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.