January 17, 1964 Electronics
[Table of Contents]
Wax nostalgic about and learn from the history of early electronics.
See articles from Electronics,
published 1930 - 1988. All copyrights hereby acknowledged.
Here is a chart you don't see
every day - "Temperature Rise in Rigid Waveguide." The company, Engineering Antenna Systems,
of Manchester, New Hampshire, that published the chart in a 1965 edition of Engineering
magazine, does not exist anymore. They were probably bought by someone else, but I could
not even find an honorable mention of them in a Google search. Given the very low attenuation
of properly sized and installed waveguide, it is hard to imagine a temperature rise of
500°F; however, when megawatts are pumped into it even a couple tenths of a decibel
of attenuation per 100 feet results in a lot of power loss. Noted is how attenuation
- and therefore temperature rise - is greater for frequencies at the lower end of the
waveguide's operational range. Temperature rise numbers are for natural convection in
free air (no forced air, heat sink, or liquid cooling).
Temperature Rise in Rigid Waveguide
By T. J. Vaughan
Manager of Engineering Antenna Systems, Inc.
Manchester, N. H.
Designers of waveguide components must be concerned with the temperature increase
above ambient due to the average power.
Knowing the average power in watts, the temperature rise above ambient of the waveguide
can be quickly determined from the graph. The heat is generated because of the power
lost due to the attenuation of the guide. The calculations are based on a 2:1 aspect
ratio and material emissivity of 0.5.
By plotting, attenuation of that waveguide is the bracketed figure on the right. Because
the attenuation varies for different materials available, the most common material used
for the respective waveguide size have been selected from: WR 2300 to WR 650 Aluminum
6061 T6; WR 430 to WR 284 Commercial Aluminum; and WR 187 to WR 90 Brass.
Data - The temperature rise plot represents averages: on the 30°F and 70°F
plots, for example, the slash line shows how the temperature varies within a waveguide
size as a function of frequency. For example, in WR-975, operating 755 to 1120 Mc,
attenuation varies from 0.115 db/100 ft at 1120 Mc to 0.18 db/100 ft
at 755 Mc. For the higher temperature this represents a worst case since natural
convection was assumed, for example even the smallest amount of forced convection will
drop the temperature.
For any given waveguide size, from the operating frequency the attenuation per unit
length can be determined from manufacturer supplied charts or available handbooks. Once
attenuation is known, temperature rise can be determined for the average power in question
from the above chart.
Example: WR-975 at a frequency of 755 Mc and an average power of 100 kW.
From charts or handbook the attenuation is 0.18 db/100 ft. Laying a straightedge
on the chart at this attenuation gives a temperature rise of 35°F above ambient.
This data, confirmed by myself in the 0.08 db/100 ft and 1 megawatt
region and by MIT-Lincoln Laboratory in 0.3 db/100 ft and 50 kW region,
can be useful to those who are unaware that with the high average powers now available
(and required in satellite communications) there can be a serious temperature problem.
This could limit the system noise temperature on a low noise tracking system.
Posted August 28, 2018