Build a Log Periodic Dipole Array Antenna
The log periodic dipole array (LPDA) is the multi-band workhorse of the HF beam antenna world — a single rotatable antenna covering 14 to 30 MHz (20m through 10m) with consistent forward gain of 6–8 dBd and front-to-back ratio of 15–20 dB, all from a single feedpoint with no band switching, no traps, and no tuner required for any covered band. Commercial LPDAs have been the standard fixed-station multi-band beam for decades. The homebrew LPDA is a substantial but achievable build that rewards the effort with antenna performance that would otherwise require multiple single-band Yagis and a band-switching system.
Log Periodic Fundamentals
An LPDA consists of a series of dipole elements arranged along a boom, with each successive element shorter than the previous by a constant ratio called tau (τ). The elements are fed from a common transmission line with alternating polarity — each successive element is connected with reversed phase. At any given frequency, only the elements near resonance for that frequency carry significant current. As the frequency changes, the active region shifts along the boom to the elements appropriate for the new frequency:
The Transposed Feed System
The most distinctive feature of an LPDA is its transposed (crossed) feed system. The two boom conductors form a balanced transmission line — each element connects to opposite boom conductors, so adjacent elements are fed with reversed polarity. This alternating phase relationship is what makes the LPDA work:
LPDA vs Single-Band Yagis — Trade-offs
The LPDA is not the best antenna for any single band — it is the best antenna for covering multiple bands from a single rotatable structure:
Boom Construction — Two Conductors
The LPDA boom is fundamentally different from a single-conductor Yagi boom. The boom must be a balanced transmission line — two parallel conductors running the full boom length, separated by a consistent gap:
- Two-tube boom two aluminum tubes running side by side, separated by 2–4 inches, connected by non-conductive spacers every 2–3 feet. Each tube is a boom conductor. Elements connect across the gap between the tubes, alternating which tube connects to which element half. This is the standard homebrew approach and the one used in this guide.
- Split-channel boom: a single rectangular aluminum channel, split down the center with a fiberglass or plastic divider. Cleaner looking but more difficult to fabricate than two-tube construction.
- Commercial approach: commercial LPDAs often use square boom sections with the element mounting hardware providing the connection reversal. The electrical principle is identical — two conductors, alternating element connections.
- Boom impedance: the two parallel boom conductors form a transmission line with a characteristic impedance determined by the tube diameter and separation. This impedance affects the antenna's input impedance. For typical 1-inch tubes separated by 2.5 inches, the boom Zo is approximately 100–150 Ω — use a 2:1 balun at the feedpoint to transform to 50 Ω.
| Element # | Half-length (ft) | Full length (ft) | Resonant freq (MHz) | Spacing from previous (ft) | Position from front (ft) |
|---|---|---|---|---|---|
| 1 (front, shortest) | 4.79 | 9.58 | 29.0 | — | 0.0 |
| 2 | 5.33 | 10.65 | 26.1 | 2.27 | 2.3 |
| 3 | 5.92 | 11.83 | 23.5 | 2.52 | 4.8 |
| 4 | 6.58 | 13.15 | 21.2 | 2.80 | 7.6 |
| 5 | 7.31 | 14.61 | 19.1 | 3.11 | 10.7 |
| 6 | 8.12 | 16.24 | 17.2 | 3.46 | 14.2 |
| 7 | 9.02 | 18.04 | 15.5 | 3.84 | 18.0 |
| 8 | 10.02 | 20.04 | 13.9 | 4.27 | 22.3 |
| Termination resistor | — | — | — | ~2.5 | ~24.8 |
LPDA Calculator
Reverse Calculator: Achievable Range from Element Count
Use this if your element count is fixed by available material — tells you the frequency range you can actually cover with what you have, instead of starting from a target range.
Materials for an 8-element LPDA covering 14–30 MHz with two-tube boom
Element Length Calculation
Each LPDA element is a half-wave dipole at its resonant frequency. The element lengths form a geometric progression with ratio τ. To calculate element lengths for any frequency range:
Feedpoint Impedance and Matching
The LPDA's input impedance depends on the boom transmission line impedance and the design parameters. For the two-tube boom specified in this guide:
Building the 8-Element LPDA
The LPDA is the most complex antenna in this guide series — more total parts, more measurements, and more connections than any previous build. Plan for two full weekend sessions: one for cutting and preparing all elements and the boom, one for assembly and wiring. Verify element lengths before assembly. Work systematically from the front element to the rear.
Build the Two-Tube Boom
The two boom conductors run parallel to each other at a separation of 2.5 inches, held apart by HDPE spacer blocks. Cut two lengths of 1-inch OD aluminum tubing to 25 feet each. Fabricate 10 HDPE spacer blocks: each block is 2.5 inches wide, 2 inches tall, and 1.5 inches deep with two 1-inch OD holes bored through it at a 2.5-inch center-to-center spacing. The two tubes slide through the spacer holes.
Cut All Eight Elements to Starting Lengths
Cut all 8 elements to starting lengths from the table, each 6 inches longer than the target final length. Label every element clearly — with 8 elements all looking similar, mix-ups are easy and have significant consequences for antenna performance:
Understand and Plan the Transposed Connections
Before mounting any elements, plan and document the transposed connection pattern — this is the most common source of LPDA build errors. Lay out the boom with the front (element 1) at your left and rear (element 8) at your right. Label the boom tubes: call the top tube "A" and the bottom tube "B":
Mount Elements to Boom — Front to Rear
Mount elements starting from element 1 (front) and working toward element 8 (rear). For each element:
- Position the element center at the correct boom position from the table
- The element left half connects to the correct boom tube per the transposition table (electrical contact via the U-bolt saddle directly on the tube, with Noalox at the contact surface)
- The element right half connects to the opposite boom tube
- At each connection point, one element half is electrically connected to the boom tube; the other half is on an HDPE insulator plate that prevents contact with the other boom tube at that same position
- Verify the connection for each half before tightening — use a continuity meter to confirm which tube each half connects to
Build and Install the Feedpoint Assembly
The feedpoint is at the front end of the boom — at element 1. The two boom tube ends at the front are the feedpoint terminals. Install the 2:1 balun (100 Ω to 50 Ω) here:
Install the Rear Termination Resistor
At the rear end of the boom (past element 8), install a non-inductive resistor across the two boom tube ends. This termination absorbs any residual RF energy traveling toward the rear of the antenna and prevents it from reflecting back toward the front:
Ground-Level Verification Before Raising
With the antenna complete but at ground level, connect the NanoVNA to the feedpoint SO-239 and sweep across all covered bands. This initial check verifies basic functionality:
Raise and Measure SWR Across All Bands
Raise the antenna to its operating height. Connect the NanoVNA at the shack end of the feedline. Sweep all five target bands sequentially and record SWR at the center of each band:
Trim Elements and Final Documentation
If any band's SWR minimum is shifted from the band center, trim the elements resonant near that frequency to correct. For the LPDA, element trimming is less critical than for a single-band Yagi because the 10% frequency margin built into the design covers small errors. Trim only if the SWR minimum for a band is shifted by more than 500 kHz from the band center.
Weatherproof all connections: apply self-amalgamating tape to the feedpoint balun terminals, RTV sealant at the termination resistor enclosure, and self-amalgamating tape over all boom tube open ends. Apply Noalox to all element-to-boom contact surfaces annually. Document all element lengths, connection table, SWR on each band, front-to-back measurement on each band, and antenna height.
| Symptom | Most likely cause | Diagnosis | Fix |
|---|---|---|---|
| Very high SWR on all bands (above 6:1 everywhere) | Boom tubes shorted together somewhere, or feedpoint balun wiring fault | Check DC resistance between the two boom tube ends at the front — should be open circuit (or resistor value at rear) not a short | Check for metal contact between boom tubes at any spacer or element mounting point; verify balun connections |
| SWR very high on one specific band only | Wrong transposition connection on element resonant near that frequency | Identify which element is resonant at the problem frequency; check its connection against the transposition table | Reverse the connection of the incorrectly wired element; re-verify against transposition table |
| No directional pattern — antenna appears omnidirectional on all bands | Multiple transposition errors — alternating phase is not established | Systematically verify every element connection from front to rear against transposition table | Correct all transposition errors; re-verify with on-air rotation test |
| SWR uniformly 2.5–4:1 across all bands | Balun ratio incorrect for actual LPDA input impedance | Measure impedance directly at boom tube ends with NanoVNA across all bands | Select correct balun ratio based on measured impedance; typical choices: 2:1 or 4:1 |
| Coverage gap between two adjacent bands | Too few elements — frequency gap between adjacent element resonances exceeds 1/τ coverage | Calculate resonant frequency of each element; verify no gap larger than 1/τ = 1/0.90 = 1.11 (11%) between adjacent elements | Add an additional element between the two bounding the gap; recalculate required length and spacing |
| Front-to-back poor on all bands (less than 10 dB) | Termination resistor missing or incorrect value | Check rear termination — measure resistance between boom tube ends at rear | Install or replace termination resistor with 200–300 Ω non-inductive type |
Is an LPDA a good choice for a first beam antenna?
No — the LPDA is the most complex antenna in this series and is not recommended as a first beam project. The transposed connection pattern, the two-tube boom construction, and the balun selection all require careful attention to detail and a working understanding of what the antenna does electrically. Operators new to beam antenna building should start with a 2-element or 3-element Yagi — simpler, cheaper, faster to build, and easier to diagnose if something is wrong. After successfully completing a Yagi build and gaining confidence with beam antenna concepts, the LPDA becomes a natural next project for operators who want multi-band coverage from a single antenna.
Does the LPDA cover 40m as well as 20m–10m?
Not with the 8-element design in this guide. Adding 40m coverage requires adding two or three more elements at the rear of the boom — elements of 33–40 feet per half, on a boom that extends another 8–10 feet beyond the rear of the current design. A 40m-capable LPDA becomes a very large antenna (35+ foot boom, elements up to 40 feet long) that requires full tower infrastructure. Most LPDA designs optimized for 40m through 10m are commercial products rather than homebrew antennas because the mechanical demands of the larger structure exceed what most homebrew builders want to tackle. For 40m coverage alongside the LPDA, most operators add a separate 40m dipole or vertical rather than extending the LPDA to cover it.
How does the LPDA compare to a trapped tri-band Yagi?
The LPDA and the commercial trapped tri-band Yagi (covering 20m, 15m, and 10m) occupy similar roles in the antenna landscape but have fundamentally different operating characteristics. The trapped Yagi covers only three bands (vs five for the LPDA) but achieves higher per-band gain (7–8 dBd vs 6–7 dBd for the LPDA) because each element is optimized for its specific band. The LPDA covers all bands in its range continuously — including 17m and 12m WARC bands that trapped Yagis typically miss. The LPDA has no moving parts or traps to fail, while trapped Yagis are susceptible to trap failure that can detune the antenna. For WARC band operation (17m and 12m), the LPDA is clearly superior. For a pure 20m/15m/10m contest antenna, the trapped Yagi's higher gain is preferable.
What height is needed for an LPDA to be effective?
The same height rules as any HF beam apply — more height is always better, and the practical minimum for DX operation on the lowest covered band (20m) is 30–35 feet. At 30 feet the 20m radiation lobe is at approximately 35° elevation — useful for medium-distance DX. At 50 feet the lobe drops to 20° — genuinely good for worldwide DX on 20m. On 10m, the antenna's smallest effective wavelength, even 25 feet represents approximately λ/2 height and produces an excellent low-angle DX radiation pattern. Many LPDA operators find that the antenna's multi-band coverage is most valuable on the upper HF bands (15m, 12m, 10m) where any modest height provides excellent DX performance, and mount the LPDA accordingly on whatever tower they have available.
Can I modify the LPDA design to change the frequency coverage?
Yes — the LPDA design formulas allow the frequency coverage to be adjusted by changing the number of elements and the length of the longest and shortest elements. To include a lower band, add longer elements at the rear of the boom. To include a higher band, add shorter elements at the front. The τ and σ parameters remain constant — only the extreme element lengths change. The boom length scales accordingly. The element spacing table in this guide was calculated for τ=0.90, σ=0.12 — the same formulas apply for any frequency range with these parameters. Online LPDA calculators (EZNEC, 4nec2, or dedicated LPDA design tools) can generate a complete element table for any combination of frequency range, τ, and σ values in seconds.
What happens if I accidentally reverse all the element connections?
Reversing all the element connections (connecting every element in the opposite orientation from what was planned) does not damage the antenna — it simply reverses the radiation direction. Instead of radiating toward the short-element end (the front), the antenna would radiate toward the long-element end (the rear). The pattern performance would be similar — approximately the same gain and front-to-back, just pointing the opposite direction. The fix is simple: either re-wire all the elements (a significant effort) or rotate the entire antenna 180° on the mast (much simpler, if mechanically possible). Many LPDA builders discover a direction reversal only during the initial on-air rotation test when the beam peaks in an unexpected direction — and simply re-orient the mast mount to compensate.