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 trouble shooting 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 back story 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 was likely had a good bit of capacitive reactance. I am speculating on that, as I cannot find the original paper work 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 the 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 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 on 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
IO: current rating on 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 is 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, 1.2.
More on basic Smitch chart usage information on 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 bottle necks 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 wave length. 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 wave length.
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 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.
This is a Webinar video from Nautel about their Radio Coverage Tool:
Highlights of the Nautel RF tool kit:
Analyze proposed transmitter location’s coverage
Tower heights can be adjusted
Antenna gains can be changed
Transmitter power levels
Includes Terrain data
Includes population within coverage areas
Frequency Range 30 Mhz to 3GHz
Useful for general broadcast or point to point systems
This can be a useful tool for those looking to gauge realistic coverage of a station in terrain challenged areas. It can also be useful for studying STL paths, RPU coverage, etc.
One problem is the power levels and antennas are preset, with the minimum setting 200 watts into a two bay antenna. These settings are too high for use when investigating a potential LPFM. For that, the Radio Mobile Online (which is the engine behind the Nautel RF tool kit) can be accessed directly via www.ve2dbe.com/rmonline.html. Requires an account, which is very easy to set up. For most users, FM broadcast band frequencies will not be available, however 2 meter amateur frequencies (146 MHz) are the default, and for all practical purposes, will model coverage in the FM band (88 to 108 MHz) just fine.
By creating a hypothetical LP100 transmitter site, the coverages between the FCC 60 dBu contour and the actual coverage based on terrain can be compared. This is the FCC 60 dBu coverage contour:
Example 60 dBu contour, LP-100 station
According to the US Census data, this station has a population coverage of; 30,721 in the 70 dBu or 3.162 mV/m contour, 92,574 in the 60 dBu or 1 mV/m contour, and 165,183 in the 50 dBu or 0.316 mV/m contour. Courtesy of REC Network. The 60 dBu contour is considered the protected area licensed for use by the FCC.
Looking at a coverage terrain map, the picture changes somewhat:
Example coverage map, LP-100 station
This is based on predicted receiver location using terrain data; receiver antenna height 1 meter, 90% reliability, minimum signal level 10 µV (20 dBu, yellow, very good car radios) and 31.62 µV (30 dBu, green, good radios and indoor reception). Areas to the south and east of the transmitter are shaded by a large hill, thus they show low or no signal on the terrain based coverage map. UN Population data indicates the yellow has 178,573 and the green area has 72,014 persons. This map does not take into account co-channel and adjacent channel interference, which there is sure to be.
When comparing the two maps, one can see the coverage holes in the terrain map that are within the 60 dBu contour. There may also be a slight difference in populations covered because the FCC map uses 2010 US Census data and the Radio Mobile Map uses UN population data. For general planning purposes, the area shaded in green would be a safe bet on good reception, all other things being equal.
Since the LPFM stations are very limited in their ERP, finding a good transmitter site which will cover the desired area will be key to a successful operation.
Working with RF can produce some head scratchers. Most transmitter manufactures tend to use the same type of connector for things like exciter RF outputs and composite inputs. Over the years, I have become well stocked with all sorts of BNC and Type N connectors. Satellite equipment uses Type F connectors, Analyzers use Type N, Oscilloscope uses BNC, GPS equipment uses SMA and so on. Except when they don’t. As any good engineer will tell you, when they don’t will be in the middle of the night at some mountain top location while the station is off the air.
After one such incident, I invested in a TPI-3000A adaptor kit. This kit has both the male and female versions of Type N, F, SMA, BNC, TNC, UHF, UHF mini and RCA. They can be mated in any combination using the Universal interface.
TPI 3000A adaptor kit
On more than one occasion, this little kit has meant the difference of between being back on the air or driving down the mountain to look for an in between series adaptor. A couple of recommended additions include a 7/8″ and 1 5/8″ EIA flange to type N male.
They can be a little pricey, however, I have seen several for sale on eBay for less than $100.00. The key to not loosing the various little parts to this kit is to write a little note detailing the date and location where the adaptor was used, then stuff it in the empty hole. Hopefully, when permanent repairs are made, the adaptor will be retrieved.
Readers of this blog will know that I enjoy history. Old photos are great things to study, as they say, picture… thousand words… etc. Here is one that I found on the RadioMarine website:
WER radio, 192X?
Here we have three gentlemen at work at an early radio station. It seems like a posed shot, nobody can study a meter that intently. They are sitting directly in front of the transmitter and it looks like the antenna tuning coils are behind the operating position. Notice the open wire and transmission line, presumably all under power when this picture was taken. There seems to be no concern about RF or electrical safety, I suppose it was trial and error back then, with a heavy price paid for error. Meter boy should be careful not to back up too far, if he does, he’ll get a little behind in his work.
We’ve been a little busy this last week, I’ll catch up on the blogging this weekend, there are many things to tell.
We have a Harris Z5-CD transmitter for one of our FM stations. Brand H is not my preferred make, however, it was already installed when we bought the station, so I have to live with it.
This particular site gets hit by lightning strikes often. Normally, it does not affect anything until the transmitter gets turned off for maintenance. Then, almost invariably, when turning the transmitter back on one of the modules will fail. Most often this is manifest when one of the two power supplies shut down causing the transmitter to run no more than 20% power.
The way this is trouble shot is to slide each module out and turn the transmitter back on. When the power supply stays on, the bad module has been located. A confirmation test is to check the MOSFET for a short circuit between Drain and Source. This short circuit condition puts a direct short on the power supply causing it to crow bar and turn off.
So, once the bad module has been located, and the spare module is installed in the transmitter, then what? Most engineers call Harris and ship the module back for repair. Most engineers don’t want to mess with unsoldering a surface mount MOSFET and soldering a new one in. I find it moderately entertaining to fix things myself, so I do not do what most engineers do.
NXP BLF177 MOSFETS
The MOSFET in this particular module is the BLF177, made by NXP. Harris will sell you one for quite a bit of money. You can also buy one from Mouser for about half the cost.
Harris FM Z series transmitter PA module with cover removed
Once the parts are obtained, the worst part of the entire job is unsoldering the old MOSFET. This takes some patience and skill. What I found works best is to melt some solder on the foil leads and get them good and hot. Since this MOSFET is already destroyed, we don’t have to worry about heat etc. The one thing you do not want to do it actually break the MOSFET open. That is because it contains beryllium oxide, a known carcinogen. Once all the solder is liquid, carefully pry the foil up with a small screw driver. There are several components that have to be moved to work on this.
Harris Z series PA module with MOSFETS removed
After the old MOSFET is removed, clean up the solder pad with a solder pump and solder wick. I like to use a little liquid flux on the solder wick, it makes things go faster.
Once all the old solder is cleaned off the solder pads, I brush a light coat of liquid flux in the pad. Again, this makes things go faster.
Harris Z series FM transmitter module new MOSFETs waiting to be soldered
The new MOSFETS are very sensitive to static discharge, so I always use a static drain wrist band when handling. I place both MOSFETs on to the circuit board. I then solder them on using as little heat as possible from the soldering iron. Again, the MOSFETs are sensitive to heat and one can easily be destroyed if it gets too hot.
Harris Z FM series PA module repaired
This is the module with the new MOSFETs soldered in. I use defluxing compound to remove all the extra flux. Once it cools off, I test the new module with a DVM:
Harris Z series FM PA circuit board under test, resistance is 3.3 Mohm
If the MOSFETS are good, they will have an internal resistance of around 3.3 MΩ. If the module is bad the MOSFETS will read only a few ohms if shorted:
Harris Z series FM PA module under test, DVM reads 1.6 ohms
That is how you do it. I think Harris charges $775.00 per module to repair. I fixed this one for $240.00, but that is not the reason I did it. I did it for the fun that was in it.
A pessimist sees the glass as half empty. An optimist sees the glass as half full. The engineer sees the glass as twice the size it needs to be.
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~1st amendment to the United States Constitution
Any society that would give up a little liberty to gain a little security will deserve neither and lose both.
The individual has always had to struggle to keep from being overwhelmed by the tribe. To be your own man is hard business. If you try it, you will be lonely often, and sometimes frightened. But no price is too high to pay for the privilege of owning yourself.
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~Universal Declaration Of Human Rights, Article 19
...radio was discovered, and not invented, and that these frequencies and principles were always in existence long before man was aware of them. Therefore, no one owns them. They are there as free as sunlight, which is a higher frequency form of the same energy.