by Harry Lythall - SM0VPO

Have you ever had a low power transmitter, with an output that was too low to measure? I used to use a "chat-box" for 145MHz repeater work which gave out a minimum signal to just access GB3PI. The path losses between myself and GB3PI were such that I only needed 20uW (not a missprint) to gain access. But how do you go about measuring such powers?

I recently described a SWR-POWER meter which was capable of giving a "rough" indication down to 10mW. I also had a VHF FM rig that had a variable output power from 10mW to 10 watts. This is all that is required, save for a couple of attenuators. (An RF signal generator may also be used, instead of the rig with a variable output power)

Connect the QRP transmitter into the junction of a "T" connector, via a 20 dB attenuator. Connect the "variable power" TX into the second port of the "T" connector via a 30dB attenuator. Connect the third port of the "T" connector to a VHF receiver via a 20dB attenuator. The attenuator should have a pair of "back-to-back" 1N4148 or 1N914 protections diodes across the receiver antenna port. An SWR-POWER meter may be left in the known TX antenna lead to indicate power as the attenuators are (or should be) resistive. Many "black box" TX's have a circuit, which which detect the RF voltage at the antenna to produce a DC voltage used to control the power. A pot feeding a small amount of DC into the "DC SIDE" of the detector diode will often turn a fixed power rig into a variable output power transmitter. The known transmitter should NOT be placed beside the monitor receiver and all leads must be screened coaxial cables.

Switch ON both the unknown and known transmitters tuned to the same frequency and you will hear a heterodyne from the monitor receiver. Adjust the known TX power level for maximum heterodyne audio level. Keep the known TX below 1watt or the TX attenuator will burn.

You may have to increase the KNOWN TX attenuator if the "QRP TX" is less than 1mW. Increase the UNKNOWN TX attenuator if the QRP TX is greater than 100mW output. The maximum beat-note occurs when the known and unknown power levels at the junction of the "T" connector are identical. The unknown TX is 10dB BELOW the known transmitter power level with the values quoted above.


The resistive attenuator is made using resistors with these resistor values (assuming a 50 ohm coaxial system):
(dB, R1, R2, R3)

5dB, 178.6, 30.4, 178.6
10dB, 96.2, 71.2, 96.2
15dB, 71.6, 136.1, 71.6
20dB, 61.0, 247.5, 61.0
25dB, 56.0, 443.1, 56.0
30dB, 53.2, 789.6, 53.2
35dB, 51.8, 1400, 51.8
40dB, 51.0, 2500, 51.0
45dB, 50.5, 4445, 50.5
50dB, 50.3, 7900, 50.3
55dB, 50.2, 14060, 50.2
60dB, 50.1, 24000, 50.1

All resistors should be non-inductive "carbon composition" and awkward values may be made up with combinations of preferred values eg:

71.2 ohms = 68 + 3.3 = 71.3 = (near enough)

The 20dB attenuator between the "T" junction, and the monitor receiver should be fitted with a pair of back-to-back 1N4148 diodes at the junction of the attenuator and receiver. This will remove any chance of the monitor receiver accidentally being overloaded with RF power. This attenuator should ALWAYS be in the receiver antenna lead during such tests. In spite of this precaution the RF power level at the monitor receiver should NEVER be allowed to exceed 1mw (it will be lower due to the impedance mismatch in the "T" connector).

Have fun, de HARRY, Upplands Vasby, Sweden.

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