Microwave Power Density Estimation for Parabolic Dish Antennas
Peter DeNeef, AE7PD
Microwave Exposure Safety with Parabolic Dish Antennas
Summary
In the article “Microwave Power Density Estimation for Parabolic Dish Antennas” in QEX (Jan/Feb 2022, pp 23-25) I compare methods amateurs can use to estimate the maximum antenna input power not exceeding the FCC maximum permissible exposure limits.
The far field model used in the ARRL RF exposure calculator [1] gives accurate estimates for power densities and separation distances in the far field. The FCC formula for the distance to the far field boundary is Rff = 0.6 D² / λ, where D = dish diameter, and
λ = wavelength [2]. For example, Rff = 24 ft for a D = 2 ft, 10 GHz dish antenna.
An analysis of the near field of parabolic dish antennas published by George Kizer includes a table for the maximum antenna input power not exceeding the FCC maximum permissible exposure limits [3],[4],[5]. The analysis is for commercial dish antennas, where power loss is usually negligible. These estimates are shown in Table 1 below. They are convenient, conservative (low) limits you can use for amateur radio antennas, which typically have a more uniform aperture illumination than commercial antennas, and non-negligible power loss [6]. The maximum input power does not depend on frequency, and it increases with dish size D because the power is distributed over a bigger area.
Table 1. Maximum antenna input power not exceeding the FCC maximum permissible exposure limits in the near field. Dish diameter is D (ft), and aperture illumination efficiency is 0.55. Data from [3],[4],[5].
D (ft) General Population Controlled
Limit (W) Limit (W)
1 0.1 0.5
2 0.4 2.0
3 0.9 4.5
4 1.6 7.9
6 3.6 18
8 6.3 32
10 9.8 49
References
[1] http://www.arrl.org/rf-exposure-calculator
[2] Evaluating Compliance with FCC Guidelines for Human Exposure to RadioFrequency Electromagnetic Fields, OET Bulletin 65 (1997); Currently under review.
https://www.fcc.gov/bureaus/oet/info/documents/bulletins/oet65/oet65.pdf
[3] G. Kizer, “Microwave Antenna Near Field Power Estimation,” Proceedings of the 4th European Conference on Antennas and Propagation, 12-16 April 2010 pp 1-5. https://ieeexplore.ieee.org/document/5505204
[4] G. Kizer, Digital Microwave Communication: Engineering Point-to-Point Microwave Systems , IEEE, 2013, pp. 249-319.
[5] G. Kizer, “Antenna Near Field Power Density Public Safety Limits,” NSMA Annual Conference, May 17-18, 2016.
https://www.nsma.org/wp-content/uploads/2016/06/NSMA-RF-Public-Safety-Antenna-Near-Field_George-Kizer.pdf
[6] P. Wade, W1GHZ, The W1GHZ Online Microwave Antenna Book, 2006. https://www.w1ghz.org/antbook/contents.htm
Author Information
Peter DeNeef, AE7PD, is an Extra Class amateur radio operator in the USA. This website has no ads or conflicts of interest.
email: HamRadioAndVision@gmail.com
rev. 3/3/2022