David G. Nagel wrote:
Ken Bessler wrote:
I just put up an inverted V for 30 meters.
I started out with each leg being 24'0". This
gave me a low SWR at 9.5665 mhz which
works out to 229.6 instead of the usual 234/F.
As I trimmed, I decided to keep track of how
much I trimmed and what the nnn/F number
would be. As I got closer to my goal of 10.15,
the number went down, eventually ending up
at 227.28/10.1955=22.292' Also, the 2:1 swr
bandwidth went up - it started at 567 kc and
ended up at 655 kc.
Either way, I got the antenna up and it's working
fine - I'm just curious why the formula for length
and the bandwidth changed as the antenna got
shorter.
Ken KG0WX

234/f is just a starting point.

The "starting point" in question was a low 40m dipole, strung in the
back alley of the old ARRL HQ building.

For any other antenna, anywhere else in the universe, the "magic number
234" is going to be slightly different.

The difference in SWR bandwidth between 2:1 points is a bit more
complicated, and probably can't be explained in a one-liner. It will be
mostly determined by the interplay between two factors:
1. What the resonant impedance is (in relation to 50 ohms), which
determines the minimum SWR.
2. How quickly the reactive part of the feedpoint impedance changes with
frequency, for different dipole lengths.



--
73 from Ian G3SEK 'In Practice' columnist for RadCom (RSGB)
http://www.ifwtech.co.uk/g3sek