Switchable RF Probe-Watt Meter

Here is one of the most trusted homemade test instruments to have, once you get your FM transmitter up and running. For sustained loads of up to 2 watts. This simple little device is capable of finding your FM transmitters: unloaded RMS output voltage, oaded RMS output voltage, output impedance, and output wattage.

Switchable RF Probe/Watt Meter Construction

First, print out the PCB template to the left. Send the picture to a graphics program, such as Paint, and squeeze and/or stretch to get a printout of the said dimensions. Once that is done, drill your hole for a SPST Sub-mini switch (Radio Shack part # 275-645A). Then mimmick the same routing as to the template. Use a single sided PCB. If you only have a double-sided PCB, etch away the backside.

Once you have etched your PCB, then go ahead and look at the picture below for soldering and wiring all of the components to the PCB.

Once you have completed with all the soldering/wiring of the components to the PCB. You are now ready to understand how this little beauty works ! Do take a look at the picture to the right, it shows my completed RF Probe/Watt Meter. Yours should look similar to that. Check to make sure all copper routings show continuity and that all soldering joints are good. Also make sure your germanium diode is oriented correctly into the circuitry. That is, the cathode (negative) side goes toward the output of the meter.

Begin by hooking up your transmitter, RF Probe/Watt Meter and DVM/Analog Meter as laid out in the picture below.

A DVM (Digital Volt Meter) will always give detailed readings to the nearest hundredths, but it usually isn't that reliable in the VHF arena. The analog meter will have a more reliable measurement...although the reading is taken directly upon where the needle rests on the scale therefore giving a less detailed reading. A DVM seems to always give higher readings then the analog meter, when doing tests with our homemade device. If you have the benefit of having both meters. I would strongly suggest making readings with the analog meter.

Source: The Switchable RF Probe/Watt Meter Project

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40W Broadband VHF RF Power Amplifier 88-108 MHz

This RF power amplifier design is to boost the output power of low power FM broadcast band exciters based on Motorola MRF171A MOSFET with power output 40W. To reduce the harmonics to an acceptable level, Integrated 7 pole Chebyshev low pass harmonic filter (LPF) is added at the final section.

Component Choices
As the input power is only half a watt, standard ceramic capacitors and trimmers were used in the input matching circuit. L1 and L2 (refer to schematic) could have been made much smaller, but were kept big for consistency with the inductors used in the output network. On the output network, mica metal clad capacitors and mica compression trimmers were used to handle the power and keep component losses to a minimum. The wideband choke L3 provides some lossy reactance at lower RF frequencies, C8 takes care of AF (audio frequency) decoupling.

The use of an enhancement mode N-channel MOSFET (a positive voltage biases the device into conduction) means the bias circuitry is simple. A potential divider taps off the required voltage from a low voltage stabilised by a 5.6V zener diode. The second 5.6V zener, D2, is fitted as a precautionary measure to ensure excessive voltage are not applied to the gate of the FET, this would certainly result in the destruction of the device. Purists would temperature stabilise the bias current, but as the bias is not critical in this application, this was not bothered with.

A BNC socket had been used for the RF input, due to the low RF input power. I've used N type for the RF output, I don't use BNC for above about 5W and I don't like UHF style connectors. Personally, I don't recommend using UHF connectors above 30MHz.

RF Power Amplifier Schematic-Printed Circuit Board

Power Amplifier Construction
The Power amplifier was constructed in a small aluminium diecast box. RF input and output connections are made by coaxial sockets. The power supply is routed through a ceramic feedthrough capacitor bolted in the wall of the box. This constructional techniques results in excellent shielding, preventing RF radiation escaping from the amplifier. Without it, significant amounts of RF radiation could be radiated, interfering with other sensitive circuits such as VCOs and audio stages, also significant amounts of harmonic radiation could occur.

The base of the power device sits through a cut-out in the floor of the diecast box and is bolted directly onto a small extruded aluminium heatsink. An alternative would have the base of the power device sitting on the floor of the diecast box. This is not recommended for two reasons, both concerned with providing an effective path to conduct heat from the FET. Firstly the floor of the diecast box is not particularly smooth, which results in a poor thermal path. Secondly, having the floor of the diecast box in the thermal path introduces more mechanical interfaces and hence more thermal resistance. Another advantage of the chosen constructional technique is that it correctly aligns the device leads with the top face of the circuit board.

Using the specified heatsink will require the use of forced air cooling (a fan). If you plan not to use a fan, a much bigger heatsink will be required, and the amplifier should be mounted with the heatsink fins vertical to maximise cooling by natural convection. RF Power Amplifier Part List

Source: A Design for a 40W broadband VHF RF Power Amplifier for FM broadcast

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VHF RF Power Meter

Here's a simple instrument for the homebrewer to measure RF power well through VHF. interests. The reference instrument incorporated a built-in analog meter and provision to connect an external DVM. It also utilized a conversion chart to relate
meter readings to RF power.

The DVM (Digital Volt Meter) also displays the correct polarity sign. There’s neither rocket science nor smoke and mirrors here folks: It’s all done simply with op amps and resistors! The schematic tells all.

In the process of designing these basic of power meter circuit functions, it occurred to me that other simple additions would add a lot to the utility of the instrument. Thus, the project grew “on the fly.” One of these extras is a gain-change option that includes an external OFFSET control for the analog display.

For more information about component parts list, power meter process design,  and calibration-operation, please download An Advanced VHF Wattmeter (in Pdf).

Source: RF power meter project by Wes Hayward, W7ZOI and Bob Larkin, K7PUA that appears in QST, June 2001.

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