tech stuff – Page 41 – Engineering Radio

The ERI LPX2E Rototiller FM antenna

This was in the back room of one of the radio stations we work for:

ERI LPX2E FM antenna on the ground — ERI LPX2E FM antenna with RADOMES on the ground

It is a burned-out ERI LPX2E antenna. The manager was complaining that it took up too much space and he didn’t know what to do with it. Could I get rid of it? Sure, no problem. I could at least cut it up and scrap it.

When I first looked at it, it seemed complete and undamaged, however, upon further examination it seemed that some of the inter-bay line had overheated and one of the tuning sections that goes from the power divider out to the bay was missing. Therefore, I took it apart and separated the copper from the brass. Most of the antenna is made from yellow brass, due to its hardness. The inner line sections are copper and the mounting hardware is all stainless steel. I will perhaps break even time-wise, but it is one of those projects that can be done on my time in between other paid work, so it will be fine.

I am going to keep one bay intact with the RADOMES on as some sort of modern art project. My daughter thinks we should install it in the yard as a part of a fountain-fish pond-bird bath contraption. The idea is to mount the bay facing up as in the picture above and run a hose up the inside of the transmission line to the T section. A hole will be drilled there and some type of fountain head installed to spray water up over the RADOME. The system will be run by a solar-powered pond pump. I’ll have to take pictures when it is done.

Implementing MCDL (Modulation Dependent Carrier Level)

Since the FCC waved some of its rules regarding carrier power and carrier shift on the AM broadcast band, AM stations are now able to implement MCDL or DCC (Dynamic Carrier Control) technology to save money on their electric bills. This technology has the potential to save tens of thousands of dollars for higher-powered AM stations (high power=greater than 10 KW carrier level).

On a standard AM broadcasting station, the carrier represents two-thirds of the energy being transmitted, with the modulation index containing the other one-third. The carrier contains no information; it is simply there on the center frequency at the power level authorized by the station’s license. Thus, if the carrier can be reduced without affecting the quality of the broadcast reception, it will reduce to the overall power consumption of the transmitter. In areas where electric costs are high, the savings can be substantial.

There are various ways to accomplish this. The first is called Dynamic Carrier Control (DCC), where the carrier voltage is reduced during moderate modulation levels (between 20-50%) and restored during peaks. This reduces the output power during average modulation, restoring most of it during quiet periods and peaks. The next is Dynamic Amplitude Modulation (DAM), which is similar to DCC. The most savings will be noted with less heavily processed programming such as talk radio because the higher the average modulation density is, the less the MDCL circuit reduces the carrier voltage level. The little graph in the diagram shows the reduction in the carrier voltage vs. modulation levels.

Nautel DAM block diagram, courtesy of Nautel, Ltd.

Finally, Amplitude Modulation Companding (AMC) reduces the voltage in both the carrier and modulation product during peaks. This results in better savings for higher-density modulation indexes. It is also the most transparent of the three schemes, as the carrier is restored to full power during periods of low or no modulation levels. During peak modulation, the reduction does not drop the power level below the un-modulated carrier level. The little graph in the diagram shows the reduction in the carrier voltage vs. modulation levels.

Nautel AMC block diagram, courtesy of Nautel, Ltd

Nautel has done extensive work on MDCL and includes several algorithms in their NX series transmitters. For older Nautel transmitter models such as ND, XL, XR, and the J-1000, there is an outboard exciter, which is in a one-rack unit chassis. Older transmitters may need a simple field modification to create a DC-coupled audio input. The cost for the upgrade is approximately $5,000 USD, however, check with the regional Nautel sales rep.

Once the system has been installed, there are several things to be aware of:

Modulation monitors may not work properly, especially older units, which will show significant carrier shifts and have carrier alarms. Belar AMMA-2 modulation monitor is specifically built to work with MDCL transmitters.
When making field strength readings, the MDCL circuitry must be turned off to get accurate readings.
For stations running IBOC, the amount of carrier power reduction may need to be experimented with, as the effect of the carrier reduction may cause the transmitter to exceed the NRSC mask.

Currently, only Nautel and Harris are selling MDCL transmitters. I spent several minutes poking around the Harris website and looking through their product brochures for the DX series transmitters and no mention of DCC o MDCL was found. I’d be happy to include any information from Harris if it were made available.

Andrew A909D type 78AGM 3 inch connector

I figured if I have this problem, someone else probably has it too. We have a backup antenna on one of our towers. The station has a TPO of 28 KW, which is starting to get into the semi-serious level. This antenna is connected to Andrew 3-inch heliax that was installed in 1971. It has a spiral inner and outer conductor, which is no longer made by any manufacture of heliax.

We completely rebuilt the transmitter site a few years ago, moving a lot of things around. One part of the project was installing a coax port on the wall and moving all coaxial cables to that entrance. The main antenna is connected to Cablewave H50J coax. I ordered a new connector for that transmission line, no worries. When I cut the back up transmission line, I figured I could re-apply the old Andrew connector.

Andrew A909D type 78 AGM 3 inch coax connector — Andrew A909D type 78 AGM 3-inch coax connector

That is all fine, however, I removed the connector without reverse engineering it, that is to say, I didn’t pay close attention to how the inner and out conductors where cut, or how the jack was cut back. I will have to reverse-engineer the thing now.

Here are the steps I followed:

Check out the CommScope – Andrew website for documentation. A search shows they only have the current connector, which is nothing like this one and will not work with spiral conductors
Call Andrew and spend many minutes on hold or explaining to various helpers what I want. I was met with a universal “That is not an Andrew Part number,” or “Gee, I wish I could help but…”
Take the thing apart and begin measuring stuff with a ruler. Write everything down and draw out a diagram.
Trim off the excess cable then practice putting the thing together once.
Make the final cut and put the re-used connector back on the Andrew transmission line.

You can skip steps 1 and 2 since I already did them for you.

A few things to note:

The inner and outer conductors should be cut flush and as close to perpendicular as possible.
The inner conductor slug has a left-hand thread. This makes the slug tighten against the bushing.
The outer jacket is cut back about 2.5 inches
Place the EIA flange on the cable first, then thread the back nut onto the outer conductor, then thread the rubber gasket onto the outer conductor. The gasket is a tight fit, use petroleum jelly to lubricate it. This is a gas block connector, so special attention is needed with the gaskets.
The inner conductor has triangular pieces 1/8 inch deep cut around the diameter, the depth of the inner slug is critical to the connector going together correctly.
The inner conductor is folded inward over the end of the slug. Bushing, dielectric spacer, and EIA bullet are connected to the inner conductor slug and snugged down with a standard screwdriver
The outer conductor is nipped 1/8 of an inch around the diameter
The outer conductor is folded outward over the collect ring
Use some petroleum jelly on all the O rings
Carefully screw the connector together
Final tightening requires a special spanner wrench or attachments for a socket wrench. The tower crew had these in their shop.

If you have a spectrum analyzer, check its return loss and see what it looks like before slamming a full load on it. If not, turn things on and bring them up slowly. Feel the connector to make sure it is not getting warm. If there are problems, heat will be the first warning sign.

Once together, I ran the transmitters for a combined output of 10 KW and got about 50 watts return, which was much the same as it was before.

Documentation and labeling wire and cable

There are a myriad of details involved in building a studio, not to mention an entire facility. Getting everything down on paper before a single wire is pulled is one way to ensure that a neat, logical, and orderly product ensues. For wire run documentation, I like to use Excel spreadsheet templates that I came up with.

There are several different types of cable, from 25-pair ATT style to 16 or 24-pair shielded audio cable, to miscellaneous control cable, all of it has different color codes. I found the Belden Technical info website to be an excellent source for various color codes.

Doing neat work is the best way to keep things in order. Notice all the wires are labeled. All the ground conductors have heat shrink, which is required on insulation displacement terminations like 66 blocks, 110 blocks, and ICON terminations.

ADC ICON termination block

Once all the work is done, the wire run sheets are updated with changes and additions (there are always changes and additions) which will keep the documentation accurate.

I made up several templates with the wire color code, pair number, and cable information on each wire. This allows the wire man to quickly enter changes to the wire information on the sheet. At the end of the wiring project, these forms can be saved in several places, printed out, and placed in a book or however, the engineering manager wants to keep the information.

The excel spread sheet for this is here.

For 16 pair Gepco cable on 66 blocks, click here.

For 16/24 pair Gepco cable on ADC ICON Termination blocks, click here.

I say Gepco cable, any audio cable that is color coded with standard resistor color codes will work with these sheets, or the sheets can be adapted for use with other cables.

66 blocks audio and control for nextgen installation

This is a good installation. The company I work for has several wiremen that are artists and do excellent work. Notice there is adequate room and light to work on the wall. A dark, cramped area will lead to hurried work, poor workmanship, and mistakes in wiring.

Automation computer on slide out rack with cable management system — Automation computer on slide-out rack with cable management system

All the cables to the rack mount computers are neatly dressed, which allows easier service.