n4aeq wrote:
I need to build a portable load to carry on the roof or tower to check
transmission lines, I know for
intermitten use a resistor will stand alot more then its rated for so
does anyone know the factor
for intermitten use?

You can connect several resistors in a series-parallel combo to get
higher wattage while keeping the 50 ohm resistance. There is a limit as
a non-resistive complex impedance will grow larger as you add more
resistors due to stray inductance and capacitance. House the resistors
in a small can and fill the can with mineral oil. This will increase
the wattage rating. In fact resistors submerged in oil will handle
10-20 times their wattage rating for short periods. Paint the outside
of the can black to increase radiative cooling.
(Ever see a Heath Cantenna?)

Hint: get 10 120 ohm 2 watt resistors, and connect them in
series-parallel. (IE: 5 sets of 2 series connected resistors in
parallel). This will give you a 48 ohm 20 watt resistor. With the
resistors heat sinked in oil they will handle 200-400 watts for short
periods.