My way of thinking about this problem and others like it is to reduce it to the simplest possible problem, and then look at the general case as a linear extrapolation. In this case I look at the interaction of the direct and single bounced ray during a site attenuation measurement on a perfect OATS, and then extrapolate from that to the reverberant chamber via linear superposition. I say this by way of an attempt to establish a common point of reference to enhance mutual understanding.
If the time delay between arrival of direct and bounced ray is much shorter than the pulse duration, then there is interference but insignificant spread or smearing. If the delay is a sizable fraction of the pulse duration, the received pulse will appear longer than the transmitted pulse. This is multi-path interference. If the time delay is well in excess of the pulse duration (but shorter than the pulse repetition period), then there is no constructive/destructive interference effect, but a second pulse appears at the receive antenna. Given the speed of light at roughly 300 meters per microsecond, if the bounce path is say 30 meters or less longer than the direct ray, there should be insignificant spreading (30 meters would spread the pulse from 1 to 1.1 us). In the general case of a reverberant chamber pulse spreading would be contained if the difference between the shortest and longest paths of commensurate power density were equal to or less than 30 meters. My initial supposition was that the limitation on pulse length would be room size, since as room dimensions increase delays increase. It was pointed out by members of the list that there is a strong dependence on chamber "Q" since a high "Q" means less power loss and longer delays (higher order modes) can still have the same intensity as more direct paths. That is where my understanding lies at this point, and the test I described earlier is my only way to gauge the extent of the problem. > From: Cortland Richmond <[email protected]> > Date: Tue, 29 Jul 2003 14:05:16 -0400 > To: Ken Javor <[email protected]>, ieee pstc list > <[email protected]> > Subject: Re: pulse modulation in reverb chambers > > > Ken Javor wrote: >>> My thinking is just the opposite. The duration of the pulse should be > long > relative to the time it takes to travel from transmit to receive antennas. > Then there is no smearing << > > It seems to me that you may be overlooking the effect of the reflected wave > on a received pulse's shape. Consider the step function, which is > certainly longer than the time to propagate across a chamber. Won't its > leading edge be reflected, and subtract from the pulse? Won't subsequent > reflections either (momentarily) increase or decrease it? And, given an > imperfectly symmetrical chamber, won't there be a confusing multitude of > such reflections? Of course, half the reflected energy incident on the > antenna would be re-radiated, and so on for each reflection, diminishing > their effect with time, but still, there's distortion not compensated for > by a long pulse. An analogy might what happens with a two dimensional > cavity -- an unterminated transmission line -- when we send a fast > rise-time pulse down it. > > We are not, I think, precisely in disagreement; I do agree a short pulse > will be smeared (after some time). It's just that I think reliance on a > longer one won't insure that it remains undistorted. If rise and fall times > were longer than the chamber transit time, then I'd expect no problem. > > I've not studied the chamber/pulse problem, mind. I did -- almost 20 years > ago -- set up two bicons a meter apart in a 4 meter long reverberant > chamber, and look at site attenuation. Nulls over 40 dB; high Q, indeed! > See the May issue of the Transactions on Antennas and Propagation for a > similar look (with regard to antenna impedance) at "Statistical Properties > of Linear Antenna Impedance in an Electrically Large Cavity.*" That may > have some relevance to your query. > > Ad astra per aspirin. > > Cortland > > *Statistical Properties of Linear Antenna Impedance in an Electrically > Large Cavity, L.K. Warne, K.S. H. Lee, H.G. Hudson, W.A. Johnson, R.E. > Jorgenson, and S.L Stronach. Volume 51 Number 5, page 978. > This message is from the IEEE EMC Society Product Safety Technical Committee emc-pstc discussion list. Visit our web site at: http://www.ewh.ieee.org/soc/emcs/pstc/ To cancel your subscription, send mail to: [email protected] with the single line: unsubscribe emc-pstc For help, send mail to the list administrators: Ron Pickard: [email protected] Dave Heald: [email protected] For policy questions, send mail to: Richard Nute: [email protected] Jim Bacher: [email protected] Archive is being moved, we will announce when it is back on-line. All emc-pstc postings are archived and searchable on the web at: http://www.ieeecommunities.org/emc-pstc
