TECHNICAL NOTE / COAX / CONNECTORS
As frequency increases, coaxial feed loss usually becomes more important. The actual loss depends on cable type, length, frequency, connector quality and condition. A nominal low-loss cable cannot compensate for poor terminations or moisture ingress.
Loss expressed in power
The surviving power fraction is 10−L/10, where L is positive insertion loss in dB. A 3 dB loss leaves about half the input power at the antenna. A 1 dB loss leaves about 79%. The receive signal is attenuated by the same passive feed line.
| Cable/system insertion loss | Approximate power delivered | 100 W example |
|---|---|---|
| 0.5 dB | 89% | 89 W |
| 1 dB | 79% | 79 W |
| 2 dB | 63% | 63 W |
| 3 dB | 50% | 50 W |
| 6 dB | 25% | 25 W |
Why preamp location changes the picture
A low-noise preamplifier at the antenna can reduce the effect of downstream line loss on receiver noise figure. The loss between the antenna and preamp remains damaging. The receive system must also survive accidental TX and nearby strong signals.
A defensible cable comparison
- Specify band, total length, nominal cable model and exact connectors.
- Include rotor loops, inside jumpers, arrestors and relays in the system budget.
- Use manufacturer data at the relevant frequency, not HF loss estimates extrapolated to 1296 MHz.
- Verify VSWR and insertion loss with an appropriate instrument and calibration.
Especially on 902 and 1296 MHz, treat a few short jumpers and adapters as real RF components rather than bookkeeping details.