transmission line varia

To: mail11: ;
Subject: transmission line varia
From: "Dave Pierson: I am the NRA" <pierson-at-msd26.enet.dec-dot-com>
Date: Wed, 29 May 96 17:58:36 EDT
Cc: pierson-at-msd26.enet.dec-dot-com
>>     sqrt(L/C) is a SIMPLIFIED VERSION.  some references do not point
>>     this out.  The full form (from memory) is more like SQRT ((L+R)/(C+g)).
>>     R is the series reistance (per unit length in tx line work).
>>     g is a 'conductance' related to resistive leakage thru the insulation.
 
>>     For the USUAL range of RF intrest (say 500KHz to 500GHz) the R is so
>>     small relative to the Xl and the g is so 'small' realtive to XC that
>>     the simplified form works out.  (Yeah.  I switched for L to XL.  Bear
>>     with me...).  AS THE FREQS GO DOWN, THE SIMPLIFIED FORM BECOMES
>>     PROGRESSIVELY LESS ACCURATE.  (I had cause some years ago, to
>>     xperimentally verify this.  got paid for it.  NOT new knowledge,
>>     tho.  First recognized at the turn of the century....)
	(References, for one, Transmission Lines, Antennas & Wave Guides,
	King, Mimno & Wing, p 13, eq 14.2, etc.
	[Std disclaimer, i am working from an RF perspective.  Many of the
	std rf approximations don;t apply, but many do....  imo...

>I realize there are simplifications in the L/C thing. Interestingly,
>Schelkunoff doesn't take the distributed parameters into account 
>either.
	I don't know Schelkunoff wel enuf to debate, but my recollection
	is that he was ca 1930s, short wave antennas 'classical rf' where the
	approximations work.  (nb:  this is NOT an adhiminem on S., simply
	my perception that HIS approximations worked where HE was using
	them.  Take the extreme case of an 18" 1/4 wave on my scanner and
	compare the 'r' in that to the 'r' of a 1/4 wave at 200 Khz...  The
	r in one case is negligible, in the other maybe not...

>It simply derives a value from the geometry of the system.
>In general, the answers are comparable in a lot of cases but I do
>wonder about using formulae that are right in only some situations.
>It'd be nice if one or other was applicable to the lot.
	For Tx line work (and, i think, antenna work, which can be handled,
	muchly, as a specialized form of tx line work).  The 'complete'
	form should work all the time, but for the cases where the simplified
	form WORKS, its mucho easier to handle.

==========================
Transmission lines, as such, are more or less inapplicable to to Tesla coil
work, as near as i can tell.  For 'regular radio' with a 'transmitter' (power
amp/osc) "over there' and an antenna 'over the other way' they are critical.
The usual coiling config puts the 'load' (i am trying NOT to say
antenna....8)>>) 'in' the Power Oscillator.  When used as Tx lines, yeah,
keeping things matched, unlsees designed not to is RF practice.

Howsomever, fo rth special case of the antenna-handled-as-transmission line,
the portion wich IS the anteann is not 'matched' in the usual sense and is
'designed' (most cases) to have a high VSWR ON ITSELF.  (handwaving,
not rigor, follows....).  The open end is an open circuit, so MUST have a
voltage peak.  The base (sticking to quarter waves) will have current flowing
in so must have some, relatively low, impedance.  If worked 'grounded'
(as usual in coiling) a variety of 'inlooking' impedances are availble, mostly
lowish.

Or i could be wrong...

	regards
	dwp