Welcome to EV's FM & HD Radio Antenna Guide!
Just getting started folks so please be patient. I am not the world's authority on FM Technology/Broadcasting, Antennas or Antenna Theory & Design, nor HD Radio. There are many others more knowledgable and experienced than I regarding these topics. I am just an enthusiast that decided to write a helpful guide with the help of many others that have gone before me. The guide is and always will be imperfect...and I am still working on it daily....however, I think the effort will prove useful. If you have recommendations for additions to the list, they are most welcome, especially vintage or discontinued antennas. Seeking photos of vintage, discontinued, and current antennas as well.
Exhaustive overview of commercially available antennas
Reporting hands on testing and comparisons of antennas on FM analog and HD Radio broadcasts
Helping the Average Consumer make better antenna purchase decisions
Discussing antenna and reception issues for better antenna/system performance
Springboard of links and sources for further research for those so inclined
Historical record of vintage and discontinued FM antennas
Learn about FM Radio and HD Radio
Please join the forum and the discussion. Look forward to seeing you here. Please let me know if you liked the guide and found it helpful and how I can improve it.
Work in Progress....
Topics for further discussion and expansion....
Proper Lengths for telescoping or otherwise dipoles for FM Reception
AM/FM Combo Antennas and their Compromises
Winegard Mystery Antenna from the 70s
FM Band - Where is it? What's it all about?
FM Broadcast Band Wikipedia In most of the world, the FM broadcast band, used for broadcasting FM radio stations, goes from 87.5 to 108.0 MHz. The name "FM band" is misleading, since one can transmit FM on any frequency. All of these bands mentioned are in the VHF band which extends from 30 MHz to 300 MHz. In some countries FM broadcast radio is referred to as VHF.
In the United States, the twenty channels with center frequencies of 88.1–91.9 MHz (channels 201–220) constitute the reserved band, exclusively for non-commercial and educational (NCE) stations. All the other channels, with center frequencies 92.1–107.9 MHz (and 87.9 MHz, where used) may be used by both commercial and non-commercial stations. (Neither Canada nor Mexico observe this reservation.)Very High Frequency VHF Wikipedia: VHF (Very high frequency) is the radio frequency range from 30 MHz to 300 MHz. Frequencies immediately below VHF are denoted High frequency (HF), and the next higher frequencies are known as Ultra high frequency (UHF).
The frequency allocation is done by ITU. Common uses for VHF are FM radio broadcast, television broadcast, land mobile stations (emergency, business, and military), Amateur Radio, marine communications, air traffic control communications and air navigation systems (e.g. VOR, DME & ILS).
United States and Canada
Frequency assignments between US and Canadian users are closely coordinated since much of the Canadian population is within VHF radio range of the US border. Certain discrete frequencies are reserved for radio astronomy. The general services in the VHF band are:
30–46 MHz: Licensed 2-way land mobile communication. 
30–88 MHz: Military VHF-FM, including SINCGARS
43–50 MHz: Cordless telephones, 49 MHz FM walkie-talkies and radio controlled toys, and mixed 2-way mobile communication. The FM broadcast band originally operated here (42-50 MHz) before moving to 88-108 MHz.
50–54 MHz: Amateur radio 6 meter band; 50 MHz is an amateur radio band used for a variety of uses including DXing, FM repeaters and radio control, which usually takes place on a "set-aside" band between 50.8 and 51 MHz.
55-72 and 77-88 MHz TV channels 2 through 6, known as "Band I" internationally; a tiny number of DTV stations will appear here. See North American broadcast television frequencies
72–76 MHz: Radio controlled models, industrial remote control, and other devices. Model aircraft operate on 72 MHz while surface models operate on 75 MHz in the USA and Canada, air navigation beacons 74.8-75.2 MHz.
88–108 MHz: FM radio broadcasting (88–92 non-commercial, 92–108 commercial in the United States) (Known as "Band II" internationally)
108–118 MHz: Air navigation beacons VOR
118–137 MHz: Airband for air traffic control, AM, 121.5 MHz is emergency frequency
137-138 Space research, space operations, meteorological satellite 
138–144 MHz: Land mobile, auxiliary civil services, satellite, space research, and other miscellaneous services
144–148 MHz: Amateur radio band 2 Meters
148-150 Land mobile, fixed, satellite
150–156 MHz: "VHF Business band," the unlicensed Multi-Use Radio Service (MURS), and other 2-way land mobile, FM
156–158 MHz VHF Marine Radio; narrow band FM, 156.8 MHz (Channel 16) is the maritime emergency and contact frequency.
160-161 MHz Railways 
162.40–162.55: NOAA Weather Stations, narrowband FM
175-216 MHz television channels 7 - 13, known as "Band III" internationally. A minority of DTV channels may appear here.
174–216 MHz: professional wireless microphones (low power, certain exact frequencies only)
216–222 MHz: land mobile, fixed, maritime mobile, 
222–225 MHz: 1.25 meters (US) (Canada 219-220, 222-225 MHz) Amateur radio
225 MHz and above: Military aircraft radio (225–400 MHz) AM, including HAVE QUICK, dGPS RTCM-104
The large technically and commercially valuable slice of the VHF spectrum taken up by television broadcasting has attracted the attention of many companies and governments recently, with the development of more efficient digital television broadcasting standards. In some countries much of this spectrum will likely become available (probably for sale) in the next decade or so (June 12, 2009, in the United States).
FM Stands for Frequency Modulation. How does that work? How do you get FM Stereo sound?
FM Broadcasting Wikipedia: FM broadcasting is a broadcast technology invented by Edwin Howard Armstrong that uses frequency modulation (FM) to provide high-fidelity sound over broadcast radio. continued.....
HD Radio: HD Radio the marketing name of iBiquity's IBOC technology - In Band on Channel....an offset carrier standard used to broadcast digital audio and data. HD Radio is a bit misleading because it isnt High Definition Audio, but rather its quality is variable between lower quality to mid quality MP3. However the advantages of digital are nearly absent backround hiss and distortion. I dont want to get into the technical aspects of HD Radio, nor the arguments over its advantages and disadvantages between it and analog FM as well as other digital broadcast standards here. The main point I want to get across here, is that HD Radio does NOT require a different or special antenna....standard, regular FM analog antennas work for HD Radio. Discussion HD Radio is encouraged in the thread.
One note about HD Radio, I would like to make is that because of its digital nature, it exhibits what is known as a digital cliff. What does that mean? It means that point of decoding or not decoding an HD Radio broadcast is a cliff, you either have enough to get clear audio or you dont. Its not a slow fade like analog. If you are familiar with OTA digital television, you may be familar with the digital cliff...its similar with HD Radio. This (along with its current low 1% broadcast power which may be changing soon) may have people scrambling for better antenna solutions. If you are on the digital cliff with HD Radio, what usually happens is that it reverts to the primary analog signal if its the HD1 signal. It can switch back and forth between these like FM Mono and FM Stereo of yor, which is very annoying. One solution is a Forced Analog switch, like the forced FM Mono switch of the past.
Youtube Video of Chicago ABC News 7 report on HD Radio, Autumn 2008.
HD Radio: Are you listening?
Important Characteristics of Antennas
There are several characteristics that are important to know in order to choose the appropriate antenna for your situation.
Gain: This number reflects the antennas ability to capture and transform radio wave energy into electrical energy that can be used by your tuner which then decodes the signal that your amplifier can use to drive your speakers to make sound waves that travel to your ears which convert sound wave energy into electrical pulses that your brain then decodes. Yeah! The most common figures used are dBi and the much more historically common dBd. Both represent a gain figure that relates the antennas ability to the ability of a reference antenna. In the case of dBd, that reference antenna is a 1/2 wave dipole cut to the length of the frequency/channel....dBi relates to an isotropic radiator, which is a theoretical point source of waves which exhibits the same magnitude or properties when measured in all directions. It has no preferred direction of radiation. It radiates uniformly in all directions over a sphere centred on the source. It is a reference radiator with which other sources are compared. These 2 figures relate to each other as dBd + 2.2 = dBi. 0 (zero) dBd means that the antenna is equal to a 1/2 dipole....indeed many of these antennas are 0 dBd and many are indeed 1/2 wave dipoles! It should be noted that a 1/2 wave dipole is a pretty good antenna, that is why it is used as a reference for judging other antennas....in other words 0 dBd aint too shabby! One more note, 3 dBd equals a doubling of capture ability over 0 dBd, 6 dBd...a doubling again or 4 times the capture ability of 0 dBd, and 9 dBd yet another doubling for 8 times the capture ability.
Front to Back Ratio, Directivity, Beamwidth, (and Multipath): As antenna theory generally works out, to increase gain, you usually have to direct the ability of the antenna to capture radio wave energy more effeciently in one direction at the expense of others. This is where the Front to Back Ratio number comes from...its the relationship of Forward Gain to Rear Gain. (Not getting into lobe discussion here.) This also can be thought of as Rear Signal Rejection. The higher the number the greater the rear rejection (and usually this corresponds to greater forward gain). As stated, dBd is the preferred historically used term, that is grounded in antenna theory....often dBi is used for psychological reasons to inflate the numbers for the unwary...but it is a valid modern standard often used in computer antenna modeling software.
Why would this be of interest to us? Well, if you have FM transmissions in mulitiple directions...then you may want to go with an Omni-Directional antenna with extremely low F/B ratios. It may be desireable to have a lot of rear rejection to help with multipath interference problems. Beamwidth is also related to F/B Ratio, in that high F/B ratios (which generally indicate high Forward Gain) also indicate tight Beamwidths. Beamwidth as a numerical value is generally described as the off axis point where gain is down 3 dBd compared to maximum forward gain. As you off angle from the direction the antenna is pointing, gain drops off as well, when you reach 3 dBd down, that angle is the measured Beamwidth. This is also important for multipath rejection and reducing or eliminating possible co-channel or adjacent channel interference.
What is this multipath and why should I be concerned about it? Multipath happens when objects in the environment reflect radio waves in different directions. Large geographical objects like hills and mountains and also smaller objects like buildings and trees can be sources of deflected/reflected signals. These reflected signals can be picked up with your antenna along with the direct transmitted signal and be passed along to your tuner which may be confused by these out of phase signals and thus reduce its ability to decode them cleanly...resulting in poor quality music reproduction. The tuner can jump back and forth from the primary and reflected signals. Tuners are designed to deal with this to varying degrees of success, but you can help the tuner by understanding mulitpath...identifying it as a source of trouble...and selecting a proper antenna to reduce it. Co channel and Adjacent Channel interference is similar except that the source of the objectionable or interfering signal is another FM broadcast, either right next to the frequency that you are attempting to listen to, or on the same frequency/channel at another distant location/direction.
Bi-Directional, Figure 8...........Directional...........Omnidirectional
Horizontal Dipole, Helical.......Yagi, Rhombic.......Whip, Vertical Dipole, Turnstile
Here you can visualize the directional nature of differing antenna designs.
Lets look at a 3D visualization of a Vertical Dipole...this may help you visualze what is going on, better.
The antenna is in the center standing up or in a vertical orientation. As you can see the round donut shape if cut in a horizontal cross section corresponds to the circular pattern of the Omni-Directional above. Now if you take your Vertical Dipole and turn it sideways to a horizontal position, then the donut rotates with it, now half of the donut is below ground with the rest arcing over the top fo the dipole. If you look at the Horizontal Dipole plot above, you will see that the Figure 8 pattern is a horizontal crossection of this 3D donut rotated on its side. In the horizontal position the dipoles nulls point off of the antenna ends, instead of up and down now side to side, thus it doesnt have a circular pattern.
Polarization: Circular, Vertical, and Horizontal: What you need to know!
Without getting to deeply into the physics aspect of polarization, what you really need to know is that FM is broadcast in both vertical and horizontal orientation, simultaneously most often. Some stations only broadcast on horizontal only, so you may need to look into a specific station you want or are having trouble with. In the past when vertical whips were the most common vehicle antennas, some broadcasters boost the power to their vertical broadcast antenna, at the expense of the horizontal broadcast antenna during rush hours....then later brought them back into equal or favoring the horizontal component of the broadcast antenna. Im not sure most FM users should concern themselves with all of that. What the average consumer may concern themselves with is atmospheric propagation issues. Horizontally oriented antennas may perform better than verticals at longer distances depending on the atmospheric propagation conditions. A good discussion of this can be found on Wikipedia, TV and FM DX. For this reason, you may favor a horizontally oriented antenna, depending on your needs and wants.FM radio
The term "circular polarization" is often used erroneously to describe mixed polarity signals used mostly in FM radio (87.5 to 108.0 MHz), where a vertical and a horizontal component are propagated simultaneously by a single or a combined array. This has the effect of producing greater penetration into buildings and difficult reception areas than a signal with just one plane of polarization.
Amplification: Is it useful? Generally speaking the answer is a qualified NO. Amplification is useful for overcoming signal loses in long cable runs of over 100 ft or so at FM frequencies....or splitter loses. It can be useful in systems that use an undersized antenna, which starts off with a much lower signal and then boosts it to useable levels. However amplifying the signal introduces its own noise into the system and can degrade analog reproduction quality. Generally speaking amplification is much more useful (while still not being a panacea) for television reception....one reason being the noise of television tuners themselves is higher than a low noise amplifier....thus increasing the Signal to Noise ratio. FM tuners are generally less noisey and thus do not give this benefit. You will see that many antennas that offer amplification also offer selectable power gain or the ability to turn it off completely, which is a good feature for reasons stated....these selectable gain amplifiers do offer some flexibility in the system....but are not as good a solution as going with a larger higher gain properly placed antenna.
Types of Antennas and How They Compare to One Another Performance Wise
Here we will discuss the various types of antennas what characteristics they exhibit and how they relate to one another.
Location & Building Materials: Where to place the antenna?
What's the most optimal place to put your indoor antenna?
FM Interference Sources:
The FCC Interference Handbook (courtesy of AARL) gives advice on how to isolate the sources of electrical interference.