Methods for Calculating Loading Coils
Table of Contents
- Method A: K1PLP’s Lookup Chart
- Method B: 66Pacific’s Online Calculator
- Method C: Calculate
from and - Comparison
So far, I’ve seen three methods for calculating shortening coils for dipoles. I’ve successfully used the first, K1PLP’s lookup chart, for the design of a homebrew 40/30/20/17m trapped dipole, measuring 50 feet. My results were good, although I intend to build a version 2 of my antenna. The 20m section is quite short, while both the 30m and 40m were longer than I intended by about a meter or so.
I haven’t technically tried Method C, however I get similar reactance values (and subsequently similar inductance and capacitance values) from both Method C and A.
Method A: K1PLP’s Lookup Chart
I simply used the loading coil chart from Chapter 9 of the ARRL Antenna
Book, Vol 2 to calculate
I highly recommend ARRL’s Antenna Book. Chapter 9 in particular has several sections dedicated to dipoles and loading techniques. Here is a link for the full ARRL Antenna Book.
Method B: 66Pacific’s Online Calculator
The math to calculate inductance, given
Where:
= inductance required for resonance = frequency, megahertz = overall antenna length in feet = distance from center to each loading coil = diameter of radiator in inches
Fortunately, 66pacific.com has published a utility for this calculation.
Method C: Calculate from and
Luiz Duarte Lopes, CT1EOJ , has published an article, Designing a Shortened Antenna in the October 2003 edition of QST.
Example 1: Loads for a full length 17m/20m trapped dipole
For a resonant two-band dipole on 17m and 20m, the inner part of the
antenna will be 8.5m (halfwave on 17m). Traps will be placed at ends of
the 17m section and since we want at least 1 meter extending past the
traps for the 20m section, this means we want very little loading (none
if possible) and we would expect a low inductive reactance,
This is the basic equation which we will use to calculate the inductive reactance required by the loading coils for our antenna:
We need to calculate reactance at the point on the antenna where the
loading coil begins
From
or
Starting with our antenna dimensions for 20m:
= 98% (shorten antenna by 2% on 20m) = 85% (because traps will be placed at edge of 17m, and 17m/20m = 0.85)
We must calculate the distance from the end of the antenna to the loading coil point, in degrees.
Next we must calculate the characteristic impedance of a one-wire
transmission line,
, where = electrical height above ground and = wire diameter
Now to calculate
Finally, we can calculate the inductive reactance required by our shortened antenna: ((\X_L\).
WARNING
This example gives us the wrong answer for a 17m/20m dipole, b/c it uses
the general band lengths as inputs to
See my calculator for an example. Calculating using
resonant frequencies, 18.118MHz and 14.055MHz gives an
Comparison
It’s helpful to see the inductive reactance,
I have only attempted trap dipole design and construction using K1PLP’s lookup chart. I can also verify that Method 3 produces reactance values similar to Method 1. 66pacific’s calculator (Method 2) produces reactance values that are a bit off from my calculations and it’s difficult to evaluate the math, since their calculator only provides inductance and skips over reactance.
Method 3’s main difficulty is with calculating
Method 1 is just a lookup chart, which seems accurate enough. But when dealing with extreme inductances/capacitances (at the extremes of low or high inductances), small errors can produce unacceptable deviations in antenna dimensions.