On Sun, 12 Jul 2015 13:17:53 -0400, rickman wrote:

On 7/12/2015 6:25 AM, Jeff Liebermann wrote:
On Sun, 12 Jul 2015 09:58:00 +0100, "Cottaging by the sea ..."
wrote:

going by adverts the k40 must be the best antenna on the market....can I cut
it for 15m ????

The K40 is a CB antenna made for 27 MHz. 15 meters is 21 MHz and is
therefore lower in frequency than what the K40 is tuned. You would
need to lengthen, not "cut" the antenna in order to lower the
frequency.

I don't get it. I've cut this antenna three times and the frequency is
*still* too high.

The more you cut, the higher the antenna operating frequency.

The K40 is just a base loaded vertical antenna. The inductance of the
base loading coil is fixed, so the only way to adjust it is by either
cutting or stretching the whip. If you cut it, it DECREASES the
operating frequency. That's the common way to move it from the
original CB (27 MHz) to 10 meters (28 MHz). If you want to go the
other direction, you'll need to somehow lengthen the antenna.

Plug the original dimentia into this calculator to obtain the value of
the inductor (or just measure it with an RLC meter).
http://www.daycounter.com/Calculator...lculator.phtml.
Once you've established the value of the inductor, do some trial and
error guesses for the whip length at 21 MHz.

--
Jeff Liebermann
150 Felker St #D http://www.LearnByDestroying.com
Santa Cruz CA 95060 http://802.11junk.com
Skype: JeffLiebermann AE6KS 831-336-2558