When you encounter an antenna system where the SWR appears properly tuned but the impedance remains incorrect, you need to adjust impedance. This article Setting Impedance in Tuned Antenna will show how to adjust impedance without effecting SWR. The SWR reflects the relationship between the feedline and transmitter, it does not always expose what is happening at the actual antenna feed-point.
As a result, the antenna may be resonant and show a low SWR while still presenting resistance or reactance values far from what the radio expects. Because of this, the system might operate inefficiently even though the meter suggests everything is fine.
Why the Issue Occurs and What It Means
In many cases, impedance problems hide behind a clean SWR curve because feedline length, matching components, or transformer ratios mask the true feed-point values. Additionally, the antenna might be resonant at the correct frequency yet still sit at an undesirable resistance level, which causes poor power transfer and reduced efficiency.
As you examine the system more closely, you’ll notice that the low SWR simply means the radio is seeing an acceptable load, not that the antenna itself is operating optimally. Consequently, you must measure impedance directly at the feed-point and adjust matching networks or feed-point placement to correct the imbalance while maintaining the favorable SWR reading.
Once you understand what’s actually happening at the feed-point vs. what the meter displays on the line.
Here’s the simplest, clearest way to break this down.
How Impedance and SWR Interact
SWR is only a measurement of how well the antenna system matches the feedline and the transmitter.
It is not a direct measurement of true feedpoint impedance.
Because of that, you can change the actual impedance at the antenna while the SWR appears unchanged, especially if:
- The coax length masks impedance variations
- A tuner or matching network hides mismatch
- The antenna has a narrow SWR dip but still isn’t at the desired impedance
- You’re checking SWR at the radio instead of directly at the antenna
So the question becomes:
How can you fix the impedance itself without affecting SWR too much?
Below are the correct methods.
Adjust the Feed-point, Not the Radiating Length
Changing the antenna length shifts resonance which will shift SWR.
Instead, adjust the feedpoint position:
- Move the feedpoint slightly inward on a dipole → raises impedance
- Move it outward toward the end → lowers impedance
These changes often leave SWR almost untouched because resonance doesn’t move, only resistance does.
Use a Matching Transformer (Unun or Balun)
This is the cleanest method.
A 1:1, 4:1, 9:1, or 6:1 transformer can alter the feed-point impedance dramatically without requiring any antenna length change.
Because it transforms resistance but preserves the resonant point, SWR often stays stable.
Use when:
- The antenna is resonant but the feedpoint resistance is not 50Ω
- You want to correct 100Ω, 200Ω, or higher mismatches cleanly
Example:
A 4:1 balun converts 200Ω → 50Ω and the SWR barely moves because resonance doesn’t shift.
Add or Adjust a Matching Network
You can use:
- L-network
- T-network
- Gamma match
- Hairpin match
These adjust impedance, not resonance.
Therefore, SWR remains nearly the same unless the network introduces reactance.
This is why many antennas use:
- A hairpin match for Yagis
- A gamma match for verticals
- A shunt inductor on mobile antennas
It fixes the feed-point resistance without altering electrical length.
Use Coax as an Impedance Transformer
A properly chosen length of coax can transform impedance based on transmission-line theory.
Because the SWR is measured at the radio end, not at the feedpoint, you can:
- Change the impedance at the antenna
- Use line length to make SWR appear unchanged
This does not fix the antenna electrically, but it does adjust the presented impedance at the radio without changing the SWR at the feedpoint.
This method is common in:
- J-poles
- End-fed half-waves
- Off-center-fed dipoles
- ¼-wave matching sections
Add a Parallel (Shunt) Capacitor or Inductor
A shunt reactive component can bring impedance into correct balance while barely affecting SWR.
Use when:
- The antenna is resonant
- The feed-point impedance is too high or too low
- You need correction without altering physical length
For example:
- A shunt coil can lower high impedance
- A shunt capacitor can raise low impedance
Again, because resonance stays the same, SWR barely moves.
Methods That Change Impedance but Don’t Change SWR
Use these:
- Feedpoint repositioning
- Impedance transformers (balun/unun)
- L-network or gamma/hairpin matches
- Coax impedance transformers
- Shunt reactive tuning (capacitor or inductor)
Avoid these if you want SWR unchanged:
- Changing antenna length
- Changing height
- Changing spacing near metal
- Trimming elements
Conclusion for Setting Impedance in Tuned Antenna
Setting impedance in tuned antenna ensures that the system transfers power efficiently, maintains a stable match, and performs reliably across its intended operating band.
Although tuning resonance often receives the most attention, achieving proper impedance at the feed-point ultimately determines how well the antenna and transmitter work together. When you adjust impedance thoughtfully, you reduce losses, prevent unnecessary stress on your equipment, and create a more predictable and consistent radiation pattern.
As you refine the antenna, focus on maintaining resonance first and then shaping the impedance through careful adjustments such as matching networks, feedpoint positioning, or reactive components. Because each change influences the entire system, take measurements directly at the feed-point whenever possible and verify performance across the full bandwidth.
By combining methodical tuning with accurate measurement, you create an antenna that not only resonates at the correct frequency but also presents the ideal load to your transmitter. With this balance in place, the antenna becomes both efficient and dependable, delivering the performance you expect every time you take to the airwaves.
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