TheEC: SatDish Heater 2 - SON OF DISH HEATER!

Mar 19, 2018

They say sequels must always be bigger, flashier, and MORE than the original.  But in this case, technically it's LESS: less of a kludge!   Read on to learn why less is more with RIPR's Custom SatDish Heater ver2.0!

Five years ago I put together a custom-built satellite dish heater for our PRSS/NPR satellite downlink.  This is so whenever Providence gets wet, heavy is the norm for our winters...the heater melts it before too much builds up on the dish and blocks the signal carrying NPR, the BBC, etc etc etc.   When the satellite in question is in orbit over 22,000 miles away, it doesn't take a lot of snow to block it - even when the dish is twelve feet in diameter!

Now this system was unique, and it's proven quite functional, but it was also a serious kludge:

  1. Our primary, current-generation SFX4104 satellite receiver is connected directly to the dish outside.
  2. An old, previous-generation SR2000pro (PDF) satellite receiver is also connected, but a broadband RF noise generator is inserted between the receiver and the dish, via a T-junction to prevent blowback onto the main receiver.  The noise generator intentionally degrades the received signal by the SR2000pro, meaning it will "fail" much sooner than the SFX4104 will.
  3. The SR2000pro is tuned to two content channels that always have audio: the BBC World Service, and the NPR Breaking News Channel (when there's no breaking news, they have an audio recording that tells the exact time).
  4. A Broadcast Tools Silence Sentinal Basic silence sensor monitors the two audio outputs of the SR2000pro.  As snow builds up on the dish, the SFX4104 will keep working but the SR2000pro will lose "lock" to the satellite, and go silent.  The silence sensor eventually alarms after about three minutes (to prevent false positives).
  5. The silence sensor alarms and closes a relay.  This relay is paralleled across the old-school Honeywell thermostat that controls the air conditioner / furnace for the building.
  6. The ceiling vents are closed at the beginning of winter, and the main furnace hot air output vent is re-routed into insulated ductwork that goes out through the building's wall, through a series of PVC piping, and into a Walton De-Ice "Snow Shield" dish cover.
  7. Warm air circulates inside the dish cover, melting the snow.  Eventually enough melts that the SR2000pro re-acquires lock to the satellite, and audio starts playing again.  The silence sensor stops alarming, and opens the relay, so the furnace can turn off.

The problem is that PRSS is changing which satellite carrier the whole system uses.  This new "combined carrier" is incompatible with the SR2000pro, and the legacy carrier is being turned off at the end of April. 

So a new system is needed.  Enter SNMP!

SNMP is Simple Network Monitoring Protocol; a blatant lie as there's nothing simple about it.  While quite complicated and not intuitive, it's also quite powerful.  RIPR recently purchased a new remote control / warning system for our studios from Burk (based in nearby Littleton MA) called the ARC PLUS TOUCH with SNMP PLUS.  I also recently discovered a Bulgarian company called Denkovi that sells all sorts of fascinating little SNMP-based gadgets for remote control and monitoring. 

Now our workflow is as follows:

  1. Via the existing data path over a T-1 line, the ARC Plus Touch at the studio uses SNMP to constantly monitor the "Carrier to Noise" (C/N) ratio of the SFX4104 directly.  Normally between 10 and 14dB...when snow falls, this number drops fast.
  2. If C/N drops below 8, the ARC Plus Touch sends a SNMP command back down the T-1 line to a Denkovi DAEnetIP2 two-relay ethernet board. 
  3. The command tells the Denkovi's relay to latch closed.  This parallels the contacts in the Honeywell thermostat in step 5 above, and the process continues from there.  The Denkovi uses SNMP to report back to the Burk that its relay is closed.
  4. When enough snow melts, the C/N number will rise on the SFX4104.  Eventually it'll get high enough for the Burk to send another SNMP command to the Denkovi to open the relay and let the furnace turn off.

The Burk ARC Plus Touch remote control, as seen on your Intrepid Engineer's iPhone. Circled are the relevant measurements, although meters 1 thru 3, and status 7 thru 18 are all derived using SNMP.
Credit Aaron Read RIPR

You may ask, and it's a fair question, why do I care so much about this?  Because most dish heaters work by inferring that there is a problem.  They use a thermometer to detect that it's cold enough to snow, and a water sensor to detect the presence of snow.  That's what turns the heat on.  But in Rhode Island's weather this is fantastically wasteful.  We routinely get cold, wet snow even when it's nearly 40F outside.  That means that thermostat has to be set pretty high.  But we also routinely get cold wet rain...not snow...even when it's below freezing.  So the dish heater can run a'plenty even when it's not needed.

hate wasteful systems!  Especially systems that're wasteful because they rely on inference rather than proper measurement.

This system doesn't infer anything. It measures what matters: the received satellite signal at the SFX4104.  If it's too low, it turns the heater on.  If not, it doesn't.   Since this is a forced-air heater from a gas-fired furnace (instead of an electric-element heater) it also warms things up quickly, so we can afford to wait until the snow has already started building up before we start to melt it.

While the Burk is a relatively expensive piece of equipment, it's also purpose-built for radio broadcasting and it's quite versatile.  I use it to monitor our Nielsen Portable People Meter encoding monitor (aka "MCEM": Multi-Channel Encoding Monitor), also via SNMP, that tells us if there's a problem with our PPM encoders for ratings.  It monitors silence sensors for each of our broadcast signals.  And it keeps an eye on the temperature in the server room, too.

But the cool part is the Denkovi; that device happened to be perfect for this application and it was just $60.  You can't argue with that!