I am also planning to put up an AV-18HT in my yard, which is very small. I also had the same questions, because both of the smallness of my yard, the closeness of the neighbors, and the large amount of power that I intend to use (I am getting a 1500 W linear amplifier (actually 2500! With an input of 65 W, it will crank out 1500, and I figure, in emergencies of life-and-death, would be most useful for really punching the signals out there with 2500 W) the Ameritron AL-1500.
This all sounded like it might be a bit dangerous, especially being a beginner and being clueless and all, but I did some research into the exact numbers and came up with the following:
1. Using the ARRL Ham Radio License Manual (Tech entry level manual), p. 7-9 to 7-10, I calculated the AVERAGE power, which is the number that you need to plug into most calculators to get correct total numbers. The average power is "Average power = PEP * operating duty cycle * (time transmitting / averaging period). For example, let's say that your 100-watt transmitter was generating conversational SSB without speech processing, Table 7-4 (my note: on p. 7-10 - this is a very important table - it shows that most of the time, especially on the HF bands, you are not using much average power) shows an operating duty cycle of 20% for that mode. During your operating period you transmitted for 1 minute out of every three. Your average power during the evaluation period is: 100 W * 20% for conversational SSB * (1 min / 1+2 min) = (100 * .2 * .33 = 6.33 W." This is much lower than the initial 100 W you put in and is your AVERAGE power, which you will now use to put into an RF Safety Calculator.
One good example of a calculator is: http://www.dxzone.com/cgi-bin/dir/jump2.cgi?ID=815
. I'll get back to this later, but this is the calculator I love - very simple to use and very nice.
I will also include the Table 7-4 from the ARRL book, so that you can get an idea of what the duty factor of the most common modes is:
Operating Duty Factor of Modes Commonly Used by Amateurs
Mode Duty Cycle Notes
Conversational SSB 20% 1
Conversational SSB 40% 2
SSB AFSK 100%
SSB SSTV 100%
Voice AM, 50% modulation 50% 3
Voice AM, 100% modulation 25%
Voice AM, no modulation 100%
Voice FM 100%
Digital FM 100%
ATV, video portion, image 60%
Conversational CW 40%
Carrier 100% 4
Note 1: Includes voice characteristics and syllabic duty factor. No speech processing.
Note 2: Includes voice charaacteristics and syllabic duty factor. Heavy speech processor employed.
Note 3: Full-carrier, double-sideband modulation, referenced to PEP. Typical for voice speech. Can range from 25% to 100%, depending on modulation.
Note 4: A full carrier is commonly used for tune-up purposes", ARRL Ham Radio License Manual, p. 7-10.
2. Now for a little information from the AV-18HT manual itself. On p. ? (pages are unnumbered), Table 1, about in the middle of the manual, different scenarios are listed as to number of radials one is willing to put in, and then the corresponding λ of each radial, and for our purposes here, especially the power gain in dB. Depending on the number of radials one is willing to put in, the power gain can be anything from 3 to 6 (16 radials (in which the radial λ would be .1) to 120 radials (in which the radial λ would be .4))
3. So now that you have the basic numbers, you can put them into the calculator and see how far away you must be for everything to be "safe" as it is termed.
If we take one of the most dangerous situations, it might look something like this:
1500 W * 100% (e.g., voice AM, no modulation) * .5 (you talk 50% of the time only) = 750 W AVERAGE power.
If you put in 120 radials for 6dBi and have 100 ft. of coax, even if you use one of the best types at this λ, e.g., type 9913, you will still lose about 1 dBi per 100 ft. (taken from the ARRL book again, p. 3-17, Types of Coaxial Cable", Table 3-3. So you would have, say 5 dBi of gain.
The frequency of operation could be, say 30 MHz, since it seems like the more dangerous HF frequencies are the higher ones.
When I run these numbers through my calculator, here is what I get:
AVERAGE power at the antenna: 750 W
Angenna gain in dBi: 5 dBi
Distance to the area of interest: (doesns't matter what you put in; it's your guess as to how far away you will be from the antenna)
Frequency of operation 30 MHz
Are Ground Reflections Calculated?: Check yes, if not automatically checked
Click on "Calculate RF Power Density"
In a "controlled environment" or basically your yard and property, the number I come up with is 14.30 feet, a really VERY small distance, I think. 31.92 feet is the distance to the neighbors in the "uncontrolled environment". In the former number, the maximum permissible exposure (MPE) is measured in mW/cm2, and comes out to be 1.005 mW/cm2 in the controlled environment. The second number, or uncontrolled environment, comes out to be .205 mW/cm2.
So, as you can see, this is probably one of the worst case scenarios. I think 90% of what we all do on the radio will be much better cases than this. Even if the duty cycle was 100% across the board, so that the AVERAGE power was 1500 W, the distances would still be 20 feet and 45 feet for the controlled and uncontrolled environments.
You can plug in your own numbers and come out with a more realistic scenario for yourself based on what you actually do with your own rig, mode type, how many radials you intend to put in, what type of coax cable you will use as feed line, etc.
Hope this helps. It has definitely helped me to get some solid numbers in my mind and ease my mind about putting this type of an antenna into my small yard.
Take care. Any more questions, just e-mail me, Shon Edwards, KO3U, firstname.lastname@example.org
73 de Shon, KO3U