Hi Rolf, NR0T
The fiberglass rods made of "Isophthlic Polyester Resin" is a good choice of material at UHF frequencies and it is the material I have used for my RHCP 10 turns Helical Antenna for 435 MHz in use on the roof since 1976 beginning from OSCAR-8 Mode-J and actually working since than for VO-52
In a separate email I have sent to you a zipped file with all mechanical drawings to built it in all details.If you like to increase the numbar of turns from 10 to 15 no problems because the only adjustement required is to change a little bit step by step the distance between the reflector and the first turn in order to get the lower possible VSWR.
In my Helical Antenna the boom is a very hard plastic tube called Moplen, outside diameter 42 mm ,inside diameter 34 mm and so 5,5 mm thick.
The turns of the Helical are made with 8 mm diameter solid annealed aluminum rod and the spacers between the turns are 8 mm outside diameter fiber glass rods.
For best performance I have mounted the supporting steel horizontal harm on the rear of reflector in order to prevent interactions between the EM field of the turns and the mechanical metallic support.
In order to balance the weight of the Antenna I have mounted in the rear of reflector a round counterweight made of lead.
Have fun !
73" de
i8CVS Domenico
----- Original Message ----- From: "Rolf Krogstad" rolf.krogstad@gmail.com To: amsat-bb@amsat.org Sent: Monday, May 13, 2013 6:22 PM 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] _______________________________________________ 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