[TowerTalk] trap antenna performance

[Joe Subich, W4TV]

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