Antenna questions

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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that $22 RadioShaft antenna is working pretty well for me in my attic @60 miles out. It's probably only ~25' off the ground. I have one balun up top to convert the 300 ohm to 75 ohm RG-6 coax which the tuner accepts dirctly. I'm too afraid to mount it, or any antenna, outdoors anymore. Two really unfortunate lightning strikes have taught me how to compromise.

SEan, thanks for the refresher on Q. The Hallicrafters has SIX selectable bandwiths (0.5kHz for code up to ultra-wide for "phono"...HA!), and an amazing selectable Q-adjustable "notch" filter to move in to squash a strong adjacent broadcast. Results in amazing bimodal bandwidth curves! Didn't know they could do that in the 50s...and with tubes....
So the APS must be a lot of similar-length elements all stacked in a plane, as 88-108 megs is only a 20% change in frequency. Interesting. But isn't the Rat Shack also a tight-Q for FM (but with less gain, of course)? Gotrta get back to Radio Rwanda with its 10dB S/N ratio! Talk about low fi....

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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Hi gang,

Thanks for the good info, keep the comments coming!

I bought the $22 Radio Shack antenna and played with it extensively last night in my attic, to compare with the current roof-mount omnidirectional antenna. The public radio station in question now sounds "good" but not "very good" or "excellent". I can listen to it without going crazy. A few other stations now sound "very good" (good bass, very little static, good stereo separation) and one station verges on "excellent" (you forget you are listening to the radio).

OK, so a few more questions:

1. I guess I expected even better. I will have to decide whether I can stomach the size of the ALPS antennas. Any votes for Wineguard or others?

2. I wonder if my Adcom GTP-506 and my old Technics receiver are part of the problem. How much better can a tuner do at rejecting multipath, in particular? Anyone know the Adcom tuners?

3. There are a few stations that are much worse off with the directional antenna (they're in the other direction). How bad an idea is it to attach two antennas to the same tuner? An omni or a dipole in addition to the main antenna? Can you just connect two atennas to the same terminals, or would you need some sort of "blending" device?

Thanks for all the interest!

- Eric