Estimate the physical distance two antennas need to be spaced apart for full-duplex repeater operation, based on vertical or horizontal placement.
| Time (UTC) | TX (MHz) | RX (MHz) | Target (dB) | Vertical (ft) | Horizontal (ft) |
|---|---|---|---|---|---|
| 2026-08-25 02:19:29 | 146.9400 | 146.3400 | 80.00 | 133.8 | 5,327.9 |
| 2026-08-24 16:44:14 | 147.2250 | 147.8250 | 80.00 | 133.0 | 5,295.9 |
| 2026-08-24 16:42:23 | 442.5250 | 447.5250 | 80.00 | 44.1 | 1,755.6 |
| 2026-08-24 16:38:05 | 442.5250 | 447.5250 | 80.00 | 44.1 | 1,755.6 |
| 2026-08-24 16:37:22 | 442.5250 | 447.5250 | 80.00 | 44.1 | 1,755.6 |
| 2026-08-24 16:31:08 | 442.5250 | 447.5250 | 80.00 | 44.1 | 1,755.6 |
| 2026-08-24 16:30:33 | 146.9400 | 146.3400 | 80.00 | 133.8 | 5,327.9 |
| 2026-08-24 16:25:04 | 442.5250 | 447.5250 | 80.00 | 44.1 | 1,755.6 |
| 2026-08-24 16:24:47 | 442.5250 | 447.5250 | 90.00 | 78.4 | 5,551.6 |
| 2026-08-24 16:24:39 | 442.5250 | 447.5250 | 120.00 | 441.0 | 175,558.3 |
| 2026-08-24 16:23:39 | 146.9400 | 146.3400 | 80.00 | 133.8 | 5,327.9 |
| 2026-08-24 15:37:00 | 146.9400 | 146.3400 | 80.00 | 133.8 | 5,327.9 |
| 2026-08-24 15:09:35 | 146.9400 | 146.3400 | 136.99 | 3,558.7 | 7,516,891.9 |
| 2026-08-24 15:08:57 | 146.9400 | 146.3400 | 80.00 | 133.8 | 5,327.9 |
| 2026-08-24 15:00:59 | 146.9400 | 146.3400 | 80.00 | 133.8 | 5,327.9 |
To run a repeater full-duplex (transmitting and receiving at the same time on different frequencies) without relying entirely on heavy cavity filters, you need enough physical separation between your TX and RX antennas to keep your own transmitter from desensitizing your own receiver. Vertical stacking is far more efficient than horizontal spacing for the same physical distance.
Vertical separation (antennas stacked directly above one another):
Horizontal separation (antennas side by side):
Where I is isolation in dB, d is the center-to-center antenna distance, and λ is the wavelength at your operating frequency. These are the standard ITU-R Report M.2244 antenna-separation formulas for low-gain, omnidirectional antennas. This calculator solves each equation for distance, given a target isolation figure. Antenna and system gain (ERP) is not factored in here — that belongs on a separate ERP calculator, since gain affects link budget independently of physical antenna spacing.
Where does the target isolation number come from? If you know your transmitter's power output and your receiver's desense threshold, this calculator can estimate it for you (Required Isolation ≈ TX power in dBm − RX desense threshold in dBm − any existing duplexer/filter isolation). Otherwise, enter a target isolation directly — as repeater-builder.com's notes on isolation point out, there's no single universal number that works for every site: isolation is cumulative, built up from antenna spacing plus duplexer cavities, front-end filtering, and site-specific noise sources. Antenna separation is one piece of the puzzle, not the whole answer.