Posted by
Edward R Cole on
Dec 10, 2011; 11:41pm
URL: http://elecraft.85.s1.nabble.com/New-K3-built-can-I-reset-the-5w-calibration-tp7076112p7082529.html
I've been reading the comments about Bird43 accuracy for awhile, but
the claim that the error is fixed amount for all levels of power
measured on a given element just does not agree with my
experience. (Just about) ALL of my RF power measurements have been
using the Bird and what I see is a scaled error and not a fixed
one. If the full scale reading is off 5% then the quarter power is
likewise 5% off. In other words 100w measured with a 100E element at
432-MHz might actually be 95w. If I measure 50w with the same
element the actual power will be about 47.5w. 25w will be about
24w. I'm not saying the accuracy tracts exactly linearly with power
level but it seems approximately so.
That being said, the accuracy for measurements at under 25% of the
full power rating of the element is not of much value except as a
relative number. I try to chose an element so the meter reads >70%
full scale for best accuracy and resolution if I am trying to obtain
absolute power measurements.
If I want better accuracy I use calibrated attenuators with my HP432A
microwave power meter (10-MHz-18-GHz) which measures -20 to +10
dBm. I have two high-power 20-dB coax attenuators: 100w NARDA (good
to 4-GHz) and 10w Bird (good to 1-GHz). I measured the Bird on a
steady 2m signal yesterday and it is exactly 20 dB within the 0.2 dB
resolution of the HP meter. For measuring 300w on 1296 I have some
40 and 50-dB directional couplers from the cellular industry (rated
for high power). That permits power measurements to be made with the
HP power meter (300w = +55 dBm => -50 dB = +5 dBm) 2:1 SWR reflected
power would be 30w (+ 45 dBm +> -40dB = +5 dBm). But this also
allows me to check the accuracy of the Bird power meter. I suspect
this is "good enough" for ham radio purposes.
73, Ed - KL7UW, WD2XSH/45
======================================
BP40IQ 500 KHz - 10-GHz www.kl7uw.com
EME: 50-1.1kw?, 144-1.4kw, 432-QRT, 1296-?, 3400-?
DUBUS Magazine USA Rep
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