[Top] [All Lists]

Re: [TenTec] OT: SWR & Coax

To: Discussion of Ten-Tec Equipment <tentec@contesting.com>
Subject: Re: [TenTec] OT: SWR & Coax
From: Stuart Rohre <rohre@arlut.utexas.edu>
Reply-to: Discussion of Ten-Tec Equipment <tentec@contesting.com>
Date: Mon, 07 Jul 2008 09:28:04 -0700
List-post: <tentec@contesting.com">mailto:tentec@contesting.com>
Coax consists of three conductors:   Center lead, inner shield, and 
outer shield conducting surfaces.

The SWR of 1:1 refers to the shield inner surface current and the center 
conductor current encountering no reactance, and the source impedance 
being matched to the load impedance.

In normal operation the currents on center lead and inside the shield 
are equal and opposite.
If the length of the coax makes a resonant structure such as an odd 
multiple of a quarter wavelength, or if the coax is not lead away at 
right angles to the antenna, current may be induced onto the outer 
surface of the shield.  Thus, the formerly balanced shield current will 
divide between the inner of the shield and the outer surface, and shield 
radiation may result.

Many hams forget that coax actually has THREE conductors as detailed above.

-Stuart Rohre

Marinus Loewensteijn wrote:
> When Coax is terminated at the transmitter end and the far end with an 
> impedance that is equal to its defined impedance we will have a SWR of 1:1.
> Only when the currents on inner conductor and on shield are equal and the 
> current / voltage nodes coincide at the same locations then they'll cancel 
> radiation.
> If we have anything else that creates a SWR that is not 1:1 then waves will 
> be reflected from the ends.  Hence no longer will the current and voltage 
> nodes be coinciding. 
> When nodes are no longer coinciding then we will have radiation.
> What is wrong about this, what am I missing here?
> Thanks, 73, marinus, ZL2ML
> _________________________________________________________________
> Get inspired - dream, research, plan and book your next holiday online with 
> MSN NZ Travel 
> http://a.ninemsn.com.au/b.aspx?URL=http%3A%2F%2Ftravel%2Emsn%2Eco%2Enz&_t=771497011&_r=MSN_NZ_travel_hmtagline&_m=EXT
> _______________________________________________
> TenTec mailing list
> TenTec@contesting.com
> http://lists.contesting.com/mailman/listinfo/tentec
TenTec mailing list

<Prev in Thread] Current Thread [Next in Thread>