01-SEP-2020 – Palomar

Palomar Mtn. Site

The past two weeks have been focused on our Palomar Mountain site. In advance of the installation of a Motorola MTR-2000 repeater as an analog upgrade, the TECHTEAM traveled to the site to investigate the poor coverage of the analog repeater (P10). Two years ago we upgraded the UHF antennas on the site to improve the coverage of both the analog and DMR repeaters. The coverage of the analog repeater has never met expectations.

The analog antenna system at the site consists of about 40ft. of Andrew LDF4-50 (1/2″ hard-line) and a Cellwave DB-408 6dB gain antenna at about 20ft. above ground level. Using a Bird Watt meter, we measured both the forward and reflected power to quantify the performance of the antenna system. The forward power measured 65W and the reflected power was 11W. This was significantly more reflected power than expected. Calculations revealed that the VSWR was 2.4:1. This is not good antenna performance. We measured the forward and reflected power of the DMR antenna and that system had a measured VSWR of 1.35:1. The DMR antenna system was performing well. The analog antenna system needed some work. Before leaving the site, I updated the programming code in the repeater controller.

Since the analog antenna system was made up of only two elements (feedline and antenna) we devised a plan to fix the problem during the drive down the hill. That afternoon, a replacement DB-411 antenna was ordered. We have plenty of new feedline and connectors in stock. The problem could be the feedline, the antenna, or both the feedline and antenna. We decided that we’d return next week with everything needed to solve the problem.

In the days before the replacement antenna arrived, I configured a Motorola MTR-2000 repeater to replace the aging DX repeater at the site. A few days later, we drove back to the site with a new antenna, Motorola MTR-2000 repeater, and new feedline.

The first thing I did after we arrived at the site was connect a scalar network analyzer to the problematic antenna system to aid in troubleshooting. The analyzer display indicated that the antenna was absorbing power in the UHF T-Band (470-512 MHz) and was useless as an antenna in the ham band. It looks like we had installed the wrong type of antenna two years ago. The return loss measurement made on the network analyzer matched the readings taken days earlier with the Watt meter. Open circuit and terminated circuit measurements made with the analyzer indicated that the coax cable was in good condition. We were one step closer to resolution.

We dismantled the existing antenna and replaced it with the new antenna. The spectrum analyzer showed that the new antenna was working great in the ham band with a return loss of -17 dB or a VSWR of 1.33:1. As the connection between the feedline and antenna was being weatherproofed, we noticed that the antenna was intermittently disconnecting. A quick check of the female N to female N adapter that was needed to connect the antenna to the feedline showed that the adapter was damaged. One side has the center pins splayed open so that the center pin on the mating connector would make/break contact. A quick check of the male N connectors topside indicated that both connectors were in good condition. It was obvious that not only was the wrong antenna installed two years ago, but the installation used a bad coaxial adapter too. The adapter was replaced with a fresh adapter and the connection was taped up to keep the weather out.

Next on the agenda was the removal of the legacy DX repeater. This was quickly accomplished and the MTR-2000 was placed in the rack. The controller code was once again updated to compensate for some electrical interface differences between the DX and Motorola repeaters. As I was setting up the IFR service monitors to set repeater and network audio levels, one of the instruments did not power up. I did a close alignment using a single service monitor. I installed two CAT5 surge suppressors on the data cables from the two microwave dishes that provide IP connectivity to Mt. Woodson and San Marcos Peak.

The ride down the hill showed that the new antenna made a tremendous improvement in the coverage of the analog repeater. This was a good lesson in how important a durable high-performance antenna is for a repeater site. Four days later we returned for the third time. With two working service monitors, an accurate audio alignment was accomplished.

With the installation of the Palomar Mtn. Motorola repeater, we are approaching the end of the upgrade program. None of this work could be accomplished without the continued financial support of the PAPA membership and the dedication of the PAPA technical team and steering committee. The site upgrades will be complete in the next 30 days. Immediately after the completion of the program, work will begin to harden sites which become inaccessible during the winter months.


Author: Chris AA4CD

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