View Full Version : Field Strength Meter
05-19-2009, 12:14 PM
Can someone give me some basic info. on field strength meters? Do I need to check my field strength to make sure I'm not exceeding permissible exposure limits? I'm only operating with 100 watts from my 718, but one lobe points almost directly at the house. If I should test it, can I buy a reasonably priced meter? Or better yet, can I rent one, or borrow one?
It's not absolutely necessary, but having one around is a very good idea.
I use mine to verify that I don't have any stray RF in the shack, that my daughters bedroom isn't a hot spot from my antenna placement, and it's best use:
To tune my antenna for maximum output.
Remember, low SWR does not mean maximum output.
05-19-2009, 03:02 PM
I have a Micronta 23-126 SWR/FS meter available. I don't think many affordable meters are actually calibrated, just give relative readings. Send me a PM or email if interested.
tlamb at mesa dot net
TOM K8ERV Montrose Colo
05-19-2009, 05:36 PM
At 100 watts, it's hard to violate the RF exposure guidelines on most bands.
The area of concern would be 10/12, 6, and 2 meters. At power levels over 50 watts, you might be pushing it. But, unless you are running the rig key-down at 100 watts for a long period of time, you are would still be within the guidelines in most cases, so long as you are not standing right next to the antenna.
You are not required to do a study of your RF exposures if you are running less than 50 watts, and there are other exemptions, too. If you are at all concerned, reduce your power to 50 watts on FM and don't worry if you're using a low duty-cycle mode like SSB. You probably shouldn't be running over 50 watts on PSK31, anyway.
The most egregious violations would include running high power to an indoor antenna, particularly in an apartment building where your neighbors would be exposed to it.
You can use the online calculator at http://n5xu.ae.utexas.edu/rfsafety/
Or you can take your time averaged (http://www.arrl.org/news/rfsafety/hbkrf.html)* radiated power, multiply by antenna gain and derive at power per square meter on the surface of a sphere (http://en.wikipedia.org/wiki/Sphere#Surface_area_of_a_sphere) whose radius is the distance to the antenna. (For free space, in the far field (http://www.rfcafe.com/references/electrical/near-far-field.htm), FS in V/m is the square root of 377*Watts per square meter.)
Check out this article at VK1OD's site (http://vk1od.net/antenna/SmallUntunedSquareLoop/ssulNEC.htm). If you can build and calibrate, or own a radio frequency voltmeter, then, using an antenna built as he specifies, adding the dB from the chart to the dB it reads relative to one volt across 50 ohms gives the absolute magnetic field strength in db relative to one Amp per meter, which can be compared to the limits in FCC OET Bulletin 65 (http://www.fcc.gov/oet/info/documents/bulletins/Welcome.html#65).
Calculating is easier, just not as much fun!
07-07-2009, 04:51 PM
The other problem with measuring it is that you are responsible for both magnetic and electric fields. The method described only measures the magnetic. It is possible to be in spec for one parameter and not for the other.
Again, as a practical matter, it's not that tough to calculate unless you're trying to run a KW RTTY station on a postage stamp sized lot.