The purpose of this article is to examine the operation of coax traps through modeling. If you need to read up on parallel LCR circuits and coax traps before proceeding, refer to the references at the end of the article. Some useful software tools are also listed.
What are coax traps? They are parallel LCR circuits made by winding turns of coax on a air-core form (see Figure-1) and interconnecting the shield and center leads. The inductance and capacitance (LC) values are chosen so the trap resonates on one or more frequencies of interest. The circuit values required for resonance are supplied by the physical properties of the coiled coax.
What does a trap do? It provides impedance at and on both sides of resonance. If a resonant frequency is correctly chosen and the resulting impedance curve is wide enough, the trap will electrically isolate sections of an antenna’s radiators. The “isolation effect” allows a single antenna to operate on multiple bands as if the antenna radiators were individually cut for those bands.
Before proceeding, let’s take a quick look at predicted SWR curves for regular 10m and 20m dipole antennas to provide some reference data to compare to a 10m-20m trap dipole antenna. By looking at the predicted SWR curves (Graphs 1 & 2) for standard dipole antennas, we see that the curves are pretty flat and indicate good SWRs.
Now let’s take a look at the predicted SWR curve (Graph-3) for the 10m band using the 20 meter antenna model. We see the predicted 10m SWR curve for the standard 20m antenna is flat but the SWR is high indicating poor antenna performance. The appeal of trapped antennas is reasonable multi-band performance can be obtained from a single antenna.
Let’s evaluate a 10m – 20m trap dipole. See Figures 2 & 3. We want the traps to resonate at a frequency that provides sufficient impedance to isolate the 10m portions of the radiators (1 & 2) from the rest of the antenna. Sections (1 & 2) act as a 10m dipole (28 – 29.7 MHz) and sections (1 + 3 & 2 + 4) act as a 20m dipole (14 – 14.35 MHz). Due to the inductive properties of the traps, the radiators will be slightly shorter than those of separate non-trapped 10m and 20m antennas.
Important: Coax traps are generally designed for resonance at or slightly below the lower limit of the trapped band to minimize trap losses. Take a look at predicted trap losses using the Load Dat button in EZNEC. You will see that trap losses are higher at resonance. Due to the relatively broad impedance curves of coax traps, it is possible to have an effective antenna by using traps that are resonant at or just below the trapped band lower limit.
10m – 20m trap dipole antenna
By evaluating a model of the antenna depicted in Figure-3, we are able to observe the predicted SWR and trap impedance values for the 10m band at different frequencies. The trap values were selected to achieve resonance at ~28.0 MHz which is the lower limit of the 10m band.
Note: click to download the EZNEC model. 10m-20m trap dipole
10m trap parameters
Table-2 was compiled from the trap data provided by the model using traps designed for resonance at 28.0 MHz.
|Freq. (MHz)||SWR||Trap Z (ohms)|
frequency – SWR – impedance
Graph-3 depicts the predicted SWR and trap impedance for the 10m band using the data from Table-2. Notice the trap provides the highest impedance at resonance and decreasing impedance across the band. Take note of the broad impedance curve provided by the trap.
Graphs 4 & 5 depict the predicted SWR curves for the 10m and 20m bands. Notice the differences in the SWR curves for the trapped 10m dipole and a standard 10 meter dipole. Compare Graph-4 to Graph-1. We see that the trapped 10m antenna exhibits a higher SWR curve than the standard 10m dipole. The noticeably different SWR curve for the trapped 10m antenna is due primarily to the loading properties of the trap. By comparing Graph-5 to Graph-2, we see that the 20m trapped dipole SWR curve is very similar to the SWR curve of a standard 20m dipole antenna.
Now let’s take a look at how effectively the traps isolate the 10m portion of the antenna. From Table-2, we see that the predicted trap impedance is 17,571 ohms @ 28.0 MHz. Figure-4 shows the antenna at 28.0 MHz. Notice that almost no current passes through the traps effectively isolating the 10m portion of the antenna from the rest of the antenna.
Figure-5 shows the antenna at 28.85 MHz. From Table-2, we see that the predicted trap impedance is 2,179 ohms @ 28.85 MHz with a noticeable amount of current passing through the traps.
Figure-6 shows the antenna at 29.7 MHz. From Table-2, we see that the predicted trap impedance is 1,118 ohms @ 29.7 MHz with a significant amount of current passing through the traps.
By studying Figures 4-6, we find that at resonance the traps provide a high degree of isolation and noticeably less but still effective isolation across the rest of the band. Trap losses in band are minimized by setting trap resonant frequency to the lower limit or just below the lower limit of the trapped band.
3. Bootstrap Coax Traps for Antennas (VK1OD)
4. Traps (W8JI)
5. Two New Multi-Band Trapped Dipoles (W8NX, SK)
1. free coax trap design tool (courtesy of VE6YP)