[TowerTalk] Grey PVC Loading Coil

Terry Gerdes terry at ab5k.net
Tue Dec 5 15:24:46 EST 2006

Hi All,

I have a Force 12 EF180S rotatable dipole for 80 meters.  The 1 3/4" dipole 
element is insulated from the mast plate using gray PVC tubing.   A matching 
box allows coils to be placed in series with both ends of the dipole to 
lower the resonance down into the CW band.

This system works fine for a while but eventually the gray PVC starts 
breaking down.  Its worst when working in the CW part of the band as the 
extra coils increase the voltage on the PVC insulator.     Pic at:

Since it failed in the middle of the 5A DX-Expedition, I fabricated a new 
insulator from some scrap PCV.   A more permanent fix is needed but will 
wait till warmer weather.  If anyone has any ideas, please let me know. 
Dennis, K5YA has had the similiar issues with his Force 12 EF180S.

After experiencing this failure, I'm not sure its the best material to wind 
a coil around.

73 Terry - AB5K

----- Original Message ----- 
From: "K4SAV" <RadioIR at charter.net>
To: <towertalk at contesting.com>
Sent: Tuesday, December 05, 2006 1:50 PM
Subject: Re: [TowerTalk] Grey PVC Loading Coil

> It's not immediately obvious as to what is going on with these coils.
> The additional loss is from the dissipation factor of the dielectric
> which is a function of the electric field strength.  Also the dielectric
> increases the capacitance across the coil which lowers its self
> resonance frequency. The low frequency value of inductance is a constant
> and independent of the dielectric. The shunt capacitance forms a
> parallel circuit with inductance and its effective impedance goes up, so
> the effective inductance goes up. You get just the opposite effect for a
> series capacitor. In the previous post, I just manually adjusted the
> number of turns to keep the desired value of inductance.
> Here is what happens when holding the turns at 30 for both, and at 1.85
> MHz. With an air dielectric, L=83.6 uh, C=5.96 pf, Rs= 1.2, self
> resonant at 7.4 MHz. Low frequency inductance of this coil is 78.4 uH.
> With PVC, L=89.3 uH, C=11.6 pf, Rs= 2.5, self resonant at 5.3 MHz. Low
> frequency inductance of this coil is the same as for the air coil, 78.4 
> uH.
> I'm not sure what the program assumes for the location of the
> dielectric, but since this program is intended to be used for
> calculating the inductance of a coil would on a form, I would hope that
> the calculation is not for being immersed in the dielectric. Also I do
> not know what the program assumes for the dielectric properties of PVC.
> I guess you could derive them based on some of the calculation results.
> Jerry, K4SAV
> Jim Lux wrote:
>> At 10:02 PM 12/4/2006, K4SAV wrote:
>>> 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.
>> How does the program calculate the loss and change in resonance with
>> the form?
>> I assume that the loss would be from the parasitic C, since there's
>> low loss from the magnetic fields?  The change in turns, presumably
>> due to the change in parasitic C that "cancels" the L, although I
>> would think that you'd need "more" turns (since the C is bigger with
>> the form), is only a few percent.
>> The resonant frequency change (smaller by a factor of .74) implies
>> that LC is 1.8 times bigger with the form than without.  The reduction
>> in turn count (29 vs 30) actually makes the L smaller by a factor of
>> 0.93, so the C must be bigger by a factor of 1.94, that is, nearly
>> doubled.  That's a pretty big change, for a form material that
>> probably has an epsilon of something like 2-3, and with the physical
>> size of the coil slightly smaller.
>> Does the program assume the coil is "immersed" in the dielectric (that
>> is, the dielectric is "between" the turns) or that the coil is wound
>> on the surface (or in a shallow groove), in which case only half the
>> flux is in the dielectric.
>> What does the program assume for dielectric properties of PVC?
>> {I'm curious because this is an interesting problem that would make a
>> good homework problem for a class in electro/magneto statics, and I
>> know someone who's teaching such a class and is always looking for
>> good practical problems.}
>>> 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
>>> know.
>> Jim, W6RMK
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