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Part 101, Private Fixed Microwave Service

I have been tasked with installing one of these systems for a sixteen channel bi-directional STL.  This system was first mentioned here: The 16 channel bi-directional STL system.  As some of you pointed out, the unlicensed 5.8 GHz IP WLAN extension was the weak link in this system.  It was not an interference issue, however, which was creating the problems.  The problem was with layer two transparency in the TCP/IP stack.  Something about those Cambium PTP-250s that the Wheatstone Blade hardware did not like and that created all sorts of noise issues in the audio.   We installed the Wheatstone Edge Routers, which took care of the noise issue at the cost of latency.  It was decided to go ahead and install a licensed link instead of the license free stuff as a permanent solution.

Thus, a Cambium PTP-820S point-to-point microwave system was purchased and licensed.  The coordination and licensing took about three months to complete.  We also had to make several changes to our network architecture to accommodate the new system.  The PTP-820 series has a mast mounted radio head, which is the same as the PTP-250 gear.  However, for the new system, we used three different ports on the radio to interface with our other equipment instead of the single port PTP-250 system.  The first is the power port, which takes 48 VDC via a separate power cable instead of POE.  Then there is the traffic port, which which uses Multi-Mode fiber.  Finally, there is the management port, which is 1GB Ethernet and the only way to get into the web interface.  The traffic port creates a completely transparent Ethernet bridge, thus eliminating all of the layer two problems previously encountered.  We needed to install fiber tranceivers in the Cisco 2900 series switches and get those turned up by the IT wizards in the corporate IT department.

Andrew VLHP-2-11W 11 GHz microwave antenna

Andrew VHLP-2-11W 11 GHz microwave antenna

The radios mount directly to the back of the 24 inch 11 GHz Andrew antenna (VHLP2-11) with a UBR100 interface.  The wave guide from the radios is a little bit deceptive looking, but I tried not to over think this too much.  I was careful to use the O ring grease and conductive paste exactly where and when specified.  In the end, it all seemed to be right.

Cambium PTP-820S mounted on antenna

Cambium PTP-820S mounted on Andrew antenna

Not wanting to waste time and money, I decided to do a back to back test in the conference room to make sure everything worked right and I had adequately familiarized myself with the ins and outs of the web interface on the Cambium PTP-820 radios.  Once that was done, it was time to call the tower company.

Cambium PTP-820S on studio roof

Cambium PTP-820S on studio roof

One side of these are mounted on the studio building roof, which is a leased space.  I posted RF warning signs around the antennas because the system ERP is 57.7 dBm, which translates to 590 watts at 11 GHz.  I don’t want to fry anybody’s insides, that would be bad.  The roof top installation involved pulling the MM fiber and power cable through a 1 1/4 inch EMT conduit to the roof.  Some running back and forth, but not terrible work.  I used the existing Ethernet cable for the management port.  This will be left disconnected from the switch most of the time.

Cambium PTP-280S 11 GHz licensed microwave mounted on a skirted AM tower

Cambium PTP-280S 11 GHz licensed microwave mounted on a skirted AM tower

The other side is mounted at about 85 feet AGL on a hot AM tower.  I like the use of fiber here, even though the tower is skirted, the AM station runs 5,000 watts during the daytime.  We made sure the power cables and Ethernet cables had lighting protectors at the top of the run near the dish and at the bottom of the tower as well as in the transmitter room rack.  I know this tower gets struck by lightning often as it is the highest point around for miles.

PTP-820S RSL during aiming process

PTP-820S RSL during aiming process

Aligning the two dishes was a degree of difficulty greater than the 5.8 GHz units.  The path tolerances are very tight, so the dishes on each end needed to be adjusted in small increments until the best signal level was achieved.  The tower crew was experienced with this and they started by panning the dish to the side until the first side lobe was found.  This ensured that the dish was on the main lobe and we were not chasing our tails.  In the end we achieved a -38 dBm RSL, the path predicted RSL was -36 dBm so close enough.  This means the system has a 25 dB fade margin, which should be more than adequate.  While were were aligning the transmitter site dish, a brief snow squall blew through causing a white out and the signal to drop by about 2 dB.  It was kind of cool seeing this happen in real time, however, strangely enough, the tower crew was not impressed by this at all.  Odd fellows, those are.

Currently brushing up on FCC part 101 rules, part C and H.  It is always good to know the regulatory requirements of any system I am responsible for.  As AOIP equipment becomes more main stream, I see many of these type installations happening for various clients.

The Gates Air FAX-10, Numero Dos

This is the second Gates Air FAX-10 that I have installed. This one is in the shipping container transmitter site from the previous post of the same name.  In this case, we dispensed with the equipment rack that came with the transmitter and installed it in a standard Middle Atlantic rack.   The Harris rack configuration wastes a lot of space and since space is at a premium, we decided to do it our own way.

Gates Air FAX-10 in Middle Atlantic rack

Gates Air FAX-10 in Middle Atlantic rack

The bottom of the rack has the transmission line dehydrator. The top of the rack has the Dielectric A60000 series 1 5/8 inch coax switch, a Tunwall TRC-1 switch controller and the Burk ARC-16 remote control.  I cut the rack panel top to accommodate the coax switch.  The racks were removed from an old studio site several years ago and were in storage since that time.

Gates Air FAX-10

Gates Air FAX-10

The Gates Air FAX-10 transmitter on the air, running a sports-talk format.

Dummy load and Broadcast Electronics FM10B transmitter

Dummy load and Broadcast Electronics FM10B transmitter

View from the other side showing the test load and BE FM10-B transmitter.  This transmitter had a problem that I have run into before with BE FM transmitters.  The jumper between the exciter and IPA had the wrong phase rotation causing reflected power.  I added a foot to it’s length and that problem disappeared.

Hoth

Alternate title: Winter in the Northeast

For all you southerners and west coast people, we have been having an average winter here in the Northeast. While many of our transmitter sites are drive ups, we have several located at ski area mountain peaks.  Technically, those mountain top transmitter sites are a fantastic way to get the Height Above Average Terrain (HAAT) way up there.  Logistically, they are much more difficult to deal with.  Installing a new transmitter or even refueling a generator takes major effort.  Working in the cold and wind is much more fatiguing and requires paying special attention protective clothing, hydration, exposure, etc.

Here are a few pictures from Killington and Pico mountain ski areas in Vermont

Your ride is here.

Your ride is here.

The snow grooming machine is the only way to bring anything up to the top of the mountain during the winter time. In this case, I needed to replace a BW Broadcast TX 1500 watt transmitter.

Trail from ski lift to tower

Trail from ski lift to tower

Even with the snow grooming machine, the last few hundred yards needs to be walked. Fortunately, the snow is packed and not too deep here.

Tower on Pico Mountain

Tower on Pico Mountain

Tower is encrusted with ice. I can tell the tower climber is having a great day:

Tower climber working on ice encrusted towe

Tower climber working on ice encrusted tower

Riding the chair lift back down the mountain gets plenty of strange looks from those skiers coming up:

Pico chair lift

Pico chair lift

Over on Killington Peak, conditions are actually worse.

Killington Peak tower

Killington Peak tower

The ERI antenna heaters cannot keep up with the ice buildup.

ERI two bay antenna with ice.

ERI two bay antenna with ice.

The general manager insists that this winter is not too bad and everything should be working right. My statement to her: Based on my 27 years experience, your shit is fucked up. But if you know how to fix this, come on up and show me.  She deferred.

FM transmitter building and antenna

FM transmitter building and antenna

What the fire tower looked like last winter.

Killington peak fire tower

Killington peak fire tower

Train from the Gondola to the tower

Train from the Gondola to the tower

Shipping Container transmitter site

Shipping container transmitter site from the early 1990's.

Shipping container transmitter site from the early 1990’s.

I do not particularly like these. I know, they are relatively inexpensive, easy to come by, easy to install, etc. However, a shipping container was not designed to house a transmitter, they have certain drawbacks. These are, in no particular order:

  • Air conditioning.  Using a traditional Bard type equipment shelter HVAC unit requires cutting through a lot of fairly heavy gauge steel.  What’s more, the steel walls are uneven, requiring filler.
  • They are by necessity, fairly narrow.  Arranging racks and transmitters along the length of the unit restricts access to either the front or the back of the equipment.  Meeting NEC clearance requirements for electrical panels, transfer switches and disconnects can pose problems.
  • They are not very tall.  Mounting overhead equipment can be problematic as one does not want to drill through the top of the container.  Crosswise unistrut is one solution, but it lowers the overhead considerably.
  • Electrical work is slightly more dangerous.  Doing any kind of electrical work, trouble shooting, repairs, etc is a little more nerve-racking when everywhere around you is a metal surface at ground potential.
  • They are difficult to insulate against cold and heat.
  • The door latching mechanisms bind, wear out or otherwise fail over time.

All of those things being said, I am now rebuilding a transmitter site in one of these shipping containers.

Inside view of shipping container transmitter

Inside view of shipping container transmitter site

Fortunately, the original electrical work was not bad.  The transmitter is a twenty year old BE FM10B, which will be retained as a backup.  The new transmitter is a Gates Air FAX-10.  We have installed several of these Gates Air transmitters in the last two years or so and they seem to be pretty solid units.  This is the second 10KW unit I have installed.

Gatesair FAX-10 transmitter in Middle Atlantic Rack

We decided to install the FAX-10 in a Middle Atlantic rack, since we did not have a whole bunch of extra room for a separate transmitter rack.  The 1 5/8 inch coax switch is installed in the top of the transmitter rack along with a Tunwall TRC-1 switch control unit. The other rack will have the STL and all other ancillary gear.  My idea is to have nothing in between the door and the FM10B so it can be easily removed when that day comes.  Something, something about planning ahead since it will be likely myself removing the FM10B.

Ef the listeners, full speed ahead!

Norway is intent on carrying out its digital radio transition, listeners or not.  I wrote about this two years earlier: Norway says “Goodbye, FM” It seems that their minds are made up, because

Norway’s parliament voted in favour of switching off FM radio after hearing it would lead to a greater choice of radio stations, as well as clearer sound.

Source: Norway warned plan to switch off FM will cut off millions

Now where have I heard that before?  I know that this is in the heart of Socialist Europe, but could this be what is in store here?  I wonder how much longer the US radio stations will survive with shrinking revenue and lack of entertaining programming.

One bright spot; I have been thoroughly enjoying the unintentional humor of NPR’s (National Public, not Radio) “AHHHHH, TRUMP!” coverage.  The Albany, NY outlet has really outdone themselves in this regard.

On a personal note; I have been feeling the urge to write more, so stay tuned!

Commercial Radio Networks changing Satellites

Lockheed Martin A2100 series satellite

Lockheed Martin A2100 series satellite

Westwood One, Premiere, Skyview Networks, et al. will be changing their satellite from AMC-8 at 139° W to AMC-18/SES-11 at 105° W longitude.  More from AMC8transition.com. There are several considerations for this move:

  • Dish design and two degree compliance
  • Obstacle clearance
  • Transponder frequencies
  • Timing

Two degree compliance is going to be an issue for many stations.  Those old 2.4 and 2.8 meter mesh dishes are going to have issues with 105º West because that is a very crowed part of the sky.  From New York, it looks something like this:

Satellite Longitude Inclination Azimuth Elevation Distance
TELSTAR 12 (ORION 2) 109.21° W 0.491° 227.46° 31.09° 38596.91 km
TELSTAR 12 (ORION 2) 109.21° W 0.491° 227.46° 31.09° 38596.91 km
MSAT M1 107.72° W 7.430° 231.14° 38.16° 38011.55 km
ANIK G1 107.33° W 0.013° 225.25° 31.96° 38518.62 km
ANIK F1 107.31° W 0.020° 225.22° 31.95° 38513.76 km
ANIK F1R 107.28° W 0.052° 225.22° 32.02° 38510.37 km
ECHOSTAR 17 107.11° W 0.019° 225.01° 32.08° 38503.29 km
AMC-15 105.07° W 0.025° 222.76° 33.28° 38400.67 km
AMC-18 104.96° W 0.027° 222.64° 33.34° 38400.16 km
GOES 14 104.66° W 0.198° 222.21° 33.38° 38394.57 km
AMSC 1 103.44° W 9.810° 228.37° 43.31° 37616.42 km
SES-3 103.01° W 0.041° 220.41° 34.42° 38307.12 km
SPACEWAY 1 102.90° W 0.032° 220.25° 34.43° 38299.87 km
DIRECTV 10 102.82° W 0.017° 220.17° 34.51° 38292.86 km
DIRECTV 12 102.78° W 0.035° 220.12° 34.50° 38292.93 km
DIRECTV 15 102.71° W 0.009° 220.05° 34.56° 38290.50 km
SKYTERRA 1 101.30° W 3.488° 219.07° 36.33° 38131.32 km
DIRECTV 4S 101.19° W 0.011° 218.24° 35.35° 38228.26 km
DIRECTV 9S 101.15° W 0.014° 218.18° 35.36° 38228.57 km
SES-1 101.00° W 0.016° 218.02° 35.45° 38217.56 km
DIRECTV 8 100.87° W 0.036° 217.88° 35.54° 38211.02 km

Generally speaking, dishes need to be 3.7 meters (12.14 feet) or larger to meet the two degree compliance specification.  For many, this means replacing the current dish.  This is especially true for those old 10 foot aluminium mesh dishes that were very popular in the 90’s because of the TVRO satellite craze.

If the existing dish is acceptable, then the next issue may be obstacle clearance.  Generally speaking the 105 degree west slot (south of Denver) will be easier to see that the 139 degree west slot (south of Honolulu) for much of the United States.  Still, there may be trees, buildings, hills, etc in the way.  Site surveys can be made using online tools (dishpointer.com) or smart phone apps (dishalign (iOS) or dishaligner (Android)).  I have found that I need to stand in front of the dish to get the best idea of any obstacles.  While you are there, spray all the dish holding hardware with a penetrating oil like WD-40, Rostoff or something similar.  Most of these dishes have not moved since they were installed, many years or decades ago.

Transponder frequencies will not be the same, so when the dish is aligned to the new satellite, those frequencies will need to be changed.  The network satellite provider will furnish this information when it becomes available.  This generally requires navigating around various menu trees in the satellite receiver.  Most are fairly intuitive, but it never hurts to be prepared.

The window of opportunity is from February 1, 2017 (first day of AMC-18) until June 30, 2017 (last day of AMC-8).  Of course, in the northern parts of the country, it may not be possible to install a new dish in the middle of winter.  It may also be very difficult to align an existing dish depending on how bad the winter is.  Therefore, the planning process should begin now.   A quick site evaluation should include the following:

Network Satellite Receive Location Evaluation

Satellite:

Satellite Location:

Dish is 2°compliant? (Y/N)

Distance to receiver location:

Dish Latitude:

Dish Longitude:

Dish Azimuth (T):

Dish Azimuth (M)

Dish Height AGL:

Dish Elevation:

Observed Obstacles:

(permanent or removable? Owned or not owned?)

Comments:

A .pdf version is available here. Based on that information, a decision can be made on whether or not to keep the old dish or install a new one.  We service about 25 studio locations and I am already aware of three in need of dish replacement and two that have obstructive trees which will need to be cut.  This work cannot start too soon.

The Energy Onix Pulsar transmitter

Engineering Radio: The Oh Dear God Edition.

I have been tasked with fixing one of these glorious contraptions. Aside from the usual Energy Onix quirks; design changes not reflected in the schematic diagram and a company that no longer exists, it seems to fairly simply machine. Unfortunately, it has spent its life in less than ideal operating conditions.

Energy Onix Pulsar 1000 in the wild. Excuse the potato quality photo

Energy Onix Pulsar 1000 in the wild. Excuse the potato quality photo

Upon arrival, it was dead in the water.  Found copious mouse droppings, dirt and other detritus within and without of the transmitter.  Repaired the broken start/stop switches, fixed the RF drive detector, replaced the power supply capacitors and now at least the unit runs.  The problem now is the power control is unstable.  The unit comes up at full power when it first switched on, then it drops back to 40 watts, then after it warms up more goes to about 400 watts and the audio sounds distorted.  This all points towards some type of thermal issue with one of the power control op amps or other composite device.

After studying the not always accurate schematic diagrams, the source of the problem seems to be carrier level control circuit.  This is based around a Fairchild RC4200AN (U10 on the Audio/PDM driver board) which is an analog multiplier chip.   That chip sets the level of the PDM audio output which is fed into the PDM integrator circuit.  Of course, that chip is no longer manufactured.  I can order one from China on eBay and perhaps that will work out okay.  This all brings to mind the life cycle of solid state components.  One problem with the new technology; most solid state components have a short production life, especially things like multiplier chips.  Transmitters are generally expected to last 15-20 years in primary service.  Thus, transmitter manufactures need to use chips that will not become obsolete (good luck with that), or purchase and maintain a large stock of spare parts.

In the mean time, the chip is on its way from China.  Truth be told, this fellow would be better off with a new transmitter.

The Realtek 2832U

In my spare time (lol!) I have been fooling around with one of those RTL 2832U dongles and a bit of software.  For those that don’t know, the RTL 2832U is a COFDM demodulator chip designed to work with a USB dongle.  When coupled with an R 820T tuner a broadband RF receiver is created  There are many very inexpensive versions of these devices available on Amazon, eBay and other such places. The beauty of these things is that for around $12-30 and a bit of free software, one can have a very versatile 10 KHz to 1.7 GHz receiver.  There are several good software packages for Windoze, Linux and OSX.

The one I recommend for beginners is called SDR-Sharp or SDR#.  It has a very easy learning curve and there is lots of documentation available on line.  There are also several worth while plugins for scanning, trunking, decoding, etc.  At a minimum, the SDR software should have a spectrum analyzer, water fall display and ability to record audio and baseband PCM from the IF stage of the radio.

Some fun things to do; look at the output of my reverse registering smart (electric) meter (or my neighbor’s meter), ACARS data for the various aircraft flying overhead, a few trips through the EZPass toll lanes, some poking around on the VHF hi-band, etc.  I also began to think of Broadcast Engineering applications and a surprising number of things came to mind:

  • Using the scanner to look for open 950 MHz STL frequencies
  • Inexpensive portable FM receiver with RDS output for radio stations
  • Inexpensive Radio Direction Finder with a directional antenna
  • Inexpensive Satellite Aiming tool

Using SDR sharp and a NooElec NESDR Mini+ dongle, I made several scans of the 945-952 STL band in a few of our markets.  Using the scanner and frequency search plugin, the SDR software very quickly identified all of the in use frequencies.  One can also look at the frequency span in the spectrum analyzer, but this takes a lot of processing power.  The scanner plugin makes this easier and can be automated.

950 MHz STL frequencies, Albany, NY

Analog and digital 950 MHz STL frequencies, Albany, NY

I also listened to the analog STLs in FM Wideband mode.  Several stations are injecting their RDS data at the studio.  There is one that appears to be -1500 Hz off frequency.  I’ll let them know.

Next, I have found it beneficial just to keep the dongle and a small antenna in my laptop bag.  Setting up a new RDS subcarrier; with the dongle and SDR# one can quickly and easily check for errors.  Tracking down one of those nasty pirates; a laptop with a directional antenna will make quick work.

Something that I found interesting is the water fall display for the PPM encoded stations:

WPDH using RTL 2832U and SDR Sharp

WPDH using RTL 2832U and SDR Sharp

Not only can you see the water marking on the main channel, you can also see the HD Radio carriers +/- 200 KHz from the carrier frequency.  That is pretty much twice the bandwidth allotment for an FM station.

WDPA using RTL 2831U with SDR Sharp

WDPA using RTL 2831U and  SDR Sharp

Those two stations are simulcasting.  WPDA is not using Nielson PPM nor HD Radio technology.  There is all sorts of interesting information that can be gleaned from one of these units.

Aiming a satellite dish at AMC-8 can be a bit challenging.  That part of the sky is pretty crowed, as it turns out.  Dish pointer is a good general reference (www.dishpointer.com) and the Dish Align app for iOS works well.  But for peaking a dish, the RTL 2832 dongle makes it easy to find the correct satellite and optimize the transponder polarization.  Each satellite has Horizontal and Vertical beacons.  These vary slightly in frequency, thus, but tuning to the correct beacon frequency, you can be assured that you are on the right satellite.  All of the radio network programming on AMC-8 is on vertically polarized transponders, therefore,  the vertical beacons are of interest.  Here are the vertical beacons for satellites in that part of the sky:

Satellite Position C band Vertical beacon (MHz) L band (LNB) Vertcial beacon (MHz) Comment
AMC-8 139W 4199.5 949.25
AMC-7 137W 3700.5 1450.25
GOES15 135.4W 2209.086 N/A NOAA WX
AMC-10 135W 4199.5 949.25
Galaxy 15 133W 4198 949.00
AMC-11 131W 4199.5 949.25
Galaxy 12 129W 3700.5 1450.25

For those in the continental United States, there is not much else past 139W, so AMC-8 will be the western most satellite your dish can see.  Of course, this can be used in other parts of the world as well, with the correct information. Bringing a laptop or Windows tablet to the satellite dish might be easier than trying to drag a XDS satellite receiver out.

AMC8 vertical beacon output from LNB

AMC8 vertical beacon output from LNB

In order to use the RTL-2832U, simply split the output of a powered LNB, install a 20-30 dB pad in between the splitter and the dongle.  Using the vertical beacon on 949.25 MHz, adjust for maximum signal.

Some other uses; look for the nearest and best NOAA Weather radio station.  Several times the local NOAA weather station has been off the air for an extended period of time.  Sometimes, another station can be found in the same forecast area.  Heck, couple these things to a Raspberry Pi or Beaglebone black and a really nifty EAS receiver is created for NOAA and broadcast FM.  One that perhaps, can issue an alarm if the RSL drops below a certain threshold.

I am sure there are plenty of other uses that I am not thinking of right now…

And… Hockey!

That season is here again; long practices, long drives to out of the way places, hotel rooms, cold arenas, and smelly locker rooms.  Why do we do all this?  Hopefully there is a life lesson in there somewhere.  There will always be somebody faster, bigger, better with the puck, meaner, dirtier, etc.  It is the competition that matters, teamwork may or may not overcome those obstacles.  In the end, the reward will be equal to the effort put into it.

In honor of the disappearing role of enforcer:

Enjoy every sandwich.

The isocoupler and the SX2.5

Second post in the series, “things to do with a truck body tool box.”

We have this client who, several years ago, moved their translator to their AM tower. All is well for a few months, then the much beloved Harris SX2.5 transmitter begins burping.  The SX2.5 transmitter being of an age when, apparently, VSWR fold back circuits were just a gleam in Hilmer Swanson’s eye.  The correct description of the sound made over the air during this event would be “motor boating,” because that is what it sounds like.  Obviously, very undesirable.

Thus, the isocoupler was removed from the tower, dried out, water proofed and replaced.  That lasted about six months.

Once again, the isocoupler was removed from the tower, a capacitor was remounted, drain holes and a small vent added to the top of the unit and it was replaced.  That lasted about a year.

I am getting a little tired of this and so is the client.  Time to rethink the entire set up.

We had several left over parts from various AM decommissionings over the last few years which included these nifty sample loop isolation coils:

AM antenna system sample loop isolation coil

AM antenna system sample loop isolation coil

Why not repurpose one of these to make an isocoupler for the translator?

Enter; the truck body tool box.  This one is slightly smaller than the last one, measuring 23.5 x 18 x 16 inches (60 x 45 x 40.5 cm).

The isolation coil consists of 35 turns of 3/8 coax on an 11.5 inch diameter form.  The coil length is 15 inches.  I calculate the length of the coax on the coil to be out to be right around 100 feet using the π x D x (turns) formula.  I measured the inductance with my analyser, which came out to 200 μH.  Not to shabby.

Checking length of cable with TDR

Checking length of cable with TDR

The coax is Cablewave FCC38-50J which has a velocity factor of .81 and the TDR shows it to be 100 feet also.

Coil impedence and reactance

Simple coil impedance and reactance

At 860 KHz, the isolation coil presents 1,200 impedance.  I don’t think that will be good enough for that cranky old SX2.5.  I decided to make a parallel LC circuit (AKA a tank circuit) to bring up the impedance some.

Tank circuit formula:

tank_circuit

Where:

FR = Resonance frequency in Hertz
L = Inductance in Henrys
C = Capacitance in Farads

Given that I have two left over capacitors, one is a .001 μF and the other is a .0012 μF, those values determine where the coil needs to be tapped.  I also wanted to have a good bit of coil in the circuit on the tower side before the capacitor tap to dampen any lightning strikes on the tower.  Thus the inductance needs to be about 28 μH.

Using Wheeler’s coil inductance formula:

L= (d2 x n2)/(18d+40l)

where:

L = inductance in micro Henrys
d = coil diameter in inches
l = is coil length in inches
n = is number of turns

I removed a small portion of the outer jacket on the coil at approximately the 28 μH point (12 turns) then installed a .0012 μF capacitor.  I used a small variable capacitor to tune for resonance on the carrier frequency.  With this set up, at 860 KHz, there is >47,500 impedance.  That goes down to about 16,000 ohms +/- 10 KHz.

That should make things better.

Then I mounted the coil and capacitor in the truck body tool box.  There is a fair amount of stray capacitance from the box itself, which raised the resonant frequency by 5 KHz.

Device Under Test, initial testing of isocoil after fabrication

Device Under Test;  initial testing of isocoil after fabrication

Resonance is slightly above the carrier frequency with the permanent fixed .0012 μF capacitor.  I think this will change once the unit is connected to the station ground plane.  The network analyzer indicated there is too much capacitance in the circuit.  Unfortunately, this may be as good as it gets, however, the analyzer shows the impedances are still pretty high:

Frequency (KHz) Impedance (Ohms) Deviation from Carrier (KHz)
850 9,950 – 10
855 14,720 – 5
860 28,590 0
865 59,580 + 5
870 24,780 + 10

The base impedance of this tower is 34 ohms on the carrier frequency, so the isocoupler should be invisible to the transmitter across the 20 KHz occupied bandwidth of the station.

The FCC38-50J cable has a loss of 1.04 dB per 100 feet at 100 MHz, which is the figure I will use to calculate the insertion loss on the FM translator antenna system.

The old isocoupler is made with RG-214, but likely a somewhat shorter length.  RG-214 cable has a loss of 1.9 dB per 100 feet at 100 MHz.

Installation:

Isocoil mounted on back of ATU

Isocoil mounted on back of ATU

Isocoil mounted on back of ATU

Isocoil mounted on back of ATU

Before and after measurements with the network analyzer show a very slight change in the reactance at the tower base.  Nothing major and easy enough to tune out with the series output inductor of the ATU.

If I where to do this again, I would simply tap the coil at ten turns from the bottom, measure the inductance and install the proper value capacitor.  Since this had to be constructed with the parts on hand, less the truck body tool box, it because a bit cumbersome to get close to the resonant frequency.

All this got me thinking; there are other possible uses for such a design.  Crossing a base insulator with Ethernet cable always presents some unique problems.  I know the WISP forum that I read, they are always talking about how difficult it is to mount an antenna on an AM tower.  What if… armoured Cat5e or Cat6 cable was used with water proof RJ-45 jacks?  Something like that could carry Ethernet data and DC voltage past the base insulator to a three or four around sectorized access point and an edge switch or router mounted on the tower.

Armoured category cable specifications

Armoured category cable specifications

just thinking…

Anyway, it would not be hard to make coils and install capacitors for the right frequency