How does cable loss between the antenna and LNA affect system noise temperature?
Feed Line Loss in Noise-Critical Systems
In any receiving system, the cable or waveguide connecting the antenna to the LNA introduces loss that degrades the system noise temperature. The magnitude of this degradation depends on the antenna noise temperature: the colder the antenna, the more damaging the cable loss becomes.
| Parameter | Superheterodyne | Direct Conversion | Digital IF |
|---|---|---|---|
| Image Rejection | 60-90 dB (filter) | 30-50 dB (mismatch) | N/A (digital) |
| DC Offset | No issue | Major issue | No issue |
| LO Leakage | Low | High | Low |
| Integration | Difficult | Easy (single chip) | Moderate |
| Dynamic Range | 80-120 dB | 60-90 dB | 70-100 dB |
- Performance verification: confirm specifications against the application requirements before finalizing the design
- Environmental factors: temperature range, humidity, and vibration affect long-term reliability and parameter drift
- Cost vs. performance: evaluate whether the application demands premium components or standard commercial grades
Frequently Asked Questions
Does waveguide have less impact than coax?
Waveguide typically has lower loss per unit length than coaxial cable at the same frequency, so yes. At Ku-band and above, waveguide feed runs of a few inches add only 0.05 to 0.1 dB, whereas a similar length of coax might add 0.3 to 0.5 dB.
Can I compensate for cable loss with more LNA gain?
No. More LNA gain improves the noise contribution of stages after the LNA, but it cannot recover the SNR lost in the cable before the LNA. The cable loss is before the first gain stage and adds to the system noise figure directly.
Is this why satellite dishes have LNBs at the feed?
Exactly. The LNB (low-noise block downconverter) is mounted directly at the focal point of the dish to eliminate cable loss before the LNA. The IF output of the LNB can then travel through long coax cables to the indoor receiver with minimal impact because the signal has already been amplified.