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

i8cvs domenico.i8cvs at tin.it
Mon May 13 21:49:15 PDT 2013

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 at gmail.com>
To: <amsat-bb at 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
> 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
> 15 meter high tower and it isn't very accessible.
> Thanks
> Rolf   NR0T
> [EN34]
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