Low Pass Filter design

Every good transmitter, tube transmitters in particular, require harmonic filtering.  The last thing any good engineer or broadcaster wants is to cause interference, especially out of band interference to public safety or aviation frequencies.  All modern transmitters are required to have spurious emissions attenuated by 80 dB or greater >75 Khz from carrier frequency.  In reality, 80 dB is still quite high these days, especially in the VHF/UHF band, where receivers are much more sensitive than they used to be.  A good receiver noise floor can be -110 dB depending on local conditions.

The principle behind a low pass filter is pretty easy to understand.  The desired frequency is passed to the antenna, while anything above the cut off frequency is restricted and shunted to ground via a capacitor.

Low pass RC filter
Low pass RC filter

In this case, the resistor is actually an inductor with high reactance above the cut off frequency.  Often, these filters are lumped together to give better performance.  This is a picture of an RVR three stage low pass filter:

RVR three stage low pass filter
RVR three stage low pass filter

RVR is an Italian transmitter maker that sells many transmitters and exciters in this country under names like Bext, Armstrong, etc.  The inductors are obvious, the capacitors consist of a copper strip sandwiched between teflon insulators held down by the dividers in between the inductors.

Schematically, it looks like this:

Low pass filter schematic diagram
Low pass filter schematic diagram

For the FM broadcast band, a good design cutoff frequency would be about 160 MHz. This will give the filter a steep skirt at the first possible harmonic frequency of 176 MHz (88.1 x 2 = 176.2).

Values for components:

Capacitors Value Inductors Value
C1 20 pf L1 74.7 nf
C2 54 pf L2 75.1 nf
C3 54 pf L3 73.9 nf
C4 20 pf

The inductors are wire, or in this case copper strap, with an air core.  It is important to keep the transmitter power output in mind when designing and building these things.  Higher carrier powers require greater spacing between coil windings and larger coil diameters.  This particular filter is rated for 1 KW at 100 MHz.

Shocker: LPFMs have little or no impact on commercial FMs

The long awaited report, required by the NAB as a part of the Local Community Radio Act has concluded that LPFMs have little or no impact on commercial FM stations. No kidding?

The executive summary states that:

LPFM stations serve primarily small and rural markets and have geographic and population reaches that are many magnitudes smaller than those of full-service commercial FM stations. In addition, LPFM stations generally have not been in operation as long as full-service commercial FM stations, have less of an Internet presence, and offer different programming formats. We also found that the average LPFM station located in an Arbitron Radio Metro Market (“Arbitron Metro”) has negligible ratings by all available measures and has an audience size that lags far behind those of most full-service stations in the same market.

Followed by:

Although each of the stations differs considerably in its individual characteristics, the results of the case studies show that the selected LPFM stations generally broadcast a variety of programming continuously throughout the day, operate with very small budgets, rely on mostly part-time and volunteer staff, do not have measurable ratings, have limited population reach, and do not generate significant underwriting earnings. All but one of the station managers that we interviewed stated that the LPFM station is not competing directly for listeners with any specific full-service stations.


We conclude that, given their regulatory and operational constraints, LPFM stations are unlikely to have more than a negligible economic impact on full-service commercial FM stations.

Forgive my excessive block quoting of the FCC report titled: Economic Impact of Low-Power FM Stations on Commercial FM Radio, I found those portions of text far better than anything that I could write on the subject.

The NAB is reportedly “reviewing” the results, which the cynical me thinks is just another way of stalling a potential LPFM window later this year.

Micro Radio goes to Occupy Wall Street

I read a very interesting article from John Anderson regarding the Occupy Wall Street movements use of media, specifically low powered radio.  Being a native New Yorker, the demonstrations are of some interest to me.  To date, the demonstrators have placed a wide variety of grievances at the feet of “Wall Street,” some justly and some not.  What I found interesting about it is this:

Last week, the Occupy Wall Street encampment established a microradio station at 107.1 FM. The station simulcasts the 24/7 live stream which provides coverage of life inside Zuccotti Park, as well as street-level reportage of daily protest actions in New York City’s financial district.

One of the reasons for this is the City’s ban on use of amplified speakers and or public address systems. By using a micro radio station, persons in the crowd too far away to hear orator can use a small FM radio or even their smartphones to listen to the speech.  Another reason is the idea that large corporate media has been controlling the narrative for far too long, to the detriment of the average citizen.

Zuccotti Park is in lower Manhattan, about two blocks away from Wall Street itself. It is described as 33,000 square feet, which makes it about 3/4 of an acre.  A part 15 FM radio station (47CFR 15.239) can easily cover this area and more.  Even with the station limited to 250 µV field at 3 meters from the radiating element, generally though to be 100 mW TPO,  the reliable coverage area would be a radius of approximately 200 feet, depending on local interference.  That makes the coverage area approximately 125,600 square feet or more.  There are several other stations licensed to 107.1 in the greater NYC area; WXPK, WWZY are the closest and most likely to cause problems.

I am not sure how they are generating their live stream, but when listening to it for several hours over the weekend, I found it interesting and technically well done.  They seem to be running circles around others, who are only grudgingly admitting that there might be something going on in some forty odd cities across the US.

Micro Radio is a creative way to use the available technology and keep the public and protesters informed.

NAB thinks Translators offer more value than LPFM

Alternate title: “I love Stupidity,” somebody else’s, usually not my own.  It’s a bit hard to reconcile the NAB’s desire for translators against the need and strong community support for local radio.  The original intent of translators was to fill in coverage areas of existing FM licenses within the parent stations protected contour.  Very few translators are actually used for that purpose today.  They have, instead, morphed into vast over the air relay networks for NPR and religious stations or are relaying programming of HD-2 channels which would otherwise not be heard.  Why we would need more of that, I don’t know.

The unfortunate part of all that stupidity is the side effects.  Think of the stupid driver who cuts of a tractor trailer on the interstate and causes a big pile up.  There are the potential injuries to those involved in the accident but also the inconvenience to all those stuck in miles of backed up traffic.  That is a fairly minor occurrence.

With big corporate government, the size and scale of stupidity can reach epic proportions. To wit:  During the natural disasters that overtook the northeast, indeed other areas of the country as well, local radio was proven to be a reliable, sometimes life saving means of communications time and time again.  Yet, in spite of all that, the NAB seems to think that LPFM stations (community radio) should be second to cross band translators broadcasting AM stations and HD-2 channels.  Regarding FM translators on AM stations, the NAB says:

NAB first commends and supports the Commission’s proposal to eliminate the restriction on the use of FM translators by AM stations to translators that were authorized as of May 1, 2009. FM translators enable AM stations to overcome inherent technical disadvantages that limit audio quality compared to other services, thus limiting their service to the public and even threatening their economic viability.

Oh where to begin. First of all, AM stations do not have inherent technical disadvantages, that is a myth.  Off the shelf AM receivers are of inferior quality and make a well designed, well executed AM station sound like a telephone. If one were to listen an older AM radio or AM on a receiver with variable bandwidth IF, you would find that it can sound quite good, if not very good. The problem is that the receiver manufactures never carried through with the promise to open up the bandwidth following the implementation of NRSC-2 in 1991.  One should wonder why.

Second, there are many AM stations out there that are economically viable. Those stations that have local programming and serve the community of license and have not been neglected or turned into a automated syndicated radio repeater.  Now, could a class C or class D AM station benefit from a translator at night, sure. That may not be a bad distinction to draw, especially for those class D stations with no night time operating authority.

Regarding more translators in general, it is difficult to imagine what all those new signals will be used for, other than more of the same (relaying distant, out of market religious stations, NPR stations or HD-2 programming which nobody cares about).    The FM band is already full of such things and could actually use less, not more.

While unfortunate, the NAB’s position is not surprising.  They do the bidding of their dues paying members, after all.  The anti-competition we are a monopoly stance of the NAB members is not new either. Remember the required economic impact study required by the LCRA on the LPFM vs full power commercial FM stations.  To think that a 100 watt LPFM could significantly impact the business of a class A, B or C FM station is laughable.  Yet, it was a requirement stuck into the bill at the behest of the NAB.

It is up to the broadband minded FCC to see how to slice the remaining FM spectrum up and whether the corporitist NAB’s argument holds water, or the rising call of the people who want a return of local radio and local community service will be heard.

This is a video of what happened during Tropical Storm Irene in Ulster County, where I live:

We are truly fortunate that no one here was killed. In the mean time, the waters around here are still receding, we had some additional flooding Wednesday (9/7) with another 6 inches of rain from Tropical Storm Lee with flood warnings still in effect for several local creeks.

In my neck of the woods, we have nine radio stations licensed within about a 16 mile radius.  One is religious, one is a college station, the other is a classical music format programmed from Albany, 90 miles away, one is a LPFM run by a local high school and two are commercial AM or FM station.  The commercial stations used to be located in downtown Ellenville but moved to Poughkeepsie, about 30 miles away in 1999.  The religious, college, and classical stations are small and have no backup systems or interest in emergency programming.  That leaves the high school LPFM, WELV-LP.

In the height of the storm, 11.53 inches of rain had fallen in the previous 8 hours, the power was out, cable was out, the internet unavailable, the Verizon telephone company office in town was almost underwater, we had two sources of local Ulster county information; WDST (100.1 MHz, class A) in Woodstock and WELV-LP in Ellenville.  WDST  studios are located in Bearsville, which is about 25 miles north of here.  They are a locally owned, locally programmed station with a good record of community support.  They did a good job updating emergency information, flooded roadways, emergency shelter information, power restoration information, dry ice, alternate emergency numbers in case 911 went out, rallying points for local fire departments, etc.  Ellenville Central School district’s WELV-LP also did a good job, although much more confined to the local area around Ellenville and have a much smaller coverage area.  Still, they were live on the air with up to date information.  Thankfully.

Next time, who knows?