EMR - Home, Portable and Mobile Station

by Gil Sones VK3AUI

This article is substantially similar to a previous article published in the May 2001 WICEN Victorian newsletter and is reproduced with permission. Some updated material and information for home stations has been added.

New rules for Electro Magnetic Radiation exposure have arrived on the scene. These have some important consequences for mobile and portable operation as well as for our home stations. The voluntary self assessment trial from 15th September 2000 to 15th January 2001 has ended for amateurs to conduct EMR assessments using the tables and information provided. The full implementation is scheduled for the end of the year.

I was reminded of "The Hitchikers Guide To The Galaxy" by the way the EMR rules have crept up on us. They are real nevertheless and we need to consider them. The material is on the ACA site on the internet. There was a significant modification in the self assessment material in December 2000. This related to the relationship between peak envelope power and the average power used in the assessment. The materials are available from the ACA website (www.aca.gov.au) .

Modes such as FM and PSK etc where the carrier is continuously at full power use the output power in the assessment. SSB which uses varying power uses a factor to reduce the PEP to a lower figure for the calculation. Similarly CW operation uses a reduced power.

For a VHF/UHF mobile using a centre of the roof quarter wave whip the distance required between the antenna and a person means that a power of between 10 and 15 Watts is all that is possible. A gain antenna such as a 5/8 th wave reduces the power possible. Antennas on the bumper bar or on the edge of the boot lid or bonnet or on the gutter may require an impossibly low power to comply. However as vehicle sides bulge outwards the distance between the mount and antenna and the outer extremity of the vehicle may be enough to allow low power operation.

Portable operation on VHF/UHF using a portable mast will allow higher power or a gain antenna or both. Here the height of the mast is the key to gaining sufficient distance between the antenna and people. For 10 watts and a quarter wave ground plane a 2.7 metre pole would suffice. Raising the power to 50 Watts the mast would need to be 3.6 metres which is not out of the question. In both cases 2 metres has been allowed for the height of a person. If a gain antenna was used the use of a 6 dB antenna would raise the height of mast required for the 50 Watt case to 5.6 metres. Still not impossible. For a 50 Watt 2 m and 35 Watt 70 cm radio and again antenna of 6 dB on 2 and 8 dB on 70 the same mast would do since the extra feedline loss on 70 would compensate for the extra height (20 cm) otherwise required. The figures all so far make no allowance for feedline loss as the feedlines are so short as not to have significant loss if thick cable is used.

A beam antenna at VHF/UHF raises the gain somewhat and a 5 element beam with a 50 Watt mobile would need a 7 metre mast. This is possible but does require some thought and also some help with erection. The 10 watt mobile with such an antenna would need a 4.25 metre mast.

HF operation with a standard 100 Watt transceiver is another situation commonly encountered. A bumper mounted or bull bar mounted antenna may be difficult to use. However if the mounting is sufficiently inboard of the outer extremity of the vehicle some form of operation may be possible. Once again a centre roof mounted antenna is the only one which can make the separation distances required for all HF bands. The distances are slightly less for 80 and 40 but are constant for 30 through 10 metre operation. Use of a speech processor with SSB increases the distance required as does CW operation. The distance for a standard transceiver is just possible for a centre roof mount whip on all HF bands using SSB. For the use of the same transceiver with a speech processor the minimum distance may impose limits above 80 metres. CW operation may need reduced power above 80 metres but is not the usual mode when mobile.

Portable operation on HF is quite possible and antenna height is the key. The use of random wires is not really possible unless you use a roped off area around the ATU or elevate the ATU sufficiently and even then this may not be possible. An antenna height of 3.8 metres for the lowest part of a dipole antenna is sufficient for a standard 100 W transceiver for SSB, Speech Processed SSB, and CW to comply with the EMR rules for operation on 10 to 80 metres. For 100 Watts and using PSK or other similar full carrier modes raising the antenna to a height of 4.5 metres at the lowest point would suffice. The antenna heights are at the lowest point of the antenna and to allow for sag the supports would need to be somewhat higher.

For your home station installation you may have to fence off a ground mounted antenna but this EMR clearance area could be disguised as a fenced garden bed or by a dense planting of shrubs. Overhead dipoles and beams should be raised sufficiently to clear the heads of people by the required distance for the band, power, mode, and antenna gain used. Inverted Vee antennas may need the ends supported on short poles to gain the clearance required. Height is the simple way to compliance and it will give you a good signal as well.

The new EMR rules mean that we will have to give some thought to our installations. Antenna height is a fairly easy way to comply just by raising the antenna well above the heads of people in the area. With the clearance distances calculated you should add 2 metres to allow for the height of a person beneath the antenna. The clearance distance is to the head of a person beneath the antenna. Operation on SSB rather than FM may be another way of increasing the effectiveness of a station where power and clearance distances are a problem. Many of the standard transceivers now provide 50 Watts of SSB in the VHF/UHF region which would be much more effective than 10 Watts of FM and with a TCXO option fitted they would be stable and resettable enough.

In order to work out your own station and situation you should obtain a copy of the EMR material from the ACA website (www.aca.gov.au). It is fairly simple to work out with only relatively simple maths even if you have to interpolate between the examples in the tables. A calculator and the dB table in a reference book helps. We will all soon have to have our own home station compliance worked out. For mobile and portable operation by adhering to some guidelines and separation distances compliance is possible also.

Some typical situations are given in the accompanying tables. The clearance distances are not impossible to achieve.
 
 

EMR distances for VHF and UHF uncontrolled access

EMR distances for HF uncontrolled access

Note 1. These are conservative figures and for most antennas the losses would be greater which would result in smaller distances and hence the figures quoted have a considerable safety margin.

Note 2. Cases marked "*" may be possible with bull bar or bumper mount depending on layout of bull bar and bumper bar with respect to antenna mounting point and outer extremity of bar.