On 3/6/19 5:33 AM, Bill Slade wrote:
My calculation was a bit hasty. Q_rad is around 123, not 7e6 (misplaced factor of 2pi). Still pretty bad, tho'. So, we have 1/24 -1/123=1/Qloss or Qloss = 25; typical of what you'd find in a lumped LC circuit. Cheers
after all, a good way to get a return loss no worse than 6dB is a 3dB pad..
________________________________ From: time-nuts <time-nuts-boun...@lists.febo.com> on behalf of Bill Slade <slade_b...@hotmail.com> Sent: Wednesday, March 6, 2019 10:32 AM To: Discussion of precise time and frequency measurement Subject: Re: [time-nuts] Smaller, and smaller antennas The Chu-Harrington limit for passive antennas (ones without active, non-Foster circuits) states that for small antennas Q_rad>lambda^3/(2pi a)^3. at 2.4GHz, lambda = 12.5cm. For an antenna of a=4mm dominant dimension, Q_rad>7e6! If a VSWR BW of 100 MHz is measured at the feedpoint (Q_tot approx 24) and we remember that 1/Q_tot = 1/Q_rad + 1/Q_loss, we see that the Q factor is dominated by antenna losses and radiation efficiency is very poor. My feeling is that the feed network on the PCB will radiate more than this antenna. It would not be the first time that I have seen electrically small antennas that exhibit suspiciously substantial VSWR bandwidth that are like resistors than antennas. Cheers, Bill
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