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.

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.

WKIP; removal of the taller tower

This is the original tower for WKIP, but not the original antenna. It was put up circa 1960 or so and like many towers from that era, has hollow legs. Thus, after 60 years or so, it is deteriorating from the inside out.

WKIP tower #1

This was part of a two-tower directional array. It is odd that a class C station on 1,450 KHz would have a directional antenna at all. Even stranger still, it was directional daytime, non-directional night, both at 1,000 watts. The reason for such an odd situation; the station was co-owned with WGNY in Newburgh and the daytime coverage contours would have overlapped without a directional array. The taller tower is 215 degrees tall with top loading. During the daytime, the pattern goes to the North and it covered very well.

Vertical Bridge, the tower owner, decided it was time to replace the aging structure with a monopole. They are completing the project this summer. Our part is to move WKIP to the shorter tower and put up a temporary FM antenna for the translator. Once the project is completed, WKIP will operate from the shorter tower (which is 85 degrees) permanently, getting rid of the now unnecessary directional antenna on a class C channel. The translator antenna will move back to the monopole, once it is put up.

Problems… Yes, we have a few of those…

WKIP tower #2 with broken guy wire

First, the short tower had a broken guy wire. Actually, the guy wire was fine, but the lowest grip connecting to the equalizing plate was rusted through. It is fortunate that this was discovered because the upper guy wire was getting ready to let go too. Northeast Towers was able to replace all of the grips on that set of guy wires and re-tension the tower. They did a full investigation of all of the other anchors prior to any climbing. This is in a swamp, which has flooded several times over the last few years.

Tower #2, guy wire repaired, Scala FMVMP translator antenna mounted

Next, the temporary FM translator antenna was hung on the tower. It was thought that the 3/8 sample line from the old AM sample system could be used as a temporary transmission line for this system. Unfortunately, that line turned out to be 75-ohm cable TV drop line and was not suitable for transmission of VHF. We had about 600 feet of leftover 3/8 sample line (Cablewave FCC 38-50J) from a decommissioned AM site, so we used that instead. It has quite a bit of loss on VHF, however, for temporary use, it will work.

Black Rat Snake, harmless and helpful
Black Rat Snake

Next, it seems this black rat snake had taken up residence in the ATU cabinet. The bottom of the ATU was full of mouse nests going back many years. One of our employees dutifully cleaned out the mouse nests unknowingly under the watchful eyes of this snake. Only after he was done, did he see the snake coiled up on the disused current meter shunt. There was a mild freakout for several minutes, but the snake left on his own and we got back to work. The black rat snakes are helpful to have around, but perhaps best if he stays outside of the ATU. We will seal up the entryway for the coax, which seems to be where all the critters are coming in.

Kintronic ISO-130-FM-N Isocoil

This Kintronic Isocoil was mounted to the back of the ATU with unistrut. Even though this is a temporary installation, I have found that sometimes temporary things can last much longer than anticipated. Besides, it was easier than trying to use pressure treated 4 x 4 lumber.

Next, we measured the ATU with the fancy machine (Agilent E5061B network analyzer). In theory, the ATU input should be 50 ohms to match the incoming transmission line. No, instead it was 38 Ohms -j20.

So, a little bit of a retune was required. With the fancy machine, we were able to get it to 52 ohms -j9 or so. This is good enough for now, there will be numerous cranes in the air and the station has an STA to run at 250 watts for the project’s duration. After the new monopole is up, we will measure the base impedance of the tower and tune up the ATU for 50 ohms and then return the station to full power at 1 KW.

Smaller crane, used to assemble the larger cranes

The old tower coming down:

Top section and top loading wires separated

Two cranes were used; one to hold and lower the tower section, the other to lift two tower workers to cut away the sections. The tower was deemed unsafe to climb, therefore it had to be removed like this. It was also unsafe to drop because of the proximity to the studio building and the other tower, which is being retained.

Top section being lowered
Next section removed and being lowered
Next section removed

You get the idea. These tower sections and guy wires were cut up and put in a scrap metal dumpster. They will be recycled into something else.

Now, they will work on removing the old tower base and putting up the monopole. Once that is done, we will tune up the AM on the short tower and get it back to full power.

WSM Nashville, Tennessee

We recently went on vacation in Tennessee, which is a great state. Most of the time was spent hiking around various state parks, investigating interesting places, or eating at various restaurants. I highly recommend the state parks, there are many and they are all good. All of that being said, I could not resist the temptation to swing by WSM on the way back to the airport. I am happy I did.

Historical Marker, WSM Brentwood, Tennesee

The site is right off of I-65 and easy to get to. There is a public dog park that is behind it, which is a convenient place to park.

WSM Blaw-Knox tower, transmitter building

The Blaw-Knox tower is impressive. The site is well-maintained overall.

WSM tower base, transmitter building and Aux tower

The main tower is fed with open wire transmission line. The aux tower is off the right. It was nice to stop and walk around the site taking pictures. The Brentwood Police Department even stopped by and welcomed us to the neighborhood.

WSM open wire transmission line, looks like 450 ohms

It is always interesting to stop by some of these more famous stations. It would be nice if more sites were recognized as historical places, given the role that radio played in 20th-century US society. Both of my parents grew up during the Great Depression. According to their stories; life was tough, it was a struggle to feed the family and pay rent, but they did have a radio in the living room which was switched on every night after dinner.