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Re: [TowerTalk] Grey PVC Loading Coil

Subject: Re: [TowerTalk] Grey PVC Loading Coil
From: K4SAV <>
Date: Tue, 05 Dec 2006 00:02:31 -0600
List-post: <>
You are going to need something more than just a loading coil to match 
it.  A 40 ft vertical with an 80 uh coil at the base, using #14 wire, 
should be resonant on 160 meters but the feedpoint impedance will be 
around 2.2 ohms plus coil loss and whatever ground resistance you have 
from your radial system. I didn't calculating a matching network for 
you, sorry.

By using a coil program I calculated some coil data. With #10 wire, 5 
inches in diameter, and 5 inches long, you would need about 30 turns to 
get 83 uH at 1.85 MHz. The resistive losses of this coil would be about 
1.2 ohms (2 ohms for aluminum) for an air core. This coil should be self 
resonant at about 7.4 MHz. For a PVC form, you would need about 29 turns 
to get back to 83 uH and the resistive loss would be about 2.2 ohms (3 
ohms for aluminum). This coil should be self resonant at about 5.5 MHz.

This should give you a hint of the amount of loss contributed by the 
coil with the two different forms.  This doesn't give you any 
information on whether white PVC is better than gray PVC, sorry, I don't 

When you do calculate a real matching network, something other than 60 
uH may be used. A realistic number for feedpoint impedance with a good 
radial system should be something around 12 ohms. This isn't going to be 
easy to match with a single LC network so the network is likely to be a 
coil plus a couple of caps, or maybe a wideband transformer. The SWR 
curve will also be very sharp.  You may want to estimate this to see if 
it will be acceptable, or otherwise plan a method of adjusting the 

You can also estimate the efficiency.  The radiation resistance is 
approximately 2 ohms and the resistive losses will probably be around 10 

Since your vertical is already up and you already have the radial 
system, it shouldn't be much trouble to try it, and measure what it 
does. A couple of variable caps, a variable inductor, and an antenna 
analyzer is about all you need to figure out the matching network and 
what the bandwidth is going to be. Of course it's getting cold now (at 
least here) and sitting out in the yard at the base of the vertical may 
not be a lot of fun. It's a lot warmer by the computer.

Jerry, K4SAV

Dan Zimmerman N3OX wrote:

>In the spirit of "some antenna is better than no antenna at all," I'm
>going to add a base loading coil to for 160m to my 40 foot vertical
>I know it's a substantial compromise over other forms of loading, but
>I'm willing to accept the efficiency penalty for convenience in this
>particular case.  What I want to know about is whether or not grey PVC
>electrical conduit is suitable for coil forms at 1.8MHz.  I searched
>the archives and found a discussion on PVC formers for HF and that
>they're not unacceptable unless you're running many close-spaced
>turns, but I wonder if people have tried grey vs. white and what
>they've found.  I know the microwave trick is routinely used but I
>don't think absorption at 2.4GHz says much about the MF properties of
>the material.
>I've got access to some 5" conduit and a lathe; a few minutes and I
>can have a beautiful coil form like the 80m one on the page above, but
>I wonder if the grey PVC is lossier than the white stuff?
>There are plenty of other approaches, but if grey PVC is decent at
>1.8MHz, I'm just going to cut a nice  groove in it and go.  It's going
>to be something like 20-30 turns of #10 wire and having a grooved form
>makes for a quick and solid coil.
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