A theorem is not, indeed, a fact. It is rather, an idea that is deduced and supported by other proven facts. Thus, a theorem is generally believed a truth. It should be of interest to the “All Digital” AM (AKA Medium Wave) proponents that noise on the digital channel will reduce data throughput as a function of channel bandwidth and Signal to Noise Ratio. This is known as the Shannon-Hartley theorem:
Where: C is the channel capacity in bits per second; B is the bandwidth of the channel in hertz (passband bandwidth in case of a modulated signal); S is the average received signal power over the bandwidth (in case of a modulated signal, often denoted C, i.e. modulated carrier), measured in watts (or volts squared); N is the average noise or interference power over the bandwidth, measured in watts (or volts squared); and S/N is the signal-to-noise ratio (SNR) or the carrier-to-noise ratio (CNR) of the communication signal to the Gaussian noise interference expressed as a linear power ratio (not as logarithmic decibels).
With this equation, one can discern a fundamental flaw in all digital logic. One of the main issues with AM Medium Wave broadcasting is the ever-increasing noise floor. Our society has changed drastically in the last one hundred years or so since AM was invented. Electrical noise generators; computers, plasma screen monitors, mobile phones, appliances, energy-efficient lighting, data over power line, street lights, poor utility line maintenance, and even electric cars, it seems, generate a cacophony of noise in the Medium Wave frequency band. A digital modulation scheme, be it HD Radio or DRM, will mask the noise to a certain extent, that is true. However, once the SNR exceeds the ability of the receiver to decode the necessary bits, the receiver will mute. While it is true, the listener will not hear noise, they may not hear anything at all.
I will also note; none of the current “AM improvement” schemes under consideration by the FCC addresses the noise issue on the AM band. Without addressing the noise issue, any digital modulation scheme will be a temporary fix at its very best. The noise floor will continue to rise and after it gets high enough, the all-digital modulation will simply not work.
It will be interesting to see the data from the all-digital HD Radio testing that is being done in various locations. That is, if the NAB, et al. does not decide to treat that data like some kind of state secret; they have become reticent of late. When somebody acts like they have something to hide, it makes me think they have something to hide…
More and more wireless LAN links are being installed between the transmitter and studio. Often these links are used for network extension, remote control, site security, VOIP telephony, and sometimes even as a main STL. These systems come in several flavors:
Moseley LAN link or similar system. Operates on unlicensed 920 MHz (902-928 MHz) band. Advantages: can use existing 900 MHz STL antennas, can work reliably over longer distances, transmitter, and receiver located indoors. Disadvantages: slow, expensive
ADTRAN TRACER or similar system with indoor transceivers and coax-fed antenna systems. Operates on unlicensed or licensed WLAN frequencies. Advantages: fast, transmitter and receiver located indoors, can be configured for Ethernet or T-1/E-1 ports. Disadvantages; expensive
Ubiquiti Nano bridge or similar system where the transceiver is located in the antenna, the system is connected via category 5/6 cable with POE. Operates on unlicensed or licensed WLAN frequencies. Advantages; fast, relatively inexpensive. Disadvantages; equipment located on the tower, difficult to transition base insulator of series fed AM tower.
Ubiquiti Rocket or similar system where the antenna and transceiver are separate, but the transceiver is often located on the tower behind the antenna and fed with category 5/6 cable with POE. Operates on unlicensed and licensed WLAN frequencies.
For the first two categories of WLAN equipment, standard lightning protection measures are usually adequate:
Good common point ground techniques
Ground the coaxial cable shield at the tower base and at the entrance to the building
Appropriate coaxial-type transmission line surge suppressors
Ferrite toroids on ethernet and power connections
For the second two types of WLAN equipment, special attention is needed with the ethernet cable that goes between the tower and POE injector or switch. Shielded, UV-resistant cable is a requirement. On an AM tower, the shielded cable must also be run inside a metal conduit. Due to the skin effect, the metal conduit will keep most of the RF away from the ethernet cable. Crossing a base insulator of a series excited tower presents a special problem.
The best way to get across the base insulator of a series excited tower is to use fiber. This precludes the use of POE which means that AC power will be needed up on the tower to power the radio and fiber converter. This may not be a huge problem if the tower is lit and the incandescent lighting system can be upgraded to LEDs. A small NEMA 4 enclosure can house the fiber converter and POE injector to run the WLAN radio. Some shorter AM towers are no longer lit.
Another possible method would be to fabricate an RF choke out of copper tubing. This is the same idea as a tower lighting choke or a sample system that uses tower-mounted loops. I would not recommend this for power levels over 10 KW or on towers that are over 160 electrical degrees tall. Basically, some 3/8 or 1/2-inch copper tubing can be wound into a coil through which a shielded ethernet cable can be run. Twenty to twenty-five turns, 12 inches in diameter will work for the upper part of the band. For the lower part, the coil diameter should be 24 inches.
In all cases where CAT 5 or 6 cable is used on a tower, it must be shielded and the properly shielded connectors must be used. In addition, whatever is injecting power into the cable, ether POE injector or POE switch must be very well grounded. The connector on the shielded Cat5 or 6 cable must be properly applied to ensure the shield is grounded.
In addition to that, some type of surge suppressor at the base of the tower is also needed. Tramstector makes several products to protect low voltage data circuits.
Transtector APLU 1101 series data line protector
These units are very well made and designed to mount to a tower leg. They come with clamps and ground conductor designed to bolt to a standard copper ground buss bar.
Transtector APLU 1101 series data line protector
There are various models designed to pass POE or even 90 VDC ring voltage.
Transtector APLU 1101 series data line protector
This model is for POE. The circuit seems to consist mostly of TVS diodes clamping the various data conductors.
As more and more of these systems are installed and become a part of critical infrastructure, more thought needs to be given to lightning protection, redundancy and disaster recovery in the event of equipment failure.
Most ordinary field engineers will not need to design an ATU in the course of their normal duties. However, knowing the theory behind it can be very helpful when troubleshooting problems. Also, fewer and fewer people understand RF these days, especially when it comes to AM. Knowing a little bit can be an advantage.
We were working on an AM tower recently when several discrepancies were noted in the ATU:
WFAS ATU, 1230 KHz, 1 KW, N-DA
This was connected to a 202° tower. There were several complaints about seasonal shifts and narrow bandwidth. The VSWR meter would deflect slightly on high-frequency audio peaks, always a bad sign. A little bit of backstory is in order. WFAS signed on in 1930 using a four-legged self-supporting tower. This tower was used until about 1986, when it was replaced with a series excited, guyed tower. The ATU in use was initially designed for the replacement tower, which likely had a good bit of capacitive reactance. I am speculating on that, as I cannot find the original paperwork for the replacement tower project. At some point, somebody decided to ground the tower and put a skirt on it, likely to facilitate tower leasing. The skirt was installed, but the ATU was never properly reconfigured for the high inductive reactance from the skirted tower. The truth is, the Collins 820-D2 or Gates BC-1G tube-type transmitters probably didn’t care. They were probably like; bad load, meh, WHATEVER! Although the audio quality likely suffered. That all changed when Broadcast Electronics AM1A was installed. To fix the bad load problem, a BE 1 KW tuning unit was installed next to the transmitter.
Technically, there are several problems with the above circuit, starting with the capacitor on the wrong side of the base current meter. This capacitor was installed outside of the ATU between the tower and ATU output. Was the base current meter really measuring the base current? I don’t know, maybe? The shunt leg was lifted but both of the inductors of the former T network were left in the circuit.
We reconnected the shunt leg and moved the capacitor inside the ATU and to the correct side of the base current meter. After several hours of tuning and fooling around with it, the ATU is still narrow-banded, although now at least the input is 50Ω j0. I believe the current design has too much series inductance to be effective.
Thus, a redesign is needed. I think, because of the inductive reactance of a skirted tower, a phase advance T network will lead to best bandwidth performance. The basic design for a +90-degree phase advance looks like this:
WFAS +90 phase advance ATU, 1230 KHz, 1 KW, N-DA
To calculate the component values for the ATU, some basic arithmetic is required. The impedance value for each leg in a +/- 90 degree T network can be calculated with the following formula:
Z = √(inputZ × outputZ)
Where Z = impedance per leg Input Z = the ATU input impedance, 50Ω Output Z = the antenna resistance, 58Ω
Thus: Z =√ (50Ω × 58Ω)
Z = 53.85Ω
Formula for Capacitance: C = 1/(2Π × freq × XC)
Thus for the input leg: C = 1/(6.28 × 1.23MHz × 53.85Ω)
C (input) = 0.0024 μF
Formula for Inductance: L = XL/(2Π × freq)
Thus for the shut leg: L = 53.85Ω /(6.28 × 1.23 MHz)
L (shunt) = 6.97 μH
For the output leg, we must also consider the inductive reactance from the tower which needs to be cancelled out with capacitance. Thus, the output capacitor needs to have a value of 53.85Ω + 580Ω = 633.85Ω
Thus for the output leg: C = 1/(6.28 × 1.23MHz × 633.85Ω)
C (output) = 0.000204 μF
The amazing thing is, all of these components are available in the current ATU, they just needed to be rearranged. The exception is the vacuum variable capacitor, which I salvaged from an MW-5 transmitter many years ago. I donated that to the project, as I am tired of looking at it in my basement. The reason for the vacuum variable capacitor will become evident in a moment. The input capacitor will be slightly over value, which will require the inductor to tune out the excess capacitance. A good design rule is to use minimum inductance to adjust the value of a fixed capacitor, thus the capacitor should be not more than 130% of the required value.
About the Vacuum variable output capacitor; in the existing ATU had a 0.0002 μF capacitor already. With a +90° phase shift, this capacitor is likely adequate for the job. The vacuum variable may be pressed into service if something other than a +90° phase shift is needed for optimum bandwidth. That will be the topic of my next post.
Final consideration is the current and voltage ratings of the component. As this is a re-build using existing components, chances are that they already meet the requirements. On a new build or for replacing parts, one must consider the carrier power and modulation as well as any asymmetrical component to the modulation index. For current and voltage each, the value is multiplied by 1.25 and then added to itself. For a 1,000 watt carrier the input voltage on a 50 ohm line will be approximately 525 volts at 10 amps with 125% modulation. A good design calls for a safety factor of two, thus the minimum rating for component in this ATU should be 1050 volts at 20 amps, rounded up to the next standard rating. The capacitor on the output leg should be extra beefy to handle any lightning related surges.
The current rating for a capacitor is usually specified at 1 MHz. To convert to the carrier frequency, the rating needs to be adjusted using the following equation:
IO = IR√ FO
Where: IO: current rating on the operating frequency IR: current rating at 1 MHz (given) FO: operating frequency in MHz
The vacuum variable output capacitor is rated for 15,000 volts, 42 amps. Adjusted for frequency, that changes to 46 amps. The calculated base current is 4.18 amps carrier, 9.41 amps peak modulation. Thus, the capacitor on hand is more than adequate for the application.
I have been fooling around with Smith Charts lately. They look complicated, but are really pretty easy to understand and use, once you get around all those lines and numbers and stuff. Smith charts offer a great way to visualize what is going on with a particular antenna or transmission line. They can be very useful for AM antenna broadbanding.
The first thing to understand about a Smith chart is normalization. Impedance and reactance are expressed as ratios of value units like VSWR. A ratio of 1:1 is a perfect match. In the center of the Smith chart is point 1, which expresses a perfect match. To normalize, the load resistance and reactance are divided by the input resistance. Thus, if the input resistance is 50 ohms and the load impedance is 50 ohms j0, then the normalized Smith chart point would be 50/50 or 1. If the load impedance is 85 ohms and the reactance is +j60, then the normalized Smith chart point would be .58 1.7,1.2.
More basic Smitch chart usage information in this video:
I touched on the black art of AM antenna broadbanding before. It is a complex topic, especially where directional antenna systems are concerned, as there are several potential bottlenecks in a directional array. To explain this simply, I will use an example of a single-tower non-directional antenna.
Below is a chart of base impedance from a single tower AM antenna on 1430 KHz. The tower is skirted, 125.6 degrees tall. An AM tower that is expressed in electrical degrees is denoting wavelength. A 1/4-wave tower (typical for AM) is 90 degrees tall. A 1/2 wave tower is 180 degrees tall. Thus this tower is slightly taller than 1/4 wavelength.
Frequnecy(khz)
Reactance
Reactance (normalized)
Resistance
Resistance(normalized)
1390
-j 139
-2.78
405
8.1
1395
-j 143
-2.86
400
8.0
1400
-j 147
-2.94
350
7.0
1405
-j 146
-2.92
310
6.2
1410
-j 142
-2.84
270
5.4
1415
-j 132
-2.64
236
4.72
1420
-j 125
-2.50
210
4.2
1425
-j 118
-2.36
190
3.8
1430
-j 112
-2.24
170
3.4
1435
-j 106
-2.12
155
3.1
1440
-j 100
-2.00
138
2.76
1445
-j 93
-1.86
125
2.5
1450
-j 86
-1.72
114
2.28
1455
-j 79
-1.58
104
2.08
1460
-j 75
-1.50
95
1.9
1465
-j 70
-1.4
92
1.84
1470
-j 65
-1.3
85
1.7
The base impedance is not too far out of line from what is expected for a tower this tall. Plotted on a Smith Chart:
1430 base impedance plotted on a Smith chart
One of the first principles behind broadbanding an AM antenna is to distribute the sideband energy evenly and have a symmetrical VSWR. The antenna tuning unit will match the line impedance to the load impedance and cancel out the reactance. Having the proper phase advance or phase retard rotation will distribute the sideband energy symmetrically about the carrier. To determine phase rotation, the cusp of the plotted graph is rotated to face either the 3 o’clock or 9 o’clock position (0° or 180°). The cusp is where the direction of the line changes, which in this case is the carrier frequency, 1430 KHz. The above example, the line is fairly shallow, which is typical of a skirted tower. Thus, the best phase rotation to start with is +79°. This will likely be close but will need to be tweaked a bit to find the optimum bandwidth. After looking at the plotted Smith chart, my first inclination would be to reduce the rotation, more tower +75° as a first step in tweaking.
When working with AM systems, the bandwidth of the entire system needs to be examined. That means that final bandwidth observations will need to be made at the transmitter output terminal or in some cases, the input to the matching network. It varies on system design, but things like switches, contactors, mating connectors, ATU enclosures, etc can also add VSWR and asymmetry. Broadbanding even a simple one-tower AM antenna can require quite a bit of time and some trial and error.
