Building helical antenna to span 470 to 850MHz (UHF TV)

[Robert Casey]

Any ideas on how to build a helical antenna that can cover this
wide of bandwidth? 470 to 850MHz (UHF TV channels 14 -69).
Looking at the ARRL Handbook, 1978 edition, page 635 is
a helical antenna for 432MHz. "Each turn is a wavelength long,
with a pitch of 1/4 wavelength". Now could I build a helical
with varying diameters of the turns? Imagine I have a cone
that I wind the turns on, starting at a 4 inch diameter and
tapering up to 8 inches. And the wire of the antenna spirals
around the cone, the turns getting bigger as the cone gets
wider. And then the base of the cone sits on a screen mesh.

Would this work, give me some directionality and gain,
and bandwidth? Figure it would feed 75 ohm coax to feed
the TV set UHF tuner.

[Spencer Webb]

Forget making the helix conical. Pick the geometric mean of your frequency
edges, 632MHz, and build your helical to those dimensions. It will work,
and will perform well over that broad bandwidth. Forget impedance matching,
you'll have a 2:1 SWR with 75-ohm cable (about 140-ohm impedance when
peripherally fed).

Helicals on a conical surface provide interesting tradeoffs in gain and
bandwidth, but it is a complex issue. Standard helicals perform well over a
2:1 range, but not too much more.

Build it. I think you will be delightfully surprised.
--
-Spencer Webb
President
AntennaSys, Inc.
www.AntennaSys.com

Robert Casey <wa2...@ix.netcom.com> wrote in message
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