Bird 43 reading different on 100w vs 1000w slug
Model 43 Bird on tune-up using 1000W 1000H slug 2-30Mhz 90 Watts output. 28 Mhz Tune-up
Model 43 Bird on tune-up using 100W 100H slug 2-30Mhz is 68 Watts output.
Do not have another working meter at this time so do I have a problem or is this a normal for the different power slugs?
Wattmeters are rated, in tolerance, as a percentage of full scale readings. As such, with the lower power slugs, which have a lower full scale, the absolute value of the tolerance is going to be less.
Always, for the best accuracy, use the slug with the lowest full scale reading that can handle the actual power output of the transmitter.
As such, assuming that the slugs have not been damaged, the 100-watt element should give a much more accurate reading.
The Bird 43 is spec'ed for 5% of full-scale accuracy. 5% of 100w is 5w. 5% of 1000w is 50w. With the 1000w slug, the Bird 43 is actually reading 90w plus or minus 50w, i.e. somewhere between 40w and 140w. With the 100w slug, it is reading 68w plus or minus 5w, i.e. somewhere between 63w and 73w. Anything between 63w and 73w is also between 40w and 140w so the Bird 43 reading is meeting its accuracy specifications. Hint: go with the lowest power slug that will measure what you need measured since it has the highest absolute accuracy.
Originally Posted by WB5UMG
P.S. Dang Glen, you beat me by one minute - not fair.
73, Cecil, www.w5dxp.com
The maximum power transfer theorem works just as well for a non-resonant antenna as it does for a resonant antenna.
On top of all that you've already been told, also consider the fact that the analog meter on the Bird is not accurate for the first half of scale as it would be in the second half of scale. Most D'arsonval type meter movements are like that. So the 1000W slug is making you read the power in the least accurate area of the meter as well. It is likely to be most accurate when the meter needle is somewhere around 60 to 80% mark. Which your 100W radio can't reach with the 1K slug...
They're rated 5% of full-scale accuracy, which is why to measure "reflected" power you need a more sensitive element to get any kind of realistic reading.
If you're running about 100W output and flip the slug around to read reflected power it's still +/- 5% of 100W accuracy, which means at a 2:1 SWR (11% reflected power) you could read anything from 6W to 16W reflected power and that would all be within the specified tolerance of the meter. I always use about a 10:1 more sensitive element for reflected power readings, to see what they "really" are.
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-- George Bernard Shaw
Yep. You want to that the meter reading as close to full scale as possible.
Originally Posted by K9STH
"The more you know, the less you don't know."