RF Radiation Hazards

[RobertW]

I'm still working on getting my Mobile radio into my Corolla with a trunk mount.

Trunk mount on a Corolla huh? Just don't use your 440 very much!

VERY bad idea.

Seriously, don't use high power on 440 with a gain antenna mounted on the back deck or trunk. I actually have a slide about this very thing in the new show. 5  watts is okay.

I've been interested in learning more about the dangers of RF Radiation at 440MHz.  In asking around on some of the HAM nets this is typical of the responses I've been getting...

The risk from 440 mHz RF fields is comparatively low, the more so because of the way Amateurs use the band. Your trunk mount is an omni directional antenna some distance from the driver and passengers.

If there is a "risk," it goes up as the square of the ratio of frequency, the cube root of the distance to the antenna, and directly with the length of time a transmitter is in use and the RF output from the transmitter.


I just ran the numbers through an RF Safety Calculator [1][2]. At 440 MHz, 50 watts average power, 2.2 dBi, considering ground reflections - the threshold for maximum permissible exposure in a controlled environment is at a distance of 3' 7".

You, the driver, should be fine. If you have folks in the back seat, you'll want to dial down the power (or keep your transmissions short).

[1] http://www.wirelessconnections.net/calcs/rfsafety.asp
[2] http://n5xu.ece.utexas.edu/rfsafety/


I found the RF Safety Calculators very interesting.  What is the difference between a "controlled enviroment" and an "uncontrolled enviroment"?

[RobertW]

Some other interesting comments to think about:


Before you believe any RF exposure calculator figures, measure the SWR at the PL259, and play with a transmission line loss program such as TL (packed with late editions of the ARRL Antenna Handbook) until you determine how much loss you have in the line.

Most 440 and dual band antennas come with 17 feet of RG58 or equivalent, with 2.2 dB of loss into a matched load at 445 mHz. 50 Watts into the coax gives you 30 watts out. But very few 440 mobile antennas really match their coax. So the loss is greater - and the RF exposure is less.

A typical 50/50 TX/RX cycle will give an average power output of 15 Watts to a matched antenna, correspondingly less with a mismatch at the antenna.


Unless you spend a considerable amount of time on the radio with passengers in the back seat - an hour or more pretty much constantly - there is little to be concerned about, if at all.

Law enforcement vehicles have used 1/4 wave HF and UHF antennas on the package shelf in undercover vehicles for years with little, if any, reported ill effects.


Personally I still worry about Cellphones.  I don't care that they transmit with very little power.  At 800 - 900MHz right next to your brain, I'm sure we'll find out down the road that is wasn't such a good idea.   

[RennMan]

I was wondering about this very topic, and am glad I found it again.

I have a different scenario in mind.  I'm thinking about getting the Yaesu FT-8900R, which is a quad band radio - 2M, 440, 6M, 10M.  The issue is with mounting the antenna on my Ford SportTrac.

The recommended antennas are a Diamond CR-8900, or a Comet UHV-6.  Both are trunk-mounted types, not recommended for a center-roof mount like I think I want to install.

Another option would be to install the antenna on the left front fender, matching the regular AM-FM antenna on the right front fender.  Depending on where I mount it, it may or may not be in the visual line-of-sight from the drivers seat.

The question is, what about RF hazards?  The radio in question has a max power of 50W on 2M/6M/10M, and 35W on 440MHz.

Another option that the Diamond antenna folks (Wayne) told me was to use the Yaesu ATAS-120 motorized tuning antenna.  It is quite a bit more expensive, and I would have to figure out a different location to mount it, most likely on the left rear quarterpanel or bumper.

Any ideas or discussions welcome.

Peace
N4MAN

[SkierBob]

Personally I still worry about Cellphones.  I don't care that they transmit with very little power.  At 800 - 900MHz right next to your brain, I'm sure we'll find out down the road that is wasn't such a good idea.   

How about those 5.8GHz 1 watt cordless telephones...

[clint]

RennMan,

Those are all poor antenna choices depending on what you really want the radio to do. Call or come by and we'll chat about it.

Clint