[Date Prev][Date Next][Thread Prev][Thread Next] - [Date Index][Thread Index][Author Index]

Re: 1.2ghz helix



Thanks, Howard.

Always nice to have the strip line calculations to back your 
efforts.  The end at the coax connector should be as low to the 
ground plan as can be done without shoritng it.  The other end 
increases to about 3/8 inch.  If you use the helix wire as one end 
support then mainly it will be air dielectric which is the lowest loss.

Support the helix itself with a low loss dielectric.  You can see 
1/4-inch plywoood used on one helix.  On the other helix I used 
polyethylene tubing which I tested in the microwave oven for two 
minutes (with a cup of water to act as load for the magnetron).  It 
didn't get warm so was fine as a support.
http://www.kl7uw.com/sat.htm

I tested the 3-1/4 turn helix at 1296-1300 MHz for SWR.  I didn't 
have an RF source at 2400 so that helix is not tested, but it worked 
just fine in receive-only on AO-40.

Ed - KL7UW

At 02:00 AM 10/29/2008, Howard Long wrote:
>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
>
>_______________________________________________
>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

_______________________________________________
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



AMSAT Top AMSAT Home