[amsat-bb] Re: Helical Antenna on 70 cm

[Bob- W7LRD]

the helix is a nice antenna...however...once wound for either RHCP or LHCP there's no going back. So switching from R to L or L to R is out of the question. Unless of course the chosen polarity is always compatible with the satellite in use. Unless you have L and a R helix and switch between them, that can get a "little" involved. Many times it is very convenient to be able to switch between L or R during a pass, at times it can make the contact happen. This is of course is easy with a L/R CP yagi. I have used a helix for L band (1.2ghz ) as well as feeds for a 2.4ghz dish. They are a fine antenna with a fair amount of bandwidth. For a 16+ turn helix I have used stainless steel "U" channel for the center support. If Clair, VE3NPC is listening, he's the resident helix guy. Just my opinion, your mileage may differ. 
73 Bob W7LRD 
Seattle 

----- Original Message -----
From: "Rolf Krogstad" <rolf.krogstad at gmail.com> 
To: amsat-bb at amsat.org 
Sent: Monday, May 13, 2013 9:22:57 AM 
Subject: [amsat-bb] Helical Antenna on 70 cm 

I am new to satellites and new to the design requirements for UHF and am 
looking for advice. 

I am looking to build a Helical antenna for the 70cm Band. I am 
considering the plans for such an antenna from the 2010 edition of the ARRL 
Satellite Handbook. The plans call for a 235 cm (7' 8") long, 2.5 cm (1") 
diameter center support made of either a wooden dowel which has been 
treated or a fiberglass tube. It also requires multiple 7.6 cm (3") 
spacers for the helical element. 

My question is would fiberglass rods made of "Isophthlic Polyester Resin" 
be a good choice of material at UHF frequencies? 
I hesitate to use treated wood because I plan to mount this on the top of a 
15 meter high tower and it isn't very accessible. 

Thanks 

Rolf NR0T 
[EN34] 
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