Durable Transmitter Plant Design

In a modern transmitter plant, whether broadcast AM, FM, TV, paging, or repeater, a circulator is a valuable part of the installation and often omitted to save costs. The circulator, in combination with SWR bridges, enables a site manager to keep the transmitter on the air during difficulty, as well as prevent minor issues from becoming large ones.

A circulator is valuable for two important reasons. First, a circulator assures that the transmitter will always see a 50-ohm match, within a few percent of that impedance. Second, the circulator prevents mixing products from being generated as a result of rectification and amplification in the last stage of the amplifier. Any amplifier designed to operate as a broadband device is subject to this noise generating effect. The circulator acts to insure the investment in transmitter plant, at the cost of the feedline and antenna in the event of fault.

In the case of lightning and other associated arc-over events in the coax, the SWR bridge provides useful feedback to the transmitter plant controller to temporarily cease transmitting, and/or lower and ramp up transmitter output. For instance, combining these functions with flow and pressure monitoring of the dry nitrogen or dry air being fed into the feedline can immediately identify the cause of failure, and shift the source of pressurization into a higher-flow mode or to a more plentiful source such as an air dryer rather than bottled gas. If dry air is used as the source, or excessive gas flow is noted, the transmitter power can be backed down since the air-dielectric insulation will no longer be as strong or effective as it was when it was dry nitrogen under a few pounds of pressure.

A self-diagnosing transmitter needs four SWR bridges. One SWR bridge should be installed between the transmitter and the circulator, another bridge between the circulator and the feedline, yet another bridge located between the feedline and antenna (equipped with high-voltage clamping diodes), and the fourth bridge sampling the RF to the circulator dummy load.

Useful temperature monitoring points are outside air temperature, temperature at the feedline-antenna junction, the middle of the feedline, the bottom of the feedline, the circulator temperature, the circulator dummy load temperature, the transmitter output temperature, the transmitter input temperature, generator input temperature, generator output temperature, generator body temperature, battery temperature, day-tank and storage tank temperature

If the RF is getting to the antenna and you have a bad match, then the antenna is the problem. If the transmitter has a bad match at the ground, then the feedline and/or antenna are compromised. If transmitter isn't making enough RF into a good match, then the transmitter is experiencing internal problems. The circulator lets you deal with bad stuff happening after it, at the cost of continuing whatever arc welding is presently happening in the feedline without extensive monitoring. The feedline is a great heatsink, so if it gets warmer than the outside air, that is an indicator that something is wrong. A fault at the antenna or in the feedline that is noticed can be mitigated by dropping the RF power level, which keeps the station on the air but may not cover as much contour or footprint on the ground. According to the FCC Parts, the engineer would have to file an STA in the case of a broadcast transmitter but it is not turning $100K of feedline into expensive, oxidized scrap.

There is a caveat to this concept: the failure of a single antenna bay in a four-element or larger array may result in a noticeable rise in SWR which, depending on the transmitter power level, may not constitute an alarm. For instance, a 1.5:1 match may be typical of a cheap antenna for a 100 to 200 watt transmitter. In a four-bay "ring dipole", this SWR may be indicative of a single bay failure. In the northern parts of the world, ice accumulation may result in antenna detuning by adding additional capacitance to the antenna. The effect compares to slug tuning an antenna with a block of PTFE. It is therefore important to keep a record of the system SWR response. Regard variations from initial install with careful suspicion, and balance the cost of the tower climber for testing with the cost of replacing the antenna. Additional factors are possible loss in revenue for replacement or inability to transmit, plus filing fees for a "silent" STA.

Other noteable points of protection inside the shelter and around it:
1. A water shield should be placed over the transmitter to prevent water ingress in the event of roof or other failure. This is largely dependent on electrical codes, but can be the difference between rebuilding a transmitter and resetting one that tripped off.

2. The air conditioner drain line should be fitted with a float switch. For example:


In the event of the switch being tripped, an engineer should be dispatched to the site. To resolve possible false trips, three switches can be placed in physical parallel with each other on the same level, and the combined or individual switches fed into the site controller. If two or three switches trip, one can safely assume that there is an irregularity in the air conditioning drain system.

Depending on the climate and likelihood of entry, the air conditioner drain line should be fitting with a screen, or a series of increasingly smaller screens. Insects and rodents may enter the drain line and nest in the drain line, causing a blockage for liquids. Additionally, molds and fungi may form in the drain liquid and introduce turbulence in the drain line as well.

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This page contains a single entry by Kris Kirby published on November 1, 2016 9:58 PM.

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