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FAQ (Frequently Asked Questions)

The Chain Home Low radar system

Type 11 (CHL/GCI)

Radar Type 11 (photo - "Watching the Skies")The Type 11 radar CHL/GCI was a 50 cm decimetric radar working in the 500 - 600 Mhz band that came into operation with the R.A.F. in 1942.

In an effort to avoid enemy jamming the Type 11 was designed to operate in the same band as German radar equipment. The performance of this radar was somewhat disappointing though, due mainly to a large gap in the vertical cover that could not be removed by tilting the aerial. Some improvement in the performance was gained by digging out the ground on which the trailer stood in order to lower the aerial!

Early models featured twin truncated paraboloid aerials with a 4.5o beam width. The transmitter consisted of a pulse amplifier chain and had a coherent output. The Mk VII version produced a peak power of 50 Kw, with a pulse length of 4 us and a pulse repetition frequency of 500 pps. Performance was limited to around 60 miles on a medium bomber sized target.

Typical performance:


500-600 MHz band

Tx peak power

50 Kw

Tx pulse length

4 us


500 pps


(Source - "Watching the Skies")






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Updated 31/10/00

Constructed by Dick Barrett
ęCopyright 2000 - 2005 Dick Barrett
The right of Dick Barrett to be identified as author of this work has been asserted by him in accordance with the Copyright, Designs and Patents Act 1988.