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?

"n4aeq" ) writes:
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?

But if you're just checking transmission lines, wouldn't it make sense
to get by with a small dummy load, and just reduce power into the
transmission lines?

Michael VE2BVW

Solder a three watt 50 ohm non-inductive resistor directly into a
PL-259 and there you are. Use a double-female barrel adaptor as a
gender changer when needed. That will stand five watts momentarily, or
better yet, get an SWR analyzer such as the MFJ units. They are much
handier than an SWR bridge anyway. I wouldn't be without one.

73, Bill W6WRT


------------ ORIGINAL MESSAGE ------------

On 10 Nov 2006 10:50:48 -0800, "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?

On Fri, 10 Nov 2006 10:50:48 -0800, 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?

There is no formula for calculating how much you
can exceed the maximum ratings of a component.

I have found that most resistors can withstand several
times their maximum rating for brief periods. A lot
more if you submerge them in oil.

You don't want to do any high power tests while you
are up the tower or on the roof. High power = high
voltage = a long fall if you get a shock!

73, Ed. EI9GQ.

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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.

No one has ever gotten a shock from high power RF. Burns, yes.

Bill, W6WRT


------------ ORIGINAL MESSAGE ------------

On Sat, 11 Nov 2006 14:46:53 GMT, "Eamon Skelton"
wrote:

You don't want to do any high power tests while you
are up the tower or on the roof. High power = high
voltage = a long fall if you get a shock!

Lemme tell you, the last (and first) time I got a burn from RF it certainly
did shock me. Would not have felt comfortable getting the burn on top of a
tower.
Joe vk3bki

"Bill Turner" wrote in message
...
No one has ever gotten a shock from high power RF. Burns, yes.

Bill, W6WRT


------------ ORIGINAL MESSAGE ------------

On Sat, 11 Nov 2006 14:46:53 GMT, "Eamon Skelton"
wrote:

You don't want to do any high power tests while you
are up the tower or on the roof. High power = high
voltage = a long fall if you get a shock!