilitate the addition of the two coaxial transformer. ---73 de du1vss