In amateur radio circles, countless myths, half-truths, and real tricks are circulating. One of the most frequently debated topics: does the length of the coaxial antenna cable really matter? When is cable tuning useful, and when is it unnecessary?
The Cable Length Myth – And What’s Behind It
The Basics of Impedance Transformation
A quarter-wavelength cable behaves as an impedance transformer. This means:
If the impedance at one end of the cable is Zl, and the characteristic impedance of the cable is Z0, then the “seen” impedance at the other end is:
Zin = Z0² / Zl
So, for example, if you want to match a 50-ohm transmitter to a 75-ohm antenna, you can use a ¼λ (quarter-wavelength) 61-ohm coaxial section to perform the matching.
The example is not perfect, as coax with exactly this impedance is not generally available. In practice, we often use a 50-ohm quarter-wavelength section, which still provides a better match than directly connecting a 75-ohm antenna to a 50-ohm radio (approx. 1.5:1 SWR mismatch), or alternatively use a 4:3 balun.
When Does Impedance Transformation Not Occur?
If the cable is half a wavelength (λ/2) or an integer multiple of it, no impedance transformation occurs. This means the radio “sees” exactly the impedance presented by the antenna. If you have a perfectly tuned antenna and/or cut the coax accurately for resonance, this is ideal.
What Is SWR, and Why Does It Matter?
SWR (Standing Wave Ratio) indicates how much power is delivered to the antenna and how much is reflected. The ideal SWR is 1:1, but most equipment tolerates mismatches up to 2:1.
| SWR | Reflected Power (%) | Loss (dB) | Remaining Power (%) |
|---|---|---|---|
| 1:1 | 0% | 0 dB | 100% |
| 1.5:1 | 4% | 0.18 dB | 96% |
| 2:1 | 11% | 0.51 dB | 89% |
| 3:1 | 25% | 1.25 dB | 75% |
| 5:1 | 44% | 2.6 dB | 56% |
| 10:1 | 67% | 4.8 dB | 33% |
Coaxial Cable Losses: RG-58, H155, H500, H1000
Loss depends greatly on cable type, length, frequency, and condition. Let’s look at some popular types over 10 meters at three frequencies:
| Cable Type | Velocity Factor | 14 MHz | 145 MHz | 435 MHz |
|---|---|---|---|---|
| RG-58 | 0.66 | 0.5 dB (~11%) | 2.1 dB (~35%) | 4.8 dB (~68%) |
| H-155 | 0.79 | 0.3 dB (~6%) | 1.1 dB (~22%) | 2.5 dB (~44%) |
| H-500 | 0.85 | 0.15 dB (~3%) | 0.6 dB (~12%) | 1.3 dB (~25%) |
| H-1000 | 0.85 | 0.08 dB (~1.8%) | 0.4 dB (~9%) | 0.9 dB (~18%) |
What Is Velocity Factor, and Why Is It Important?
The signal does not propagate at the speed of light in the cable.
Inside coaxial cables, RF signals do not travel in a vacuum but within the dielectric material (e.g. PE, PTFE, foamed polyethylene). This slows down the signal.
This phenomenon is described by the velocity factor (VF), which indicates what percentage of the speed of light the signal travels in the cable.
Examples:
| Cable Type | Velocity Factor |
|---|---|
| RG-58 | 0.66 |
| H-155 | 0.79 |
| H-500 | 0.85 |
| H-1000 | 0.85 |
For example, if the free-space wavelength for a frequency is 2 meters, the wavelength in RG-58 will only be 1.32 meters.
That’s why you must consider velocity factor in all cable length calculations – otherwise, the cable will not resonate properly at the desired frequency!
Total Losses – A Real-World Example
| Parameter | Value |
|---|---|
| Frequency | 145 MHz |
| Cable type | RG-58 |
| Cable length | 10 meters |
| Number of connectors | 2 |
| Transmitter power | 10 W |
| Antenna impedance | not 50 ohms → SWR ≠ 1:1 |
Loss factors included:
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Coaxial cable loss (from manufacturer data)
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Connector loss (typically 0.1 dB per connector)
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Reflection loss due to poor SWR
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Extra loss from non-resonant cable length
| SWR | Reflection Loss (dB) | Reflection Loss (%) | Extra Loss (non-λ/2) | Cable Loss (dB) | Connector Loss (dB) | Total Loss (dB) | Output Power (W) |
|---|---|---|---|---|---|---|---|
| 1:1 | 0 dB | 0% | 0 dB | 2.1 dB | 0.2 dB | 2.3 dB | ≈ 5.9 W |
| 1.5:1 | 0.18 dB | ~4% | 0.2 dB | 2.1 dB | 0.2 dB | 2.48 dB | ≈ 5.6 W |
| 2:1 | 0.51 dB | ~11% | 0.3 dB | 2.1 dB | 0.2 dB | 3.11 dB | ≈ 4.9 W |
| 3:1 | 1.25 dB | ~25% | 0.4 dB | 2.1 dB | 0.2 dB | 3.95 dB | ≈ 4.0 W |
| 5:1 | 2.6 dB | ~44% | 0.5 dB | 2.1 dB | 0.2 dB | 5.4 dB | ≈ 2.9 W |
Conclusions:
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RG-58 is a thin, inexpensive coaxial cable that already causes 2.1 dB of loss over 10 meters at 145 MHz — meaning your antenna system starts with a disadvantage. Use lower-loss cables such as H155, or preferably H500 or H1000.
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Even with a perfectly cut cable, if the antenna is not well tuned and SWR rises, significant losses occur. For example, with a seemingly harmless 1.5:1 SWR, more than 40% of power can be lost in RG-58 cable. Take the time to tune your antenna properly!
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If the cable is not a half-wavelength, additional loss occurs due to impedance distortion (reflections don’t just occur once but continue to “travel” back and forth, being absorbed). A non-resonant cable length adds 0.2–0.5 dB extra loss, which often goes unnoticed — but the power delivered to the antenna drops drastically. If you can’t install an ideal antenna (which rarely exists in reality), at least cut the coax to a half-wavelength or an integer multiple of it to avoid impedance distortion losses.
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5:1 SWR + RG-58 = 70% of your power is lost! But that’s horror territory — let’s not even go there…
So, Is This an Urban Legend or a Real Trick?
Cable length really does matter — but not always.
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With a half-wavelength cable, the antenna impedance reaches the radio undistorted.
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With a quarter-wavelength cable, you can match a non-ideal antenna.
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In other cases, losses should be quantified — a poor cable and/or SWR can lead to up to 50% power loss.
This is not an urban legend — it’s simple physics.
Bonus Tip
Created a calculator that helps you calculate the ideal coax cable length.
