That means UK amateurs are explicitly permitted to allow for feedline
loss (very handy at UHF and higher) and abnormal transients aren't
counted. Given our 400W PEP output limit, we need all the concessions we
can get.

73 from Ian G3SEK
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It's also very handy on 160 meters.

For example, if the antenna is just a 15-feet length of wire and 400 watts
PEP are fed into it over 115 feet of 600-ohm single-wire transmission line,
who needs concessions?

A 15-feet length of wire, all by itself, is quite efficient on 160 meters.
Nearly all of the 400 watts fed into it will be radiated and the licensing
regulations are not violated.

Furthermore, because a transmission line of that particular length
accurately does the impedance matching, a tuner becomes redundent.
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Reg.