tech stuff – Engineering Radio

Locking AM station carriers to GPS

This is not a new idea, many people have discussed it in the past. The National Radio Systems Committee (NRSC) has a guidance paper, NRSC-G102 which gives a detailed explanation of why synchronized AM carriers are beneficial. There was even a move by some to have it included in the AM revitalization plan of a few years back. The NAB opposed this idea, saying it would be too expensive. That is unfortunate because out of all of the revitalization initiatives, GPS locked carriers had the best potential for an actual technical improvement. While it may be expensive for some very old tube type transmitters, for more modern solid state transmitters, GPS referenced carriers can be implemented as little as $200.00 US.

The FCC rule (73.1545(a)) for AM Carrier frequency specifies:

AM stations. The departure of the carrier frequency for monophonic transmissions or center frequency for stereophonic transmissions may not exceed ±20 Hz from the assigned frequency.

40 Hz is quite a bit of movement on a 20 KHz AM (18 KHz in ITU region II and III) channel. The reason for trying this is simple; there are many co-channel and first adjacent channel AM stations which at night, interfere with each other.

Above is a typical spectrum analysis of an AM station on 940 KHz. This was a 10 minute peak hold for an NRSC-2 spectrum mask measurement. The carrier is approximately 20 dB greater than the audio, which means that most of the interference between co-channel AM stations is created by the carriers beating against each other. By locking carriers to the same reference, that carrier interference will be greatly reduced. NRSC-G102 goes into great detail on the listenability of interfering stations with synchronous AM carriers (Page A-3).

Stations drifting off frequency also cause greater adjacent channel interference.

Almost all transmitters made in the last 30 years have an option to use an external frequency generator or 10 MHz reference. The required drive levels vary. The easiest way to implement this is by using a GPS locked programmable frequency source such as the Leo Bodnar LBE-1420. It can be programmed to any frequency from 1 Hz to 1.1 GHz, has a frequency stability of 0.000001 PPM (10^-12), and an output level of 3.3 V peak-to-peak. This drive level is not enough for some transmitters. For those situations an additional amplifier such as a Mini Circuits ZHL-3A+ is needed.

Here are a few AM transmitters that except an external RF source including a GPS disciplined oscillator.

Nautel J-1000

An external RF source can be plugged into the EXT RF IN connector (J-6) on the Remote Interface board. The source must be on the carrier frequency ± 5 Hz and have a peak-to-peak voltage of between 5 – 15 V (sine wave or square wave), 50 ohm impedance.

An external 10 MHz signal can be connected to the RF synthesizer board 10 MHz REF INPUT (J2). The external 10 MHz frequency reference must be precisely 10.00 MHz and have a peak-to-peak voltage of between 2.2 – 8.0 V (sine wave or square wave).

Jumpers on the Remote Interface Board and RF synthesizer board need to be configured appropriately for each source. Consult the manual pages 3-9 and 3-11.

Nautel ND series

An external RF source can be connected to ABA1J1 on the external interface board. The RF drive must be on the carrier frequency ±5Hz with level of between 5 – 12 V peak-to-peak (sine wave or square wave) and have a 50-ohm impedance.

Do not remove the crystal from the RF Drive board as the PDM frequency for the modulator is derived from it.

To select the RF drive source for the transmitter the links on the RF Drive board need to be changed. Consult the manual pages 3-3 and 3-14.

Nautel XL series

An external RF frequency source can be connected to the Exciter Interface board, J7. The external drive signal must be between 5 – 12 volts peak-to-peak (sine wave or square wave). Consult manual pages A1 and B4.

Nautel XR series

An external RF source can be connected to the remote interface board’s digital EXT RF IN (J6). This replaces the internal carrier frequency oscillator for one or both exciters (A/B). The external RF source must be the carrier frequency, within ± 5 Hz, have peak-to-peak voltage between 5 – 12 V (sine wave or square wave). Consult the manual pages 7-1.

Broadcast Electronics AM2.5 – AM10A, AM5E

The transmitter has an external RF input on the top of the unit (EXTERNAL RF INPUT). The input is designed for an external stereo generator or reference oscillator with a signal level from 5 to 15 volts peak-to-peak. To use this input, program jumper P7 on the exciter circuit board in position 1-2. Consult the manual page 2-19.

Broadcast Electronics AM500 – AM1A

The transmitter has an external RF input on the ECU rear-panel (EXTERNAL STEREO RF INPUT (J1). The input is designed for an external stereo generator or reference oscillator with a signal level from 5 to 15 volts peak-to-peak. To use this input, program jumper P7 on the exciter circuit board in position 1-2. Consult the manual page 2-20.

Harris Gates AM series

An external RF source can be plugged into J-1 on the Oscillator board. The source must be on the carrier frequency ± 20 Hz and have a peak-to-peak voltage of 5 volts. Frequency source selector P-6 must be set to external. Consult the manual, Oscillator Board Schematic.

Harris DX series

An external frequency generator can be connected to J2 on board A3. Jumper P5 should be set to either 20K ohms or 50 ohms depending on the source impedance. Jumper P6 can be set to either external source or automatic source selection. The drive level needs to be 4 to 4.5 volts peak-to-peak square wave for high impedance inputs or 0 to +25 dBm for 50 ohm impedance sources. Consult the manual, page A-2.

DX-50 oscillator board, A-17 external source connected

Newer DX series oscillator boards which have automatic source selection will fail over to the internal oscillator if anything happens to the externally generated RF signal.

Harris DAX

An externally generated carrier frequency or 10 MHz reference signal can be connected to connector J11 for the external carrier or J10 for the 10 MHz reference on the External I/O board. External carrier or 10 MHz reference must then be enabled via the VT100 screen. The external carrier frequency or 10 MHz reference must be above 2.0 volts peak-to-peak. Consult the manual page 3-15.

Harris 3DX

An externally generated carrier frequency can be connected to J12 (RF CARRIER) jack on the external IO board. The drive levels need to be 4 to 5 volts peak-to-peak, square or sine wave. On the carrier frequency +/- 5 Hz. The input is impedance is selectable for either 50 ohms or 10 K ohms.

An externally generated 10 MHz reference frequency can be connected to J10 (10 MHz REFERENCE). 10 MHz reference level needs to be 1 to 5 volts RMS, square or sine wave. The input impedance is selectable for either 50 ohms or 10 K ohms.

Programming for these options is done on the exciter setup page. Consult manual page 2-43.

Harris SX series

SX series transmitter have either an oscillator board or a frequency synthesizer board. Both will accept an external frequency source. The oscillator board is A16J1 and it needs a 5 volt peak-to-peak carrier frequency signal. Frequency source selector P-6 must be set to external.

The frequency synthesizer board external frequency input is also J1, however, it requires a 10 volt peak-to-peak signal. Frequency source selector P-6 must be set to external.

Conversion table for various RF power levels into a 50 ohm impedance

Volts, Peak-Peak	Volts, RMS	dBm	mW
2.2	0.77	10.8	12
3.3	1.23	14.35	27
5	1.76	17.9	62
10	3.5	23.9	250
12	4.24	25.5	360
15	5.3	27.5	562
20	7.07	30	1000

A 10 MHz reference input is preferred over direct carrier frequency generation simply for the ease of implementation. With direct carrier frequency generation, the frequency output of the GPSDO needs to be double checked. One misplaced digit and severe damage to the transmitter can result.

AM Tube type transmitters, plus early solid state transmitters such as the Harris MW1A may have instructions for implementing AM stereo. Since the AM stereo exciters generated the carrier frequency, those instructions would be a good guide on how to connect an external frequency generation source. However extensive modifications may be needed to the oscillator section depending on the transmitter.

Honestly, this is cheap enough that I think all new AM transmitters should come with this from the factory.

Audio Processing

Any radio station’s on air signal is its biggest marketing tool.

What sounds bad:

Over use of compression (gain reduction)
Over use of high frequency EQ
Over “equalization” on all frequencies
Over modulation
Overly aggressive composite clipping
Improper use of FM pre-emphasis
Poorly tuned transmitters (tube type)
Poorly matched antenna systems (all types)
Poor quality audio input
Over use of bit reduction on the STL
Analog STL’s that are off frequency
Playback of bad audio recordings

What sounds good:

Moderate use of compression to bring up audio levels for in car listening
Using equalization that suites format (e.g. more mid-range for all talk, more bass for urban, etc.)
Properly adjusted processor output levels for the correct modulation levels
Setting the pre-emphasis correctly
Tuning tube type transmitters for minimum distortion
Tuning antennas for adequate impedance and bandwidth
Making sure that audio input levels are correct, the audio is properly distributed and terminated with the correct impedance
Using STLs that have enough throughput that either no bit reduction or minimum bit reduction is used
Regularly check analog STL frequencies and re-adjust as necessary
Get rid of all bad audio recordings in the automation/playback system. Make sure that new files are from good sources and/or are re-recorded correctly

I took a little road trip between Christmas and New Years (Happy New Year!). I cannot help myself, I ended up tuning around the radio to see what was on. Suffice to say, I found the usual formats and a few locally focused stations. What struck me was the sound of some of the stations. While most sounded acceptable, if not somewhat generic, there were a few that had ear splitting, headache inducing audio. These stations were often over modulating and way over processed. It would have been better if there were no processing at all.

That got me thinking, what is or rather what should be the point of audio processing? Way back in the day, there were loudness wars. These were often program director ego induced efforts to sound louder than the competition because if you were louder, it meant you had more power. As listeners tuned their analog car radios from station to station, the signal that “jumped out” was mostly likely to attract more listeners. At least that was the way it was explained to me in the by a program director in the late 1980s.

We are no longer living in a listening environment where loudness is of huge importance. The number of audio sources has increased greatly; iTunes, Amazon Alexa, Spotify, Tune in, Pandora, YouTube Music, Sirius XM, iHeart, and AM/FM radio. Audio levels can be anywhere and listeners have gotten into the habit of raising or lower the volume as needed. Outside of program directors (or whatever they are called these days) offices, loudness means next to nothing. If you asked an average audio consumer how loud their program sounded, they would not likely know how to answer you.

I believe what most people are looking for is an enjoyable listening experience. The most important quality of any type of audio processing is that the product sounds good. The problem is “sounds good” is very subjective. Perhaps a better term would be technically sounds good. The audio should be free from distortion and artifacts of CODEC bit reduction. Overdone AAC or HE-AAC has this strange background swoshy platform behind everything which is headache inducing. Instruments should sound as they do when heard live. In other words, Susan Vega’s voice in the original Tom’s Diner should sound like Susan Vega.

Next would be compensating for difference levels in program material. A bit of gain reduction so that those in mobile listening environments can hear all of the program material. Finally, some format specific equalization can be useful. That is it. Moderate use of various audio processing tools can certainly accomplish those things. Like everything else, too much of a good thing is bad.

WKIP; tune up on shorter tower

This tower was previously part of a two tower DA. The taller tower was taken down and slowly replaced with a monopole to facilitate vertical real estate development. The shorter tower was retained as the radiator for WKIP-AM, 1,450 kHz, Poughkeepsie, NY.

Knowing that the tower is 85 degrees at 1,450,000 Hz, I calculated the height above the base insulator to be 48.816 meters. The tower face is two feet or 0.6096 meters (this becomes important). Using the chart, we can see that the theoretical resistance should be about 25 – 30 ohms:

The bottom or X axis on this graph is the ratio of the antenna height over the antenna diameter or 48.816/0.6096 meters or 80.

The reactance is slightly less clear according to this chart:

Between 80 and 90 degrees, a large phase shift occurs due to resonance. That means the reactance could be either negative or positive, but will likely be a low number, say +/- 5 ohms. That may be why this height was chosen for the second tower in this system.

And now for a bit of reality; all of that theoretical information is nice, but a measurement under power is where the rubber meets the road. Using the trusty OIB-3, I obtained a reading of 48 ohms base resistance and +j 37.6 reactance. Thus the base current should be 4.56 amps at 1,000 watts.

It was a little tricky setting up the OIB-3. The only place for it was far back in the ATU meaning I had to be careful reaching around active components while getting a reading. That being said, it is only 1,000 watts and in the end, no new RF burns were acquired.

The new Delta Electronics base current meter confirms the measured base resistance with the use of Ohm’s Law; I = √(P/R).

The theoretical information is useful for checking the component ratings in the ATU. The series capacitor on the output leg needs to handle the full carrier current plus 125% modulation. I calculate that to be 10.125 amps, so the 12 amp capacitor is sufficient. In the end, the actual base current was about half of the theoretical, so all good. The ATU is a standard T network with a capacitive leg to ground.

While construction was underway both taking down the old tower and putting up the new monopole, the base impedance of the radiator changed several times. Thus, we waited until all of the construction was completed and the monopole was detuned.

The skirt wires on the monopole are doing double duty. They are first, detuning for the AM tower located about 57 meters (186 feet) away. Next, they are a backup antenna system in case that main tower becomes unusable. This can happen from time to time as the swamp floods or if any type of tower work is needed. To do that I installed another J plug with the detuning network, which will be the normal position. To switch to antenna, it is moved to the antenna position. The base current meter is on the output leg, so it can be used to detune the monopole or measure the station output power.

I used the analyzer to get the detuning network close to resonant. The second step involved using the base current meter to touch up the tuning with the transmitter running into the tower 57 meters (186 feet) away. This is necessary because the two structures are close together. The skirt wires on the monopole pick up a lot of RF, therefore the stray capacitance on the inductor coil plays a role in the circuit. The net result is less inductance is needed when the transmitter is on. The resonance point will shift somewhat with ground conditions, but as long as the monopole impedance is high (above say 2K ohms) the structure should be invisible to the nearby 1,450 KHz radiator.

Monopole detuned for 1,450 kHz; impedance is 4.07 K ohms, at or close to resonance

The ATU for the monopole looks like this:

The operating impedance measurement shows a 47 ohm impedance, making the daytime base current 4.61 amps. It is coincidental that the two tower impedances are that close.

The new base current meter agrees with the impedance measurement.

Distance to Fault Measurements

The ability to do a Distance to Fault Measurement can greatly speed up the troubleshooting of potential antenna and/or transmission line problems. DTF measurements take on one of two forms; Time Domain Reflectometry (TDR) and Frequency Domain Reflectrometry (FDR).

TDR is the traditional method of measuring Distance to Fault. The test equipment sends short DC pulses down the cable and measures any return loss or SWR. Energy reflected back toward the instrument will plotted based on the time difference between the transmitted signal and the received reflection, similar to RADAR. This works well finding opens or shorts, but may not see lesser faults that could still be causing problems.

FDR is now common in most field models of Vector Network Analyzers. An FDR sends a frequency sweep down the cable then uses an Inverse Fast Fourier Transform (FFT) function to convert the information into a time domain. FDR can more reliably detect smaller issues with cables such as kinks, sharp bends, water in the cable, poorly applied connectors, or bullet holes. The piece of dented cable above would not have given a large reflection on a TDR, but on an FDR it would show up very nicely.

Like a VNA, an FDR needs to be calibrated for the sweep frequencies in use. The frequency span or bandwidth of an FDR has a major role in DTF measurements. A wider span will result in more precise fault information, however, it will reduce the over all length that the instrument can test. For most broadcast RF applications, cable lengths are less than 670 meters (2200 feet). Many instruments will adjust the maximum distance automatically based on the chosen span and velocity factor.

For my equipment, a Siglent SVA 1032X, the maximum distance for any frequency span can be found with this formula:

Maximum Distance (meters) = 7.86 x 10⁴ x Velocity Factor/Span (MHz)

Thus, to get best resolution sweeping a cable that is 670 meters long with a velocity factor of 86%: 7.68 x 10,000 x .86 / 95 MHz = 695 Meters maximum distance.

The resolution for any frequency span can be calculated with the following formula:

Resolution (meters) = 1.5 × 10² × velocity factor/Span (MHz)

In this case, the resolution would be +/- 1.35 meters. For shorter cable lengths a larger span can be used for better resolution.

My preference is to center the sweep frequency around the channel or frequency of the system under test.

To use a DTF function, a few inputs are needed:

The velocity factor of the transmission line
Cable attenuation for the swept frequency in dB/Meter
The approximate length of the line under test

The cable velocity factor and attenuation can be obtained from the manufacturer’s data sheet. Keep in mind that the manufacturer’s data is an estimation. These are usually pretty close to the actual number, but may vary due to tight bends in the cable, any splices, transitions to different cable, etc.

I had this used 1/2 inch RFS LCF12 50J cable in the barn, left over from project. Fortunately this is newer cable and it had the length marked out in meters. The beginning number was 0980 meters, the end was 1021 meters. Each meter marking has an asterisk before the number. I used a meter stick to measure out the distance between the asterisks and they are exactly one meter apart. I then measured the distance between the asterisk and the connector on each and ended up with 1.49 meters (4.9 feet) additional length, making the total length 42.49 meters (139.4 feet). The manufacture’s specification on velocity factor is 0.87 or 87% of the speed of light.

When I test this line with a 50 termination, it looks like this:

FDR DTF, 50 ohm termination at 42.49 meters

When I test the line either open or shorted, it looks like this:

I swept the cable at HF frequencies (3-30 MHz) since I think that is what this is going to be used for. At 3 MHz, the cable has a loss of approximately 0.003 dB per meter, which is inconsequential for this test. The velocity factor of 0.87 is pretty close. A longer run might indicate that it is actually 0.875 or 0.88.

Velocity factor and cable impedance are very important when using the Moment of Methods (MOM) system for AM antenna work. In that situation, both need to be obtained with a VNA for the FCC application.

The best practice is to sweep into a terminated line. In an AM system, a termination can most often be applied at the ATU input J plug. Sweeping into an antenna is possible, however there are several things that may lead to poor results. Most often, an FM antenna will look like a short on a DTF measurement. A UHF slot antenna will look open. In addition to that, the DTF measurement may be corrupted by any signals being received by the antenna while the system is under test.