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.

Take a ski lift to work day, WBEC style

One thing that I like about the radio engineering job, every day is different from the last, at least when doing field work. Access to transmitter sites can be a challenge, especially in the Winter months. Last Wednesday, I was doing work for WBEC in Pittsfield, MA. Their FM transmitter site is atop Mahanna Cobble in the Bousquet Ski Area.  The summit is about 1,800 feet AMSL and since the terrible rains last month, not accessible by vehicle.  No worries though, somebody built this nice chair lift for us to use:

Bousquet Ski Area chair lift going down hill
Bousquet Ski Area chair lift going down hill

Going down!  I forgot to take pictures on the ride up.

Bousquet ski area chair lift
Bousquet ski area chair lift

The steep part of the hill.

Top of the hill radio tower
Top of the hill radio tower
American tower site top of bousquet ski area
American tower site top of bousquet ski area

One of two towers, both owned and managed by American Tower Corporation.  This is the older tower that has the radio station, somebody’s translator, some paging and two way stuff and sprint PCS.  The other tower is to the right, out of the picture and holds cell carriers.

View to the north over the valley, WBEC-AM’s two tower directional array can be seen in the lower left hand side of the picture.

View to the north
View to the north

WBEC-FM’s transmitter, it’s in there somewhere:

WBEC Nautel VS 2.5 transmitter
WBEC Nautel VS 2.5 transmitter

Polyphasor surge protector has seen better days.  It suffered some serious damage and was removed from the circuit.  Now, it makes a convenient home for rodents.  Reduce, reuse, recycle:

Polyphasor surge suppressor
Polyphasor surge suppressor

Something is missing…

N2 regulator with nothing to do
N2 regulator with nothing to do

Now where did I put that Nitrogen tank?  The site needs a little work and that’s okay, we came to work and get paid.

A tale of two air conditioners

It was a hot day, it was a cold day. The tube transmitter was running, the solid state (HD-1) transmitter was off the air. The books show that the company has deep pockets, but the accountant has short arms.  And so it goes.  In a sordid, yet familiar tale of leaping three quarters of the way across a river, the builders of this transmitter site seemed to think of everything except the cooling requirements for a 35 KW FM transmitter.

Instead of installing real commercial AC units, someone decided that 34,000 BTU window units were the way to go.  At one time, there were eight of those units, all single phase 240 volts sucking down gobs of power and freezing up when the outside temperature dropped below 40°F.  This was always a problem, but became more so when we took over the maintenance of this site.  When there was a full time engineer, his time, apparently, could be wasted running back and forth turning the window units on or off in the winter as required.  Now that a contract company is doing the work, it becomes cost prohibitive to require such things.

Therefore, the time had come to make a change.  To that end, six of the 34,000 BTU window units were removed from the building.  Two of the existing holes in the wall where used to create an emergency cooling system, consisting of a 4,292 CFM fan and a couple of shutters.  Two other holes were used for the new air conditioners and two holes were blocked up.  The remaining two window units were left in place in the combiner room, which is a separate cooling zone.

Bard 5 ton wall mount AC unit
Bard 5 ton wall mount AC unit and cooling fan shutter

The new AC’s are five ton wall mount Bard units.  These are three phase and should be more than enough to keep the transmitters cool.  Here is how I arrived at that conclusion:

  • The entire building load when the main transmitter is running at full power, without the transmitter room air conditioning, is 60 KW.
  • All of the building loads except the transmitters go through a single phase panel.
  • The load on the single phase panel is 10 KW, thus the transmitter load is 50 KW (this 10 KW is mostly the single phase AC units in the combiner room)
  • The TPO is 32 KW, therefore the transmitter is generating 18 KW of waste heat.
  • One watt hour = 3.412 BTU of energy, thus
  • 18,000 watt hours equals 61,416 BTUs
  • One ton = 12,000 BTU, thus
  • 61,416 BTU ÷ 12,000 BTU = 5.118 tons

That will take care of the main transmitter waste heat.  The HD transmitter generates another 4,000 watts of waste heat or 1.37 tons.  The combiner is an another room and doesn’t factor into the calculation.  The rest of the equipment is inconsequential, adding up to less than 100 watts.

The solar gain is more difficult to calculate, as it is based on the building structure, the type of construction and the heat gain (loss) through the walls and doors.  This building is concrete block, insulated, and has no windows.  It is unshaded, however it is painted a light color.  All in all, the solar gain should be less than two tons on a hot day.  Therefore the total AC load should be 8.25 tons or less.

Bard 5 ton wall mount AC unit
Bard 5 ton wall mount AC unit

All that is left now was to install the things.  Just pull up the truck and use a crane to lift them in place, except, no; that plan won’t work.  This is the transmitter site at the power plant and the 138 KV lines overhead precluded any lifting with a crane.  We instead had to build ramps and move the things around on large hand trucks.  One unit is installed on the rear of the building, the other on the front.  It required several days to make the ramps and four people to muscle the things into place.

The bottom air intake holes needed to be cut out for the new units.  Cutting into the concrete block while the BE FM 35A was running proved to be another challenge.  We used several sheets of plastic, shop vacs and extra air filters on the transmitters to keep the concrete dust out of the PA cavities and motor bearings.

Plan B cooling consists of a 4,292 CFM Venturi fan mounted on the rear wall of the building.  The fan is controlled by a ceiling mounted thermostat set to 95 degrees.  If the AC’s fail, the ceiling temperature will rise and the fan will turn on.

Transmitter site emergency cooling fan
Transmitter site emergency cooling fan

The room volume is about 3600 cubic feet, therefore this fan will change the room air about once every 60 seconds or so. It is not the best plan to move humid, potentially dirty outside air through a building, but it it keeps the station on the air while the main AC units are being repaired, then so be it.

The entire system went on line last week and is working well.

The Nautel VS-2.5 FM transmitter

This is cute. A small (VS allegedly stands for “Very Small”) integrated 2,500 watt FM transmitter.  This one we just finished installing as a backup transmitter for WSPK, on Mount Beacon, New York.

Nautel VS-2.5 FM Transmitter
Nautel VS-2.5 FM Transmitter

This site has a Nautel V-7.5 as the main transmitter.  That unit is very reliable, however, this transmitter site is non-accessible 4-5 months out of the year due to ice and snow.  The last time we had an off air emergency due to a crippling ice storm, it took an entire week to clear away all the downed trees so we could gain access to the site via snowmobile.  As such, every system needs dual or even triple redundancy.  Lack of said redundancy has lead to several prolonged outages in the past.

WSPK signal flow diagram
WSPK signal flow diagram

Last year, we were finally able to install a backup antenna after 63 years without one.  This year, it is time to upgrade the rest of the backup equipment.  The new auxiliary transmitter is connected directly to the auxiliary antenna via a five port coax switch.  This allows for use of the dummy load for testing when we are present, but removes a potential failure point in the coax switch.  There have been at least two incidences of the disk jockey accidentally transferring the transmitter into the dummy load when taking transmitter readings.  Hopefully this configuration will be fairly idiot proof.  I am making an interlock panel that will prevent both transmitters from being on the air at the same time.

Nautel VS 2.5 connections
Nautel VS 2.5 connections

This site is a work in progress.

The backup processor is at the transmitter site, the main processor is in the rack room at the studio.  This works well because the main processor occasionally looses its mind and needs to be rebooted.  It would be a significant pain to drive all the way up to the transmitter site just to reboot the processor.  It might not happen at all during the winter.  The back up processor has no mind so it is not an issue.

The VS transmitter is attractive because it has a built in exciter that accepts composite, AES or IP audio.  The exciter also has a built in Orban processor as an option.  Thus, if it really hit the fan, we could use the LAN extender to get the audio to the site.  Further, it could be addressed by any studio in the company WAN.  Which is cool, when you think about it.

Nautel continues to crank out innovative, dependable products and there is nothing wrong with that.