L-band for search radars? I'm making my way through the Linesman-related section's of Gough's Watching the Skies. Linesman was developed to counter the carcinotron jammer.
The main solution was the Type 85, which had 12 frequencies switching each pulse at random.
However, they also deployed a second radar, the Type 84. This was a normal single-frequency pulse radar, but in the L-band rather than the S-band. This was to be used as an early-warning system.
I'm trying to understand why... for one, as a single-frequency system it would be highly vulnerable to jamming, and for another, the longer wavelength would mean lower resolution or much larger antennas.
Gough mentions a single possible reason for this, in passing, that L-band was less susceptible to clutter. I'm not sure why this would be.
Does anyone have any ideas on why the L-band would be better for early warning?
 A: Ahh, found the answer in a totally unrelated document.
The L-band has less interaction with rain, hail, snow and clouds, and therefore reduces clutter in the long-range role.
So it was a physics reason.
A: S-Band is used for weather radar (signal for NWS, clutter for DOD)--that's in long range application. It's used for final approach in air traffic track surveillance. L-band is used for air-route surveillance (ARSR) and defense application (e.g. AN/FPS-117) in the long range (200 nautical miles).
Range resolution is determined by bandwidth, which is typical around 1 MHz, and that can easily be achieved in L or S band.
Regarding frequency hopping, not having it is a weakness (in military applications).
Aside: L-Band is currently allocated for Earth observations, surveillance, and geo-navigation (GPS), so if you want to do Earth science from space, you have to worry about RFI from the other applications.
A: If the radar’s mission is to search a given solid angle in a given period of time, then there is no real advantage (other than accuracy) to using a higher frequency, because a narrower beam will not be able to dwell as long on any given position.  If the design features a phased array antenna, it is less costly to build it from lower-frequency modules, since the number of modules required per unit area scales as the square of frequency.  L-band technology offers a nice compromise between cost and accuracy.  
