Also known as: front-to-back ratio, F/B ratio
Front-to-back ratio (F/B) is the ratio, in decibels, between the power a directional antenna radiates in its forward main-lobe direction and the power it radiates directly to the rear.1 A high F/B means the antenna is nearly deaf behind it — it favours signals coming from where it points and rejects those from the opposite bearing. It is a key figure of merit for a Yagi or any beam antenna, read straight off the radiation pattern as the difference between the main lobe and the back lobe.
How it works
A directional antenna gets its shape by adding several elements whose radiated fields reinforce forward and largely cancel to the rear. In a Yagi, a reflector element sits behind the driven element; a director (or several) sits in front. Their spacings and lengths are tuned so that re-radiated currents arrive in phase along the boom axis but out of phase toward the back, suppressing the back lobe. The residual rearward radiation, compared with the forward peak, is the front-to-back ratio.
Two subtleties matter:
- F/B is not the same as gain. Gain measures how much the forward lobe exceeds an isotropic reference; F/B measures forward-versus-rearward for the same antenna. An antenna can have high gain but mediocre F/B, or vice versa, and designs are often optimized for one at the expense of the other.
- “Back” can mean a range, not a point. Some datasheets quote the ratio to the single 180° back direction; others quote the worst case over a rear angular window (sometimes called the front-to-rear ratio), which is the more honest number because a null exactly at 180° can flatter the figure.
Typical values run from about 10 dB for a small two-element beam to 20–30 dB for a well-designed multi-element Yagi. F/B is also sharply frequency-dependent: an antenna optimized for maximum F/B usually shows a deep, narrow rear null that drifts off the design frequency, so a broadband log-periodic trades some F/B for its wide bandwidth.
Relevance to SDR
Front-to-back ratio is what lets a directional scanner antenna reject interference from behind. If a strong local transmitter or a co-channel simulcast site sits roughly opposite the distant system you want, aiming the beam forward puts the interferer in the back lobe, improving the carrier-to-interference ratio by the F/B figure — often the difference between a decode and a stream of errors. For direction finding, a clean rear null also resolves the front/back ambiguity that a symmetric pattern would leave. GopherTrunk has no awareness of the antenna’s F/B, but a high-F/B beam raises the effective signal-to-interference ratio at the SDR input, which is exactly what the demodulator and error-correction stages need to lock a marginal control channel.
Sources
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Front-to-back ratio — Wikipedia, for the decibel definition and its role in directional-antenna evaluation. ↩