STANDARD:Antenna mounting height issue for ET

The movers and shakers in the IAG have been the New York tollsters, especially the Triborough Bridge and Tunnel Authority, the Port Authority and the NY State Thruway – all of which have quite low height standards for vehicles – 13' and 14' (3.96m and 4.3m). They have many low bridges and tunnels, so placing antennas in the height range 14’6 to 15’6 (4.4m to 4.7m) comes naturally to them. There’s already a lot of signage and wires in that height zone.

But other roads have higher height standards. Truck heights are going up. And many road managers want a greater margin of safety to protect equipment from overheight strikes. Most new bridges in the US are being built to a clearance of 16' or a few inches over (5m).

The Penn pike for example has an internal rule that nothing must be hung over any of its facilities below 16’6" (5.03m), so to conform with the IAG contractual commitment for the ETC antenna, they would have to break their own corporate rule by at least a full foot (0.3m).

A particular hazard in mountainous country like Pennsylvania is the occasional heavy snow or ice storm. Parked trucks can accumulate up to a foot of snow on their roofs. It won’t harm bridge girders, but if there has been a combination of strong sun and sub-zero temperatures it can easily form an icy mass atop the trailer strong enough to smash a more sensitive ETC antenna.

The maximum 15’6 (4.7m) antenna height requirement in the IAG-Mark IV contract doesn’t seem to make any technical sense. The distance at which the transponder and antenna communicate varies within the cone of the antenna beam. And indeed the higher the antenna the larger coverage of the cone, so unless the signal was on the very edge of insufficient strength the higher mounting should increase the probability of a good read, not decrease it. The vertical distance of the transponder to antenna also varies by 2' to over 3' (0.6m to 1m) depending on whether the transponder is mounted high on a windshield or low on a bumper.

A Mark IV staffer we asked about this agreed there need not be any problem with higher mounting. He said that most of the testing has been done in the 14’6 to 15’6 range so “our knowledge is not as good and we are just not as certain about performance” outside the range.

“As the pattern (of the radio beam) spreads out, different things could happen.”

In the highway speed or ET-express (ETX) mode antennas are usually being mounted in the range 16' to 18' (4.9m to 6m) and there do not seem to be any operational complaints from Delaware (Biddles Corner) or the New Jersey turnpike (IC-6A on the spur) where ETX operates. At least we haven’t picked up any.

The guffuffle illustrates a problem with standards, often touted as an unalloyed benefit. A good antenna height standard for the NY agencies and their midget vehicles turns out to be a bad standard for those who deal with vehicles of greater stature and mountainous snow/ice effects. Of course the NY agencies could say the others should have spoken up about their need for higher antenna mounting, so testing could have been conducted of higher antennas. (Contact Peter Oomen Mark IV 905 624 3025)