CW Broadcast

Information for LPFM Licensees
LPFM stations have some special requirements when considering certain equipment. 

FCC Approved FM Transmitters
Transmitters are an area of special consideration.  Unlike other classes of station, LPFM transmitters are required to be "TYPE CERTIFIED" by FCC rule § 73.1660(a).   

A relatively small number of transmitters are FCC "Type Certified".  Many FM transmitters are either "Type VERIFIED" or have no official testing whatsoever.  All transmitters, other than Type Certified transmitters are absolutely illegal for an LPFM station to use. 

What's all this about and why is it important?
Type certification means that the transmitter has been tested, usually by both the manufacturer and by an independent FCC testing lab, and found to not cause interference when used in the service that it has been tested in.  Additionally, the results of the  testing have been submitted to the FCC and the FCC has issued a type certification number for that transmitter.   Only transmitters that bear an FCC registration number are considered type certified.  A type certified transmitter is supposed to be designed not to cause interference when operated normally.  This type certification requirement is the FCC's way to make sure that an LPFM licensee won't get stuck with junk that will cause interference to other services.  THERE ARE A LOT OF UNCERTIFIED JUNK TRANSMITTERS ON THE MARKET, EVEN THOUGH THEY ARE ILLEGAL.

What's the difference between "Type Certified" and other similar devices, like "Type Verified" or "Type Approved"?
Basically, the difference is that type certified units have been reviewed and approved by the FCC itself.    Other transmitter, like the Type Verified units have been tested by the manufacturer and shown to not cause interference, but this testing information has not been reviewed or approved by the FCC.  Type verified transmitters are legal for all services, except LPFM.  Our own transmitter line contains a Type Certified transmitter, the FM300ES, but our higher powered transmitters are Type Verified because there's no reason why an LPFM station needs more than 300W.     

The FCC strictly enforces the Type Certification rule as one unfortunate LPFM station found out.   The type certification protects the LPFM operator if there is interference.  If interference occurs where a type certified transmitter is NOT used, the FCC holds the LPFM station responsible for that interference.   When a type certified transmitter is used, the FCC presumes that the LPFM licensee is innocent of wrong doing.  Even if no interference occurs, an LPFM station using an un-certified transmitter can face significant fines that are far greater that the cost of buying a certified transmitter in the first place.

How do I know that my transmitter is type certified?
First, a type certified transmitter has to have a permanent label attached bearing the FCC certification number.  Even if the transmitter's manufacturer says it is FCC certified for LPFM use, unless the transmitter is appropriately labelled, it's not legal.  Second, the transmitter certification has to be in FCC part 73, not in part 15.  Part 73 embodies the licensed broadcast service.  Part 15 applies to micro-power unlicensed broadcasting. 

So, you found a great deal on eBay, Amazon, etc.  Why not buy it?
There are at least two main problems with uncertified transmitters:

1.  Legality.  As we've already mentioned, transmitters that don't bear a proper FCC certification number are illegal to use and can get the unwary LPFM licensee in a lot of expensive trouble.  On thing to keep in mind is that most FCC part 73 type certified FM transmitters are made in North America.  A few are made in various European countries such as Italy, Spain and the United Kingdom (England).   As of this writing, there are no FCC Type Certified transmitters made in Asia, Africa, Australia or South/Central America.  Commonly found online brand names like Warner, FM User,  CZH, CZE, YXHT, Giant Communication (Alpha), Elite/Elit, RS Radio, Axon and others are not only uncertified, most of them perform too poorly to ever be certified.  This brings us to the second issue.

2.  Quality.   Besides uncertified transmitters being illegal in the USA, most are also of varying and unpredictable quality.   Many do not make the power displayed on the front panel. You might think you're getting 500W when in fact the transmitter is incapable of putting out even 200W.   Some of these Chinese wonders use sub $5 consumer grade chips to generate the RF signal (this is called the exciter).  These consumer grade chips were never meant to be used in high powered transmitters.  While fine for micro-powered Fm modulators, the quality is rather poor and they drift frequency because they don't have an AFC lock, as required by the rules.  We've also noticed that a lot of these so-called transmitters have little or no output filtering.  Without an effective low pass filter, these units are very likely to interfere with other services, such as the aircraft band which is immediately adjacent to the FM broadcast band.



Case Study:  1kW Transmitter from Asia vs CW Broadcast 1kW Transmitter
The transmitter shown below was taken from a popular overseas manufacturer of uncertified transmitters.  This transmitter is supposed to be rated at 1,000W.  We can see that the DC power supply appears to be adequate.  The main RF output amplifier is a typical design for a 1kW unit, but there are a few items of concern:  First, the output transistor form factor differs slightly from standard 1kW to 2kW transistors.  This particular device may either be rated in the 350 to 600W range or it may be a Chinese attempted reproduction of a higher powered transistor.  It looks a little bit like the STAC2942, a common 350W transistor. Second, the output low pass filter is VERY small for the power that they claim this transmitter makes.  The LPF appears to be adequate for under 200W.   At 1kW, this LPF would run extremely hot and be prone to arcing.  The trace for the directional coupler is also pretty thin for the amount of power supposedly being put out.  It appears that every possible corner has been cut.

We cannot see the exciter here, but it appears that the exciter and main controller are on the front panel.  The exciter might be hidden in another box, but, the cable routing suggests that it's on the front panel.  If so, it's almost certainly one of those $5 consumer grade chips with a few RF amplifier chips following.  if this transmitter operates at all, it will have a short life and likely cause interference, too.  What will you do when one of these foreign-made wonders blows up?  Will you ship it back to China for costly repairs?   

Below is the RF Deck and Low Pass Filter from one of our CW Broadcast 1kW transmitters.  Notice that two amplifiers are used for power levels of 1kW or more.  Also, the Low Pass filter is many times bigger.  This transmitter is built to last and provide many years of interference free performance.


Below, the RF Deck from a CW Broadcast FM300ES FCC Type Certified transmitter.  Notice that the coils in the low pass filter are larger than the coils in the foreign made 1kW transmitter.
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About buying parts from foreign suppliers: 
Be extremely careful when buying replacement parts from unknown sources.  We have experimented with buying parts overseas and found that the supply chain quality is very unreliable.  It's sometimes possible to get good parts at a fair price, but it's impossible to predict if you'll get good parts or not.  Often parts are sold through many different sources and in a lot of cases, these sellers all have bad parts.  They may not even know that they're selling fake parts, but they are. The parts below are supposedly both MRF1K50hr5 transistors.  The left part is counterfeit.  We tested this part to destruction, which didn't take much.  Aside from the obvious failure damage, you can easily see that the transistor on the left has a tiny die compared to the legitimate transistor on the right.  The counterfeit part produced less than 50% of the rated output before it failed.  This part cost about 70% as much as the legitimate part, but was probably really designed to make about 30% of the power of the legit part.  No bargain there.  If this transistor had been used, even at lower power, it still could have voided the type certification of your transmitter because using parts that are different than the design used could easily cause interference.  We did not find even one legitimate MRF1K50 or similar transistor, for sale on AliExpress and only a few good ones on eBay.

Comparison of real MRF1K50hr5 vs conterfeit
                MRF1K50

Other Considerations
Antennas
Be careful about your choice of antennas.  There are two main factors to consider when buying antennas:

1.  Life Span. - These things get hung in the air and are hard to service. If they fail, the cost to get them down often far exceeds the cost of buying them.  Aluminum antennas are cheap to make, but tend to have problems, unless special care is taken.  While there are some aluminum antennas that are good, there are more bad ones than good ones.  They will end up costing money because they just tend to not be very rugged.  This is especially true for sub-$500 products.  When buying antennas that will be hung on towers, generally try to use antennas made of stainless steel or marine brass, whenever possible.  Yes, they cost more and are heavier, but they will probably last many times longer.
Generally, avoid knock down kits.  There are some good antennas that require partial assembly.  However if it come in a bag, there could be reasons to avoid it.  If the device uses screws and hose clamps to put it together, then it's not suitable for unattended outside use.  The antenna below is a commonly available Chinese made aluminum antenna that costs about $200.  This antenna does work, but is too fragile to survive outdoor use.  Of three tested, one blew up at 150W, while the other two survived 300W.  The screws/hardware are not stainless steel.

A common FM antenna from China
Below:  The assembled antenna. This is a so-called "Penetrator" style.   It does radiate correctly in both horizontal and vertical planes.  
However, this antenna will not survive long outdoors.  If it was used in an indoor penthouse setting, it might last for a while.

Assembled antenna example


2.  Coverage.   Antennas come in various configurations and they all can radiate a signal.  In a very broad sense, antennas can be divided into Horizontal Polarization, Vertical Polarization and "Circular" Polarization.  There are variations of these three, but for simplicity's sake, we are limiting the discussion.  The ability to receive a signal is affected by the orientation of the antennas.  When antennas share a common orientation, the best transfer of signal occurs.  So, when the receiving antenna is oriented in the vertical plane, as in the rod antenna on a pickup truck, then a vertical radiator has the best chance of being well received.  If the radiator is  horizontal and the receiver has a vertical antenna (or vice-versa), there will be a significant loss of coverage.  To overcome this problem, circularly polarized antennas were invented.  These antennas put part of their power in each plane, so no matter which way the receiving antenna is oriented, there will be adequate coverage.  Because circularly polarized antennas divide their power between each plane,  their gain in each plane is only 1/2 of what a horizontal or vertical polarized  antenna would be.  The lower gain of a circularly polarized antenna means that twice as much power must be put in to achieve the same coverage.  However, the coverage that you do get is more universal. 

Which antenna type should you choose?  Generally, Circular polarization is preferred, but you need more transmitter power to get the same coverage.  If available power is limited, then a single polarization antenna may be needed.  In this case, a vertical antenna will usually perform better than a horizontal antenna.  However, it can't be overstated that circular polarization is preferred whenever possible. 


Below:A horizontal only antenna Below:  A stacked vertical antenna
Below:  2 examples of circularly polarized antennas