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SBE Chapter 22 Shindig

I am mulling over attending this year’s SBE Chapter 22 Expo in Verona, NY.  It has been quite a while since I have attended any of these events, so it may be nice to walk around and look at the Exhibition hall and see what is new.  There also appears to be some engineering presentations.  Verona is between Utica and Syracuse and is also noted for the Turning Stone casino.  Looks to be about a three hour drive from my house.

If you are attending and want to meet up for a few beers or something afterwards, shoot me an e-mail.

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.

A Whole Lotta Silence

That is what we hear coming from NOAA All Hazards radio station WXL-37 these days.  It has been off the air for the last thirty days or so.  I suppose a 91% (or less) up time is acceptable in these circumstances.  This is the NOAA radio station that is supposed to cover the mid-Hudson Valley area just north of NYC.  It’s a good thing there are no potential hazardous conditions out there this time of year.  Nope;  no need to worry about tornadoes, hurricanes or even severe thunderstorms.  Not to mention any of the other potential hazards in the Hudson Valley.  It is not like a tornado ripped through an elementary school and killed a bunch of kids.  Nobody from the area was ever stuck by lightningWe do not get hurricanes, ever, ever, ever.  No earthquakes, forest fires, there are no nuclear power plants nearby.  No child abductions, never ever any terror attacks, hijackings, civil unrest, wide spread power outages, etc.  It is not like any public schools, private schools, fire departments, county EOC’s, broadcast LP-1 and LP-2 stations, or cable head ends are monitoring this station anyway.  Nope.  Not at all.

The question that should be asked is why do we even need NOAA All Hazards radio station WXL-37?  That is a good question in light of the fact that the station has been off the air since July 26th.  After all; we, the tax payers, are paying for this.  So… what are we paying for?

To answer this question, one needs to understand how government employment works.  We are paying for some bureaucratic, paper pushing pencil neck geek to check another day off the calendar on the road to his or her retirement.  Whether or not any work gets done on that day is strictly coincidental. Working for the government means that the ultimate supervisor is a nameless, faceless, grinding bureaucracy that does not tolerate procedural deviation and wields ultimate power by threatening the loss of pension.  Original thought is to be scrubbed out of the system.  Continued original thought will activate the explosive chair wheels which will then propel the former employee out of the door with the aforementioned loss of pension condition.

When the station first went off the air, we took the network analyzer up to the site and basically fabricated an antenna to work on 162.475 MHz.  We were able to get the station back on the air with 200 watts, however; we did the wrong thing.  Even though this is enough power to hit the cable head end, the LP-1 stations and at least two of the county EOC’s, turn it off.  This was not a part of the plan.  It makes no difference that the primary source for emergency weather information is off the air for four weeks in the middle of thunderstorm season, must stick to the plan, no exceptions.

Makes sense to me.

Transportation

Broadcast Engineering from a contracting stand point requires a lot of driving. I mean a lot of driving.  Since switching from full time Director of Engineering to a contracting field engineering position, I have already worn out two vehicles.  Having reliable transportation is a key component of this job. Of course, the other consideration is the price of gasoline which can range from expensive to horribly expensive depending on the warring or not warring that is currently taking place.

Thus, when it came time to replace my strange looking but roomy and reliable Scion xB, I did some research.  My complaint about the xB, other than the looks, was lack of ground clearance and lack of all wheel or four wheel drive.  After a bit of reading, it seemed the Subaru Crosstrek XV was a good choice.  Long story short, I got my car last week and got a pretty good deal, as the car dealer was looking to get rid of all their 2014 stock.

2014 Subaru Crosstrek XV

2014 Subaru Crosstrek XV

As I was leaving the dealership, the salesman had one final question.  The conversation went something like this:

Sales guy: “Mr. Thurst, can I ask what it was that sold you on this car?”

Myself: “Sure, it was the oil filter.”

SG: “What?”

Me: “The oil filter.”

SG: “No, I heard that, I just don’t understand.  It wasn’t the price or the fuel economy or the features?”

Me: “Nope.  To be honest, you did give me a good price, I like the all wheel drive, the ground clearance, the gas mileage and all that.  But when I popped the hood to look at the engine and saw the oil filter, I was sold.”

SG: “No one has ever said that before.  Welp, good luck and thanks for buying your car from us.” (now walking backwards into the dealership,  smile fixed on his face and nodding slowly)

Here is a picture of the Subaru FB20 boxer engine:

Oil filter location on a Subaru FB20 engine

Oil filter location on a Subaru FB20 engine

See the oil filter right next to the oil fill plug, up right and easy to get to.   Not only that, some design engineer put a catch basin around the filter mount, knowing that when the filter was unscrewed, all the oil would run out of it.  Without the catch basin, that oil would run down the engine block creating a mess that would get worse with each oil change.

Little things.  Little things mean a lot.

Transmitter repair

Sometimes it is obvious and relatively easy, other times not so much.  This summer we have had wave after wave of afternoon thunderstorms.  It is almost like living in Florida; almost, but not quite.  Anyway, with the storms occasionally comes some lightning damage.  At most of the transmitter sites we service, every step has been taken to ensure good grounding and adequate surge suppression.  This is especially true of sites that have been under our care for a few years.  Even so, occasionally, something gets through.  After all, those five hundred foot steel towers do attract lightning.

Broadcast Electronics AM5E output tuning section

Broadcast Electronics AM5E output tuning section

This is the output section of the BE AM5E transmitter at WROW.  The transmitter got pretty trashed; a bad PA module and power supply and this capacitor in the output section.  This particular transmitter is 14 years old and this is the first major repair work we’ve had to do it.

Broadcast Electronics AM5E output tuning capacitor

Broadcast Electronics AM5E output tuning capacitor

The capacitor was fairly easy to change out.  As a general precaution, both capacitors were changed.  There was a spare PA module and power supply on the shelf, thus the transmitter was returned to full power relatively quickly.

Broadcast Electronics AM5E output forward and reflected power meters

Broadcast Electronics AM5E output forward and reflected power meters

The rest of the antenna system and phasor were inspected for damage, a set of common point impedance measurements taken, which showed that no other damage was sustained.

Next, the 30 year old Harris SX2.5 A transmitter at WSBS.  This failure was slightly more exotic; the transmitter started randomly turning itself off.  The culprit in that case was this:

Harris SX2.5 remote control interface bypass capacitor

Harris SX2.5 remote control interface bypass capacitor

Literally, a two cent part.  The transmitter remote control uses opto-isolators.  The inputs to these opto-isolators are RF bypassed to ground on the back of the “customer interface board.”  After determining that the remote control was not malfunctioning, it was down to either a bad opto-isolator or something really silly like a bypass capacitor.  This capacitor was on the ground side of the remote off terminal.  It shows short on the capacitance meter and 4.1 K on the ohm meter, just enough to randomly turn the opto-isolator on and shut down the transmitter.  Being a Harris transmitter, removing and replacing the “customer interface board” was no easy matter.  Overall, it took about three hours to find and repair this problem.

Fewer owners means more diversity!

Alternate title: Less is more (and other non-sense)!

The NAB has come out with their latest interesting opinion on radio station ownership in comments to the FCC regarding the 2014 Quadrennial Regulatory Review.  They state that “Retaining the local radio ownership rule unchanged would be arbitrary and capricious” because the audio market place has changed radically over the last ten years.  The introduction of online listening via Pandora seems to have created competition that can only be adequately dealt with by further consolidation, it seems.  Also, the Commission cannot demonstrate that the current rules promote localism or viewpoint diversity.  That last sentence is a fair statement.  What the NAB does not say is that there is no evidence that further consolidation will promote localism or viewpoint diversity either.

The comment then goes into a lot of information and statistics on smart phone usage; who has them, what they are using them for et cetera.  It is very interesting to note that there is no reason given for the sudden and alarming upswing in mobile online listening.  But, let us examine a few interesting data points first:

  • Mobile data is not free.  There are very few unlimited mobile data plans out there anymore, most everyone now has some sort of data cap.  Extra data can be purchased, but it is expensive
  • On line listening uses data at a fast rate.  According to Pandora, they stream at 64 kbps, or 0.480 megabytes per minute or 29 mega bytes per hour.   Spotify uses quite a bit more, 54 megabytes per hour.

Let us assume that the average commute to work these days is one hour.  That would mean two hours per day of driving and mobile listening.  That adds up to 1.16 GB of data per month just in on line listening.  Assuming that the smart phone functions as more than just a radio and will be used for email, maps, news, web browsing and other downloads, a fairly hefty data plan would be required of the smart phone user to accommodate all this data.  Why would somebody pay considerably extra per month just to listen to online radio?

Do you get where I am going with this?  Good, compelling programming is what people are searching for.  If they cannot find it on the radio, they will go elsewhere.  Nature abhors a vacuum.  Want to compete against Pandora, Spotify, XM or whoever?  Offer up something good to listen to.  These days, competition seems to be a dirty word.  Yes, competition requires work, but it, in and of itself, is not bad.

The NAB seems to be saying that relaxing ownership rules and thus, presumably, allowing more consolidation will promote diversity.  In my twenty five years of broadcasting, I can say that I have never seen this to be the case.   Some of the most diverse radio stations to be heard are often single stations, sometimes an AM/FM combo, just out there doing their thing.  Stations like WDST, WHVW, WKZE, WHDD, WJFF, WTBQ, WSBS, WNAW… I am sure that I am forgetting a few.

You can read the entirety of the NAB’s comments here.