A 160 m Inverted-V: Open-Wire Line, Coax and the Load Your Tuner Sees
A 160 m Inverted-V: Open-Wire Line, Coax and the Load Your Tuner Sees
Open-wire line does not stabilise SWR, and coax does not merely punish it. Both transform impedance. The practical difference is how much loss, balance sensitivity and tuner stress each complete installation creates.
A 160 m inverted-V is a fine low-band workhorse. It is also electrically close to the ground in many gardens, so its feedpoint impedance changes with apex height, leg angle, soil, nearby conductors, weather and common-mode participation. The feed line then presents a different complex load to the tuner. That is the real reason line choice and line length matter.
Choose the feed system from the load the tuner must handle—not from a low SWR slogan. A low-loss balanced line can carry a large mismatch efficiently and rotate the measured R + jX into a more comfortable tuner region. Coax performs the same impedance transformation, but with a different characteristic impedance, attenuation, routing behaviour and common-mode boundary.
Line Length Changes Input Impedance, Not the Antenna
At the antenna feedpoint, call the installed load ZL. A uniform line has characteristic impedance Z0 and propagation constant γ = α + jβ. At a distance ℓ from the load, the input impedance is
Zin = Z0(ZL + Z0 tanh(γℓ))/(Z0 + ZL tanh(γℓ))
That equation applies to coax and balanced line. Changing length changes the impedance and phase seen at the shack; it does not change the antenna feedpoint impedance unless the routing or common-mode current changes the antenna system itself.
On an ideal lossless line, the magnitude of the reflection coefficient—and therefore SWR on that line—does not become smaller merely because the line is cut to a different length. The impedance point rotates around a constant-SWR circle. With loss, the reflection magnitude decreases toward the source because part of each travelling wave is dissipated. A better-looking shack-end SWR can therefore mean either a useful impedance transformation, more line loss, or a changed antenna/current path. The meter alone cannot choose among them.
Keysight’s RF Design Software Learning Kit shows this Smith-chart rotation explicitly. NIST’s distributed transmission-line treatment derives the same behaviour from line voltage, current, characteristic impedance and propagation constant.
Why Open-Wire Line Often Helps on 160 m
Well-built open-wire line can have low conductor and dielectric loss because most of its electric field is in air and its conductors can be relatively large. That makes high differential-mode SWR a workable design condition when conductor spacing, supports, routing, wet-weather behaviour and voltage clearance are controlled.
The useful trick is not “stable SWR.” It is selecting an electrical length that presents a tuner-friendly R + jX over the part of 160 m you actually use. A tuner can struggle when transformed resistance is extremely low or high, or when reactance demands large circulating current or voltage. Another metre of line may move the load into a much easier region. It may also move it into a worse one.
A physical 16 m line with velocity factor near 0.98 is only about 0.10 wavelength at 1.8 MHz and 0.11 wavelength at 2.0 MHz. Those figures are illustrative: the actual velocity factor, characteristic impedance and antenna load must be measured. The right length cannot be inferred from a universal “add two metres” recipe.
Open-wire line is valuable because it can combine low loss with a deliberately chosen impedance transformation. It does not make the load resonant, guarantee balance, remove tuner loss or establish a safe power rating.
Coax Loss on 160 m Needs Real Numbers
Coax is not automatically a poor 160 m feed line. It is shielded, mechanically convenient and relatively insensitive to nearby objects in its intended differential TEM mode. A mismatched coax run does dissipate more power than its matched-loss value, but the result must be calculated from the actual cable, frequency, length and load reflection.
Belden’s current 8267 RG-213 data sheet specifies nominal matched attenuation of 0.17 dB per 100 ft at 1 MHz and 0.55 dB per 100 ft at 10 MHz. A 16 m run is about 52.5 ft, so those endpoints correspond to roughly 0.09 dB and 0.29 dB respectively. The 1.8 MHz matched-loss value lies between them; it is not automatically “multiple dB.”
For a matched source, let k = 10−a/10 be the line’s one-way power ratio for matched attenuation a, and let ρ be the magnitude of the load reflection coefficient. The fraction of net line-input power accepted by the load is
ηline = k(1 − ρ²)/(1 − k²ρ²)
Using the data-sheet endpoints and a 2.5:1 load SWR (ρ ≈ 0.429), the idealised total line dissipation is about 0.13 dB at the 1 MHz endpoint and 0.41 dB at the 10 MHz endpoint. The 1.8 MHz result lies between those values for that cable under the stated assumptions. Connectors, ageing, moisture, a balun, current on the outside of the shield and a different cable construction are separate losses.
Do not transfer this worked example to another cable or power level. Use the manufacturer’s attenuation and power data, the measured complex load, the actual line temperature and a full voltage/current calculation. A tuner achieving 1:1 at the radio does not lower the SWR already present between tuner and antenna.
The Balun and Tuner See the Transformed Load
A centre-fed inverted-V is intended as a balanced two-terminal antenna. Coax provides an unbalanced port, so a current balun or common-mode choke is normally part of the feedpoint boundary. That device does not see a universal 50 Ω load. It sees the antenna’s installed complex impedance, and its differential voltage, current, flux, loss and temperature depend on frequency and power.
Open-wire line moves the balanced-to-unbalanced boundary to the tuner or station entrance. A genuinely balanced tuner may accept the line directly. An unbalanced tuner with a current balun can also work, provided the balun is qualified at the transformed R + jX, voltage, current, duty cycle and common-mode impedance it actually encounters.
Place common-mode impedance where the unwanted current path is measurable. A choke at the feedpoint, tuner or station entrance can help, but no fixed location is automatically correct. ITU-R Report SM.2158-2 is useful here because it separates differential and common-mode currents rather than treating every feed-line current as one phenomenon.
A Practical Line-Length Method
I would tune the system in this order:
- Measure the installed antenna load. Record complex feedpoint impedance across the intended segment of 160 m with a characterised balanced fixture or transition. Declare the calibration plane and uncertainty.
-
Characterise the line. Use its measured or defensible
Z0, velocity factor, attenuation and physical length. Include wet and dry conditions for open-wire line. - Calculate the shack-end load. Transform every measured frequency through the candidate line lengths. Plot resistance, reactance, current and voltage—not SWR alone.
- Check the tuner envelope. Confirm component voltage, circulating current, capacitor spacing, inductor loss, balun stress and repeatability at the chosen power and duty cycle.
- Check balance and routing. Keep both conductors similarly exposed, maintain spacing, avoid prolonged close runs beside metal or wet foliage, and measure common-mode current at several positions.
- Compare A/B/A. Try the original length, a controlled length change and the restored original without moving unrelated geometry. Record accepted power, tuner settings, temperature, exterior current, local RFI and stable remote field or SNR observations.
A line length is good when the complete system remains low-loss, controllable and repeatable. A 2:1 or 3:1 reading before a competent tuner can be entirely acceptable. A 1:1 reading can still hide loss, high internal voltage or common mode.
| Observation | What it supports | What it does not prove |
|---|---|---|
| Tuner finds an easy repeatable match | The presented R + jX is inside that tuner’s practical range |
Low feed-line or tuner loss |
| Open-wire line remains cool | No obvious heating at the tested power and duty cycle | A calibrated efficiency figure |
| Low current around both line conductors | Low vector-sum common-mode current at that position | Low common mode everywhere |
| Lower shack-end SWR through coax | Lower reflection at that reference plane | A better antenna or more radiated power |
My 160 m Recommendation
Use open-wire line when the site can route it symmetrically and safely, and when its low loss lets you choose electrical length for a manageable tuner load. That is a strong 160 m architecture. Use coax when its shielding, route and mechanical simplicity matter more, but calculate the real mismatch loss and qualify the feedpoint balun instead of assuming disaster.
The winning system is not “ladder line at any length” or “coax plus a magic balun.” It is the one whose measured antenna impedance, line transformation, tuner loss, voltage/current stress, common-mode path and thermal behaviour remain controlled across the band segment you operate.
Primary and official technical sources
- Belden 8267 RG-213 technical data: nominal characteristic impedance, velocity, attenuation and power data for the named cable.
- NIST transmission-line theory: distributed parameters, characteristic impedance, propagation and lossy-line voltage/current relationships.
- Keysight RF Design Software Learning Kit: Smith-chart impedance transformation and electrical line length.
- Keysight Network Analysis training: reference planes, complex impedance, S-parameters, calibration and error correction.
- ITU-R Report SM.2158-2: differential/common-mode current decomposition and mode conversion on two-conductor lines.
Mini-FAQ
- Does open-wire line make SWR more stable? No. It can carry high differential-mode SWR with low loss and transform the antenna load into a tuner-friendly impedance. On an ideal lossless line, changing length rotates impedance without changing SWR magnitude.
- Does coax only dissipate mismatch as heat? No. Coax also transforms impedance with electrical length. Its attenuation makes mismatch loss larger than matched loss, but the result must be calculated from the actual cable, length, frequency and load.
- Is 16 m of RG-213 several dB lossy on 160 m? Not for the Belden 8267 example under the stated conditions. Its official matched-loss data imply a small fraction of a decibel for 16 m at low HF; connectors, baluns, damage and common mode require separate checks.
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How should I choose the open-wire line length? Measure the installed feedpoint
R + jX, characterise the line, transform the load through candidate lengths and choose a region the tuner can handle with acceptable voltage, current and loss. - Where should the common-mode choke go? At the boundary where measured unwanted common-mode current needs impedance. That may be the feedpoint, tuner, station entrance or another transition; no fixed position is universal.
- Should I chase 1:1 SWR before the tuner? No. A manageable complex load, low accepted-power loss, safe component stress, low common mode and repeatable operation matter more than one SWR number.