Tuning Your Antenna System Using Coax Cable Length
When tuning antennas, most hams focus on trimming the radiator, adding traps, or adjusting matching networks. But there’s another powerful, often overlooked tool in your shack: coaxial cable.
Yes, the length of your coax can significantly affect the impedance seen at the transmitter end—sometimes enough to bring a high SWR into a tunable range or optimize power transfer.
Let’s explore how it works and how to use it to your advantage.
The Basics: Transmission Line Transformation
Coax isn’t just a wire—it’s a transmission line, and its length matters when dealing with mismatched loads (i.e., antennas that aren’t a perfect 50 ohms).
When the antenna’s feedpoint impedance isn’t 50Ω, the coax can transform this impedance along its length depending on:
- The electrical length of the coax (in wavelengths at the frequency of interest)
- The characteristic impedance (usually 50Ω)
- The velocity factor (VF) of the coax
This is governed by transmission line theory—especially the Smith chart—but you don’t need to be a math wizard. Here’s what matters in practice.
Practical Takeaway: Coax Can Transform Impedance
Let’s say your antenna feedpoint is 25Ω. Using a quarter-wave (λ/4) length of coax, that impedance will be transformed closer to:
Z_in = (Z₀²) / Z_load
If Z₀ is 50Ω and Z_load is 25Ω, then:
Z_in ≈ (50²) / 25 = 100Ω
That’s still not 50Ω, but depending on your tuner, it might be easier to match than 25Ω directly.
And here's the trick: any odd multiple of λ/4 (e.g., 3λ/4, 5λ/4) will perform the same transformation. Each added half-wavelength (λ/2) just repeats the original impedance seen at the antenna feedpoint.
This also applies very well to multiband verticals using a 4:1 or 9:1 UNUN. These antennas often present different complex impedances across bands, and a quarter-wave length of coax—or a multiple thereof—can shift those impedances into a more tuner-friendly range.
This trick is especially useful for getting better performance out of the RF.Guru EFHW, EFOC, and EFLW antenna lines, as well as the IronWave 6 and IronWave 9 verticals, and the DeltaRex and QuadLoop (all of them are using a 4:1 or 9:1 UNUN).
Suggested Lengths to Try (λ/4 Coax Sections)
To use this in practice, try these quarter-wave lengths of coax for different bands. Multiply the free-space quarter-wave length by the velocity factor (VF) of your coax type.
| Band | Freq | Free-space λ/4 | EF Bury 7 (VF 0.83) | EF Bury 10 (VF 0.87) | EF Bury 13 (VF 0.86) |
|---|---|---|---|---|---|
| 160m | 1.8 MHz | 41.67 m | 34.59 m | 36.25 m | 35.83 m |
| 80m | 3.5 MHz | 21.43 m | 17.79 m | 18.85 m | 18.43 m |
| 40m | 7.0 MHz | 10.71 m | 8.89 m | 9.32 m | 9.21 m |
| 20m | 14.0 MHz | 5.36 m | 4.45 m | 4.66 m | 4.61 m |
| 15m | 21.0 MHz | 3.57 m | 2.96 m | 3.11 m | 3.07 m |
| 10m | 28.0 MHz | 2.68 m | 2.22 m | 2.33 m | 2.30 m |
| 6m | 50.0 MHz | 1.50 m | 1.25 m | 1.31 m | 1.29 m |
To shift impedance in smaller steps, try adding or subtracting multiples of λ/4, or use a tuner-friendly length near 3λ/8 or 5λ/8 to nudge impedance in the right direction.
How To Use This Trick
1. Measure the SWR at the Shack
Use your analyzer or radio’s SWR meter at the operating position.
2. Cut and Test
Add or remove coax in 1–2 meter steps. Observe how the SWR curve shifts.
You may find that just adding a 2-meter jumper of Bury 10 can significantly improve matching on 40m or 20m.
3. Use Your RigExpert Analyzer
A great way to figure out if coax length might help is by using your RigExpert AA analyzer:
- Measure at the shack, then again at the antenna feedpoint if possible
- Compare impedance and SWR between both points
- If the feedpoint shows a manageable impedance but the shack reading is poor, coax transformation is likely the cause
- Try modeling quarter-wave shifts or add coax jumpers while monitoring SWR
RigExpert tools like Time Domain Reflectometry (TDR) and Smith chart view can help you visualize exactly how your transmission line is behaving:
- TDR Mode: This feature shows reflections along the transmission line, letting you detect where mismatches (discontinuities) occur. If you see a large reflection before the end of the line, your coax is transforming the impedance in a non-ideal way. By watching how reflections shift as you add coax, you can tune for smoother transitions.
- Smith Chart View: This graphical tool lets you see the impedance transformation caused by your coax. If your antenna impedance is far from the center (50Ω), you can observe how coax length moves the impedance around the chart. Use this to guide coax additions to bring the load closer to the center, ideally within your tuner’s range.
- Impedance Sweep: Perform a wide-band sweep and monitor how impedance and SWR behave across frequencies. Look for frequencies where the impedance crosses near 50Ω—this can guide optimal coax lengths for multiband use.
Together, these tools let you see impedance movement, instead of just guessing, allowing you to experiment with precision rather than trial-and-error.
When This Helps
- With non-resonant wire antennas like EFHW, EFOC, and EFLW antenna lines, as well our QuadLoop antenna!
- With multiband verticals using, such as IronWave 6 and 9
- With multiband DeltaLoops as our DeltaRex
- When using Baluns or UNUNs that create odd impedances (all the above)
- When your ATU struggles to match certain bands
- To minimize RFI, since better matching often reduces common-mode current
When This Won’t Help
- If your antenna is wildly off-frequency, coax won’t magically fix it
- Using too much lossy coax might hide high SWR but waste power
- Complex reactances may still require a tuner or matching network
Real-World Example
We are using this coax length tricks to flatten the response of our EFOC29 antenna. (and the EFOC17, and EFOC8 as well)
By adjusting coax length and choke position, we bring the entire 10–80 meter range into a <3:1 SWR envelope, tunable with modest internal ATUs.
Final Thoughts
Your coax is more than a signal path—it’s part of the matching system. With just a little planning (and a few coax jumpers), you can optimize your antenna setup, improve SWR, and make better use of your tuner.
So next time you're faced with a tough match—try a meter of ExtraFlex Bury 10 before trimming anything. It might just save the day!
Written by Joeri Van Dooren, ON6URE – RF, electronics and software engineer, complex platform and antenna designer. Founder of RF.Guru. An expert in active and passive antennas, high-power RF transformers, and custom RF solutions, he has also engineered telecom and broadcast hardware, including set-top boxes, transcoders, and E1/T1 switchboards. His expertise spans high-power RF, embedded systems, digital signal processing, and complex software platforms, driving innovation in both amateur and professional communications industries.