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Re: [TowerTalk] 80/40 coax trap dipole design

To: Dan Zimmerman N3OX <>
Subject: Re: [TowerTalk] 80/40 coax trap dipole design
From: jimlux <>
Date: Fri, 17 Dec 2010 07:51:56 -0800
List-post: <">>
Dan Zimmerman N3OX wrote:
>> Sure, a parallel LC circuit (trap) will do it but not a coil alone
> Not true.
> An EZNEC model shows that a 110ft dipole 30 feet above ground* resonates at
> 3.64MHz and 7.13MHz when 39uH coils are added 11 feet from each end.   The
> 2:1 SWR bandwidths are narrow (100kHz on 80 and 150kHz on 40) even with
> realistic inductors (Q=100). There is no possibility of stray capacitance
> because in the simulation they're mathematically idealized objects.
> This is just one example.  I did not try to optimize it, just tried to get
> it to resonate on two bands.
> Now, real antennas do not use pure inductors, and probably stray capacitance
> is not negligible for most coils that would make this technique work.  But
> this is an example that, in principle, it is possible to get dual resonance
> on a "second harmonic" band using ONLY inductors and wire.

For coils of "reasonable" length to diameter ratios, you can get a 
pretty good approximation of parasitic C (for use in a NEC lumped load) 
by using Medhurst's approximation.  There's a table lookup involved, so 
I usually do it with a spreadsheet, but I suspect that you could come up 
with some equation that approximates it, and then dump it into a 4nec2 
SY card (or the equivalent in EZNEC).

Capacitance (pF) = K * Diam (cm)

where K is an empirical function of Length/Diameter
L/D     K
5.0     0.81
4.5     0.77
4.0     0.72
3.5     0.67
3.0     0.61
2.5     0.56
2.0     0.50
1.5     0.47
1.0     0.46

-- I haven't tried it, but NEC should have no problem calculating the 
self C, if you model the loading coil as a wire with diameter and length 
equal to that of the coil, and with distributed or lumped inductive 
loading (depending on if you use multiple segments over the length of 
the coil).

  Capacitance is all about electric fields, and NEC calculates electric 
fields.  NEC should also take into account the C from the coil body to 
the other parts of the antenna.

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