Ham Radio vs Pave Paws Radar - Round 2

The saga of the government's Pave Paws radar system versus a number of UHF ham radio repeaters in Northern California and New England continues. This, as the ARRL Lab sends out a letter to those systems to bring them up to date on where interference mitigation now stands.

Amateur Radio Newsline's Bruce Tennant, K6PZW, is here with some of the details made public by one repeater owner.

The following is from a letter to an unnamed California repeater owner from ARRL Lab Manager Ed Hare, W1RFI, posted to the Internet. It says that back on July 16th a teleconference was held among ARRL representatives, the staff of the US Air Force Space Command and civilian contractors from the Department of Defense that operate of the Pave Paws radar sites.

During the meeting Department of Defense representatives confirmed that specific interfering signals were measured on a number of frequencies used by the Pave Paws Radar system. The research was done using a calibrated antenna and receivers at both Pave Paws sites during August, 2006.

The Air Force stated that its concerns about interference are based on these actual measurements. Also, factors such as tower shielding or other antenna patterns have been fully taken into account. In other words, the claims of interference from UHF repeaters operating near the two Pave Paws sites is real. Also that it is based on solid data and good engineering practice. -- end quote.

So what's to be done to eliminate the problem? According to Hare's letter to the repeater owner, to work towards resolving this with his repeater, the ARRL Lab recommends that he take the following steps,. Again in part we quote:

In some cases, it may be possible to reduce power. The reduction in power from 50 watts to 5 watts, for example, is a reduction of 10 dB. In some cases, however, where tens of dB of power reduction is required, it simply is not practical to do that much power reduction to that extent.

Power reduction can also be combined with a change in antenna pattern. If the antenna used on your repeater is omnidirectional, installing a directional antenna with a null in the direction of the Pave Paws Radar site should further reduce the signal transmitted in the direction of the radar system.

In cases where significant attenuation by use of nulls is required -- more than 10 to 15 dB, for example -- it will be necessary to "aim" that null carefully. One way to do this could be to listen to the radar signal, using a receiver with an S meter, and adjust the orientation of the antenna until the received radar signal is at its minimum value.

You may be able to relocate the repeater, either significantly farther away, or perhaps lower in elevation, if you can take advantage of terrain shielding in the direction of the Pave Paws Radar and still get reasonable coverage for the repeater. Terrain shielding can help, but that, too, has its limits. -- end quote.

W1RFI continues by stating that there is a practical limit to how much interference mitigation can be accomplished. While there is no limit to how far power can be reduced, at some point the repeater becomes unusable. In some cases, especially those where significant mitigation is being required, the only practical solution may be to shut the repeater down, or locate it significantly farther away, or in a location where significant terrain shielding can further reduce the signal at the Pave Paws radar site.

For the repeater owner operators of the affected systems and their users, none of this is very good news.

Bruce Tennant, K6PZW
Amateur Radio Newsline

• The situation currently involves 15 repeaters in an area of less than 100 miles of Otis Air Force Base near Cape Cod, Massachusetts, and more than 100 repeaters within some 140 miles of Beale Air Force Base in California.

And FM voice repeaters might not be the only ones eventually affected. Department of Defense officials indicated to Hare that there could be other sources of interference identified in the future including other amateur operations in the 70cm band. However, for now they are only working only with the original list of repeaters provided to them last March.

Source: ARNewsline, QRZ.com, VHF Reflector, others