Antenna questions


Hi, I have a roofmounted FM-only omnidirectional antenna. It works great except for one station (important to my wife) which suffers from multipath distortion/static, so I am looking to install a directional in it's place.

1. APS is mentioned here, but their website says the antennas are non-returnable. I think it's pretty important to be able to try the antenna in your attic before installing on the roof, and return if necessary! Does anyone sell these antennas with return privileges?

2. The Radio Shack website has a chart that shows specs for their various antennas. "FM Gain" is 2.2 on the largest of the their antennas, and only 1.0 on the antennas (including a directional FM-only) that I am considering. What is up with that? Aren't directional antennas also high-gain? Should I worry about it? Since signal strength isn't a problem, maybe just a unity-gain directional is enough?

Thanks for any ideas.
ehart
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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Sean, thanks, that is the exact Radio Shack model I was considering. It turns out the gain of this antenna isn't listed anywhere. The other Radio Shack models all seem to have some extra elements for picking up UHF, which doesn't help with FM, so I am inclined to stick with this barebones model.

I will probably do as you suggest and just try it.

Any thoughts on Wineguard and ChannelMaster antennas? I believe I can buy them with option to return.
Sean, you nailed this one down nicely!
Yes, I'd try the RS $22 one. You can fairly easily estimate relative gain by counting element count and geometry. If the others have significantly more elements than they'll probably have more gain.....
On a similar tack I just decided to relive my preteen years as a Ham by buying a 1957 Hallicrafters SX-100 receiver, and found on the net a couple of guys who sell "universal dipoles" using two long slinkies and a T connection and downlead! Amazing stuff. You simply string it up in your attic, and pull the slinky ends out to match wavemength. It'll go to 130 feet (gulp!), but works nicely at 15-16 feet for the SW bands normally used. For FM youd only need a few feet...hence normal dipoles. The point here is to confirm Sean's: height IS might. Even a lowly dipole, if set high enough, will offer great FM reception. Adding a few more elements (as in the $22 RS unit) simply makes it better. Don't worry too much about needing to spend more. DO use a good low-loss twin-lead instead of coax for a down-lead, though. I was surprised to hear from all the Hams that good foam twin-lead easily outperforms coax. Hmmm....
Have fun on the roof...I settled for the attic, as a couple of slinkies blowing in the wind could get a SWAT-team over to my house, I fear.
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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