Just watch the power dissipation. You wouldn't
want to risk melting the coax. There's next to no
cooling - conduction, convection or radiant from a
sealed paint can.
My rule of thumb - figure what the attenuator is
going to dissipate - let's say that you are going
to take a 20 watt transmitter and knock it down
2 watts. That's 18 w of dissipation. Double the
number to account for some safety factor (makes
36w), then round the number up to the next larger
light bulb (60w in this case).
Mount a ceramic socket to the underside of the lid
using a screw into a hole you can re-use later as
one of the coax connector mounting holes.
Use a coax connector center hole for the18ga
wires to the light bulb socket then stuff some steel
wool into the hole to plug it. Use a variac to
set the bulb to dissipate 36w. Run it overnight.
Once it stabilizes can you place the palm of your hand
flat onto the side of the can and count to 20? If not, it's
too hot and you need to go to a larger can (with more
surface area). Maybe a 2.5 gallon can (rare, but they
exist), or a 5 gallon can.
A coat of flat black paint will help radiate the heat.
Once your attenuator configuration passes the palm
test, remove the lamp socket and the wiring, then
mount the coax connectors and the coax.
Mike
At 06:31 PM 05/04/09, you wrote:
Thanks Mike. I'll file that trick away for future use!
Mike
WM4B
----------
From: Repeater-Builder@yahoogroups.com
[mailto:repeater-buil...@yahoogroups.com] On Behalf Of Mike Morris WA6ILQ
Sent: Monday, May 04, 2009 7:10 PM
To: Repeater-Builder@yahoogroups.com
Subject: RE: [Repeater-Builder] Re: desense question
At 02:22 PM 05/04/09, you wrote:
Mike,
I assume the purpose of the paint can is to act as a Faraday cage?
Yep. Cheap coax is lossy and leaky.
Is it attached to the common-point ground system or left freestanding?
If I remember the situation (it's been over 15 years since I shot the photo)
it was freestanding (but the DC continuity went from the shield of coax #1
through the connector to the paint can lid to the connector #2). Of course
it also went through the coax braid.
The can was just in the coax line from the exciter to the PA deck. Nothing
fancy, just two superflex jumpers and the attenuator can.
73,
Mike
WM4B
From: Repeater-Builder@yahoogroups.com [
mailto:repeater-buil...@yahoogroups.com] On Behalf Of Mike Morris WA6ILQ
Sent: Monday, May 04, 2009 5:19 PM
To: Repeater-Builder@yahoogroups.com
Subject: Re: [Repeater-Builder] Re: desense question
At 06:07 AM 05/04/09, you wrote:
> >> The amp is a UHF PA off a mobile rig, and I needed about
> >> 50 feet of RG58U to attenuate the signal from the repeater
> >> into the amp module.
> >
> > Not good, probably better to bypass (not use) the 15 watt
> > amplifier and drive the external amp direct from the exciter.
>
> Not a thing wrong with using a length of coax as an attenuator if it's
>done right. Have used this technique often when a little attenuation is
>needed.
>
>Al, K9SI
Somewhere I've got a couple of photos of attenuators made from a paint
can with coil of coax inside it. A pair of hooded SO239s on the lid and
you're done. One is of a 5 gallon paint can, the other of a 1 gallon.
Mike
