FM DXing Tools
I have read in various publications that one does not need sophisticated equipment to successfully catch good FM DX. While this is is technically correct, it really doesn't tell the whole story. When the band is really open, especially with sporadic E propagated signals, good results can be obtained with just about any receiver/antenna combination. Even a table radio with a line cord antenna will pull in astounding DX. However, "blockbuster" sporadic E openings don't come along every day. Most of the time, the avid FM band DXer will be trying to sniff out weak, tropospherically enhanced signals. To be successful in this endeavor, you need good equipment. We are going to get a little technical here, so hold on to your shorts.
The Antenna - Let's start with the antenna system. It's probably the most important weapon in an FM DXer's arsenal. Here's why:
These days, the FM spectrum is full of stations. The FCC allocates new FM station licenses based mainly upon the proposed new transmitter's location and its distance from existing stations operating on the same and adjacent frequencies. Just about every "hole" in the FM station coverage contour jigsaw puzzle has already been filed under the current rules. Yet, the FCC's philosophy seems to be, the more stations there are, the better it is for the listening public.
The FCC recently passed new rules which "downgrades" the contour protection criteria for a large number of stations. This, the FCC says, "...gives greater flexibility in the location of transmitter sites." What it really means is more stations will be allowed to come on the air! Also, the FCC recently "identified" a large number of areas where new stations can be located. They are in the process of auctioning off these allocations to the highest bidder. As if things weren't crowded enough, the FCC introduced a new class of "low power" FM stations a few years ago. These are stations serving small areas of a community, operating with up to 1000 watts. As more of these low power power stations come on the air, those relatively quiet, good DX frequencies in your area will probably be occupied by new signals.
The net result of these various actions will be more stations crowding into the already packed FM band. You need a good, directional antenna to sort it all out.
I'm certainly not saying that more stations is a bad thing. Indeed, the nice thing about FM stations operating from just about every corner of the United States is that when there is a geographically selective opening, there will always be someone transmitting from the "open" area. Because there are stations operating from just about every corner of the United States, it's easy to detect the various kinds of band openings. If you hear stations where you normally don't hear any, the band is open!
What this means is that when you aim the antenna at the station you want to hear, it will greatly enhance that station's signal and effectively reject signals from stations in other, unwanted directions. As an example, we have a local station on 107.9 MHz, WBTF-FM, Midway, KY. Yet, by rotating the APS-13 to null out WBTF, I have picked up several other distant stations on the same frequency.
The APS-13 is a very "flat" antenna, meaning it exhibits essentially the same performance characteristics across the entire FM broadcast band. It also means that the APS-13 provides a good impedance match to the receiver on all the FM frequencies, giving maximum signal transfer from the antenna to the receiver. Most other directional antennas are optimized to perform their best at the approximate center of the FM band. Their performance will fall off at the upper and lower ends of the band. The APS-13 works as well at 88.1 MHz as it does at 107.9 MHz.
With the highly directional APS-13, I can usually pick up three, four or more stations on each FM frequency at any given time. In the first month of using the APS-13, I logged ten different stations on 89.5, one of them a 190 watt "educational" station, 97 miles away. You just can't do that without a good, highly directional antenna. And, when that blockbuster opening does come, a good directional antenna helps a lot. If it's a major opening, you may have two, three or more stations "fighting it out" on the same frequency. A highly directional antenna can help you sort them out. It'll also help you to pull the DX out from around the locals and semi-local stations.
I realize that people living in apartments or restricted by local zoning laws or covenants are probably not going to be able to put up a tower and big antenna. Unfortunately, you'll be handicapped without it, no matter what the band conditions might be.
Advanced Antenna Techniques - On October 5, 2003 I installed a second APS-13 antenna and Andy Bolin's FM Phaser box. Put very simply, the technique of antenna phasing allows you to null out one FM station and hear what's "under" the nulled out station.
I'm not going to go into all the technical details because Andy has already done a great job of explaining the technique of antenna phasing. You should read his paper at: http://pages.cthome.net/fmdx/phase.html before proceeding.
With the proper installation of Andy's phase box, you can achieve truly astounding results! To illustrate this point I've prepared a one minute audio clip that demonstrates the effectiveness of Andy's FM Phaser. For this demonstration I aimed both the main and "phase" antennas toward WZZZ (107.5), Portsmouth, OH. WZZZ is 95 miles away, and operates with only 2,600 watts. WIOK is on the same frequency. They are only 42 miles away, operating with 1,350 watts. WZZZ's antenna is at 495 feet, while WIOK's antenna is much higher at 696 feet. All things considered, both stations are equal in terms of coverage area. Both stations are northeast of my location, but since it's much closer, WIOK's signal is always very strong here.
Click HERE to hear the demonstration clip. The religious station is WIOK, and the rock station is the more distant WZZZ. The clip starts out with WIOK. Then, as I "null out" WIOK with Andy's phase box, WZZZ comes in perfectly. I repeat the process a couple of times. No, I'm not moving the antennas or changing frequency! I'm simply adjusting the phase box to null out WIOK. After listening, I think you'll agree that the technique of antenna phasing can produce some truly amazing results!
The Receiver - With band crowding being what it is, probably the most important characteristic of a good FM DX receiver is its selectivity. That is, it's ability to reject strong, adjacent channel (200 KHz away) and alternate channel (400 KHz away) signals. This is a function of the filters employed in the receiver's intermediate frequency (IF) amplifier stages. The "perfect" filter would exhibit uniform, absolutely flat response out to just under 100 KHz on either side of the center frequency, and no response at all beyond that. Unfortunately, there is no such thing as a "perfect" filter, but there are some pretty good ones. Pick a tuner that does a good job of rejecting strong, adjacent channel signals, keeping in mind that expense doesn't always translate to performance. Unfortunately, most tuners or receivers designed for home use exhibit rather poor selectivity.
The next most important receiver performance characteristic is sensitivity, the receiver's ability to pull in weak signals. A sensitivity of under 3 micro volts to produce a listenable mono signal is usually good enough. Receivers with sensitivity figures under 1 micro volt for a usable signal are not uncommon. Related to this is the receiver's front-end dynamic range, which is also very important. This is the receiver's ability to handle very strong signals without becoming overloaded and producing "garbage" that you'll hear all over the band. A low front-end noise figure is another characteristic of a good receiver. If the transistors (or other amplifying devices) in the initial RF amplifier stages of a receiver (its "front-end") produce noise, it doesn't matter how sensitive the receiver may be. The weak signals will be eaten up by noise produced in the front-end and amplified by the subsequent stages of the receiver.
The capture ratio of the receiver is another important performance characteristic. Capture ratio is the receiver's ability to clearly hear a stronger FM signal, while completely rejecting a weaker signal or signals on the same frequency. A capture ratio of 1.5 db is good. This means that a signal only 1.5 db stronger than another signal on the same frequency will be heard in favor of the weaker signal. The lower the number, the better.
If you are serious about FM DX, there are many great FM tuners available. It pays to carefully research FM tuners before your buy. A higher price on a tuner or receiver does not always equate to good performance! A good resource for FM tuner information and reviews can be found at: www.fmtunerinfo.com
The performance of most tuners can be dramatically improved by careful alignment and the replacement of the tuner's stock IF filters. These operations require specialized test equipment and a good electronic skills. For this reason, it's a great idea to seek the assistance of a professional when it comes to buying a DX tuner or modifying an existing tuner. One such resource is Don Scott, former Stereophile tuner reviewer. Don presently reviews RF devices for The Audiophile Voice. More information is available in the "Products & Services" section of this web site. You can contact Don at: email@example.com
RDS and Digital FM
RDS (Radio Data Service) has really picked up momentum in the United States over the past few years. RDS is a digital data transmission on an FM station's subcarrier. With RDS, the station presumably transmits its call letters and format type digitally, and has the ability to send text messages, like the titles of songs and such. From the DXer's perspective, it's nice to see a station's call letters instantly displayed on the front of the receiver. No questions, no doubt - just log it and move on!
IBOC (In Band On Channel), better known as HD Radio is on the air in most major cities. Digital information is transmitted just above and below the analog information that makes up the FM carrier. The "hash" created by the digital IBOC carriers often prevents reception of analog FM on the adjacent channels of the IBOC station. In areas congested with HD Radio service, this can be a major source of frustration for the DXer as previously clean frequencies become noised up by the digital hash. On the other hand, HD Radio presents yet another DX target. Like RDS, the call letters of the HD Radio station will be displayed on an IBOC capable radio, making for easy identification.
There are very few HD Radios on the market that are suitable for DX use at this time. I've tried the Boston Acoustics Recepter HD and found it to be a pretty miserable radio. I acquired a JVC car HD Radio, which I run from a 12 volt power supply. Unlike my Boston Acoustics Recepter, the JVC is a respectable radio for both AM and FM reception.
In theory, it's possible to pick up distant IBOC on the same frequency as that of a fairly strong but different analog FM since the analog and digital portions of the signal occupy different segments of the spectrum that make up the FM signal.
My TV / FM DX Equipment
At the top is the Antenna Performance Specialists APS-13 FM Yagi antenna. The APS-13 is 17 feet long! Under the APS-13 is Channel Master 4228 UHF TV Antenna. Below the UHF antenna is a Delhi (Jerrold) VIP-307SR VHF TV antenna.
The antenna assembly is rotated using an old CDE Ham IV rotor. The tower is Rohn 25G, with an overall height of 50 feet.
Here's the DX equipment layout as of 07/04/06. The computer to the right is used as the server for the Live TV DX-Cam. The computer in the center is used for everything else.
DX Equipment Rack
This is my DX equipment rack as of 07/04/06. It's always evolving as I experiment with new equipment or antenna setups.
Some of the items, like the CDE main antenna rotor, have been with me for a long time. I've had this old work horse in service for over 25 years. The Crown audio amplifiers have been with me for 20 years.
A Few Last Thoughts
Up to this point, we've concentrated on the equipment. However, there are a few other tools you should have at your fingertips when the band opens and the DX starts rolling in.
It's vital to have up-to-date station lists in front of you. Toward that end, I created a series of Excel spreadsheet that have proven to be invaluable in my DXing efforts. These spreadsheets are created using the FCC's relational CDBS database, and Canada's broadcast database system. They list every FM and TV facility in the U.S. and Canada. Within the spreadsheets, I added formulas for calculating the exact azimuth (antenna aiming direction) and distance between my location and every listed station.
I sort the spreadsheets by frequency then azimuth, making it very easy to locate and target stations, based on which way the antenna is pointing.
These spreadsheets are available for download
from the DXFM.COM main page. For additional information about these files, click
THE 2009 EMISORAS DE FM
Created and published by Jim Thomas
If you are in range of Mexican, Caribbean, or Central American FM stations, you really should pick up a copy of Jim Thomas' "Emisoras de FM -- A Complete Guide to FM Radio South of the Border." Click HERE or on the image above for more information.