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[Amps] RF power metering for HF

To: amps@contesting.com
Subject: [Amps] RF power metering for HF
From: "John T. M. Lyles" <jtml@lanl.gov>
Date: Mon, 21 Mar 2005 11:01:38 -0700
List-post: <mailto:amps@contesting.com>
I have a similar distrust for Bird 43s and their clones. I do use one 
online all the time on the output of my SB220, and currently the 
DX100 that I am restoring (pair 6146B). However, for more accuracy to 
determing true efficiency, dissipation, gain, etc, in addition to the 
inline Bird directional wattmeter, I use a Werlatone directional 
coupler, an extremely broadband device (250 KHz to 30 Mhz) which is 
rated for 2 kW. I terminate the forward port with a small BNC 50 ohm 
shunt terminator (the ones from Tektronix work nicely), at the 
vertical channel of a 100 Mhz scope (that is in good condition). With 
this, I have a peak voltage reading which is accurate. By the way, 
the transmitter is terminated in appropiate low reflection dummy 
load, either homemade or my Bird 600 watt Termiline.

We use big Bird power attenuators when testing large solid state amps 
in the VHF/UHF range at work. With these and a suitable set of pads, 
one can pretty much accurately measure RF power. For power metering, 
Agilent makes nice systems, and we continue to depend on the old 
HP8900D peak power meters, which have a video output and a digital 
readout. These are some of the simplest and most reliable diode-based 
peak power meters, although they don't have a huge dynamic range.

All of these methods depend on having devices (such as coax, 
attenuators, directional couplers) which can be verified using a 
calibrated network analyzer such as the Agilent 8753 series. The 
wattmeter or oscilloscope are low power units which can be easily 
calibrated by manufacturers or cal labs. It is much trickier to get 
accurate field calibration of a device like a high power direct 
reading wattmeter without resorting to calorimetric loads and other 
transfer standards.

73
John
K5PRO
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