Build a Flag Receive Antenna
The Flag is a small rectangular terminated loop — the third design in this site's family of compact low-band receive antennas alongside the EWE and Pennant. Its rectangular shape (rather than the Pennant's triangle or the EWE's open "U") is simply a different practical geometry solving the same problem: a directional pattern that helps you hear weak DX by rejecting noise and interference, with essentially no gain of its own. This guide covers the complete build from loop dimensions through the feedpoint transformer, termination resistor, and TX/RX switching.
A Rectangular Closed Loop
The Flag is a closed rectangular wire loop, typically taller than it is wide, supported by two vertical poles at the corners on one side (or one pole plus a rope-supported far side). Two adjacent corners on one vertical edge serve as the feedpoint and termination points, with the loop closed by the wire running around the remaining three sides.
Feedpoint and Termination on One Vertical Edge
Unlike the Pennant's two bottom corners, the Flag's feedpoint and termination points sit at the top and bottom of the same vertical edge (or occasionally both at the bottom, depending on the specific variant built) — the loop's rectangular geometry changes exactly where these connections land, but the underlying principle (a closed loop with one impedance-matched feed corner and one resistively-terminated corner) is the same family of design as the Pennant.
The Termination Resistor — Found Empirically
As with the EWE and Pennant on this site, the termination resistor value is the most important variable and depends on your specific loop dimensions and local ground conditions — always treat published starting values as an experimentation starting point.
Why This Isn't a Transmit Antenna
Exactly as with the EWE and Pennant, the termination resistor and feedpoint transformer are sized for receive-only signal levels, and the thin wire used isn't rated for transmit power. A TX/RX changeover relay that disconnects the Flag from the receiver during transmit is mandatory, protecting both this antenna and your receiver's front end.
| Band | Loop height | Loop width | Starting termination resistor |
|---|---|---|---|
| 160m | 20-22 ft | 12-14 ft | ~850 Ω starting point |
| 80m | 18-20 ft | 10-12 ft | ~750 Ω starting point |
| 40m | 16-18 ft | 8-10 ft | ~700 Ω starting point |
Flag Antenna Starting-Point Calculator
Materials for a complete Flag build
Building the Flag Receive Antenna
A compact rectangular loop build — most of the effort is in termination resistor optimization, same as this site's other small receive antennas.
Install the Support Poles
Set up one or two support poles at the loop height from the dimensions table, oriented so the antenna's favored direction points toward your target listening direction.
Mark the Rectangle Corners
Mark all four corners of the rectangular loop at the height and width from the dimensions table, using rope supports for any corners without a dedicated pole.
Run the Rectangular Loop Wire
Run a continuous wire around three sides of the rectangle, leaving the near vertical edge open at top and bottom for the feedpoint and termination connections.
Install the Termination Resistor and Ground
At one open end of the near vertical edge, connect a variable resistor (for initial testing) between the wire end and a ground rod driven at that location.
Install the Feedpoint Transformer
At the other open end of the near vertical edge, wind and install the 9:1 unun on the ferrite toroid, connecting the loop wire ends to the transformer's high-impedance side and the coax to the 50Ω side.
Install the TX/RX Relay
Wire the relay so it disconnects the Flag's coax from the receiver whenever your station keys up.
Route Coax and Optimize the Termination Resistor
Run the receive coax to your shack, ideally through a low-noise preamplifier. Using a known station in the "back" direction, adjust the variable termination resistor for the setting that best minimizes back-direction signal while keeping the front strong, then replace it with a fixed resistor of that value.
| Symptom | Most likely cause | Diagnosis | Fix |
|---|---|---|---|
| No directional effect — sounds omnidirectional | Termination resistor missing, wrong value, or poor ground connection | Check the resistor value and ground rod connection | Verify and correct the ground connection; re-optimize the resistor value |
| Very weak signals in all directions | Feedpoint transformer wiring reversed, or normal expected low output | Check transformer winding sense against the reference diagram | Correct transformer wiring if reversed; otherwise add a low-noise preamp, which is normal for this antenna class |
| Front-to-back ratio worse than expected | Termination resistor not optimized for your specific site | Re-run the optimization procedure using a known back-direction station | Adjust the resistor value for your site's ground conditions |
| Loop shape distorted or sagging | Insufficient support height or tension on one or more corners | Sight the loop shape against the intended rectangle | Re-tension or raise the affected support |
| Antenna damaged after operating | Transmitter power reached the Flag, likely a relay fault | Check relay operation and switching timing relative to transmit keying | Repair or replace the relay; verify switching happens before RF is applied |
Can I transmit through this antenna if I keep the power low?
No. This is a receive-only design — the termination resistor, feedpoint transformer, and thin wire are not built to handle transmit power, and the antenna provides no meaningful radiated signal in return. Always switch to a separate transmit antenna before keying up.
How is a Flag different from a Pennant?
Both are closed-loop terminated receive antennas from the same design family — the Pennant uses a triangular loop with feedpoint and termination at the two bottom corners, while the Flag uses a rectangular loop with feed and termination on one vertical edge. Performance is broadly comparable; the choice often comes down to which shape fits your available supports better.
How is a Flag different from an EWE?
The Flag closes the loop into a full rectangle, while the EWE is an open "U" shape between two posts. The closed loop generally gives comparable or slightly better front-to-back performance for a similar footprint.
Why is my received signal so much weaker than my transmit antenna?
Completely normal — the Flag has essentially no gain, often meaningfully negative in dBi terms. Its value is the directional pattern that reduces unwanted noise, not signal strength. Pair it with a low-noise preamp if needed.
Can I build multiple Flags for different directions?
Yes, switching between two or more Flags aimed at different headings from a control box at the operating position is a common approach, the same as with the EWE and Pennant.
Do I need a preamp with this antenna?
Most operators find a low-noise preamp genuinely useful given how little signal this small loop captures compared to a full-size transmit antenna.