1. Antennas with Loads and Traps – What are these things and why are they used in our antennas?
By: Dave Brown
K3CTN

2. Impedance – The ohmic value of an antenna
feed point, matching section or transmission
line. An impedance may contain a reactance
as well as a resistance component.
(ARRL Antenna Book, 23rd Edition) R
Resistance
j XL
- j XC or
Reactance
Antenna Impedance
X is Positive for Inductive Reactance,
R + j X
Negative for Capacitive Reactance

3. Today's transmitters are designed to deliver
power to a resistive 50 ohm antenna.
Less than maximum power is transferred
to the antenna when the antenna impedance
varies from 50 ohms resistive.
Some sort of impedance matching mechanism
is used to “match” the transmitter to an
antenna.

4. Impedance Matching – The Antenna Tuner

5. Concepts discussed here apply to both
Horizontal and Vertical Antennas

6. Antenna Engineering Handbook – Jasik

7. Antenna Engineering Handbook – Jasik

8. Move part of the Antenna Tuner “into” the Antenna
Transmission
Line
Load
Antenna
RANT
j XLOAD
- j XANT
Load Inductor

9. A Dipole or Vertical (with Ground image) shorter, or longer, than a half wavelength (λ/2) can:
Have a reactive “load” built into the antenna to remove or minimize the antenna's reactance.
But where should this “load” be placed in the antenna?
(λ / 4)
(< λ / 4)
(> λ / 4)
Transmission Line Connection

10. Designing a Shortened Antenna – Lopes
Design a shortened half wave dipole with loading coils due to limited available space:
Where do you put the loading coils?
How do you determine the loading coil inductance?
Lopes solution:
You specify the antenna length and loading coil locations
Use transmission line theory to calculate the loading coil inductance

11. Calculate the line's characteristic impedance
Determine the reactance at points X1 and X2, the piece of wire that will be replaced by a coil
Determine the inductance of the coil

12. A “Trapped” Dipole (or Vertical)

13. Example: 40 / 20 meter Trap Dipole
Target Frequencies: 7.05 mHz and mHz
Trap Parallel Resonant Frequency: ~ mHz
3.3 uHy
39 pF
0 ft,
Transmission
Line Connection
16.8 ft
29.7 ft

14. Example: 40 / 20 meter Trap Dipole
SWR < 3:1
6.90 – 7.28 mHz
13.86 – mHz

15. When a Trap is Parallel Resonant between two amateur bands, the Trap will effectively provide loading where:
At a lower amateur band frequency, the Trap has an inductive reactance
At a higher amateur band frequency, the Trap has a capacitive reactance
In other words, inductive loading will electrically lengthen an antenna and capacitive loading will electrically shorten an antenna
Design of a two-band dipole with a Parallel Resonant Trap between two bands is a challenge

16. Design of a Two-band Loaded Dipole Antenna – Birnbaum
David Birnbaum, K2LYV described an innovative solution to this problem in a recent QEX article
Birnbaum's solution uses an out of band Parallel Resonant Trap where:
You choose an antenna length that will fit into your available physical space
The dipole feedline-to-Trap length must be shorter than λ/4 at the higher frequency
Continued

17. Birnbaum continued:
Using Lopes' method, calculate the value of inductive reactance for the lower frequency
Also, using a modified method of Lopes, calculate the value of the capacitive reactance for the higher frequency
Use these two reactance values in additional calculations described by Birnbaum to identify the Trap's Parallel Resonant frequency along with the Trap's L and C values

18. Birnbaum Clarification
Total Length
Trap
Inner Length
Low Freq, Total Length < λ / Trap = j XL
High Freq, Total Length > λ / Trap = - j XC

19. Example: 30 / 20 meter Trap Dipole
Dipole length = 40 ft
fLOW = mHz, fHIGH = mHz
3.0 uHy
53 pF
0 ft,
Transmission
Line Connection
14 ft
20 ft
Trap Parallel Resonance ≈ 12.6 mHz

20. Example: 30 / 20 meter Trap Dipole
SWR < 3:1
9.95 – mHz
13.93 – mHz

21. Notes “The ARRL Antenna Book,” 23rd Edition, 2015.
Jasik, “Antenna Engineering Handbook”, 2nd Edition, 1984, Figs. 4-3 and 4-4.
Luiz Duarte Lopes, CT1EOJ “Designing a Shortened Antenna,” QST, Oct. 2003, pp
David Birnbaum, K2LYV, “Design of a Two-Band Loaded Dipole Antenna,” QEX, Sept./Oct. 2017, pp
Ward Silver, N0AX, “Antenna Modeling for Beginners,” ARRL, 2012.