Okay, so, there's some fact, and a whole lotta fiction that come along with antennas, feedline, and recievers, and it is oftentimes difficult to sort it all out.
First... radio signals are electromagnetic. Light is electromagnetic, albeit at a much, much higher frequency... but for sake of comparison... the television signal you're trying to capture, is a line-of-sight phenomena. IF you can SEE it, you can receive it. If it's beyond the horizon, that signal will be attenuated, or flat-out blocked.
Back when, Lower television frequencies started at 52Mhz... and proceed up to 88mhz... after that, is the FM Broadcast band (88 to 108mhz) then the VHF aircraft band (110 to 140Mhz)... blah blah blah... until you get up further, and SOME of the UHF frequencies around high 400Mhz range, and they were allocated into the 5-600Mhz range somewhere (it's been so long, i've forgotten and would hafta look 'em up, but it doesn't matter). But for comparison, your AM Radio is at 0.540mhz up to 1.6Mhz. Your CB radio operates at 27Mhz. Your old-time cordless phone (with that long collapseable antenna) was at 49Mhz. Your little FRS two-way radio is at 462Mhz.
Now, about the SIGNAL:
Foliage WILL supress OR reflect a signal, so having it not in your line-of-sight to the horizon. Foliage Will, in general, allow passage of a LOWER FREQUENCY signal much better than a HIGHER, so don't be surprised. Solid objects will REFLECT the signal, and depending on where it's coming from, you may get a stronger signal pointing the antenna in a different direction, to catch it 'bouncing' off some other object. For example... a water tower a mile north of you may be high enough so that a distant signal source will reflect off, and down to your antenna, offering you a path AROUND your neighbor's house, and a grove of tall trees, hence, a very strong signal when you're pointing almost 90 degrees from where the transmit site is from your location.
Coaxial cable, connectors, and crimpers are NOT all equal... and coaxial cable that works well at 50-100Mhz, can become incredibly 'lossy' above 200Mhz... which means you could have good reception in the lower channels, and poop above 12.
Also, the longer the coax, and the more connections in that coax, the greater the amount of signal loss that occurs along the path. This is the reason why Lou noted that installation of a receive preamplifier at the ANTENNA is most important- the signal at the antenna's feedpoint is strong, so it is sent down the coax to counteract attenuation of the long feedline.
Unfortunately, preamplifiers are not without cost- they are wide-banded gadgets... they amplify EVERYTHING coming in their front door, and jam it down the feedline to your receiver.. so if you have a really strong signal on one channel, and a weak signal on the other, the strong signal will be crammed hard into the receiver, and the weak signal will be lost in the boiling swamp of the strong.
An omnidirectional antenna is nice in that you don't have to point it... but a directional antenna improves performance for two reasons: One, it exhibits 'gain'... meaning, it is very sensitive in it's forward direction, and more importantly for broadcast signals... it has a 'null' towards the rear... meaning... it is substantially DEAF in the opposite direction. If you have a weak signal that you really want to hear, but another really LOUD one that's eating your receiver alive, rather than pointing directly at the weak, instead, point the antenna's NULL towards the STRONG signal... to reduce the strong's swamping.
Raising up an antenna improves your view of the horizon... but it's a double-edged sword. IF you have a really big horizon, chances are good that there's gonna be more than one Channel X in your antenna's view. If you have a directable antenna, you can point the null at the strong sig, but if not, you're gonna get lousy reception on that frequency. Lowering the mast will remove the distant signal from your horizon, leaving the closer one all by itself.
By the way, sometimes, lowering the antenna will give your antenna a view UNDER an obstruction. Lowering the antenna means you can run SHORTER feedline, which equates to lower transmission line loss.
The other thing that really complicates stuff nowdays... is that the signal, being digitized, cannot be 'humanly' observed for quality, to determine what changes yield a more useable signal. Unlike analog TV, One cannot tell that they're receiving two different stations, or receiving a strong reflection from some other angle.
Finally... the trash. There's an entire culture of consumer-trap crap out there... and they LOVE to take your money. MOST antennas you buy, have little or no actual 'design science' backing them up. Television, being across such a wide frequency range, requires some pretty complicated stuff in order to function on all those channels. Also, there is nothing different in the actual radio frequency signal of an 'HD' image vs. a 'standard'... so if you have something small that boasts 'big' promises, or identifies itself as 'necessary for HD', it's a lie.
You can MAKE a fully functional, decent performing antenna out of old coathangers and chunks of stripped coax. Television is so wide-banded that you don't really even have to measure anything to any precision to get a good result, just so long as you have good connections, properly installed connectors (and the fewest quantity...), and have it mounted where it's got a good view of the horizon, and it'll work fine.
My local broadcasts come from four towers about 28 miles due south of my place. I have a clear view of the horizon in that direction, UNDER the lowest branches of a maple tree. I have a piece of RG6 coax coming from my office TV set, through the wall, and up to the vinyl rain gutter. The coax has been stripped away carefully so that the jacket goes one direction, the core goes the other, and it's held to the bottom of the rain-gutter with packaging tape. I get incredible reception with what could best be described as absolute junk... but it's simply the case that I've met all the criteria necessary, with the least amount of hardware and feedline.
If you can't get a good signal, ask around, and find a local HAM radio operator, frequently, they'll show up, check it out, and let you know what'll work best for your circumstances.
------------- Ten Amendments, Ten Commandments, and one Golden Rule solve most every problem. Citrus hand-cleaner with Pumice does the rest.
|