Towertalk
[Top] [All Lists]

Re: [TowerTalk] SWR is what SWR meter measures

To: Jim Lux <jimlux@earthlink.net>
Subject: Re: [TowerTalk] SWR is what SWR meter measures
From: Bill Coleman <aa4lr@arrl.net>
Date: Sun, 15 May 2005 12:45:08 -0400
List-post: <mailto:towertalk@contesting.com>
On May 12, 2005, at 7:00 PM, Jim Lux wrote:

> With a 2:1 mismatch at the far end the return loss is 10 dB. This is
> another way of saying that for every 1000 watts you send to the  
> antenna,
> 100 watts is reflected back towards the transmitter.  The radiated  
> power is
> reduced from 1000W to 900W (about 0.5 dB decrease)

But what happens to that 100W? It isn't dissipated at transmitter.  
Most of it is reflected back down the transmission line. And then 10%  
is reflected back. And so, back and forth it goes until all the  
energy ends up in the antenna.

It isn't the SWR can reduces the energy feeding the antenna. If we  
assume a lossless transmission line as above (you indicated that all  
1000 watts from the transmitter reached the antenna), then SWR  
doesn't matter.

Of course, transmission lines are anything but lossless. The initial  
trip to the antenna incurs some loss, each reflective trip incurs more.

The key to efficient antenna operation is eliminating loss, not  
lowering SWR. (Although, in some cases, lowering SWR can eliminate loss)

Bill Coleman, AA4LR, PP-ASEL        Mail: aa4lr@arrl.net
Quote: "Not within a thousand years will man ever fly!"
             -- Wilbur Wright, 1901

_______________________________________________

See: http://www.mscomputer.com  for "Self Supporting Towers", "Wireless Weather 
Stations", and lot's more.  Call Toll Free, 1-800-333-9041 with any questions 
and ask for Sherman, W2FLA.

_______________________________________________
TowerTalk mailing list
TowerTalk@contesting.com
http://lists.contesting.com/mailman/listinfo/towertalk

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