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#13
May 24th 07, 06:17 PM
posted to rec.radio.amateur.antenna
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Are pi networks THAT INefficient?
"Jimmie D" wrote in message ... "Owen Duffy" wrote in message ... wrote in : Ralph Hanna, W8QUR, in a brief article "Pi Networks" on page 108 of the December, 1965, issue of 73 MAGAZINE, after discussing power- supply filters and high- and low-pass TV filters, wrote: (Paraphrasing) "The most popular of all pi networks is the output circuit of a transmitter ... with which the output of almost any transmitter can be matched to almost any antenna ... another advantage is the reduction of harmonics.... (Actual quote) "The big disadvantage of this system is the low efficiency. It is not possible to run more than 50% efficiency and it tends to be more like 30%. Other methods of feeding the antenna will result in efficiencies of as high as 65% to 70%." Is that "low efficiency" of 30-50% really true? Myron, The temptation is to see that the second paragraph is about Pi networks, though it doesn't actually use the term. It does refer to a "system" and goes on to discuss efficiency in the context of "feeding the antenna". There is no doubt that practical Pi networks in transmitters operate at efficiencies much greater than 50%, and the design efficiency is a trade- off with harmonic suppression (for the low pass configuration in a typical PA). If the term "system" is to include more than just the Pi network, then lower system efficiciency will prevail, but without a clear definition of the "system", it is not possible to comment on the reasonableness. For example, if a Pi coupled transmitter feeds a full wave dipole via a substantial length of coax, system efficiency might well be much less than 10%. Does he include DC to RF conversion loss in his view of system efficiency? Owen Giving W8QUR the benifit of the doubt I thought he may be including feedline losses which could be from 1 to 2 db for coax compared to balanced line used with a balanced output network. I think something may be lost in the paraphrasing and this is probably a comparison of balanced to unbalanced systems rather than a comparison of Pi-net to other types of tuner networks. Jimmie In the 1960's it was common to refer to efficiency as relating to the entire system converting AC or DC power into RF out of the antenna. Total system efficiencies of 30-50% would have been normal taking into account the losses involved in running valve heaters, HT valve supplies and final stage cooling fans. The reference to Pi matching output circuits is a bit of a red herring. These were probably the most commonly used system used to match transmitters that were required to operate on a wide range of frequencies because they were so effective at this task, and relatively cheap to manufacture. Other more efficient matching methods could be used for fixed frequency valve transmitters. Even up to the 1980's, many shipboard emergency transmitters were valve based. Radio Officers were required to make regular checks and efficiency calculations to ensure that the emergency transmitters and receivers could operate for a minimum specified period from a bank of emergency batteries. Even with several hundred amp hours of battery capacity, only around 16 hours of full power (100watts) operation was the maximum that could be expected. Mike G0ULI |
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