Long Wire Antenna for Multiband Operation
When amateur radio operators talk about a “long wire” for multiband operation, they are usually referring to a Random Wire Antenna. Technically, a true long wire must be at least one full wavelength long at the lowest operating frequency (which would be over 260 feet for the 80m band, and massive for 160m). A random wire, on the other hand, is an end-fed wire cut to a specific, mathematically “random” length designed to avoid being a half-wavelength (or multiple thereof) on any band you want to operate.
If a wire is a half-wavelength on a given band, the feedpoint impedance shoots up into the thousands of ohms. Most tuners cannot match this, and it results in high SWR and heavy RF in the shack. Here is how to set up an effective multiband random wire, especially if you want to include lower bands like 80m and 160m.
The Core Components
- The Radiator (Wire): A single wire run horizontally, as an inverted-L, or sloped. Get it as high and clear of obstacles as possible.
- 9:1 UNUN (Unbalanced to Unbalanced): Because the impedance of a non-resonant end-fed wire is typically between 300 and 600 ohms, a 9:1 UNUN steps this down to roughly 50 ohms so your transceiver and tuner can handle it.
- Antenna Tuning Unit (ATU): Even with the UNUN, the antenna will not be perfectly resonant. A wide-range tuner is required to flatten the SWR across different bands.
- Counterpoise / Ground: Since you are feeding an unbalanced antenna, the UNUN needs something to push against. Without a good ground or counterpoise, the coax shield becomes the counterpoise, dragging RF directly into your operating position.
“Magic” Wire Lengths
To operate on multiple bands, you must choose a wire length that avoids half-wave resonances. Here are proven lengths that offer a manageable impedance across the HF spectrum:
| Wire Length | Primary Bands Covered (with a tuner) | Notes |
| 53 – 58 feet | 40m – 10m | Good for limited space, but highly inefficient on 80m. |
| 71 – 73 feet | 40m – 10m (Some 80m) | A popular compromise length. |
| 84 feet | 80m – 10m | An excellent length if you want solid 80m performance without needing excessive space. |
| 124.5 feet | 80m – 10m | Often considered the “golden length” for 80m-10m random wires. Very easy for most tuners to match. |
| 162 feet | 160m – 10m | Highly recommended if you want 160m operation. It is long enough to radiate effectively on top band while remaining tunable on higher bands. |
Managing the Counterpoise
An end-fed random wire is only half of the antenna system. The other half is the ground.
- Elevated Counterpoise: Connect one or more radial wires to the ground lug of your 9:1 UNUN. These wires don’t have to be perfectly resonant, but running a few wires roughly 30–40 feet long (laid on the ground or slightly elevated) will drastically improve efficiency.
- Ground Rod: If the UNUN is near the ground, tying it to a dedicated RF ground rod helps, but radials are generally superior for actual radiation efficiency.
Keeping RF Out of the Shack
Because random wires are inherently unbalanced, common-mode current loves to travel back down the outside of your coaxial cable.
To prevent your Raspberry Pi, transceiver, or linear amplifier from acting erratic when you transmit, install a 1:1 Current Balun (Common Mode Choke) on the coaxial cable. Place it either right after the 9:1 UNUN, or just before the coax enters your shack. This chokes off the stray RF while letting the signal inside the coax pass cleanly to your receiver.