The Continental D323C medium wave transmitter

I found a 1981 Continental Electronics equipment catalog at an old transmitter site. These finds are great if one is interested in history and looking at the way things used to be done.  This particular transmitter is a 2,000 KW (2,000,000 watt) medium wave unit:

Continental Electronics D323C, Circa 1981
Continental Electronics D323C, Circa 1981

I believe most units like this were destined for use by government broadcasters either the middle east or western Europe.  I know there were several 1,000 KW medium wave stations in West Germany at one time.   The Continental transmitter is basically two 1,000 KW units (323C) combined.  They used a modified version of Doherty modulation, that is called “Screen and Impedance,” which accurately describes how it works.  More information from the Continental Catalog can be found here: Continental D323C.  The tubes (or valves depending where you are located) used in the D323C were 4CW25000A tetrodes as modulators and IPA the final was a pair of X2159, which is an impressive tube.

EIMAC X-2159 water cooled power tetrode
EIMAC X-2159 water cooled power tetrode

The tube sat anode up.  The filament, grid and screen connections are underneath.  Cooling water was pumped through the two connections on the top at about 130 gallons per minute depending on the plate dissipation.  With a 30° C rise, that equals about 96,000 BTU per minute.  The D323C had a dissipation of 400,000 watts for the carrier tube and 240,000 watts for the peak tube (640 KW total) under 100% modulation.  That equals about 2 million BTU per hour.  Notice the lifting hook, this tube weight in at 175 pounds.  Tube date sheet here.

Continental no longer makes medium wave transmitters, their closest high powered broadcast product now is the 418/419 and 420 HF (shortwave) transmitters.  The 420D does a wimpy 500 KW using a solid state modulator section.

I remember in the early 1990’s when I was at the Harris plant in Quincy, they were working on a 1,000 KW solid state DX series AM transmitter for Saudi Arabia.  It had to be liquid cooled, which added another layer of complexity to an already complex system.

I don’t know if there is much call for 2 MW medium wave transmitters anymore as there are more efficient ways to reach remote populations and I can’t even imagine what the electric bill would be like.

Zonecasting; the Technical Details

I saw this a item many weeks ago, however, had not had time to look at it until now.  Geo Broadcasting Solutions has filed Petition for Rule Making (RM-11659) based on a system divides the coverage area of major stations into smaller zones allowing for ad targeting of specific audiences.  They have coined the term “Zone Casting” to describe the scheme. It is covered by two US issued patents filed by Lazer Spots, LLC: 20120014370 and 20110065377.  After a look at these two patents, it seems there are three possible ways to accomplish this Zone Casting Scheme:

  1. In the first described method, the main transmitter is broadcasting area wide and all the zone transmitters are muted.  An inaudible signal is transmitted to all units, the main transmitter is then muted and the zone transmitters turn on and transmit localized content.  After the local information is transmitted, the zone transmitters mute and the main transmitter resumes broadcasting.
  2. In the second described method, the main transmitter and the zone transmitters are broadcasting area wide information.  The main transmitter ceases broadcasting area wide information and the zone transmitters begin broadcasting localized information.  At the end of the localized information the main transmitter and zone transmitters transmit area wide information.
  3. In the third describe method, the main transmitter and zone transmitters are broadcasting wide area information with “capture ratio pattern.”  The main transmitter initiates an alteration, temporarily becoming a zone transmitter.  The zone transmitters then transmit localized content.  After the localized content, the main transmitter becomes a main transmitter again.

All of the transmitters are linked to the studio via digital STL systems, content for the zone transmitters is distributed via IP network.  The transmitter frequencies are synced with GPS, similar to FM on channel booster stations.  Method number three includes possibly switching the transmitter output to a lower gain and or lower height antenna.

Zone Broadcasting Conceptual Diagram
Zone Broadcasting Conceptual Diagram

Of the three methods, the first system will result in the fewest interference issues.  No matter which method is used, there will be interference issues between the zone transmitters and or the main transmitter where the signal strengths are equal and the audio is 180 degrees out of phase.  These can be moved around slightly by adding delay to the audio signal, but they will always be present.  More about Same Frequency Networks (SFN) and Synchronized FM signals can be found here.  While the zone transmitters are transmitting dissimilar localized information, standard capture effect rules apply.

The system has had limited testing in Salt Lake City, Utah (KDUT) and Avon Park, Florida (WWOJ), which according to the filing and comments, went well.

Geo-Broadcasting is applying to conduct a full test with WRMF in Palm Beach, FL.  The expected installation will include up to 22 zone transmitters.

Conceptually, tightly targeted advertising is not a bad idea.  Advertisers like it because they perceive a better return for their dollar.  The cost of such a system is not insignificant. Transmitter site leases run $1-2K per month, leased data lines, equipment, installation work, equipment shelters, etc will likely run several hundred thousand dollars or more.

If it gets approved by the FCC, it will be interesting to see how it works and whether or not the system is financially justifiable.


The Nautel NV-5 Transmitter

We are currently installing this sweet little transmitter:

Nautel NV-5 FM transmitter
Nautel NV-5 FM transmitter

Like its big brother, the NV-40 at WVPS, the NV-5 is a very cool transmitter.  I am a born sceptic, things like a touch screen displays tend to make me a little nervous, especially on a transmitter connected to a 350 feet tall steel tower right next to the transmitter building.  That is the one major difference between WVPS and this site; at Mount Mansfield there are many things between the transmitter and antenna, this place, not so much.  Even so,  Nautel makes a good product, so troubles are not expected.

The ground strap, AC power, remote control and composite audio connections were all made with out difficulty.  Result, new transmitter on the air:

Nautel NV-5 FM transmitter GUI
Nautel NV-5 FM transmitter GUI

This unit is analog only, but the information on the spectral display is still useful.  The GUI uses Linux with a touch screen, which is a neat feature.

Nautel NV-5 FM transmitter controller board
Nautel NV-5 FM transmitter controller board

In case the front panel GUI goes out, all transmitter controls can be accessed via push buttons on the remote control interface, which is the small board to the right.  The main controller board is on the left.

Part 15 Broadcasters

I am utterly amazed at the lengths that some people will go to to get on the air. Where there is a will, there is a way.  Part 15 refers to FCC Part 15 rules, which cover unlicensed operation. Such things as wireless microphones, cordless phones, garage door openers, WIFI, other intentional and unintentional RF generators like computers.  Subpart C deals with low power, unlicensed broadcasting.

There are several rules regarding unlicensed Part 15 broadcasting, the most often cited rules are 15.209, 15.219 and 15.239, which sets the signal strengths allowed for various frequencies.  For the FM band (88-108 MHz), the signal strength requirements are very straight forward;  it is 150 250 µV/m measured at 3 meters from the antenna. This results in about 200 foot radius from the radiator or antenna reception distance.  Slightly more can be gained by broadcasting in mono.

For the AM band (530-1,700 KHz) there are several different measurement criteria.  First, FCC 15.209 states that the signal strength for an unlicensed medium wave station shall not exceed the value of (24,000/F(KHz) which varies from a maximum 45 µV/m at 530 KHz to a maximum 14 µV/m at 1,700 KHz measured 30 meters from the antenna.

Medium Wave broadcasting lends itself well to power line transmission, also known as carrier current.  There used to be many carrier current college radio stations in the country as it was a very easy way to broadcast to a limited area without the expense of a license and large transmitter.   There are still some carrier current stations out there, but many have gone dark.  For carrier current stations, the signal strength requirements can be found in FCC 15.221, which states:

(a) Carrier current systems and transmitters employing a leaky coaxial cable as the radiating antenna may operate in the band 525–1705 KHz provided the field strength levels of the radiated emissions do not exceed 15 µV/m, as measured at a distance of 47,715/ (frequency in kHz) meters (equivalent to Λ/2Π) from the electric power line or the coaxial cable, respectively.


(b) As an alternative to the provisions in paragraph (a) of this section, intentional radiators used for the operation of an AM broadcast station on a college or university campus or on the campus of any other education institution may comply with the following:
(1) On the campus, the field strength of emissions appearing outside of this frequency band shall not exceed the general radiated emission limits shown in § 15.209 as measured from the radiating source. There is no limit on the field strength of emissions appearing within this frequency band, except that the provisions of § 15.5 continue to comply.
(2) At the perimeter of the campus, the field strength of any emissions, including those within the frequency band 525–1705 KHz, shall not exceed the general radiated emission in § 15.209.

Finally, there is Part 15.219, which states:

(a) The total input power to the final radio frequency stage (exclusive of filament or heater power) shall not exceed 100 milliwatts.
(b) The total length of the transmission line, antenna and ground lead (if used) shall not exceed 3 meters.

Thus, there are several different ways to look at Low Power AM (LPAM) broadcasting.  In all cases, LPAM stations are not to be employed on the same frequency of an licensed AM station within its protected contour.  Part 15.219 appears at first to be contradictory to 15.209 which sets a specific signal strength value.  On reading the FCC’s recent NOUO and NOVs it appears the 15.219 is an exception and is left deliberately ambiguous, somewhat cryptically noting:

Another exception for some transmitters operating in the 510 kHz to 1705 kHz band is found in 47 C.F.R. S: 15.219. Specifically, Section 15.219(b) of the Rules states “the total length of the transmission line, antenna and ground lead (if used) shall not exceed 3 meters” (see 47 C.F.R. S: 15.219(b)).

From FCC EB-FIELDWR-12-00001143

Thus, from a technical and legal standpoint, which criteria will an FCC inspector use if they are looking at a possible violation with an unlicensed LPAM station?  In at least one case, it appears to be up to the inspecting officer.

With a well designed 3 meter (9.84 feet or 118 inches) vertical antenna and good ground system, it is very likely that a 100 mW station, particularly on the upper part of the AM band, could carry up to a mile or so, depending on the local ground conductivity.  There are several cases where multiple LPAM transmitters have been chained together, creating a SFN (Same Frequency Network) which covers a significant geographical area. This is a video showing two LPAM transmitters synchronized in Sioux Falls, SD.

Pretty amazing considering all the power lines and such.  Then of course, there is this, which shows that they might not be operating at 100 mW after all.  I don’t know at which point they began operating above the legal threshold, perhaps that video was taken during legal operation.

Even so, it is a very interesting concept, when one considers using a battery, solar panel, wireless LAN bridge and an AOIP device such as the Barix Extreamer to connect transmitters. There is one particular FCC certified transmitter that allows external synchronizing from a GPS source or by chaining the units together on a RS-485 buss. I have spent several days driving around and listening to static on 1,700 KHz, much to the annoyance of my wife and children.

There are several sources of information regarding LPAM broadcasting:

Those are just a few, if you know of others, leave them in the comments.  One thing to note:  If you are going to broadcast LPAM, make sure that you can demonstrate compliance with either 15.209 or 15.219.  Any type of unlicensed station that broadcasts with a regular schedule  over significant coverage area will be noticed.

If you are a licensed broadcaster and are concerned that a legally operating Part 15 station is going to cut into your market share, you are simply doing it wrong.