[time-nuts] Smaller, and smaller antennas

[jimlux]

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-bounces at lists.febo.com> on behalf of Bill Slade <slade_bill at 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