Installing another couple of these stations recently in the New York/Canadian border region. In this case, WTKJ-LD now transmitting from Cape Vincent, NY. This is owned by Sagamore Hill broadcasting and is retransmitting the NBC affiliate from Watertown, NY.
This is a pretty simple set up; BE 600 Watt UHF TV transmitter, Pro Television Exciter, 6 pole Dielectric Filter, and an 5 panel UHF antenna.
The shelter was made by Broadcast Electronics, it is somewhat small, but serviceable.
The LG window unit works well enough to keep the shelter cool. The transmitter runs at about 35% efficiency, so if the TPO is 470 watts. Thus the transmitter is putting 300 watts of heat into the room continuously.
The local cord cutters can get the following channels:
19.1
1080i
DD5.1
WVNC-NBC
19-2
480i (w)
DD2.0
Antenna TV
19-3
480i (w)
DD2.0
ION
19-4
480p (w)
DD2.0
Grit
19-5
480p (w)
DD2.0
Bounce TV
19-6
480p (w)
DD2.0
Court TV
19-7
480i (w)
DD2.0
QVC
19-8
480i (w)
DD2.0
SonLife
The weather up here is great! Cape Vincent is a nice small village with some decent local businesses. Unfortunately, summer is their main focus and many of them have closed down for the season. Still, there is a decent cup of coffee and the local market has a deli section that makes good sandwiches.
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.
Damaged six inch coax
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:
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.
Cable to test
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.
Manufacture’s markings
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:
FDR DTF, cable shorted at 42.49 meters
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.
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.
I pulled this out of the published posts and updated it.
I have been working on updating some wiring at one of our client’s transmitter sites. I noticed that an off air monitor feed was going back to the studio on a Barix box, which is fine. It was being fed from a balanced output of a DA to the unbalanced input on the Barix box. This being at the transmitter site, was susceptible to RF noise. I decided to make a passive audio BALUN.
Balanced to unbalanced audio converter using 10K to 600 ohm transformer
In any case, there are several ways to go from balanced to unbalanced without too much difficulty. The first way is to wire the shield and Lo together on the unbalanced connector. This works well with older, transformer input/output gear, so long as the unbalanced cables are kept relatively short.
simple balanced to unbalanced audio connection
Most modern professional audio equipment has active balanced input/output interfaces, in which case the above circuit will unbalance the audio and decrease the CMRR (Common Mode Rejection Ratio), increasing the chance of noise, buzz, and so on getting into the audio. In this case, the CMRR is about 30 dB at 60 Hz. Also, newer equipment with active balanced input/output, particularly some brands of sound cards will not like to have the Lo side grounded. In a few instances, this can actually damage the equipment.
A Henry Match Box or something similar can be used. I have found, however, the active components in such devices can sometimes fail, creating hum, distortion, buzz, or no audio at all. Well-designed and manufactured passive components (transformers and resistors) will provide excellent performance with little chance of failure. There are several methods of using transformers to go from balanced to unbalanced or vice versa.
Balanced to unbalanced audio using 1:1 transformer
Using a 600:600 ohm transformer is the most common. Unbalanced audio impedance of consumer-grade electronics can vary anywhere from 270 to 470 ohms or more. The 10,000-ohm resistor provides constant loading regardless of what the unbalanced impedance. In this configuration, CMMR (Common-Mode Rejection Ratio) will be 55 dB at 60 Hz, but gradually decreases to about 30 dB for frequencies above 1 KHz.
Balanced to unbalanced audio using a 4:1 transformer
A 600:10,000 ohm transformer will give better performance, as the CMMR will be 120 dB at 60 Hz and 80 dB at 3 KHz, remaining high across the entire audio bandwidth. The line balancing will be far better for the high-impedance load. This circuit will have about 12dB attenuation, so plan accordingly.
For best results, use high-quality transformers like Jensen, UTC, or even WE 111C (although they are huge) can be used. I have found several places where these transformers can be scrounged, DATS cards on the old 7300 series Scientific Atlanta satellite receivers, old modules from PRE consoles, etc. A simple audio BALUN can be constructed for little cost or effort and sound a whole lot better than doing it the wrong way.
A brief list, there are other types/manufacturers that will work also:
Ratio
Jensen
Hammond
UTC
Edcor
1:1 (600:600)
JT11E series
804, 560G
A20, A21, A43
PC600/600
4:1 (10K:600)
JT10K series
560N
A35
PC10K/600
Keep all unbalanced cable runs as short as possible. In stereo circuits, phasing is critically important, so pay attention to how the balanced transformer windings are connected.
Device Under Test; THD at 20 KHz
As for cost; I purchased the Edcor PC10K/600 transformer on eBay for $20.00 and the Hammond 1590B Enclosure was about $9.00. The audio jacks and resistor were in the parts drawer. It took about 20 minutes to layout the holes, drill, mount the audio jacks, and solder the jumper wires. I used a tie-base, wire tie, and some Gorilla glue to hold the transformer down. I used a 1/4 inch TRS jack because the enclosure was a little bit too small for an XLR jack. If a stereo pair needed be converted, it would require two of everything.
Overall, I fun project. The old Simpson 260 is still accurate!
Checking the accuracy of a Simpson 260 on audio frequencies.
