The head smasher

I have worked in hundreds of transmitter sites over the years; AM, FM, TV, HF, Two way, Paging, Cellular, etc.  So many, I have lost count.  The one thing that is always annoying is equipment that is suspended from the ceiling at just the wrong height, AKA: The Head Smasher.  It does not matter if warning signs are posted, I’ve seen them marked with black and yellow caution tap, and so on.  If it is installed low enough for somebody to hit their head, contusions will result.

3 1/8 inch motorized coax switch mounted
3 1/8 inch motorized coax switch mounted

Thus, when it came to install this motorized 3 1/8 inch coax switch, there was only one way to do it.  Installing it the other way would result in a head smasher behind the backup transmitter because the ceilings are low.  The problem with this style mounting is how to get to the motor and clutch assembly for servicing.  There is but one inch of clearance between the top of the coax switch and the transmitter room’s ceiling.  If servicing is needed, the entire switch would need to be removed, resulting in lots of extra work and off air time.

3 1/8 inch motorized coax switch cover
3 1/8 inch motorized coax switch cover

So, an idea was formed.  Why not cut the switch cover in half and put some hinges on it.  The cover itself is made of aluminum.  I was able to carefully mark it out and cut it with a jig saw.  Then, I attached a set of hinges on the back side and a set of latches on the front.  It now opens like a clam shell.

3 1/8 inch coax switch cover modification
3 1/8 inch coax switch cover modification

Now, when access is needed to either the motor or clutch, the cover can be opened up and removed.  Unless the actual RF contact fingers burn out, there should be no need physically removed the switch for servicing.

3 1/8 inch coax switch cover, modified
3 1/8 inch coax switch cover, modified

Cover replaced.  This will not have to be removed very often, in fact, I have known some coax switches that never need service.  Still, having the ability to quickly get the cover off and do some basic repairs is a good thing.

Moving the WRKI and WINE transmitter site

Blogging has been light due to work load being heavy, at the moment. We are engaged in moving transmitters out of this old house:

WINE 940 WRKI 95.1 former studio and transmitter site
WINE 940 WRKI 95.1 former studio and transmitter site

Into this new transmitter building:

WINE WRKI transmitter building at base of tower
WINE WRKI transmitter building at base of tower

The former building was the original studio for WRKI, 95.1 MHz, which signed on in 1957.  The co-located AM station, WINE 940 KHz, did not sign on until 1963.  As such, the building is a little worn around the edges, so to speak.  The FM transmitter has an auxiliary cooling device, for those hot summer days as the building itself is un airconditioned:

WRKI Harris FM25K transmitter, circa 1986
WRKI Harris FM25K transmitter, circa 1986

The rest of the building is in similar condition.  Ceiling tiles are falling off the ceiling and getting ground into the floor, junk is pile up in almost every corner, rodent feces, and the basement, don’t even get me started on the basement.

The basic floor plan for the new building is simple:

WRKI WINE transmitter room floor plan
WRKI WINE transmitter room floor plan

Right now, the preliminaries are being done, mounting the coax switch, running conduit, pulling wires, etc.

A few design notes:

  1. This building is much closer to the tower, which is sited on a high hill (715 feet, 218 Meters) and sticks up 500 feet (152.1 Meters) above that.  Basically it is the area lightning rod, thus special attention will need to be paid to grounding and bonding.  I decided to isolate the electrical ground in favor of the RF ground for lightning protection.  This involves putting toroids on the electrical ground conductors.
  2. The building itself is shielded with continuous steel plating, but that has been cut in a few areas to install air conditioners.  Those areas will have to be repaired and the AC units bonded to the steel plate.
  3. Back up cooling will be in the form of a large exhaust fan and intake louver.
  4. The tower itself is AM radiator for WINE.  It is 170 degrees tall, which means high RF fields at the base, therefore good RF bypassing is needed.
  5. The transmitter room itself is fairly small for what needs to go in there. careful design and placement is required.

Here are some in progress pictures:

WRKI backup transmitter, Harris FM3.5K, coax switch in the background
WRKI backup transmitter, Harris FM3.5K, coax switch in the background

The first order of business was retuning a Harris FM3.5K transmitter to function as the backup. The current backup transmitter is an RCA FM20E, which no longer runs. After the move is completed, that transmitter will likely be scrapped.

I attached super strut to the ceiling at four foot intervals. I used this strut to support the 4 port coax switch. All coax in the transmitter room is 3 1/8 inch hardline, which has a power rating of 40 KW.  Since the transmitter power output is 20 KW, this leaves a lot of head room for problems.  When working with 3 1/8 inch coax, it is important to remember to cut the inner conductor 1 1/2 to 1 3/4 inches sorter than the outer conductor, otherwise the stuff doesn’t go together right.

The 30 KW air cooled dummy load was moved up from the other building and connected to the coax switch.  This allowed the backup transmitter to be tested.

WRKI backup transmitter and dummy load
WRKI backup transmitter and dummy load

Three inch ground strap connects all the transmitters, racks, and dummy load to the station ground.

WRKI ground strap, new transmitter building
WRKI ground strap, new transmitter building

Electrical requirements are being met by a 400 Amp service backed up by a 120 KW generator.  Once the conduit work is finished and all the wires pulled, the coax to the old building can be cut and brought into the new building, then the station can go on the air with the “new” backup transmitter.

The melted ground wire

I found this on one of the guy wire anchor points for a 400 foot tower:

#2 solid copper wire burned open by lightning strike
#2 solid copper wire burned open by lightning strike

Had to be a pretty big hit to burn open a #2 wire. This is on one of six guy anchor points for the tower. The ground wire is U bolted to each guy wire before the turnbuckle and then goes to ground. This was noted between the last guy wire and the ground rod.

It is important to find and fix these things, as the next lightning strike on this tower would have a less than ideal path to ground at the guy anchor points, forcing the current to flow through other parts of the transmitter site, possibly through the transmitter itself, to ground.

I generally try to do a brief inspection of towers, guy anchors, lighting, painting and a general walk around the property twice a year.  That helps prevent surprises like “Oh my goodness, the guy wires are rusting through,” or “Hey, did you know there is an illegal “hemp” farm on your property?”  Well, no officer, I don’t know anything about that…

Andrew A909D type 78AGM 3 inch connector

I figured if I have this problem, someone else probably has it too.  We have a backup antenna on one of our towers.  The station has a TPO of 28 KW, which is starting to get into the semi serious level.  This antenna is connected to Andrew 3 inch heliax that was installed in 1971.  It has a spiral inner and outer conductor, which is no longer made by any manufacture of heliax.

We completely rebuilt the transmitter site a few years ago, moving a lot of things around.  One part of the project was installing a coax port on the wall and moving all coaxial cables to that entrance.  The main antenna is connected to Cablewave H50J coax.  I ordered a new connector for that transmission line, no worries.  When I cut the back up transmission line, I figured I could re-apply the old Andrew connector.

Andrew A909D type 78 AGM 3 inch coax connector
Andrew A909D type 78 AGM 3 inch coax connector

That is all fine, however, I removed the connector without reverse engineering it, that is to say, I didn’t pay close attention to how the inner and out conductors where cut, or how the jack was cut back.  I will have to reverse engineer the thing now.

Here are the steps I followed:

  1. Check out the CommScope – Andrew website for documentation.  A search shows they only have the current connector, which is nothing like this one and will not work with spriral conductors
  2. Call Andrew and spend many minutes on hold or explaining to various helpers what I want.  I was met with a universal “That is not an Andrew Part number,” or “Gee, I wish I could help but…”
  3. Take the thing apart and begin measuring stuff with a ruler.  Write everything down and draw out a diagram.
  4. Trim off the excess cable then practice putting the thing together once.
  5. Make the final cut and put the re-used connector back on the Andrew transmission line.

You can skip steps 1 and 2, since I already did them for you.

A few things to note:

  • The inner and outer conductors should be cut flush and as close to perpendicular as possible.
  • The inner conductor slug has a left hand thread.  This makes the slug tighten against the bushing.
  • The outer jacked is cut back about 2.5 inches
  • Place the EIA flange on the cable first, then thread the back nut onto the outer conductor, then thread the rubber gasket onto the outer conductor.  The gasket is a tight fit, use petroleum jelly to lubricate it.  This is a gas block connector, so special attention is needed with the gaskets.
  • The inner conductor has triangular pieces 1/8 inch deep cut around the diameter, the depth of the inner slug is critical to the connector going together correctly.
  • The inner conductor is folded inward over the end of the slug.  Bushing, dielectric spacer and EIA bullet is connected to the inner conductor slug and snugged down with a standard screw driver
  • The outer conductor is nipped 1/8 of an inch around the diameter
  • The outer conductor is folded outward over the collect ring
  • Use some petroleum jelly on all the O rings
  • Carefully screw the connector together
  • Final tightening requires a special spanner wrench or attachments for a socket wrench.  The tower crew had these in their shop.

If you have a spectrum analyzer, check it’s return loss and see what it looks like before slamming a full load on it.  If not, turn things on and bring them up slowly.  Feel the connector to make sure it is not getting warm.  If there are problems, heat will be the first warning sign.

3 Inch coax patch panel
3 Inch coax patch panel

Once together, I ran the the transmitters for a combined output of 10 KW and got about 50 watts return, which much the same as it was before.