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Art Unwin wrote:
"What exactly is inside the various slugs that one must use with this meter?" 73 Amateur Radio had an article, "Calibration and Repair for Bird Wattmeter Elements" in its April 1989 issue by Francis Kelson, K2KSY. K2KSY`s plug-in element diagram is more complete than that given by Bird under the topic, "Thruline Principle" in my Bird catalog. Bird may have been guarding proprietary information in 1989 when my catalog was issued. The Bird Model 43 accuracy is specified as + or - 5% of full scale in the catalog. So, a "slug" which would cause nearly full scale deflection should probably be used for accuracy. Accuracy results from the careful, rugged construction and calibration of the Bird. Bird says: "Broadcasters may want to order two identical Elements at the same time and keep one in a safe place after recording the meter readings obtained by each. If a question arises about recalibration (e.g. in case an Element has been dropped on a cement floor), a quick comparison with the original twin could save time, effort and inconvenience of shipping the whole wattmeter back for a checkup. For better resolution of low reflected power levels, we recommend a second Element 1/10 the power of the forward Element." From the above, Bird seems to advise broadcasters initially buy 4 Elements. Richard Clark, a metrologist, occasionally disparages Bird`s accuracy on this newsgroup. I`m no metrologist, but have experience with many Bird 43`s and have seen them to be highly repeatable and exchangeable. They usually read what you would expect and they read what you determine the power to be by other means. I doubt that cheap meters come close to the Bird`s reliability. I think the Bird Model 43 is a good value. Best regards, Richard Harrison, KB5WZI |
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