eHam Forums => Elmers => Topic started by: ROBPUR on May 13, 2011, 12:52:57 AM

Title: RF Safety Calculator
Post by: ROBPUR on May 13, 2011, 12:52:57 AM
I've been toying with the idea of putting a GMRS mobile radio on the back of my motorcycle and have been playing with an online calculator to see how far I should be away from the antenna. I haven't liked what I've seen so far, but it could be because I'm not punching in reasonable numbers for power and antenna gain, so I'm seeking advise. Here's the calculator.

For "Average power at antenna" I used 45 watts, which is probably high. I have no idea how much power a mobile radio such as an Icom F6021 puts into the antenna. It has a rated output power of 45 watts, but I don't know what the real world number into the antenna would be. Feed line loss would be small since there would be only about a foot of coax between the radio and antenna.

For antenna gain I used 0dBi. I would likely use a half wave mobile antenna without a ground plane. Again, I don't know what the real world number should be here.

With a distance from the antenna of 2 feet, and a frequency of 460Mhz, the result is 2.467 mW/cm2, and the Maximum Permissible Exposure (MPE) is considered to be 1.5383 mW/cm2. So, with those calculations, I'm way over the MPE.

I also did the calculation with a 4 watt HT (BlackBox-U or HYT TC-500) an inch away from my body (clipped to my belt), and got a really high result.

So, any idea what numbers I should use with the calculator?


Rob Purcell

Title: RE: RF Safety Calculator
Post by: N3OX on May 13, 2011, 05:28:23 AM
Rob, the average power is calculated over six minutes according to FCC bulletin OET 65.  The average power of a 45W rig during a transmission is the full 45W for an FM radio (less for CW or SSB because there isn't a carrier present all the time in those modes), but since the total averaging period is probably/possibly longer than a typical transmission you may be fine.  If you transmit 30 seconds or a minute and then listen for a few minutes, the average power on a six minute time scale will be much less than 45W which will help.  If you're too long-winded there could be a problem remaining in compliance with the regs.

0dBi is reasonable for the antenna, but you'll notice there's a selection for turning the ground reflections on and off.  With an antenna a couple feet from your body and the current maximum of the antenna up off the bike a bit, you can probably turn that off.

Title: RE: RF Safety Calculator
Post by: K2DC on May 13, 2011, 09:44:20 AM

    Dan's got it right.  I work with high power RF systems (mostly microwave) and I have performed many many RF Safety analyses.  Every personnel safety standard I've worked to, FCC, IEEE, ISO, ANSI, US MIL, EU, etc., are all based on a 6 minute averaging period.  If over a 6 minute period your duty factor (talk time divided by talk plus listen time) is over 50%, you have a VERY big mouth.  When you use an average power of 22.5W (45W at 50% duty) at 2', the exposure is .48 mW/cm sq, well within the Controlled Environment MPE.  Controlled Environment specifies informed consent, meaning that anyone to be exposed is aware of the the potential for exposure and consents to the potential exposure.  That would be you, and you know the requirements and intend to use the radio.  Uncontrolled is for the general population who are unaware of the potential for exposure.

    Unfortunately, that means that you can not use the radio if you have a passenger behind you on the bike.  Using the same average power and antenna gain at 1', you're over the limits, both Controlled and Uncontrolled.

     By the way, I also agree with Dan that in this configuration, ground reinforcement does not exist, and should be turned off for the analysis.  You may want to look at other radios with lower outputs.  And you also need to consider somone in the car next to you at a long stop light (Uncontrolled Environment)


Don, K2DC

Title: RE: RF Safety Calculator
Post by: N3OX on May 13, 2011, 05:41:47 PM
  And you also need to consider somone in the car next to you at a long stop light (Uncontrolled Environment)

I believe the averaging period for uncontrolled exposure is longer, 30 minutes, which will mostly eliminate concerns about in-motion exposure of others around the bike.

Title: RE: RF Safety Calculator
Post by: ROBPUR on May 13, 2011, 08:31:33 PM
Thanks Dan and Don for your input. Given the 6 minute average I should be fine since my transmissions are usually around 5 seconds, no longer than 10 seconds, and my conversations are short. I won't need to be too concerned about transmitting close to someone since I ride logging roads and trails. No room on the back of my bike for a passenger so that won't be an issue either.

Like I said in my original message, I'm playing with the idea of putting a mobile on my bike. The difference in power between my current 4 watt HT and a 45 watt mobile is significant, but if I'm trying to communicate with someone else using a low power HT then the extra power isn't going to help if I can't hear them. Using a mobile radio means using a mobile antenna, and I suppose that could translate into better reception. I assume that there could also be differences in the receiver performance of the radios.

I realize that there are a lot of factors involved, but would I likely see improved receive performance using an Icom F6021 mobile with a half wave mobile antenna, no ground plane, compared to a BlackBox-U (HYT TC-500) with a six inch single band rubber duck antenna, strapped to the belt of my riding jacket?

Thanks again for your help.

Rob Purcell