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Medium Frequency ATU design

This is a topic I have covered before, but it is worth doing it again for future reference.  The previous post covered downgrading an AM transmission facilities for WGHQ, Kingston, NY.

This is part II of that process.

WGHQ transmitter site, towers 1 and 2 removed

WGHQ transmitter site, towers 1 and 2 removed

The old towers have been cut up and put in a scrap metal dumpster. They are off to China to be melted down and made into a submarine or a missile or a tank or something useful like that.

Towers scrapped

Towers scrapped

The directional array had a three towers in a straight line with a common point impedance of 60 Ohms.  Dropping two towers greatly changed the electrical characteristics of the remaining tower, therefore the existing ATU needed a bit of reworking to match the 50 Ohm transmitter output.

First step, correct a few deficiencies left over from the old array.

Vise grip tower feed

Vise grip tower feed

This vise grip RF connection has to go. The problem is where the tower erectors attempted to solder the copper tubing.  That tower base plate is pretty big and I would wager they didn’t use enough heat to make the solder connection.  They were probably working in the winter time, thus the “temporary” fix.  This tower was put up in 1993, so that temporary fix lasted 23 years.

I removed the offending tool and soldered the connection to another part of the tower with silver solder.  The smaller cross bar made a good connection point.

RF feed correctly connected to the tower

RF feed correctly connected to the tower

After soldering, I cleaned up and sprayed some grey primer on it to prevent rust forming where I scraped the paint off.

Next, I made an impedance measurement:

WGHQ tower base impedance measurement

WGHQ 920 KHz tower base impedance measurement

That junk on the upper part of the graph is coming from WHVW on 950 KHz. The tower itself looks pretty good, 77.6 Ohms resistance with 130 Ohms inductive reactance.  Since this is not a part of a directional antenna system, the ATU design is pretty straight forward.  Given that WHVW on 950 KHz is located 10.41 miles away, a low pass filter design is optimum.  A basic low pass filter T network has inductive input and output legs with a capacitive shunt leg to ground.

T network diagram

T network diagram

Each leg is used to match the 50 Ohm transmission line impedance (R1) to the 77.6 Ohm tower impedance (R2) and cancel out the 130 Ohms of inductive reactance.  This is a vector impedance problem, much like a vector force problem in physics.   Some basic arithmetic is required (always include the units):

X1, X2, X3 = √(Zin x Zout)

X1, X2, X3 = √(50Ω x 77.6Ω) or X = 62.28Ω

The value of inductance or capacitance for each leg is calculated using the basic inductance or capacitance formulas:

L (μH) = XL / 2πf(MHz)

And

C (μF) = 1 / 2πf(MHz) XC

Thus the input leg, or X1 = 62.28Ω / (6.28 x 0.92 MHz) or 10.78 μH

The Shunt leg, or X2 = 1 / (6.28 x 0.92 MHz x 62.28Ω) or .0028 μF

The output leg is a little different.  The tower has 130 Ohms of inductive reactance that needs to be cancelled out with a capacitor.  Rather than cancel out all of the inductive reactance, then add an inductive output leg, the tower reactance can be used as part of the tuning circuit.  The design calls for 62.28 Ohms inductive reactance, so 130Ω – 62.28Ω = 67.27Ω, which is the value needed to be cancelled by a capacitor:

Output leg, or X3 = 1 / (6.28 x 0.92 MHz x 67.27Ω) or .0025 μF

A little Ohm’s law is used to calculate the base current for both the day and night time operations.

Ohm's law pie chart calculator

Ohm’s law pie chart calculator

Thus the daytime base current is I = √(P/R) or I = √(1000 W/77.6Ω) or 3.58 Amps.

Night time base current is I = √(38 W/77.6Ω) or 0.70 Amps

Current handling requirements:

Base current is calculated to be 3.6 Amps at 1,000 Watts carrier power.  Allowing for 125% peak positive modulation makes it 5.7 Amps.  Having safety factor of two or 11.4 Amps output leg and 14 Amps input leg.

Voltages: 353 maximum input voltage, 439 output.

Thus, 20 amp, 10 KV parts should work well.

The designed schematic for the ATU:

WGHQ ATU Schematic diagram

WGHQ ATU Schematic diagram

Putting it all together.

Since the tower looks fairly broad at 920 KHz, we are going to attempt a nice broadband ATU to match it.  This station is currently programmed with a classic country format, and I have to tell you; those old Conway Twitty, Merle Haggard, Patsy Cline, et al., songs sound pretty good on the old AM radio.  The Subaru stock radio has HD, which also has a nice broad IF section, thus allowing all those lovely mid-high range frequencies through.

This is the existing ATU, which I believe was built by Collins in 1960:

WGHQ Tower 3 ATU

Existing WGHQ T network ATU

The ATU building is a little rough, but the ATU itself is in remarkable shape for being 56 years old.  The input leg inductor is in the center and will be reused as is. The large Jennings vacuum capacitor at the bottom is a part of the shut leg.  Its value is 2000 pF at 15 KV.  The top vacuum capacitor is series output cap, its value is 1000 pF at 15 KV.  The basic plan is to move the upper cap down in parallel with the bottom cap.  The shut leg inductor will be kept in place to tune out any access capacity.  For the output leg, I have a 2500 pF mica cap and a 10-100 pF variable cap connected in parallel.  The inductor on the output leg will be removed.

After some re-work on the ATU components, I tuned everything up.  The easiest way to do this is to disconnect the legs, measure them individually and adjust them for the desired reactance, which in this case is 62.28 ohms or thereabouts.  The output leg was measured with the tower connected since the tower reactance is a part of the tuning circuit.  The input leg was right about 10 μH.  The shunt leg turned out to be about 0.002 μF.  This is often the case, theoretical values are slightly different than field values due to stray capacitance and inductance in the connecting straps, etc.

This is the load, as measured at the output terminals on the transmitter:

WGHQ tower load as measured at the transmitter output terminals

WGHQ tower load as measured at the transmitter output terminals

Slightly asymmetric on 910 KHz, but overall pretty good. There is a fair amount of phase rotation in the transmission line due to the length from transmitter to tower (855 feet, 260.6 meter), which works out to be 0.93 wave length allowing for the 86% velocity factor of the transmission line.

Time to pack up and go home.

Downgrading an AM radio station

WGHQ in Kingston, NY has been downgraded from a 5KW DA-1 to a 1KW non-DA system.  This was done because two of the three towers in the directional antenna array dated from 1960, were in very rough condition and needed to be replaced.  The remaining tower (furthest from the transmitter building) had been replaced in 1994, is in good condition and is being kept as the non-directional radiator.

Here are a few pictures:

WGHQ 3 tower directional antenna array, Port Ewen, NY

WGHQ 3 tower directional antenna array, Port Ewen, NY

More deferred maintenance

More deferred maintenance

RF and tower light feed disconnected from tower base

RF and tower light feed disconnected from tower base

Second tower base vegetation not as bad, tower disconnected

Second tower base vegetation not as bad, tower disconnected

WGHQ transmitter and original Collins phasing cabinet

WGHQ transmitter and original Collins phasing cabinet

First tower video (sorry, I appear to have no idea what I am doing with the camera):

Second tower video, this one is better:

Towers on the ground:

I made measurements on the third tower and constructed a temporary ATU with parts on hand to get the station back on the air. They are now running 1 KW day, 38 watts night, as per their CP. I will be going back up to finish the job once the brush has been removed from around the existing tower and the ATU building has been repaired.  The coverage with 1 KW is not bad, actually:

Predicted coverage map from FCC website

Predicted coverage map from FCC website

The translator is on its way.

The inglorious task of AM antenna array maintenance

AM radio stations are rough customers. They frequently operate on the margins, both in terms of ratings and revenue. Their transmitter plants are complex and very often have been on a reduced maintenance schedule for years, sometimes decades.  Those of us that understand the operation of AM transmitter plants and all their quirky behaviours are getting older.  I myself, feel less inclined to drop everything and run off to the AM transmitter site when things go awry.  Seldom are such efforts rewarded, much less acknowledged.  Station owners are also finding that their previous demands are unrealistic.  For example, time was that any work that takes the station off the air had to be done after midnight.  These days, I can tell you, I will not be working at your radio station after midnight.  You can find somebody else to do that work.

Thus, today, we took this particular AM station off the air from Noon until 3 pm to diagnose and repair a problem with the four tower daytime array.  Once again, this involved a shift in common point impedance and a drastic change in one tower’s current ratios.

Antenna Tuning Unit, mice have made a mess

Antenna Tuning Unit, mice have made a mess

In all fairness to the current owner, this ATU reflects years of neglect. At some point, mice made a home in here and created a mess. The ATU smells of mouse shit, piss and mothballs.  It is full of mouse droppings, grass seeds and fur.  All of the ATUs in this array are in similar condition.

Paper wasp, inside ATU

Paper wasp, inside ATU

It was warm enough that the wasps were active, if not a little bit lethargic.

Broken stand off insulators in ATU

Broken stand off insulators in ATU

This coil is being held up by the tubing that connects it to other components. When the ATU was built, no nylon or cork bushings were used between the insulators and the wall of the ATU they were mounted on. Heat cycling eventually did all of the insulators in.

Catwalk to the other towers

Catwalk to the other towers

Catwalks to the other towers. At least the swamp grass has been cut this year, it is only four feet tall instead of ten.

Tower base

Tower base

The tower bases are all elevated above the theoretical maximum water level. The ATUs are also up on stands with platforms build for maintenance access.

ATU Work "platform"

ATU Work “platform”

I cannot even blame the current owner, who has spend considerable money to make repairs and upgrades to this site. It is very difficult and very expensive to catch up with deferred maintenance. Sadly, most AM stations we encounter have similar or worse problems.

I think it is too late to save many of these AM stations.  The technical issues, lack of revenue, perceived poor quality, lack of good programming are all taking their toll.  At this point, the hole is so deep there is no hope of ever getting out.  The FCC’s faux interest in “revitalization” followed by two years of stony indifference seems to be a final, cruel joke.

The old school marketing campaign

I found these old drawings in the filing cabinet and thought they were kind of cool. They look like they were drawn sometime in the 50’s for the WPTR studio at 1860 Central Avenue in the Town of Colonie.

WPTR-billboard

It looks like there was a lot of Neon, including a speller, which I take to mean the sign would spell “W-P-T-R 1540” then turn off again.

This was the sign for the entrance to the studio building

WPTR sign for front of old studio building at 1860 Central Avenue

WPTR sign for front of old studio building at 1860 Central Avenue

I think this is a take off on the old KHJ sign in Los Angeles.

Something fun

So, I spent wasted several hours on this SDR website over the holiday weekend:

University of Twente SDR website

University of Twente SDR website

This is a web based SDR hosted by the University of Twente in Enschede, Netherlands. I enjoyed listening to the European medium wave and shortwave stations available.  Something that is always fun to checkout: UVB-76 on 4625 KHz.

Have fun!

PS: A special thanks to all those who have donated to the cause via the donate button on the upper right side bar.  I had enough money to buy a FUNcube dongle SDR.  I think I have all the other necessary hardware to launch one of these sites myself.  If or when that happens, I will post a link here.

Accidents, mistakes, mishaps and other tales

Lets get started:

Results of a deer vs automobile accident

Results of a deer vs automobile accident

It does not look like much, however, that is about $5,500.00 worth of damage. What you don’t see is the mashed oil cooler and radiator. This happened on my way from one place to another during the early morning hours. I was traveling at about 55 MPH when a deer bolted from the woods and entered the roadway from the right. I did not have time to break.

In a ditch

In a ditch

A momentary lapse of attention causes loss of $80.00. I think I was adjusting the defroster as I was driving down the road when suddenly, I felt the car tilt over to an alarming degree. You can see the tow truck getting ready to pull it out. Fortunately, there was no damage to the vehicle.

Troubles with the neighbor

Troubles with the neighbor

This is on the access road to one of our transmitter sites. The station has a legal right of way through this property, however, the neighbor seems to object. I spoke with him and showed him a copy of our deed, he has since changed plans.

One side of a balanced audio connection disconnected

One side of a balanced audio connection disconnected

This is the downside of using category cable to make audio connections. The wires are not as rugged as say Belden 8451. This was causing problems because it is at an AM studio/transmitter site.

Burned 30 amp three phase contactor

Burned 30 amp three phase contactor

Three phase, 30 amp, 240 volt contactor installed in a 480 volt system. Lasted a few years, anyway.

White face hornets nest

White face hornets nest

New tenants on one of our towers. This is a white faced (or bald faced) hornets nest. They are really paper wasps, but that difference aside, these beasts are nasty, aggressive and have a painful sting. Normally, I am a live and let live kind of person, but in this case, they gotta go.

Dummy load attached to plywood

Dummy load attached to plywood

This is at one of our AM clients site. Somebody, quite some time ago it seems, made this test load for a 1 KW AM transmitter. It is very nice, carbon ceramic resistors, 50 ohms and surprisingly little reactance. Then they attached it to this piece of plywood. As one can surmise, the load gets quite hot under full power, full modulation conditions. We remounted this in a cage type enclosure and bolted it to the cinder block wall.

Scala PR-950U cross polarized

Scala PR-950U cross polarized

The client at this station is complaining of intermittent STL drop outs and low signal strength at the receive end. Found this Scala PR-950U antenna mounted for vertical polarization, but the antenna element is horizontally mounted. We’ll call it “vorizontal.”

Ribbon cable from a Cummins 135 KW generator

Ribbon cable from a Cummins 135 KW generator

This was discovered during routine maintenance and thankfully not during a power outage. Mice got into the control box of a newish Cummins 135 KW generator and chewed through what looks like a data buss cable. The generator would not run and the cable and control board needed to be replaced.

Bulging capacitors

Bulging capacitors

There is more bulging capacitors removed from flat panels monitors.

And so on…

Don’t try this at home, kids

Happy New Year, and stuff.

I found this interesting little video on Youtube recently:

That has to be a fairly high powered AM radio station to have that effect. According to the video, this is in Ukraine.

Other than generating RF burns to the hands, there is also the issue of exposure to non-ionizing radiation causing body tissue heating. Then there is the potential broadband RF interference from the arcing plant matter. This can cause interference to STL’s and other receivers.

Whatever happened to the CFA?

Remember way back when, perhaps in high school or college, you met this really cool person who seemed to be wonderful in every way? Yeah, then you got to know them a little better and, well, those first impressions changed a little bit.

Crossed Field Antenna, Courtesy of Wikipedia

Crossed Field Antenna, Courtesy of Wikipedia

The Crossed Field Antenna (CFA) sort of reminds me of my first prom date.  There was a lot of promise there, but plans fell through.

From a 1999 Radio World article:

Imagine an AM antenna one–fiftieth of a wavelength long, that needs no radial ground system, occupies a small parcel of land, produces little or no RFI (Radio Frequency Interference), has great bandwidth and performs better than a full–sized vertical radiator.

This potential new antenna was all the rage during the early 00’s or whatever you call that decade.  I remember thinking to myself; I will believe it when I see the test results.  At one point, there was a battery of tests run in the installation in Egypt and China.  The test results are spotty at best, however, none of these installation performed up to expectations.  While it looks like a cool idea, and it would have been great to see it succeed, it seems that sheer will power alone will not make a particular system work outside of the laws of physics.  There are a few of these still in operation out in the wild, mostly in Egypt.

Fixing small problems

This happened recently at an AM station we were doing work for. It seems the modulation monitor was not working when connected to the backup transmitter. A quick check of the RG-58 coax showed that I had the correct cable plugged into the monitor selector relay.  Another check with an ohm meter showed the cable was okay.  Then I looked at the connector on the monitor port of the transmitter and saw this:

BNC connector pin  improperly located

BNC connector pin improperly located

Looks like the pin is too far back in the connector. This is an old style BNC connector with a solder in center pin:

BNC connector center pin

BNC connector solder type center pin

The center pin has a blob of solder on it, preventing it from seating properly in the connector body. I could have lopped it off and applied a new crimp on connector, but my crimp tool was in the car. I didn’t feel like walking all the way through the studio building, out into the parking lot and getting it. Therefore, I used a file and filed off the solder blob then reassembled the connector:

BNC connector

BNC connector

The transmitter was installed in 1986, I think the connector had been like that for a long time.

It may seem like a small detail to have the modulation monitor working on the backup transmitter, however, the modulation monitor is also the air monitor for the studio.  Switching to the backup transmitter but not having a working air monitor would likely have caused confusion and the staff might think they are still off the air.  I know in this day and age, a lot of station do not even have backup transmitters, but when something is available, it should work correctly.

I like my cool network analyzer and all that, but sometimes it is the Mark 1, Mod 0 eyeball that gets the job done.

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.

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

Free counters!