Antenna questions

Gain on an antenna is a relative thing. If i compared a 12" piece of wire to a wet noodle as an antenna, the wire would have high gain. If i compared that same wire to a well designed 1/4 wave "ground plane" antenna, it would have less gain. As you can see, it is all relative and one has to know the point of reference to know whether the antenna really has "gain" or not.

As such, most antennas are rated dBi or dBd. DBi means "deciBels over an isotropic reference" and dBd means "deciBels over a dipole". An isotropic reference is a theoretical antenna that works like garbage. As such, most any "reasonable" antenna will work better and show "gain" when using a dBi point of reference. On the other hand, a dipole is quite capable of very reasonable performance and makes for a very usable point of reference. However, since it works better than an isotropic, gain ratings based on dBd's will produce a lower, less impressive number than if one used a dBi rating instead. You can bet that dBi is what the marketing department wants to use to make their gain rating look more impressive. As such, it is possible to buy an antenna that looks to be of a high gain rating ( using a dBi point of reference ) and end up with an antenna that is equal to or lesser than one using a dBd reference. Obviously, you have to know what the antenna was referenced to in terms of how they arrived at the specific gain rating.

Having said all of that, I am 100% certain that the APS antennas would work better than any Rat Shack antenna that they have in stock. The APS antennas are tuned specifically for the FM broadcast band and are a very specialized design. While Rat Shack does make strictly FM antennas along with FM / TV antennas, they do not have the physical size / element count / gain that the APS models do.

With all of that in mind, the APS models would be the way to go IF you want the ultimate in FM reception using a mass produced antenna. Going to even this Radio Shack FM antenna and mounting it on a rotor with proper aiming would produce FAR greater signal strength than any omnidirectional antenna. We are talking about three to four times the amount of range and signal strength.

As such, you might want to try the Rat Shack design and see if it will work for you. RS has a very liberal return policy and they can be found locally in most cases. I would caution that the results that one gets in the attic will not be as good as one achieves up on the roof out in the open, so keep that in mind. You might also want to remember that "height is might" when it comes to antennas and receiving signals, so do your best to get it up high and out in the open. If the RS antenna won't do what you want it to do, you will probably have to shell out the cash for one of the APS designs. Sean
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Twin lead is lower loss than coax so long as humidity remains low and it is not mounted up against or near metalic objects. Otherwise, the impedance is altered and the loss goes way up at that point. Having said that, foamed twin-lead is more stable and lower loss than the regular "el-cheapo" twin lead that uses only plastic as a dielectric. It is also slightly more expensive.

Bare in mind that what lowers the performance of coax so drastically is the use of impedance matching baluns or "transformers". These are the devices that allow one to use 75 ohm coax and adapt it to a "split terminal" ( 300 ohm ) connection. If you use one of these at the antenna and also need one for use at the tuner ( a lot of old tuners used screw or lug terminals with no provision for coax ), you are losing quite a bit of signal.

As to Ernie's comments about element count, that is "basically" true. One can change the spacing, diameter and length of said elements and alter not only the foward gain, but the front to back ratio and the bandwidth. Bandwidth is the amount of frequency coverage that one obtains with good gain & the proper impedance. That is why a TV / FM antenna can have a "million" elements yet not have that much gain i.e. it was designed for wider bandwidth ( low "Q" ) to cover the phenomenally wide TV broadcast spectrum than it was for higher gain in a more narrow ( like the FM band ) frequency range.

Gain, front to back ratio and bandwidth are all factors that APS has played with and why you pay so much for their products i.e. they are highly specialized designs. As such, it takes technology, "know-how" and R & D ( Research & Development ) to get things dialed in for optimum performance. Since time and knowledge typically equal money, they want to be compensated for their efforts : ) Sean
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The "basic" formula for a Yagi design is that the driven element is tuned to resonance in the center of the desired frequency range. The reflector is then tuned appr 5% longer than the driven element and placed behind it. Varying the space will affect the front to back ratio and forward gain. This is the most basic Yagi that one can build and have it work well.

To increase the gain, a director is added in front of the driven element. This is appr 5% shorter than the length of the driven element, making it 10% shorter than the reflector. If one wanted to increase gain further, you can add another director, which in turn would be another 5% shorter in length. This works well until you hit about 5 - 6 elements total ( 1 reflector, 1 driven element and 3 or 4 directors ). After that point, the boom becomes longer, wind load is increased and the increases in gain are not as sizeable.

As far as bandwidth goes, 20 MHz is actually quite wide in spectrum at that frequency range. As one goes up in frequency it is FAR easier to make an antenna that covers a a wide range and maintains good tuning / good gain. At the frequency range that we are at with FM, obtaining a 5 MHz bandspread with excellent tuning would be a very reasonable goal. As one strayed further away from the center of this band, antenna impedance and gain would begin to vary. As such, most antennas are tuned with a center frequency of appr 98 MHz ( middle of the FM band ) and performance is relatively lacking at the low end ( 88 MHz ) and top end (108 MHz ).

As such, if one really wanted to pull in specific stations within a certain frequency range, having an antenna made ( or making one yourself ) for that specific center frequency could result in noticeable improvements. The fact that many people are happy with indoor dipoles or simple "stick" antennas like the Magnum / Fanfare / Metz design ( that are not very broadband at all ) shows that a simple antenna can work quite well. More advanced designs simply give you even greater range with less noise and interference.

Other than that, some of the older, well designed receivers ( HF, shortwave, broadcast band, etc.. ) can work phenomenally well. Depending on how involved one wanted to get in terms of user adjustable controls, some of the "fossils" that are out there are actually "diamonds in the rough". The biggest advantage that i see to a well designed and highly tuned tube front end is that they are less prone to front end overload and adjacent channel interference. If one lived in the city near a lot of strong stations, that is probably what i would be looking for. Sean
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Eric: Front to back ratio would come into play if you are trying to null out stations on the side of or to the rear of the direction of the station that you are trying to pick up. Gain is how much the signal is amplified once it pulls it in and front to back ratio is how well signals of to the rear of the antenna are rejected. Antennas are typically optimized for one or the other as doing both extremely well would be a tough thing to do. This has to do with the spacing of the elements. One would think that the antenna with the highest forward gain would have the tightest rejection of the back end and vice-versa, but it doesn't work that way. As "Tweed" says above, once you've gotten to this level of antenna, it is pretty much a matter of optimizing the antenna installation itself and going from there. Sean
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