tech stuff – Engineering Radio

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).

This 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. 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.

AM field strength measurements

I have been finishing up a project which required detuning a new monopole installed near an AM tower. One requirement was a series of field strength measurements along six evenly spaced radials around the AM tower. The point is to see if there is any effect in the omni-directional AM signal (there should not be).

For this, I used the venerable Potomac Instruments FIM-41. As I recall, these units are rather pricey. The frequency range is 0.5 to 5.0 MHz. The basic measurement unit is a Volt/Meter, which is an electric field measurement. Something that measures 1 V/M means that the electric potential between two objects 1 meter apart is 1 volt. The meter will also make measurements in dBm, which is a logarithmic electromagnetic field strength measurement.

Before making any measurements, it is a good idea to check the batteries. Also, the hinged lid part is a loop antenna and there are several contact fingers which can get a little dirty which may effect the measurement accuracy. These should be cleaned off with some alcohol and a q-tip. I have also seen a pencil eraser used.

The directions for meter calibration are on the inside of the lid. Even though I have done this type of measurement a thousand times, I always do a quick read through the directions just to make sure I don’t skip any of the steps. Depending on the power of the signal being measured, I like to calibrate the meter at least a mile or so away from the AM antenna system.

Check the battery with the function switch in the Batt position. The meter should read within the Batt range
Tune the signal with the function switch in the FI-Cal-Tune position. This should be done at some distance away from the antenna system. Tune for maximum meter reading.
Rotate the FIM until the signal is below 10 mV/M, switch the full scale switch to CAL and adjust the CAL OSC for maximum meter reading.
Switch the Function switch to CAL NULL and adjust the GAIN control to minimum meter reading.

To take readings put the function switch in CAL TUNE and the Full Scale switch at whichever position results in an on scale reading. On less one of the knobs gets bumped, the meter only needs to be calibrated once.

Measurements should be made three or more hours after local sunrise and three or more hours before local sunset. This is to prevent other sky wave signals from interfering with the measurements. The first measurement should be greater than five times the tower height, in this case more than 240 meters.

I used Google Maps to generate a set of points along each radial then noted the coordinates and a brief description on a spread sheet. Since everything was on Google Maps, it was easy to navigate from one point to the next:

Field strength readings follow the inverse square law. Whatever the increase in the distance factor from the radiator, the electrical field will decrease inversely by the square of that factor. Thus, if the distance increases by 3, the field will decrease by a factor of 9.

This can be seen in a field strength vs distance graph which I plotted on an Excel spreadsheet:

Field strength in mV/M, distance in Meters

You can see around 2 KM away, there is something re-radiating the signal. This was near a college campus with lots of vertical metal structures around. There are two readings which should probably be thrown out to smooth out the curve.

At one point, further out along this radial, my car was attacked by a Rottweiler. The dog owner just stood in his front yard and watched it happen. After he got the dog back under control, I rolled my window down and told him what I thought of his dog. It is for this reason, I have a dash camera in my work vehicles. Too many times things happen while driving.

Repeat this five more times and call it a day!

After 10 years, it is time to move

W277CJ, Pittsfield was originally a translator for WUPE-AM 1110 KHz. Since that license has been surrendered to the FCC, it is now a translator for WBEC-FM HD2 which is simulcast of WUPE-FM, North Adams.

Confused yet? Don’t worry, it is a reshuffling of signals because the land under the 1110 KHz transmitter site was sold and the license turned in to the FCC. Something that I think will happen many more times to many more stations in the coming years. This translator was first put on the air in June of 2015. This is the third part of a series, the first two parts are: More AM work, Part V and The Bext TFC2K broadband antenna.

The translator recently moved it off of the Holiday Inn (formerly Crown Plaza) in downtown Pittsfield, MA to the WBEC AM tower. In order to make that move, we needed to do several things;

“sufficient measurements shall be made to establish that the operation authorized in this construction permit is in compliance with the spurious emissions requirements of 47 C.F.R. Sections 73.317(b) through 73.317(d). All measurements must be made with all stations simultaneously utilizing the shared antenna.”

These are intermodulation products, or third order products, between the two signals being transmitted, in this case W277CJ on 103.3MHz and WUPE-FM 95.9 MHz. The antenna side mounted on the AM tower serves as a back facility for WUPE-FM.

WBEC-FM backup and W277CJ connected to Bext FDCSDC-2 combiner

Those measurements are as follows:

(F1-F2) + F1 or (103.3 MHz – 95.9 MHz) + 103.3 MHz = 110.7 MHz
F2 – (F1-F2) or 95.9 MHz – (103.3 MHz – 95.9 MHz) = 88.5 MHz
F2 +F1 or 95.5 MHz + 103.3 MHz = 199.2 MHz

In order to make those measurements, I used two Microwave Filter Company MFC-6367 notch filters to attenuate the carriers on 95.9 and 103.3 MHz. This keeps the spectrum analyzer from overloading, thus lowering the analyzer noise floor and giving better results.

Various tools for proofing FM installations

Over the years, I have collected various parts to assist in getting good measurements for FM proofing. Going clockwise and starting at the top, the Rhode Schwarz NRP-Z11 power sensor, the MFC-6367 FM notch filters, directional couplers with power extractor element, various attenuators including the HP 255C variable 0-12 dB unit, and in the middle are two Mini-Circuits NHP-200+ high pass filters. The high pass filters are great for measuring harmonics.

To measure the third order products noted above, I first measured the carrier without the filters and an appropriate pad to get a carrier reference level. Then installing the MFC-6367 filters to measure the third order products. In addition to that, harmonics of both FM transmitters out to the 10th harmonic. Of particular importance is anything in the cellular or mobile data bands. All of these measurements were well below the -80 dBc threshold required by the FCC.

All of these measurements were well within the limits established by FCC part 73.317.

Also, because this is mounted on an AM tower, there are some AM things that needed to be completed:

“The AM station identified below may be affected by the facilities authorized by this construction permit. Pursuant to Section 1.30004 of the Commission’s Rules, at least 30 days prior to commencement of construction of the facilities authorized herein, the permittee must provide notification of the construction to the AM station licensee. As part of this notification, the permittee must examine the potential impact of the construction of the authorized facilities on the AM station using a moment method analysis. The analysis shall consist of a model of the AM antenna together with the potential re-radiating tower in a lossless environment. The model shall employ the methodology specified in Section 73.151(c) of the Commission’s Rules, except that the AM antenna elements may be modeled as a series of thin wires driven to produce the required radiation pattern, without any requirement for measurement of tower impedances. If the AM station was authorized pursuant to a directional proof of performance based on field strength measurements, the permittee may, in lieu of the moment method analysis, demonstrate with measurements taken before and after construction that field strength values at the monitoring points do not exceed the licensed values.”

Since this station was proofed several times, we did about ten readings along the monitor point radials, both before and after.

The new isocoupler was properly mounted:

This is simply a large coil of 7/8 Coax wound inside of a PVC form.

These AM antenna systems are a regulatory nightmare. Although the Moment Method is an improvement over the system of field measurement proofs, it is still complicated. Part of the issue with AM in general is the expense of the the antenna systems, particularly anything that is directional.

No real research into Medium Frequency antennas and propagation has been done since the 1930’s. Perhaps we know all there is to know about it, then again, perhaps not. I am currently working on a project which will study Medium Frequency propagation, which I feel, is the first step into revisions of antenna design.