tech stuff – Engineering Radio

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.

The General Electric T1000C Stereo Receiver

My parents had one of these units on the side table in the dining room. My father put up an FM antenna outside on the roof so he could listen to more stations. In the early 1960s, there were not as many around as there are today. Our house was on the wrong side of a hill for the NYC stations, although Peekskill seemed to come in just fine. What is fascinating to me is the timing and cost. These stereos were made in 1963, not long after the Zenith/General Electric FM stereo system was adopted and first broadcast on WGFM (now WRVE) in Schenectady, NY (June 1, 1961). Not every FM station rushed out to install the new system.

General Electric T1000C Stereo receiver marketing

For a bit of a reference, $180.00 in 1963 is worth $1,868.65 in 2025. At that time, my father was an installer/repairman for New York Telephone. My mother was not working and six of us lived under one roof. That was quite a bit of money for an AM/FM radio.

The radio was normally tuned to 100.7 WHUD, which initially went stereo in 1972. Other stations that could be received: WGFM, WROW-FM, WSPK, WEOK-FM (now WPDH), and WGFH (later WINE-FM now WRKI).

General Electric T1000C stereo (Walnut cabinet)

I purchased this one on eBay for $70.00. It turns on (in fact, it did not turn off), there is a hum, the pots are scratchy, etc. However, if I tune it to one of the local AM stations, I can hear music under the loud 60-cycle hum. In other words, it works! So, I spent time fixing all the defects and enjoyed some nostalgia. According to this date stamp, the wood enclosure was made in January 1963. I would think the rest of the unit was made about the same time, which means this is one of the first radios in this model. This may have been manufactured in Bridgeport, CT, or Syracuse, NY. The serial number is missing from the back of the chassis.

The main source of the hum appears to be this capacitor, which clearly has seen better days.

The on/off problem was the selector switch, which stuck in the on position because it was gunked up with dried-up lubricating oil and dust. I cleaned it with denatured alcohol and DeOxit.

The parts list included about $15 worth of capacitors, $1 for a new rectifier diode, a $7 telescoping FM antenna from Amazon, and $6.32 for two PLT 12 6.3 volt miniature lamps for dial light.

All of the tubes look to be the original GE units. After the recap, I turned it on and there was nice sounding AM, but no FM. The FM RF section has a triple triode (V2) which is the AFC, 10.7 MHz Oscillator, and mixer. This tube was loose in its socket and needed to be reseated. After that, everything worked.

All of the pots were scratchy. I cleaned them with DeOxit and worked them back and forth many times. After a while, they all are working.

FM Stereo receiver MPX decoder block diagram

I found a Sam’s Photofact (basic service manual) on this set. What is very interesting is the schematic for the multiplex receiver. This section decodes the L+R/L-R signals and produces the stereo audio. Unlike modern FM stereo receivers, in which the broadband multiplex signal is fed into one side of a chip and the discrete L/R signal comes out of the other side, the signal path through the various processing stages can be followed.

The broadband MPX signal comes from the IF stage via wire #27. The signal is amplified by V6. The L+R (20 Hz to 15 KHz) or mono signal goes through a low-pass filter L17/C40; the 3dB cutoff should be around 16-17 KHz. The L-R and 19 KHz pilot goes to wire 34, thence through a high-pass filter C37/L16/C38; the cutoff should be 20 KHz or so. The L-R and 19 KHz pilot are Amplitude Modulated subcarriers on the FM signal. Wire 38 routes the MPX signal to V6 which recreates the 38 KHz subcarrier by doubling the 19 KHz pilot. This is filtered by a bandpass filter C13/L14. The L-R and the 38 KHz subcarrier are sent to the product detector.

Diode product detectors X4 and X5 (1N541) demodulate the lower sideband (23 – 37.98 KHz) and the upper side band (38.02 – 53 KHz) respectively. Those signals are summed in the matrix subassembly K4 with the L+R. Mathematically, the results are:

The Left and Right audio is then sent to the first audio stage V7 through a deemphasis network. If no 19 KHz pilot is detected, no 38 KHz carrier is recreated and this stage remains silent. In other words, you have to find an FM station in mono first, then flip it to stereo to see if there is enough signal to decode the L-R. One of the limitations of the first generation of FM stereo receivers. Newer versions of this set have a stereo light, or “Stereo Eye” so the listener knows when stereo reception is possible.

The front of the cabinet is covered with glass, which I cleaned with soapy water. The glass has part of the gold leaf trim rubbed off. I think this radio got a lot of use.

I let the knobs soak in soapy water overnight then cleaned them off with an old toothbrush. I believe that this radio was once in a smoking environment, based on the amount of yellow, gooey substance covering everything. I ended up disassembling the entire unit to clean it. I used a paintbrush and the shop vac to get all of the dust out of the cabinet.

The speakers and speaker cones are in good condition. The speaker cabinets needed a little work; in both cabinets, the fronts (the part that is seen when both speakers are “closed”) were popping off. I had to glue a bit of wood back together and fix the metal holding brackets. The cloth on the speaker side is a little faded.

General Electric T1000C restoration complete

The wood finish is in good shape with a few scratches and dings. I decided to use Howard Restore-A-Finish. This is not the same as stripping and refinishing but rather repairing the existing finish. There was a water ring on top of the cabinet, which was removed with the Restore-A-Finish and light use of steel wool.

Three power supply capacitors, held down by a ty-base glued to the chassis

Reassembly went about as expected. I glued these tie bases to hold up the new capacitors.

The receiver is fairly sensitive and the dial is accurate. There is an alignment procedure in the repair manual, but I think everything is working as it should. I have spent enough time trying to fix things that are already working to know that for a 1963 tube receiver, this is good enough. Perfection, as they say, is the enemy of everything else.

So, how does it sound? Pretty darn good, as it turns out. I am working on a brief YouTube video with some religious music (I’ll post it when it is done). On the FM side, I can get WAMK, WBPM, WKXP, WJUX, WDST, WPDH, WFSO, and WPDA clearly with the whip antenna on the radio. AM, I hear WGHQ and WJIP.

I can hear the old man now, humming along to his favorite tune…