Hi Bob.
I just ran some calculations.
Assuming 0.2mm thick copper shim:
7.6mm width
1.6mm height from the reflector at the base/feed point (use a standard PCB thickness to guide)
8.5mm height from the reflector to the helix itself, and adjust this height for minimum SWR.
This is the matching section, and as Ed said it shouldn't matter if it's straight or curved because it's not really meant to radiate. Be very careful of standoffs/supports, insulated or otherwise. The dielectric effect of standoffs can significantly detune the antenna, reduce gain due to losses and affect both the radiation pattern and circularity. The definite no-no is to use a single off-centre insulated pole through the helix. Use instead a support (insulated or conducting) that's dead centre, and insulating standoffs rotated uniformly around the support along its length. The use of a PVC (or other insulating pipe) as a permanent former should be resisted unless you know its dielectric effect.
73, Howard G6LVB
-----Original Message----- From: amsat-bb-bounces@amsat.org [mailto:amsat-bb-bounces@amsat.org] On Behalf Of w7lrd@comcast.net Sent: 29 October 2008 02:59 To: AMSAT-BB Subject: [amsat-bb] 1.2ghz helix
I am almost finished with a 16 turn helix for 1.2ghz. I am not sure of what type of matching I should use. Some of designs I have seen use a flat tab on the first 90 degrees of the helix. What size should it be? Is there a better way? I have a bird 43 with a 1.2 slug for tuning. 73 Bob W7LRD
-- "if this were easy, everyone would be doing it" _______________________________________________ Sent via AMSAT-BB@amsat.org. Opinions expressed are those of the author. Not an AMSAT-NA member? Join now to support the amateur satellite program! Subscription settings: http://amsat.org/mailman/listinfo/amsat-bb