On 2015-04-13 12:38 PM, Jim Brown wrote:
All of this makes sense to me. Also, a trapped is a shortened
antenna, so radiation resistance can be lower relative to loss
components.
And the difference need not all be dissipation. Any antenna with
"gain" concentrates its radiation in the desired direction(s) by
"stealing" it from other directions.
Not necessarily. At 1500 W from the transmitter less 1 - 2 dB of
feedline loss less 1 dB of reflected power means that there is
maybe 800 W going into a trapped antenna. If the total trap loss
is 2 dB (enough to account for the under performance of some of
these trapped triband antennas) that's less than 50 W per trap.
Given the surface area of a large metal cased trap that is rated
for 1500 W, you're not going to see a significant level of heating.
The 2 - 2.5" diameter by 12" long metal cylinder (~100 sq. in) is
an effective heat sink for that relatively small amount of heat -
the heat density is less than 0.5W/sq in. One is not likely to
notice that in free air particularly considering that is the *peak*
loss/dissipation not an average over time which could be a lot less.
73,
... Joe, W4TV
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