Short Version of Antenna Traps

Concepts of certain basic "Antenna Traps" in 5 Parts:

Dave Funk - WA7RSO

To best understand just how any kind of "Trap" behaves, we first need to look at both Inductive and Capacitive effects in any kind of circuit.
We need to realize the effect that an Inductive Element or a Capacitive Element has on the "Length of an Antenna".
We need to understand how the combination of both the Inductor and Capacitor behave as either a "Series Resonant Circuit", or as a "Parallel Resonant Circuit".
Then, we need to understand how either of these two Resonant Circuits may be used as either a "Bandpass Filter", or as a "Bandpass Reject Filter" (known as a "Trap" type of a filter).
Finally, we need to see how these are commonly used in an Antenna System as "Band Traps".

An "Antenna Trap" is simply a Resonant Circuit used to block certain frequencies

Looking at both Inductive and Capacitive effects in any kind of circuit.

Think of an Inductor as first a long coil of wire that actually interacts with each turn to effectively behave longer than if the wire was stretched out, and think of a Capacitor as just the opposite of an Inductor, therefore shortening the length of wire.

A "Series Resonant Circuit" behaves much like a simple short circuit at resonance,
and a "Parallel Resonant Circuit" behaves much like an open circuit at resonance.

Bandpass Filters and Bandpass Reject Filters:

Bandpass Filters (i.e. Passband Filters):
- An interesting point is that a "Series Resonant Circuit in series", will represent a "Series Pass Filter" which will "Pass a Band of Frequencies, known as the "Passband", and thereby be known as a "Passband Filter".
- A "Parallel Resonant Circuit in parallel", will represent a "Passband Filter".
Bandpass Reject Filters:
- A "Series Resonant Circuit in parallel", will block the passage of those frequencies, and therefore behave as a "Trap".
- However, a "Parallel Resonant Circuit in series", will represent a "Passband Reject Filter or Trap".

A "Parallel Resonant Circuit" may be used as a series element of an antenna, to behave as a High Impedance (i.e. an open circuit) to frequencies beyond the desired. An example given here for a Multi-Band Vertical Antenna:

The lower element for the 10Mtr section may be separated from the 20Mtr section by a sturdy insulator, with a Coil of wire wound around the insulator (creating an Inductive Element) connecting the two sections together. By itself, the Coil would cause the combination of the two elements to appear electrically longer than the physical, due to the Inductance of the Coil. However, if a Capacitive Element is placed across the Coil, to create a Parallel Resonant Circuit at the high end of the 10Mtr Band, the result will be effectively an open circuit at the Trap, isolating the 20Mtr section from the 10Mtr section. This added parallel Capacitive Element may simply be a conductive sleeve surrounding the coil and also encompassing a portion of the 10Mtr and 20Mtr sections (usually connected to the 10Mtr section and insulated from the 20Mtr section).
At frequencies lower than the 10Mtr Band, the Capacitance literally fades out of the picture, leaving the Coil to connect the 20Mtr section to the 10Mtr section and also act to electrically "lengthen the antenna".
Basic tuning should be accomplished by the varying the length of the 10Mtr element. It can also be tuned by changing the Capacitance by adjusting the sleeve. Keep in mind though that adjusting the sleeve will affect the resonance of the Trap itself!
Some very important considerations about troubleshooting and tuning a Trap Vertical Antenna:
- The elements physically above the lower section (10Mtr) section will "affect slightly" the resonance of the lower section.
- The lower section is "incorporated" into those sections physically above (which are used for frequencies lower than the 10Mtr Band).
- Therefore, in tuning the 10Mtr section, the 10Mtr Trap should be in place. Note that when the section above the 10Mtr Trap are physically placed, they will have a slight effect on the final tuning of the 10Mtr Resonance.
- If possible, when troubleshooting a questionable antenna, tuning should be accomplished from the "bottom up" (high frequency first) with limited sections and associated traps.

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Inductors do not like Current Changes

Capacitors do not like Voltage Changes