TV sweeps

We have been really busy this fall working on multiple projects plus the day-to-day tasks. One thing that is always fun; sweeping antennas with a VNA.

In this case, WVIT Hartford, CT needed to repair a leaking transmission line section just below the antenna. To ensure that there would be no problems with return to the air at full power, we did a before sweep and after sweep.

WVIT is the ATSC 3.0 lighthouse station for the Hartford Market. has the station listed as ATSC 3.0.

WVIT Tower, Hartford CT

The WVIT tower is 1,100 feet tall and is located on Rattlesnake Mountain near Farmington, CT. Most of the other Hartford TV stations are on the same hill.

Tower crew, hitching a ride to the top
Selfie; return loss looks good

It is always interesting to see new places and meet new people. This site has an auxiliary TV studio, which they were using during COVID.

Working with rigid transmission line

Update: This post is from several years ago (January 31, 2018), however, I did a fairly major revision and added a lot of information, so I am bumping it to the top of the pile. The header picture is from the Myat facility in Mahwah, New Jersey.

Installing transmitters requires a multitude of skills; understanding the electrical code, basic wiring, RF theory, and even aesthetics play some part in a good installation.  Working with rigid transmission line is a bit like working with plumbing (and is often called that). Rigid transmission line is often used within the transmitter plant to connect to a four-port coax switch, test load, backup transmitter, and so on.  Sometimes it is used outside to go up the tower to the antenna, however, such use has been mostly supplanted by Heliax-type flexible coax.

We completed a moderate upgrade to a station in Albany; installing a coax switch, test load, and backup transmitter.  I thought it would be interesting to document the rigid line work required to complete this installation.  The TPO at this installation is about 5.5 KW including the HD carriers.  The backup transmitter is a Nautel VS-1, analog only.

This site uses a 1 5/8-inch transmission line.  That line is good for most FM installations up to about 10-15 Kilowatts TPO.  Beyond that, 3-inch line should be used for TPOs up to about 30 Kilowatts. Above 30 KW TPO, 4 inch or greater line is required. There are a few combined FM stations that are pumping 80 or 90 KW up to the antenna. Those require 6 inch or greater line.  Even though the transmission lines themselves are rated to handle much more power, reflected power often creates nodes along the line where the forward power and reflected power are in phase.  This can create hot spots and if the reflected power gets high enough, flashovers.

This brings up another point; most rigid line comes in 20-foot sections. There are certain FM frequencies that require different lengths due to the aforementioned nodes that fall along the 1 wavelength intervals. If one of those nodes happens on a flange, that could create problems.

  • Frequencies between 88.1 and 95.9 MHz, use 20-foot line sections
  • Frequencies between 96.1 and 98.3 MHz, use 19.5-foot line sections
  • Frequencies between 98.5 and 100.1 MHz, use 19-foot line sections
  • Frequencies between 100.3 and 107.9 MHz, use 20-foot line sections

TV frequencies are much more complicated. The large channel width and much larger spectrum use means that close attention needs to be paid to line section length. Since low-power TV and translators may need to change frequency, those stations often use Heliax instead of rigid line.

Milwaukee portable band saw
Milwaukee portable band saw

Working with rigid line requires a little bit of patience, careful measurements, and some special tools.  Since the line itself is expensive and the transmission line lengthener has yet to be invented, I tend to use the “measure twice and cut once” methodology.  

For cutting, I have this nice portable band saw and table.  I bought this particular tool several years ago and it has saved me hours if not days of work at various sites.  I have used it to cut not just coaxial line and cables, but uni strut, threaded rod, copper pipe, coolant line, conduit, wire trays, etc.  If you are doing any type of metalwork that involves cutting, this tool is highly recommended.

Milwaukee 6230N Band Saw with cutting table
Milwaukee 6230N Band Saw with cutting table

There are now Lion battery types of bandsaws which are certainly more portable than this. Still, the table with the chain clamp makes work much easier and the cuts are straight (perpendicular), which in turn makes the entire installation easier.

The next point is how long to cut the line pieces and still accommodate field flanges and inter-bay line anchors (AKA bullets). 

Inner bay line anchor, aka “bullet” 3 1/8 inch, 1 5/8 inch, and 7/8 inch respectively

The inner conductor is always going to be shorter than the outer conductor by some amount.   Below is a chart with the dimensions of various types of rigid coaxial cables.

Length cut chart for various sizes of rigid coaxial cables

When working with 1 5/8 inch rigid coax, for example, the outer conductor is cut 0.187 inches (0.47 cm) shorter than the measured distance to accommodate the field flange. The inner conductor is cut 0.438 inches (1.11 cm) shorter (dimension “D” in the above diagram) than the outer conductor to accommodate the inter-bay anchors. These are per side, so the inner conductor will actually be 0.876 inches (2.22 cm) shorter than the outer conductor.  Incidentally, I find it is easier to work in metric as it is much easier to measure out 2.22 CM than to try and convert 0.876 inches to some fraction commonly found on a tape measure.  For this reason, I always have a metric ruler in my tool kit.

If you do not have a handy chart, you can estimate the inner conductor length by measuring the inner bay anchor from the insulator to the first shoulder. Then multiply by two.

Measuring inner bay line anchor

In this case, the measurement from insulator to shoulder is 11/16th of an inch (17.5 mm). If Clamp On Flange adaptors (AKA field flanges) are being used, don’t forget to account for the small lip (usually less than 1/16th of an inch) around the inside of the flange where the outer conductor is seated. If you are using unflanged couplings instead of field flanges, then you can disregard this.

Clamp on Flange adaptors in the front, flangeless couplers in the back
Altronic air cooled 20 KW test load
1 5/8 inch rigid coax run to Altronic air-cooled 20 KW test load
1 5/8 inch rigid coax and 4 port coax switch mounted in top of Middle Atlantic Rack
1 5/8 inch rigid coax and 4 port coax switch mounted on top of Middle Atlantic Rack

The next step is de-burring.  This is really critical at high power levels.  I use a copper de-burring tool commonly used by plumbers and electricians.

De-burring tool, can be found in the plumbing isle of most big box hardware stors

One could also use a round or rat tail file to de-bur.  The grace of clamp-on field flanges is they have some small amount of play in how far onto the rigid line they are clamped.  This can be used to offset any small measurement errors and make the installation look good.

Watching the weather

The weather affects many things. When the weather improves, outdoor projects like tower work can be completed. When the weather is terrible, we may need to do extra work restoring broadcast signals. Today, I am looking at Hurricane Lee, in the North Atlantic basin. Historically speaking, September is the month when we get Hurricanes in the Northeast.

As of this writing, it is too early to be concerned about Lee. Hurricanes can be very unpredictable and there is a good chance the forecast will change many times over the next week or so. That being said, this time of year is a good time to call the fuel companies and top off the generator tanks since winter is coming in a few months anyway. As the situation develops, I may need to dust off the pre-storm checklist.

The basic pre-storm checklist looks something like this:

  • 96 hours or more before the storm: Schedule fuel deliveries for generators, and top off oil and water as needed. Test generators under load if possible. Check UPS batteries. Make an off-site data backup if it does not already exist.
  • 72 hours before the storm: Coordinate with programming to have backup programs available in the event that the satellite dish is damaged, the internet goes down, etc. Inventory and restock PPE, emergency food, water, blankets, first aid supplies, batteries, etc.
  • 48 hours before the storm: Procure supplies needed to secure buildings and sites (plywood, tarps, sandbags, rope, nails, screws, etc). Work out backups for internet STL systems if possible. Work on access plans to remote sites. Make sure that you have the proper tools available.
  • 24 hours before the storm: Secure your personal dwelling, and make sure you have a plan for pets and loved ones. Secure proper shelter for everyone. Fill vehicle gas tanks, and fill portable gas tanks. Update off-site data backup and secure in a safe location.
  • 12 hours before the storm: Secure buildings, park vehicles in areas where they will not be damaged by flooding or blowing debris, and make any last-minute supply runs for emergency food and water. Have a set or two of dry clothes and shoes in your vehicle (almost nothing is worse than spending 12-24 hours in wet and cold clothes). Coordinate response with other station personnel, prioritize the order of restoration, and coordinate with local authorities on their needs.

A few years ago, I purchased one of these LiPo battery chain saws:

DeWalt battery chain saw

These are great units because you do not have to carry cans of 2-cycle gas around. This model will cut trees 12-14 inches in diameter and I get about 25-35 minutes of cutting time per battery depending on the motor load. I have used it several times to cut small trees from access roads to tower sites.

Above all else, during and after the storm, be safe. Do not take any risks involving downed wires, damaged towers, satellite dishes, etc.

The sound of an ATU

I am not generally given to nostalgia as it is often a luxury I cannot afford. However, there are some times when I think; I remember the first time I experienced that. Here is a brief video of the WABC ATU coils singing with modulation:

I believe the arc at the 23-second mark came from the Delta base current toroid sample transformer and was due to heavy modulation. Sid, shouting into the microphone again!

The current sample toroid is at the highest impedance point in the system and the voltage exceeds 5KV on the positive modulation peaks. There are also some little black flies that like to fly into the gap between the antenna output conductor and the toroid sample. When I clean up the ATU every quarter, I find many dead flies below the base current sample toroid. A 50,000-watt fly zapper. Fortunately, the DX-50 doesn’t seem to notice this and keeps chugging along.

After about a minute thirty I realized I was probably exceeding my 6-minute SAR and left the ATU building for a while.