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 spiral 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 jacket 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 are connected to the inner conductor slug and snugged down with a standard screwdriver
  • 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 its 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 transmitters for a combined output of 10 KW and got about 50 watts return, which was much the same as it was before.

Towers in the fog/repeating geometric shapes

For no other reason than they were there, I took a few shots under a couple of 300-foot self-supporting towers.

300 foot self supporting communications tower in fog
300-foot self-supporting communications tower in fog

This tower has a UHF slot antenna on the top of it fed with 6-inch hard line.

280 foot self supporting tower
280-foot self-supporting tower

Tower next to the first one. Both can be found on North Mt. Beacon, NY, about 1,800 feet AMSL.

What is the deal with those FEMA/DHS AM backup transmitters?

Back last February, it was reported that FEMA/Department of Homeland Security was mysteriously constructing prepackaged AM transmitter buildings at various PEP (Primary Entry Point) transmitter sites across the country as something call “Primary Entry Point Expansion.”  These buildings contain a 5 KW Nautel AM transmitter, EAS gear, satellite equipment (the exact equipment list is undisclosed), and a backup generator all in a shielded (Faraday Cage), prefabricated building placed inside a fenced-in compound at the station’s transmitter site.  The buildings are being put in place, but not connected to anything in the outside world.  They are planning to have about 80 (the number keeps increasing) of these structures in place by when the project is completed in mid-2013.

FEMA/DHS IPAWS expansion project
FEMA/DHS IPAWS PEP expansion project

Why, inquiring minds want to know, would they do that?

The new buildings and equipment are, of course, not provided to the government for free.  I would estimate each unit costs at least $200,000 based on the following:

  • A new solid-state 5 KW AM transmitter costs $50-55K
  • A new 35 KW generator costs $23K
  • A new, shielded communications structure costs $70-85K
  • Misc racks, equipment, wiring, shipping, installation costs, fuel tanks, fencing, etc $40K
Those prices are roughly what a private company might pay, the government procurement costs would be higher.  Multiply by 80, which equals at least $16M, perhaps double that when project administration is considered. In the distant past, through something called the Broadcast Station Protection Program (BSPP), FEMA did provide generators, fuel tanks, transfer switches, and occasionally a bomb shelter to key EBS stations throughout the country.  In the recent past, FEMA and the government, in general, have been reluctant to fund even mandated changes in the EAS system, first in 1997 when EAS was first implemented and again in 2011 when the CAP modifications were required.  Why are they now spending at least $16M to provide EMP-hardened facilities for AM radio stations?

The rationale for this current wave of government spending, as reported in several industry periodicals, is simply a matter of supplying in-depth backup facilities in accordance with Executive Order 13407. The design of the structure and manner of installation seems to indicate the primary concern of FEMA is some type of Electromagnetic Pulse (EMP).  If an EMP were to happen and it took out the station’s main transmitters, these could be connected to the existing antenna system and switched on.  They would provide emergency programming and interface directly with FEMA’s IPAWS (Integrated Public Alert and Warning System).

The interesting thing about this is that there is a coincidence with the upswing of solar cycle 24.  Back in 2008, likely when this project was likely first dreamed up, the predictions were for a great number of sunspots in this cycle.  That has not happened and in fact, this cycle is now predicted to be the weakest solar cycle since 1823.  Even weak sunspot cycles can create problems, but does that warrant supplying 80 backup transmitters, generators, fuel tanks, and buildings to various AM broadcasting stations throughout the country?  Further, solar flares and Coronal Mass Ejections (CME) are fairly slow-moving events, the sun is well monitored; alerts would be issued and precautions are taken.

One other thing to consider: HEMP (High altitude Electromagnetic Pulse from a nuclear air burst).  AM transmitters are more robust when it comes to HEMP than FM transmitters.  This is because of their modulation type and frequency of operation.  A 5 KW AM transmitter can withstand RF voltages six or eight times its nameplate carrier rating.  Tube-type transmitters are even more robust than solid state.  The FM broadcast band falls right in the middle of the HEMP fast pulse frequency (72-225 MHz), which will likely resonate in the tuning circuits of the transmitter exposed to it and destroy all of the active devices.  Not so with AM transmitters.

A HEMP event would cause catastrophic damage to the electrical grid across wide areas of the continent (see also;  Starfish Prime). The voltages instantaneously induced on computer circuit boards and power supplies would be so high, they would likely burst into flames if they were close enough to the detonation.  The same for almost all other electronic devices with circuit boards.  It would set the country back one hundred or more years, technologically, causing massive disruptions in the food supply chain.  Such an act would surely be met with massive nuclear retaliation by the US.  The military has not only hardened all of its communications and command facilities, but they have also undergone rigorous EMP testing, finding and fixing design flaws.  Thus, the US military’s capacity to wage war would continue undiminished after a HEMP event, a fact that all other members of the nuclear club are surely aware of.