VIDEO
Why a 71 ft random wire?
A single wire that covers 160 metres through to 10 metres sounds like magic, but it is practical and surprisingly straightforward when you understand the basics. The so-called random wire is not truly random. Certain lengths produce predictable impedances across the HF bands, and one of the most commonly recommended lengths is approximately 71 ft (about 21.6 m). When used with a 9:1 UnUn, that length tends to present a medium impedance (roughly 300–600 ohms) on many amateur bands, which the 9:1 UnUn converts down close to 50 ohms for the radio or tuner to handle.
How the different wire lengths behave
Understanding how wire length affects impedance helps explain why a 9:1 UnUn plus 71 ft performs so well across multiple bands.
- Quarter-wave on a band: the feedpoint is a low impedance. A transceiver can feed this directly with little fuss, but it will only work well on that frequency and possibly its third harmonic.
- Half-wave on a band: the feedpoint is a high impedance. A 49:1 UnUn is commonly used to step that high impedance down to something usable by the rig. That arrangement gives reliable coverage on the band where the half-wave resonates and its harmonics (for example 40, 20, 15 and 10 metres for a typical end-fed half-wave around 20 m).
- Medium-length wires like 71 ft: they often present a medium impedance across many bands. With a 9:1 UnUn, these medium impedances are reduced to a low impedance range near 50 ohms, permitting multi-band use from 160 through 10 metres, including 60 m and the WARC bands.
Practical setup I used
The approach was simple. Replace the 49:1 UnUn at the feedpoint of an end-fed half-wave with a 9:1 UnUn and extend the wire to about 71 ft. The wire can be bent around a garden, so a 50 ft long garden can still accommodate the full length by running the extra length across fences or along boundaries.
Feedline ran from the operating position down to the feed point. Initially there were about 40 ft of RG58 coax between the shack and the UnUn. With the 9:1 UnUn and the 71 ft wire in place, the higher HF bands (14–28 MHz) presented a VSWR usually below 3:1, often nearer 2:1, which is entirely workable.
Troubleshooting: why 40 m initially refused to match
The rig’s internal tuner struggled to match the antenna on 40 metres at first. VSWR hovered around 3:1 there, and the internal tuner on the transceiver could not find a match. A simple, old-school trick fixed this: increase the feedline length. Adding an extra 25 ft of coax in series changed the impedance presented to the tuner and allowed the tuner to match the antenna on 40 metres without affecting the other bands.
Coax length can act as part of the matching chain when impedances are reactive or when the feed point impedance is not close to 50 ohms. If you encounter tuning problems on a particular band, experiment with adding or removing a section of coax before changing the antenna itself.
Getting on 80 m and 160 m — the Pi network trick
Lower HF bands can present very low impedances with a relatively short wire. On 80 m and 160 m the 71 ft wire is well below a quarter wavelength, so the impedance at the feedpoint falls dramatically and VSWR rises. That higher VSWR is not the whole story: it is possible to deliver power even into a 4:1 or higher VSWR if the matching network can cope.
A valve (tube) amplifier with a Pi network can match a wide range of impedances. In practice, placing a valve linear between the transceiver and the antenna tuner provided the extra matching capability needed. By driving the amplifier with modest power the amplifier’s Pi network tuned the antenna for effective operation on 80 m and 160 m. This converted the 71 ft wire into a practical antenna from 160 m all the way up to 10 m.
Key points about the Pi network approach
- Valve amplifiers usually include a robust Pi network capable of matching very low or very high impedances. That is why many older amplifiers remain valuable tools for working problem antennas.
- Solid-state amplifiers tend to prefer a low, fairly stable input impedance and often cannot tune as wide a range without external matching gear.
- If a valve amplifier is unavailable, an external antenna tuner (ATU) capable of handling the impedance extremes can perform the same role for HF bands below 20 m.
Real-world performance and surprising contacts
Delivering power through the Pi network and working the antenna on 160 m produced good results. CW contacts included stations in central and northern Europe. SSB contacts inland were surprisingly solid: signals were readable with good reports from locations several hundred miles away.
The results confirm a practical truth: radiation efficiency on short antennas is not zero. Even with a 71 ft radiator on 160 m, contacts are possible because the antenna still radiates and because favourable propagation often compensates. Operating at modest power with an appropriate match produced reliable QSOs on both 160 m and 80 m.
VSWR behaviour and what to expect
The VSWR curve for a 71 ft wire with 9:1 UnUn is less “peaky” than a resonant end-fed half-wave. You will see dips where the impedance is near the 300–600 ohm “medium” range the UnUn is designed for. Those dips often fall on or near amateur bands, and those are the points where the converted impedance sits close to 50 ohms.
On the low end of the HF spectrum the impedance rises or falls sharply. Where the feedpoint impedance goes very low, the 9:1 UnUn can only make it lower, so VSWR climbs dramatically on 80 m and 160 m without additional matching. That's why either a longer coax, a Pi network in the amplifier, or an external ATU will be necessary to use those bands reliably.
Installation and tuning checklist
- Replace any 49:1 UnUn with a 9:1 UnUn at the feed point if you want broad multi-band behaviour from a medium-length wire.
- Extend the radiator to around 71 ft (21.6 m). You can route the wire around corners and along fences to fit it into small gardens.
- Run a sensible length of coax from the shack to the feed point. About 40 ft worked in this setup, but adding another 20–30 ft of coax can help the tuner find a match on problematic bands like 40 m.
- Test the high bands first (20, 15, 10 metres). Those usually present the lowest VSWR with this configuration.
- If 40 m refuses to match, try increasing the coax length in increments until the tuner can find a match.
- For 80 m and 160 m, use an external antenna tuner capable of dealing with a wide reactance range or employ an amplifier with a Pi network to match the antenna.
- If shifting the length by a few feet improves one band but hurts others, decide which bands are priorities and tune the wire length accordingly. Slight adjustment is part of the job.
Common questions and practical tips
Do I need a counterpoise?
Many end-fed and random wire installations benefit from a counterpoise or good earth. A 9:1 UnUn will work without an extensive counterpoise in many installations, but if you encounter RF in the shack or erratic tuner behaviour, adding a short counterpoise or improving the station earth is a sensible next step.
What if I do not have a linear amplifier?
An external high-quality antenna tuner is the most practical alternative. A wide-range ATU between the transceiver and the feedline or at the feedpoint will allow operation on the lower bands without an amplifier.
How critical is the exact 71 ft length?
71 ft is a commonly recommended compromise length. Other “magic” lengths exist (for example 111 ft and longer lengths up to around 200 ft are discussed on many reference pages). The shorter the wire, the poorer the efficiency will be on the lowest bands, so choose a length that fits your space and your target bands. Expect to tweak the length by a few feet depending on how the wire is routed and nearby objects affect the effective electrical length.
Coax type and losses?
RG58 is convenient and often used for testing, but higher-quality coax (lower loss, better shielding) will reduce feedline loss, particularly at HF with higher VSWR. If you plan long-term operation across the bands, consider upgrading to RG213 or equivalent for lower loss and more robust handling.
Advantages and limitations
- Advantages
- Simple, inexpensive and quick to install.
- Flexible routing around garden obstacles.
- With the right matching arrangement, covers 160 m through 10 m including 60 m and WARC bands.
- Great for contacts on lower bands when matched properly.
- Limitations
- Requires some experimentation with feedline length, UnUn, or an ATU to get reliable performance on all bands.
- Lower efficiency on 160 m and 80 m compared with full-size resonant antennas, so higher power or better matching networks may be necessary for consistent results.
- Potential for RF in the shack if the feed system is not properly grounded or if a counterpoise is absent.
Final thoughts and recommendations
The 9:1 UnUn plus 71 ft random wire is a highly practical solution for hams who want broad HF coverage without erecting multiple resonant antennas. It is a design that rewards experimentation: adjust wire length, try different feedline lengths, and use a capable matching device (external ATU or Pi network on a valve amplifier) for the lower bands.
If you enjoy hands-on antenna work and want a compact, multi-band option that fits into a modest garden, give the 71 ft random wire a try. Be prepared for a little trial and error, and remember that a simple change—adding 25 ft of coax or introducing a Pi-network match—can be the difference between a stubborn mismatch and a reliable operating antenna across the HF spectrum.
Happy operating. Enjoy experimenting and finding the sweet spots for your station.
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