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Brother, can you spare a theorem?

A theorem is not, indeed, a fact.  It is rather, an idea which 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:

 C =  B \log_2 \left( 1+\frac{S}{N} \right)

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 the 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, 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 the 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…

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.

Trends in Terrestrial Broadcasting, II

Things seem to be relatively quite these days, no earth shattering developments, no big news stories, etc.  My work load consists of mostly driving to one location and cleaning things up, then driving to another location and cleaning more things up.  Nothing really new to write about.  However, industry wide, there have been some developments of note:

  1. More AM HD radio only testing out in Seattle.  We hear that these tests are phenomenal but have yet to see any data.  The HD Radio proponents keep pushing for an all digital transition.  To that I say good, let those stations (AM and FM) that want to transition to all digital do so, provided they conform to the analog channel bandwidths and do not cause interference to analog stations.  It should also be an either/or decision: Either transmit in all digital format or revert to analog only format, no more interference causing hybrid analog digital.
  2. BMW depreciates AM radio in some models.  It seems the all electric car generates too much electric noise to facilitate AM reception.  My question; are these mobile noise generators going to cause reception problems for other vehicles too?  What if I want to hear the traffic on 880 or 1010 and one of these things roles by?  There are larger implications here and the FCC should be concerned with this.
  3. General Motors pauses the HD Radio uptake in some models.  No real reasons given, but more emphasis on LTE in the dashboard is noted.  We are reassured by iBquity that this trend is only temporary.
  4. Anxiously awaiting this year’s engineering salary survey.  For science, of course.  Here is last year’s survey.
  5. Clear Channel is no more!  They have gone out of business and a new company, iHeart Media, has taken over.  Things will be much better now, I can feel it.
  6. John Anderson finds a chilly reception at the last NAB confab: An Unwelcome Guest at the NAB radio show. This is not surprising but kind of sad. John has been a reasonable critic of IBOC and wrote a book titled: Radio’s Digital Dilemma.
  7. Not too much going on with the AM revitalization.  Tom King of Kintronics notes that the fault is in our receivers.
  8. Government shortwave broadcasters continue to sign off permanently.  Radio Exterior de Espana ceases operations.
  9. European long wave and medium wave stations are also throwing the big switch; Atlantic 252 (long wave), as well as German long wave stations on 153, 177,  and 207 KHz, medium wave stations 549, 756, 1269, and 1422 KHz also are signing off.  Those 9 KHz channel spacings look strange don’t they.  What fate awaits US AM radio stations?
  10. I am reading Glenn Greenwald’s book, No Place to Hide.  I knew this, you should know it too.

 

AM Stereo Renaissance?

At least in some quarters, there appears to be interest in reviving AM Stereo.  Perhaps as an unintended consequence of AM HD Radio, it seems.  Some people have discovered, quite accidentally, that some AM HD Radios will detect the presence of AM stereo pilot and open up with IF bandwidth automatically, making the analog signal sound much better. AM Stereo being received on an AM HD Radio receiver:

That particular brand of AM HD Receiver only allows 5 KHz audio, which still sounds much better than the typical 2.5 to 3 KHz.

A short video comparing AM HD Radio and AM C-QUAM:

As IBOC and C-QUAM are incompatible, it is an either/or situation.  Being that C-QUAM is open source and many new solid state transmitters come with AM stereo cards installed, the financial leap from AM mono to AM stereo is not nearly as steep as it would be to install AM HD Radio.  The other nifty thing;  C-QUAM is it is completely backwards compatible with existing AM mono receivers, the all digital version of IBOC is not.

It bears repeating; AM is not inherently inferior to FM sound.  Wide band AM can sound really, really good.   Something that we seemed to have forgotten over the years of listening to crappy receivers.  This has caught the attention of Tom King, owner of Kintronics, who penned the following letter to the FCC and all AM broadcasters:

Subject: Meeting with FCC Commissioner Ajit Pai and Mr. Peter Doyle,
Chief of the Audio Division of the FCC Media Bureau
at the offices of the FCC in Washington, DC on Tuesday, September 23, 2014.

To All AM Broadcasters in the USA:

Kintronic Labs is concerned about the declining position of the AM radio service in the United States, which we reflected in our Reply Comments to the FCC NPRM Docket No. 13-249 on the subject of “AM Revitalization,” issued on October 31, 2013. In the interest of preserving this great national resource for local public media, we have scheduled a meeting with FCC Commissioner Ajit Pai and Audio Media Chief, Mr. Peter Doyle, to address what we believe are the critical steps toward putting AM radio on a more competitive basis with FM as follows:

(1) FCC enforcement of regulations relative to the power distribution industry and the consumer electronics industry that are not currently being enforced, resulting in a constantly worsening electromagnetic environment for AM radio service.

(2) The need for parity between AM and FM receivers through the establishment of minimum technical standards for AM receivers that would become effective as soon as January 2016. We plan to demonstrate a comparison of full-bandwidth C-QuAM AM stereo reception with a local FM station and with a typical AM receiver in a popular consumer multi-band receiver. The effects of adjusting the AM bandwidth from 2.5 to 10 kHz in 2.5-kHz steps will also be demonstrated.

(3) The need for FCC authorization of AM synchronous boosters. Unlike FM translators, such on-channel boosters would serve to increase the AM stations’ audiences while concurrently maintaining the future viability of the band. The related technique of wide-area AM synchronization for coverage improvement will also be addressed.

Referring to Step #2, it is absolutely essential that very close to full parity be established for new AM radio receivers versus their FM radio counterparts. This includes all key AM receiver performance attributes, including:

Low internal noise floor, well below the average AM-band atmospheric noise level. This includes all internal synthesizer and DSP circuitry within the receiver (and in the immediate environment for integrated automotive applications).
High overall RF sensitivity, selectivity, and dynamic range, to provide adequate amplification of weak signals, even in the presence of significant adjacent- and/or alternate-channel signals, especially in strong-signal environments. This would incorporate typical advanced, multi-stage AGC action, with appropriate interaction between the RF and IF AGC control mechanisms to maximize overall receiver dynamic range, including adaptive front-end attenuation for signal-overload protection in very strong-signal areas. Useful typical specs include: sensitivity – 1 mV for 10-dB SNR; selectivity (adjacent-channel) – 25-50 dB (adaptive).
Highly effective noise (EMI) rejection, including staged RF and IF noise blanking, accompanied by appropriate audio blanking and/or expansion when required. Such features were developed and included in Motorola chip sets in the 1990’s in the AMAX program, and are easily integrated into modern, high-density AM/FM receiver chips.
Full 10-kHz audio bandwidth capability with low detector distortion. This would obviously incorporate dynamic, signal-controlled bandwidth control (including AMAX-style adaptive 10-kHz notch filtering) as dictated by noise and adjacent-channel interference.
Stereo capability. If the receiver has FM stereo capability, it must have corresponding C-QuAM decoding for AM.

Without fulfillment of the first three requirements (this also includes the associated AM antennas both for vehicles and for home use), basic AM reception will suffer significantly compared with FM. Without the last two, the output sound quality cannot be closely competitive with FM (i.e., 10-kHz full bandwidth on AM versus 15-kHz nominal for FM).

We therefore petition the FCC to mandate the following minimum allowable performance specifications for all AM receivers that will be manufactured and installed in new automobiles as of January 1, 2016:

Audio Bandwidth: 10 kHz typical, adaptive, with a minimum nominal bandwidth of 7.5 kHz
Signal-to-Noise Ratio: minimum 55 dB, preferably 60 dB
Sensitivity: -120 dBm for a signal-to-noise ratio (SNR) of 10 dB
Selectivity: 25-50 dB (adaptive filtering, using co-, adjacent-, and alternate-channel detection)
Dynamic Range: 100 dB
Noise Figure: 1 – 3 dB
Image Rejection: -50 dB
Intermod: IP2 , IP3 intercepts +10 to +40 dBm
IF: low with image-rejecting down-conversion, or double-conversion
Stereo Separation: minimum 25 dB

Respectfully Submitted,
Tom F. King
President

All of those technical specifications are doable with modifications to the current receiver chipset.  Currently there are very few if any AM Stereo receivers being manufactured.  One might ask, how can a typical AM mono receiver be modified to receive AM Stereo.  A great question.  For a small sum, an outboard circuit board can be purchased and installed in a typical AM mono receiver.  For most non-car radios, this modification would be fairly easy.  Car radios, on the other hand, will be very difficult to modify since most new radios will be bricked if tampered with (thanks a lot, crackhead radio thieves of New York).

And for those interested, there are also lists of radio stations broadcasting in AM stereo:

According to the Wikipedia source, there are 90 some odd station using C-QUAM AM stereo.  Using iBquity math, that is nearly the same number as are broadcasting AM HD Radio.

If you are an AM station owner, you can start by transmitting good programming.

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.

WVWA Nine Double Oh Radio

It seems branding and programming issues are a long running problem for radio stations. This is a copy of something that was made at WALL in 1974.  It has been circulated extensively in the NY metro market, but perhaps some of you from other areas or countries have not heard it yet. There is no WVWA 900 in Pound Ridge, it is a fictitious station:

What is hilarious is that the same exact this is still going on forty years later. How many times have programming consultants, program directors, corporate programming guru’s sat around and said “What we really need is a catchy name, like The Buzz or something.” I don’t know how many times I have heard “The X” or “The Eagle” or “fill in stupid name here.” Do the listeners really think “Oh wow, they changed their name, I will listen to this station now!” No, not likely.

The funniest part; “After more than 100 hours of extensive research… (the programming consultant) developed, refined, molded, polished, honed, shaped and pulled out of left field a revolutions new formatic programing concept…”  Play music, say  nothing, and scream “NINE!” between each song.

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.

Happy Labor Day

The Shively 6810 FM antenna

Update, W232AL:

The news is out; this is for the new “WFAS-FM” which is actually W232AL retransmitting the WPLJ HD-2 channel.  What do they call translators these days… Metro stations?  Something like that.  Anyway, quite a bit of work went into getting this off the ground before the start of Labor Day weekend and here it is!

We are currently working on a project that involves installing a Shively 6810 FM antenna. Since few people get to see these things up close, I thought I would post a few pictures.

This particular antenna is a four bay, half wave spaced directional antenna.  It is going to be side mounted on a 430 foot tower.  To do this, we had to lower the AM skirt wires by about fifteen feet and retune the AM antenna.

This Shively antenna came in seventeen boxes with sixty four pages of assembly instructions.  There are many parts and they need to be assembled in the order specified, otherwise things get in the way.  We found that Shively provided many extra bolts, washers, O rings, etc because things get lost.  Also, all of the parasitic locations and bay orientations were clearly marked.  One thing that the tower crew said; always check the allen screws and other hardware on the elements before installing the RADOMES.

Shively 6810 installing elements

Shively 6810 installing elements

Since this is a half wave antenna, the radiating elements are 180 degrees out of phase, bay to bay.

Shively 6810 mounting brackets

Shively 6810 mounting brackets

Stainless steel tower leg mounting brackets.

Assembled element with RADOME.  This is the top bay with the gas pressure release valve

Assembled element with RADOME. This is the top bay with the gas pressure release valve

Shively 6810 top bays staged for hoist

Shively 6810 top bays staged for hoist

We hoisted two bays at a time. The top bays are ready to go up.

Shively 6810 top two bays lift

Shively 6810 top two bays lift

The bottom two bays were hoisted next.

Shively 6810 four bays installed

Shively 6810 four bays installed

This is the antenna installed, less the tuning section and parasitic elements. It is tilted off axis from the tower by design due to its highly directional nature.

The transmission line was installed and we swept the antenna. I will snap a few final pictures once the transmitter is installed, which will happen tomorrow.

Updated Pictures: Here are a few pictures of the finalized installation:

W232AL antenna, new installation on WFAS AM tower

W232AL antenna, new installation on WFAS AM tower

The fully installed antenna, tuning unit and transmission line. We did some program testing, made sure the audio sounded good, then the station was signed on. We also had to lower the AM station’s skirt a few feet and retune the ATU. Actually, the ATU needed to be reconfigured because the shut leg had been disconnected and there was a capacitor added to the circuit after the base current meter.  All of that was fixed, along with a few other things…

W232AL transmitter, a BW Broadcast TX300 V2

W232AL transmitter, a BW Broadcast TX300 V2

The W232AL transmitter is a BW Broadcast TX300 V2. These little transmitter are packed with features like a web interface, on board audio processing, etc. They are pretty neat.

Its Friday, time to go home!

It’s Friday, time to go home!

The tower crew from Northeast Towers did a great job, as they always do.