I think there are advantage to all metal "plumber's delight" antennas for VHF &
UHF. I would go for an all copper folded dipole. Split the "far end" dipole
element & feed it there using a balun made using a 1/2-wavelength piece of
coax; run the coax feedline in from the grouinded antenna clamp. It'll give
4:1 Z transform but then you need to fiddle with the diameters of the dipole
elements & their spacing to get 200 ohms feedpoint Z. Then whole thing will be
at DC ground & vy sturdy & broadbanded too, esp. if you use thick (say, 3/4")
elements.
Cheap too. I'd model it carefully using Eznec before-hand, get it maybe a bit
long so you can cut it down when built (can't so easily _add_ pipe!) to get to
resonance. Also, don't use a ruler to measure out the 1/2-wave piece of coax.
Measure it out using an RF bridge or SWR "analyzer", always more accurate.
--John
--- On Sat, 8/29/09, wb8vlc <mas...@cableone.net> wrote:
From: wb8vlc <mas...@cableone.net>
Subject: [Repeater-Builder] Re: 2M Vertical Dipoles
To: Repeater-Builder@yahoogroups.com
Date: Saturday, August 29, 2009, 9:39 PM
I found some Plans on this groups page for a home made 4 pole
array
http://forums. radioreference. com/antennas- coax-forum/ 109144-4-
bay-vhf-dipole- array-project. html
scroll down a few responses and download the PDF document, I'm building one at
this time.
I just started to look at the spacings in EZNEC and it looks nice.
Mike
--- In Repeater-Builder@ yahoogroups. com, Eric Grabowski <ejgrabowski@ ...>
wrote:
>
> I have used schedule 40 PVC for several antennas from HF thru 440 MHz without
> a problem. It does have a dielectric effect however that you must take into
> account.
>
> There is another plastic pipe called CPVC that does absorb RF at the higher
> frequencies. You don't want to use that.
>
> To make sure your plastic pipe will work ok, do the microwave test. Put a
> short length of the plastic pipe in a microwave along with a coffee mug
> filled with water. Run the microwave for a while. Then see if the plastic
> gets hot. If it does, you don't want to use it for RF.
>
> 73 and aloha, Eric KH6CQ
>
> --- On Wed, 8/12/09, AJ <aj.grantham@ ...> wrote:
>
> From: AJ <aj.grantham@ ...>
> Subject: Re: [Repeater-Builder] Re: 2M Vertical Dipoles
> To: Repeater-Builder@ yahoogroups. com
> Date: Wednesday, August 12, 2009, 5:05 AM
>
>
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>
> Does really PVC absorb or RF or just act as a dielectric?
>
> The reason I ask is I'm looking at encasing an antenna project for the sake
> of weatherproofing and PVC would fit the bill rather easily.
>
> 73,
> AJ, K6LOR
>
>
> On Mon, Aug 10, 2009 at 9:49 AM, AA8K73 GMail <aa8...@gmail. com> wrote:
>
>
> AJ, if you replace the steel mast with a fiberglass one,
> won't you still have the metallic feed line there?
>
>
> Doesn't PVC absorb RF?
>
>
> 73,
> Mike
>
>
>
>
>
> AJ wrote:
>
>
> On this same topic of the mast-less Antennex/Laird dipole arrays, has anyone
> attempted to top mount these from a fiberglass mast to minimize interaction
> with the normal steel pole? I have quite a few surplus fiberglass poles left
> that would likely work, even for side mounting on 1/2 wave spacing from the
> tower...
>
> On that same note, does anyone have construction plans for a dipole array
> (not necessarily folded dipoles)? I remember seeing a set of plans somewhere
> quite a while ago - we're thinking of constructing one but encasing the
> dipoles in fiberglass or PVC to try to protect from the weather and debris at
> our site (top of a large farm field)...
>
> 73,
> AJ, K6LOR
>
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