----- Original Message ----- From: "Tim Goodrich" tim@timgoodrich.net To: amsat-bb@amsat.org Sent: Monday, September 28, 2009 9:45 PM Subject: [amsat-bb] quadrafilar helix for 70cm- ambiguous design specs
Hi All,
The house I just bought precludes me from even using my arrow inside as there must be something un-RF friendly in the walls. Therefore, I was looking for a roof antenna and the October QST article on the QFH inspired me to build one. However, after doing some research, I have come up with
two
sets of design specs and I'm unsure which one is correct.
I converted everything from you're the 146Mhz QST model to 436Mhz.
However,
when I plug that info into http://www.jcoppens.com/ant/qfh/calc.en.php
The values it puts out are different in the "compensated length" field.
The
only difference is this model calculates for a full turn as opposed to a half turn, but the lengths shouldn't be as different as it says. Under the QST model, I get a wavelength of 688mm, but under the website's model, it gives me a "compensated wavelength" of 729mm.
The values I used to calculate are:
436
1
1
15
2
.23
Can anyone tell me which specs to use and/or explain this compensated wavelength issue?
Thank you,
Tim
KI6VBY
Hi Tim, KI6VBY
Read please the book "REFLECTION" Transmission Lines and Antennas by M. Walter Maxwell, W2DU Chapter 22 The Quadrifilar Helix Antenna pages 22-1 to 22-23 and also "Experimental Investigation of Quadrifilar Helix Antennas for 2400 MHz" The AMSAT Journal May/June 2004 In a separate email I have sent to you both articles as an attachement.
73" de
i8CVS Domenico