I think you will find that the helically-wound vertical you are considering will behave more like a quarter-wave, even though it is wound with a half-wave length of wire.  A counterpoise or radials would be pretty important.

I played with a helically-wound vertical a long, long time ago.  It was intended for 80 meters and wound on a 12- or 16-foot length of wooden hand-rail stock which was prepared with several coats of polyurethane varnish.  Either PVC was not yet readily available at the time or I just wasn't aware of it.  The results were less than exciting, but I realize now that I didn't pay nearly enough attention to radials, etc.

73.

Mike, K9AZS

> ----- Original Message -----
>From: "Mike Duke, K5XU" <[log in to unmask]
>To: [log in to unmask]
>Date: Mon, 26 Dec 2005 11:26:54 -0600
>Subject: Re: antenna building

>CQ magazine ran a construction article about such an antenna in either 1992
>or 1993.

>I started one for 30 meters, but never got around to finishing it. I think
>the 2 5 foot sections with the wire coiled on them are still around here
>somewhere.

>My suggestion is that unless you are wanting to make it more portable, use a
>full 10 foot stalk of PVC, rather than 8 feet. That will mean less loading.
>If you need to make it for quick storage as I was doing for an apartment
>balcony installation, use 2 5 foot pieces with a coupler glued to one of
>them, and a short clip lead or terminal wire to connect the two sections of
>wire.


>The center conductor of the coax feed line connects to the antenna at the
>bottom of the coil. Since you are using a half wave of wire, radials won't
>be as critical, but will help. They connect to the shield of the coax.

>Another simple option for 20 meters and above is to go with a full size
>quarter wave wire inside a PVC tube. This would be more or less 16 feet for
>20 meters. Ground radials would be essential for this antenna to perform at
>its maximum efficiency.

>While ground-mounted verticals do work well, you will generally find that a
>vertical with the base several feet above ground will blow them away.

>Either way, build whatever fits your physical situation, and have fun using
>it!

>K5XU