Build a K9AY Receive Loop Antenna
The K9AY loop — designed by Gary Breed K9AY and published in QST in 1997 — is the most practical small-footprint directional receive antenna for 160m and 80m ever described in amateur literature. Where a Beverage antenna requires 500 feet of wire in a straight line, the K9AY loop fits in a 30-foot diameter circle yet achieves comparable front-to-back ratios through a different operating principle: a terminated magnetic loop that combines electric and magnetic field sensing to produce a cardioid receive pattern. A single K9AY support post with four loops oriented in different directions — switchable from the shack — gives four-direction directional receive coverage from a single installation. This guide covers the theory, loop dimensions, termination, feedpoint transformer, preamplifier, direction switching, and complete installation.
The Terminated Loop — Combining Two Field Components
The K9AY loop achieves its directional pattern through a fundamentally different mechanism than the Beverage. Where the Beverage uses the travelling-wave principle along a long wire close to the ground, the K9AY combines the responses of a small magnetic loop antenna and a short vertical antenna in a single structure:
Loop Geometry — The Standard K9AY Shape
The K9AY loop uses a specific triangular shape — not a circle or square. The original K9AY design uses a right-angle triangle formed by the support pole and two wire legs that spread outward and downward from the top of the pole, rejoining at ground level where the terminating resistor connects to a ground stake:
Terminating Resistor — The Critical Variable
The termination resistor is the most critical variable in the K9AY design — far more so than for the Beverage. The correct resistor value is not a fixed number; it depends on local ground conductivity and must be found empirically for each installation:
K9AY vs Beverage — When to Choose Which
The K9AY and Beverage are both excellent low-band receive antennas — the choice depends primarily on available space and the number of directions needed:
- Space requirement: K9AY fits in a 30-ft circle; Beverage needs 500+ ft in a straight line. For a typical suburban lot, the K9AY is often the only practical choice.
- Directional coverage: four K9AY loops on one pole give four directions from one installation. Four Beverages require 2000 ft of wire in four separate directions across open land.
- Performance: a long Beverage (500+ ft) typically outperforms the K9AY on 160m by 3–6 dB in signal-to-noise ratio. For most operators this difference is acceptable given the K9AY's space advantage. On 80m the performance difference is smaller.
- Installation effort: the K9AY requires careful termination optimisation and a feedpoint transformer; the Beverage requires a long wire run with many stakes but simpler electronics.
- Ground sensitivity: the K9AY's performance is more sensitive to local ground conditions than the Beverage. In very high-conductivity ground (salt marsh, wet clay), the K9AY can underperform expectations; in average suburban ground it works very well.
- Best choice for most amateurs: if you have suburban lot space (under 100 ft in any direction) and want directional receive on 160m and 80m, the K9AY is the right antenna. If you have rural acreage with 500+ ft of open land in multiple directions, add Beverages to complement or replace the K9AY.
| Pole height | Each wire leg length | Ground footprint | Total wire per loop | Best bands | Notes |
|---|---|---|---|---|---|
| 20 ft (6.1 m) | ~28 ft per leg | ~20 ft radius | ~60 ft | 80m, 40m | Minimum practical height; limited 160m performance |
| 25 ft (7.6 m) | ~35 ft per leg | ~25 ft radius | ~75 ft | 160m, 80m, 40m | Good all-round performance; fits most suburban lots |
| 30 ft (9.1 m) | ~42 ft per leg | ~30 ft radius | ~90 ft | 160m, 80m, 40m | Recommended height — best balance of performance and practicality |
| 35 ft (10.7 m) | ~49 ft per leg | ~35 ft radius | ~105 ft | 160m, 80m, 40m | Excellent — noticeably better 160m than 25 ft version |
Materials for a four-loop K9AY system covering NE, NW, SE, SW directions on one 30-ft support pole
Building the Four-Direction K9AY Loop System
This guide builds a four-loop K9AY system on a single 30-ft pole, covering NE, NW, SE, and SW directions — the four most useful compass quadrants for DX operation from North America. The switching relay at the pole base selects the active loop from the shack. Build the electronics first, test on the bench, then install the pole and wire.
Build the Feedpoint Transformer
The K9AY feedpoint transformer matches the loop's output impedance (approximately 400–900 Ω depending on termination and ground) to the 50 Ω coax feeding the preamplifier. A 9:1 unun on a Mix 43 ferrite toroid is the standard choice, identical to the Beverage feedpoint transformer:
Build one transformer per loop and mount all four in the weatherproof enclosure at the pole base. Label each transformer with its loop direction (NE, NW, SE, SW) to avoid wiring confusion during installation.
Build the Direction Switching Relay
The direction switching relay selects which of the four K9AY loops is connected to the preamplifier input. From the shack, a simple DC voltage switch controls the relay state. Several architectures are used; the simplest for homebrew construction uses individual DPDT relays for each loop:
Erect the Support Pole and Install Ground System
The support pole is the physical centre of the K9AY system — all four loops originate from its top. A 30-ft fibreglass push-up mast is the ideal choice: non-conductive, light enough to erect single-handed, and tall enough for good 160m performance. A wooden telephone pole, aluminium pipe, or tree trunk also work — the pole material matters less than the loop wire geometry.
Anchor the pole base securely — the four loop wires will exert outward tension in all directions, which loads the pole laterally. For a push-up mast, a concrete base sleeve or a heavy wooden anchor post is appropriate. Install two or three ground rods at the pole base, connected together with heavy copper wire and to the ground terminal of the switching enclosure. This feedpoint ground system is critical — poor feedpoint ground degrades all four loops equally.
Install the Loop Wires
Each K9AY loop consists of two wire legs that originate at the top of the pole (the feedpoint), spread outward at approximately 45° angles in opposite directions, and terminate at ground stakes on opposite sides of the pole. The two legs of each loop form a triangle with the pole as the vertical side:
Install the Preamplifier and Run Coax to Shack
The preamplifier mounts in the weatherproof enclosure at the pole base, after the direction switching relay. A single preamp serves all four loops — the relay selects which loop's transformer output feeds the preamp input. The preamp output connects to the single coax run to the shack:
Run the coax from the preamp output through weatherproof conduit or buried direct-burial coax to the shack entry point. Run the four control wires (for loop selection relay) in a separate small-gauge cable alongside the coax. Label all connections at both ends before burying or securing permanently.
Optimise Termination Resistors for Each Loop
With the system connected to the receiver, optimise the termination resistor value for each loop individually. This is the most time-consuming step but the most important — a poorly optimised termination produces a mediocre antenna regardless of how well everything else is built:
Verify Performance and Install TX Protection
With all four loops optimised, verify the system's front-to-back ratio and compare performance to the transmit antenna. A well-optimised K9AY should show 20+ dB front-to-back ratio on 160m and 80m and a noticeably lower noise floor than the transmit antenna on most HF bands from 1.8–10 MHz.
Install the TX/RX protection switching to prevent transmit energy from reaching the K9AY preamp during transmission. This is identical to the Beverage protection described in the Beverage build guide — a changeover relay triggered by PTT or RF sense disconnects the K9AY coax from the receiver input and connects it to a 50 Ω dummy load (or simply grounds the coax centre conductor) during transmit.
Two-Loop K9AY — Bidirectional Coverage
A two-loop K9AY installation using loops at 90° to each other (one NE/SW, one NW/SE) gives four-direction coverage with only two physical loops — each loop is reversible by swapping which end is the feedpoint and which is the termination:
- Reversing a K9AY loop: connect the feedpoint transformer and termination resistor at opposite ends of the loop via a relay. When relay is in position A: feedpoint at north stake, termination at south stake — loop receives from north. When relay in position B: feedpoint at south stake, termination at north stake — loop receives from south.
- Two reversible loops = four directions: loop 1 (NE/SW) plus loop 2 (NW/SE) gives NE, SW, NW, and SE — all four major DX directions from one pole with two loops and two reversing relays.
- Switching complexity: reversible loops require more careful relay wiring than fixed-direction loops. Each loop needs two DPDT relays — one for feedpoint/termination reversal and one for loop selection. Commercial reversing relay boxes (Array Solutions, DX Engineering) simplify this.
- Footprint: two reversible loops on one pole require only two pairs of ground stakes at 90° — slightly smaller footprint than four fixed-direction loops.
Flag and Pennant Loops — K9AY Relatives
The Flag and Pennant antennas are close relatives of the K9AY — all three are small terminated loops producing cardioid receive patterns. They differ mainly in shape and ground connection:
- Flag antenna: a rectangular loop (approximately 8 ft × 14 ft) oriented vertically, with the terminating resistor at one bottom corner and the feedpoint at the other bottom corner. Does not require a ground connection — the termination is within the loop itself. Excellent for elevated mounting (on a mast or building rooftop) where a ground connection is impractical.
- Pennant antenna: a right-triangle loop, similar to the K9AY in shape but without using the support pole as a vertical element. Smaller than the flag; slightly less sensitive but easier to support.
- EWE antenna: another terminated loop variant using a specific shape optimised for low-angle receive performance on 160m. Requires a good ground connection at both ends. Performance similar to K9AY.
- Choice between types: the K9AY is generally preferred for ground-level installation because it uses the ground return effectively and achieves better performance with a given support pole height than the flag or pennant at the same height. For elevated or rooftop mounting where a ground connection is unavailable, the Flag antenna is the better choice.
| Symptom | Most likely cause | Diagnosis | Fix |
|---|---|---|---|
| No signal on any loop — system appears dead | Failed preamplifier, open coax, or no power to relay/preamp | Check 12V supply at pole base enclosure; verify coax continuity; test preamp on bench | Restore power supply; repair coax fault; replace failed preamp |
| Poor front-to-back ratio on all loops — under 10 dB | Poor feedpoint ground system; or termination resistor values very wrong for local ground | Measure ground resistance at pole base — should be under 5 Ω; re-optimise termination values | Add more ground rods at pole base; drive deeper; re-optimise each loop's termination independently |
| One loop has no F/B but others work | Termination resistor open or shorted on that loop; or wire break in that loop | Check resistance from loop termination point to ground — should read termination value (~470 Ω) | Replace failed termination resistor; repair wire break; check ground stake connection at that loop's termination |
| F/B ratio degrades noticeably after rain | Water in termination enclosure; or saturated ground changing optimal termination value | Inspect termination enclosure seal; measure F/B in dry vs wet conditions | Re-seal termination enclosure with silicone; seasonal re-optimisation of termination values may be needed as ground moisture changes |
| Switching between directions has no effect — all directions sound the same | Relay switching not working; all loops shorted together or only one loop connected | Verify DC control voltage reaches relay box; measure resistance between adjacent loop terminals at relay box | Repair control wiring; replace faulty relay; verify relay wiring connects loop outputs to preamp input as intended |
| Preamp burned out after TX session | TX/RX changeover relay failed to disconnect K9AY; or nearby TX antenna inducing high voltage | Test changeover relay operation with voltmeter before connecting preamp | Replace preamp; repair TX/RX changeover relay; add gas discharge tube across preamp input for additional protection |
| Performance much worse on 160m than 80m | Pole too short — loop electrically small on 160m; or termination optimised for 80m not 160m | Compare F/B on 160m vs 80m using known signals in front and back directions | Increase pole height if possible; re-optimise termination at 160m operating frequency rather than 80m |
How does the K9AY compare to a Beverage on 160m?
A well-optimised K9AY on a 30-ft pole typically produces 3–6 dB worse signal-to-noise ratio than a 500-ft Beverage on 160m. In practice this means the Beverage pulls out marginally weaker signals during a contest pileup. For most operators, this difference is acceptable given that the K9AY fits in a suburban backyard while the Beverage requires a country estate. On 80m the performance gap narrows — a K9AY on a 30-ft pole is very competitive with a 300-ft Beverage on 80m. Many low-band operators install both — a K9AY for general receive use and a pair of Beverages in the two most important DX directions for contest weekends.
Does the K9AY loop need to be exactly the right size?
No — the K9AY's dimensions are not as critical as a resonant antenna's. The loop perimeter can vary ±20% from the nominal dimensions without significantly degrading performance, because the terminating resistor controls the pattern rather than resonance. What matters much more than exact dimensions is the termination resistor value relative to local ground conductivity, and the quality of the feedpoint and termination ground systems. A slightly undersized or oversized K9AY loop with an optimised termination will outperform a correctly sized loop with a poorly optimised termination every time.
Can I mount the K9AY on my house roof?
Not easily — the K9AY requires a ground connection at the termination point that must be separate from the feedpoint ground, and both grounds connect to the earth through ground rods. On a rooftop there is no practical way to make this earth connection. The Flag antenna is the correct choice for rooftop or elevated mounting — it is a similar terminated loop design that does not require an earth ground connection because the termination is within the loop structure. If you are restricted to a rooftop installation, a Flag or Pennant antenna on a short mast is more practical than a K9AY adaptation.
Why does the K9AY have a dead null rather than just reduced signal from the back?
When the termination resistor is at exactly the correct value for your ground conditions, the electric field component (from the vertical wire/ground path) and the magnetic field component (from the loop) cancel almost perfectly in the back direction — producing a very deep null of 25–30 dB or more. When the resistor is even slightly off, the cancellation is incomplete and the null shallows to 10–15 dB. This is why careful termination optimisation can make the difference between a good K9AY and an exceptional one. Seasonal re-optimisation as ground moisture changes (spring thaw, summer dry season) maintains the deep null year-round.
What wire gauge should I use for the K9AY loops?
Any wire from #28 AWG through #14 AWG works — this is a receive-only antenna and there is no RF power to consider. Thinner wire (#22–#18 AWG) is easier to handle, lighter, and exerts less tension on the support pole. Heavier wire (#14 AWG) is more mechanically robust for a permanent installation in areas with ice loading or heavy wind. Many builders use #22 AWG magnet wire for the loops because it is inexpensive, available in large spools, and lightweight. Insulated wire is preferred over bare wire — insulated wire can pass through foliage or touch fence posts without creating an unintended ground connection that disturbs the loop's pattern.
How often do I need to re-optimise the termination?
In most installations, a one-time optimisation at installation and a seasonal re-check twice a year (spring and autumn, when ground moisture changes most dramatically) is sufficient. In regions with extreme seasonal variation between wet winters and dry summers, the optimum termination value can shift by 150–200 Ω between seasons — enough to degrade front-to-back ratio from 25 dB to 12 dB if not re-optimised. The simplest approach is to install a weatherproof variable resistor (potentiometer in a sealed enclosure) at each termination point permanently, adjust it to the seasonal optimum by monitoring a back-direction signal, and note the setting for future reference.