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Re: Antenna Performance

Black is cooler than bare aluminum:
------- Forwarded message follows -------

Date sent:         Fri, 26 Apr 1996 09:50:14 -0400 (EDT)

From:                Bob Bruninga <bruninga@nadn.navy.mil>

To:                    aprssig@tapr.org

Copies to:         amsat-bb@amsat.org

Subject:            Aluminum, black and white


While building a GPS unit for mounting on my dashboard and noting the coming 
summer months, I looked up the difference in absorption and emissivity for 
Aluminum, Black paint, and white paint.  Satellite builders are well aware 
of these facts, but many of us landlubbers are not.

ALUMINUM will get 30 TIMES hotter than WHITE paint!   (in a vacuum)

The following table is for a vacuum and accounts for RADIATIVE effects.

It does not account for convective or conductive cooling (air).

                      Absorption  Emissivity  Ratio     Temp C

ALUMINUM          .4             .03         11:1          400

STEEL                  .6             .4             3:2          150

BLACK PAINT      .9             .9             1:1          110

WHITE PAINT      .25           .85           1:3            72

Most people are aware that Black gets hotter than white, but the fact that 
bright, reflective, shinny Aluminum gets 10 times hotter than BLACK is a 
surprise to most people...

So, if it sits in the sun, paint it white!  If you don't believe this, put 
an aluminum baking sheet in the sun.  I baked my first roof mount GPS 
standalone tracker thinking that the upside down baking pan would reflect 
the sun...  WRONG!  Painted it white and it is now as cool as a cucumber.

The difference in Aluminum is the POOR EMISSIVITY at infrared.  It can't 
radiate the heat away...

------- End of forwarded message -------

Bruce Paige, KK5DO

AMSAT VP-User Services & Awards Manager

ARRL Awards Manager (WAS, 5BWAS, VUCC), VE

Houston AMSAT Net - Wed 0100z on W0KIE AMC-7, T5, 7.5Mhz

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Sent via amsat-bb@amsat.org. Opinions expressed are those of the author.
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