[Antennas] coax length question

[Michael Josefsson]

On 3 Sep 2008, at 21:31, Neil wrote:

> Hello,
>
> As a new antenna experimenter, I think I may know the issue
> he's spotlighting.
>
> As I read through the multitude of antenna books I've recently  
> purchased, I see
> many references to specific lengths of coax that are usedfor  
> particular antenna
> designs. One might specify something like 'feedline length has to be  
> 1/4 wavelength
> or the swr will be astronomical" etc.

Understanding how current highs and current lows work on a feed line  
is crucial when trying to match another impedance than 50 ohms to the  
rigs 50 ohms. It takes time, but is a good investment, to get a grip  
on the smith chart. Many seemingly irrational/odd feed line lengths  
are fairly easily analyzed with this old 1930's tool.

> My poverty stricken understanding is that the feedline (coax) IS  
> part of the antenna system, and that
> for certain designs, it's length is contributory to the function of  
> the antenna.
> Often specified lengths of coax function to tune, or tame, the beast.

The 1/4 wl feed line of 75 ohms coax is just what is needed to match a  
loop's circa 120 ohms to 50 ohms. And once 50 ohm then any length 50  
ohm coax can be used to the shack. It is important that the 1/4 wl is  
75 ohms - or the "magic" won't work.

/Micke

Ps I haven't seen the W3FPR site before. Very interesting. Thanks!

>
>
> The abundance of info online, and printed, related to feedlines and  
> feedline-antenna
> interactions can be daunting. The ARRL Antenna Book always seems to  
> be a good
> place to start. Googling "antennas" and "feedlines" has led me to  
> assemble a rich
> set of links to good reference materials. Here are a few (some have  
> more than just
> antenna/feedline info):
>
> http://www.tpub.com/neets/
> http://www.w8ji.com/
> http://www.cebik.com/
> http://www.qsl.net/w3fpr/index.htm
> http://www.smeter.net/feeding/feeding.php
>
> 73, Neil
>
> Neil Murphy  KH6EAM
> www.ghostdog.com
> 650.375.0886 wrk
> 650.533.2015 mobile