Thursday, July 15, 2010

I put up a 10-meter quarter-wavelength ground plane antenna on my property in my subdivision...

Q Steve, N8UBR, asks, “I put up a 10-meter quarter-wavelength ground plane antenna on my property in my subdivision. When doing an RF safety evaluation for this system, is the distance measured from the main vertical radiator or the ground plane radials? (I plan to use 100 W.) The radials are currently above the ground, but will be buried in the spring when the ground thaws.”

A If I were doing your evaluation, I would consider radials that were within a few inches of the ground to be grounded and would do my calculations from the main antenna itself. If I had a ground-plane antenna with elevated radials, I would, to be conservative, consider them as an active part of the antenna system. The simple evaluation methods, such as the one found on the University of Texas Web page (see “The Doctor is IN,” February 2000), works very well for ground-mounted verticals. You can assume about 1 dBi of antenna gain and do use the ground-reflection factor.

Start with your 100 W output and adjust it for the operating mode and typical duty cycles.
100 W CW = 40 W
100 W SSB = 20 to 40 W, depending on speech processing,
use 30 W for average speech processing
100 W FM, RTTY, other digital = 100 W

Then, adjust it by the amount of time you might be transmitting continuously during the averaging time of 6 minutes for controlled exposure or 30 minutes of uncontrolled exposure. For “conversational” operating, you can use 100% for controlled exposure and about 67% for uncontrolled. If you wish, you can also make further adjustments for feed-line loss, but I will refer you to RF Exposure and You for more info on that.

For a 1-dBi-gain antenna on 28 MHz, this typically works out to:

Mode Controlled Distance (feet) Uncontrolled Distance (feet)

This all assumes 100 W, 1 dBi, 28 MHz, 10 minutes on, 10 minutes off, 10 minutes on and moderate speech processing for SSB. If you and your family are greater than 4.9 feet from the antenna and your neighbors are greater than 9 feet from the antenna, you can run 100W continuous duty (carrier) for an indefinite period.

The required distances are from your antenna to any point where people could actually be exposed. Most hams choose to control exposure in their backyard by instructing their families not to linger closer than the controlled distance to their antennas when they are on the air. You should also take some steps to ensure that no one can accidentally contact your antenna. Hams generally use their property line as the criterion for the uncontrolled distance because they have no way of knowing whether their neighbor might be spending time near the property line. With the above assumptions, if you operate SSB on 10 meters with 100 W and your antenna is located 4.9 feet from the property line, you are in compliance.

from QST May 2000