Mobile ant mounting

[racerman1cars]

Hi, This probably been discussed but I didn't see it. I just bought a 98 Suburban and it has one of those stainless L brackets that has 3 screws that fastens to the fender between the hood. it is lined up with the broadcast antenna on the other side, my question is will a vhf 45 watt Spectra with a Moto 5/8 wave be too close to me while in the drivers seat? Is that to close to an RF source? I am going to mostly monitor but will talk on 2 meters some. I am going to drill a hole in the roof for a NMO mount for a uhf antenna. Any thoughts on this?
Thanks
Steve

[NSPD]

I have my 2m/70cm antenna on the fender of my explorer putting out 55 watts, and I don't see a problem with it. You will get better performance with it on the roof though, so if you are going to take the steps to drill one hole in the roof, i'd go ahead and move it up there if height restrictions aren't a concern.

[Cowboy]

Hi, This probably been discussed but I didn't see it. I just bought a 98 Suburban and it has one of those stainless L brackets that has 3 screws that fastens to the fender between the hood. it is lined up with the broadcast antenna on the other side, my question is will a vhf 45 watt Spectra with a Moto 5/8 wave be too close to me while in the drivers seat? Is that to close to an RF source? I am going to mostly monitor but will talk on 2 meters some. I am going to drill a hole in the roof for a NMO mount for a uhf antenna. Any thoughts on this?
Thanks
Steve

If you go off of the FCC guidelines for MPE for the Amateur Service...

100% duty cycle (hope you're not using it that much mobile LOL) at 50 watts with 3 db antenna in "uncontrolled or general population" exposure on 2m requires 3.2 meters (10.49 feet) between the "main beam" of the antenna and an individual to meet the RF power density limits identified in OET65 Supplement B (Chart 4B by W5YI on PDF page 29 of 65)...

So technically to meet the Maximum Permissible Exposure level, you would either need to mount the antenna 10.49 feet away from the driver horizontally, or ideally, mount it above the driver on the roof so as to remove the operator from the "main beam" of the antenna.

I absolutely hate people that throw Part 97 around like it's the God's own divine word... But when it comes to whether or not I want myself or the occupants in my vehicle to get an excessive amount of RF, I lean towards following it word for word for safety's sake...

Couple down sides to a 3 db VHF antenna on the roof - one being height (hitting stuff in the woods or urban jungle) and secondly the radiation pattern. A 3 db antenna primarily radiates out of the top 1/4 wave length of the antenna. Great if you're on a trunk lid trying to get over the roof line of your car. Bad when you're doing 75 MPH and that 1/4 wave top portion of the antenna is now horizontal to the earth's surface. Your target station is vertically polarized but your mobile antenna is now horizontally polarized, resulting in a 20 db loss. Sometimes, depending upon the location of your target stations (repeaters, bases or mobiles), a standard 1/4 wave antenna may perform better, especially if these stations are essentially above you rather than at the horizon - the 3 db 5/8 wave antenna tends to have a much narrower beam width more closely described as a pancake or small stack of pancakes compared to the typical melon shaped beam (up, down and all around but not quite as far horizontally) of a 1/4 wave antenna.

Although not required by the FCC’s rules, it is advisable that mobile stations also be
considered for potential exposure before an amateur automatically applies the categorical
exemption. As an example, a 500-watt, 10-meter mobile installation with a vehicle mounted
antenna would certainly merit a closer look. On VHF, the use of a high-power amplifier
could also present problems in some cases. In general, it is recommended that in these higher
powered installations, the antenna be located such that the vehicle occupants will be shielded
from the antenna during normal use. One good location is in the center of an all-metal roof.
Locations to be avoided for high-power operation would be a trunk-mounted antenna, or
installation on a vehicle with a fiberglass roof. In general, mobile installations, even
higher-powered ones, should not exceed the MPEs if sound installation guidelines are
followed. The ARRL Handbook and ARRL antenna books, available from the ARRL, have
additional material on mobile installations and antennas (see footnote 9).

Here's a cut of the chart for MPE for 144 MHz - I've bolded the applicable fields for the antenna/radio in question:

Estimated distances in meters from transmitting antennas necessary to meet FCC power density limits for
Maximum Permissible Exposure (MPE) for either occupational/ controlled exposures (“Con”) or general
population/uncontrolled exposures (“Unc”) using typical antenna gains for the amateur service and
assuming 100% duty cycle and maximum surface reflection. Chart represents worst case scenario.
Freq Antenna
(VHF/UHF) Gain Peak Envelope Power (watts)
(MHz/Band) (dBi) 50 watts 100 watts 500 watts 1000 watts
Con. Unc. Con. Unc Con. Unc. Con. Unc.
144 (2m) 0 1.0 2.3 1.4 3.2 3.2 7.1 4.5 10.1
144 (2m) 3 1.4 3.2 2.0 4.5 4.5 10.1 6.4 14.3
144 (2m) 6 2.0 4.5 2.8 6.4 6.4 14.2 9.0 20.1
144 (2m) 9 2.8 6.4 4.0 9.0 9.0 20.1 12.7 28.4
144 (2m) 12 4.0 9.0 5.7 12.7 12.7 28.4 18.0 40.2
144 (2m) 15 5.7 12.7 8.0 18.0 18.0 40.2 25.4 56.8
144 (2m) 20 10.1 22.6 14.3 32.0 32.0 71.4 45.1 101.0

Reference Link
http://www.fcc.gov/Bureaus/Engineering_ ... oet65b.pdf


It's 0044 local time - I apologize in advance if this fails to make sense...

[Will]

I concur with the R. F. Exposure guide lines. Wish we would have followed this years ago.

I recommend the 1/4 wave broad band 2 meter antenna on the roof.
Again to keep R. F. exposure levels to a minimum, mount the antenna more than 1/4 wave from ANY edge.