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Lightning protection for WLAN links

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, VIOP 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 tranceivers 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 tranceiver 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 tower, difficult to transition base insulator of series fed AM tower.
  • Ubiquiti Rocket or similar system where the antenna and tranceiver are separate, but the transciever 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 need with the ethernet cable 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 my 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 proper 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.  A good video from Ubiquiti, which makes TOUGHCable, on application of connectors to shielded Cat5 cable is here:

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 dataline protector

Transtector APLU 1101 series dataline 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 dataline protector

Transtector APLU 1101 series dataline protector

There are various models designed to pass POE or even 90 VDC ring voltage.

Transtector APLU 1101 series dataline protector

Transtector APLU 1101 series dataline 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.

Broadbanding an AM antenna

Many articles have been written on the topic and it is still a black art to some.  Making a Medium Frequency (MF) antenna that has enough bandwidth to pass 10 KHz audio can be challenging, to say the least.  The VSWR out to +/- 15 KHz carrier needs to be kept at a minimum and the power needs to be evenly distributed between the two sidebands.  This can become problematic with complex Directional Arrays or towers that are tall or short for their operating frequency.

When we were working on the WFAS-AM tower in White Plains, NY, it became apparent to me that something was not right.  The tower is skirted and now holds the antenna for W232AL, a 250 watt translator broadcasting the WPLJ HD-2 channel.  After installing the FM antenna, some tuning of the AM antenna was required and this is the graph of the resistance and reactance curves:

WFAS 1230 KHZ, ATU output resistance and reactance

WFAS 1230 KHZ, ATU output resistance and reactance

This looked very similar to the resistance and reactance curves before the FM antenna work was done.  Red line is resistance, the blue line is reactance.  I think it had been like this for a long time.  While it is not terrible, it is not that good either.  As alluded to in a previous post, some re-working of the ATU was needed.  After some trial and error, this is the circuit that we ended up with:

WFAS 1230 KHZ White Plains, NY ATU schematic

WFAS 1230 KHZ White Plains, NY ATU schematic

Not quite what I expected, however, it was designed with the parts on hand, excepting the vacuum variable output capacitor, which was donated by me.  That part was key in making the proper adjustments.

After my redesign and tune up of the ATU, this the resistance and reactance curves at the input terminal of the ATU:

WFAS 1230 KHz resistance and reactance after ATU modification

WFAS 1230 KHz resistance and reactance after ATU modification

The graphs have a slightly different format, but you get the idea.  The red line is resistance, the blue line is reactance and the green line is overall impedance.  The resistance is symmetrical about the carrier as is the reactance.  Truth be told, I think there is a little more that can be had here, but for now, there is no reason to go any further.  I made the initial measurements at the input of the ATU and confirmed them again at the output terminals of the transmitter.  When we turned the transmitter back on, I noticed that the modulation index had dropped by about 15 percent.  I think the reflected power was getting back into the RF sample and fooling the mod monitor.  I also noticed that the high end in particular sounded much nicer.

WFAS 1230 KHz, White Plains, NY ATU

WFAS 1230 KHz, White Plains, NY ATU

The ATU building is a little cramped and it is hard to get a good picture.  The vacuum variable capacitors were salvaged from a scrapped AM transmitter years ago.  The tower is 202 degrees tall, which is also a factor.  It will be interesting to see what seasonal changes there are with snow cover, mud, etc.

Overall, this was a fun project.

Designing an ATU

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

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 lagging ATU

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 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.

The Smith Chart

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.

Smith chart

Smith chart

.pdf version available here: smith-chart.

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.

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

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.

I will touch on ATU design in the next post.

Troubleshooting an AM array

Today, there will be a quiz.

Recently, we had an AM antenna array go out of tolerance by a good margin.  This has been repaired, however, I though I’d post this information and see if anybody could identify the problem and the solution. Unfortunately, I don’t have a prizes to give away, however, you can show off your AM engineering prowess.

All of the information is pertinent:

  1. The station has two directional arrays (DA-2) using the same towers; the night time array is out of tolerance, the daytime array is not effected and is performing normally.
  2. There were no weather events connected with this event; no electrical storms, no major temperature changes, no rain events, no freezing or thawing, etc.
  3. The problem happened all at once, one day the array was performing normally, the next day it was not.
  4. Station management reports that some listeners were complaining that they could no longer hear the station.
  5. The ATU’s and phasor were inspected; all RF contactors were in the proper position, no damaged or burned finger stock, no evidence of damaged components (inductors or capacitors) was observed.  Several mouse nests were cleaned out of the ATU’s, however, this did not change the out of tolerance antenna readings.
  6. The towers are 1/4 wave (90 electrical degrees) tall.

Readings:

Tower Phase angle as licensed Current ratio as licensed Phase angle as read Current ratio as read
1 147.2 0.583 149.5 0.396
2 (reference) 0 1.00 0 1.00
3 -137 0.493 -125.8 0.798
4 107.5 0.481 92.7 0.355
5 -38.1 0.737 -60.2 0.623
6 -178.7 0.382 142.8 0.305

Licensed values for common point current is 13 amps, impedance is 50 ohms j0 and there is normally no reflected power on the transmitter.  On this day, the common point current readings were 8.9 amps, impedance 38.5 ohms +j5 the transmitter had 340 watts of reflected power.

This is the overall schematic of the phasor and ATU:

WDGJ overall RF schematic diagram

WGDJ overall RF schematic diagram, click for higher resolution

Aerial view of transmitter site, oriented north:

WGDJ aerial view showing towers as identified in schematic diagram

WGDJ aerial view showing towers as identified in schematic diagram

So, where would you begin?  Ask questions in the comments section.

Axiom


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.

Congress shall make no law respecting an establishment of religion, or prohibiting the free exercise thereof; or abridging the freedom of speech, or of the press; or the right of the people peaceably to assemble, and to petition the Government for a redress of grievances.
~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.
~Benjamin Franklin

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.
~Rudyard Kipling

Everyone has the right to freedom of opinion and expression; this right includes the freedom to hold opinions without interference and to seek, receive and impart information and ideas through any media and regardless of frontiers
~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.
~Alan Weiner

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