Originally Posted by enchant
I don't have the rf technical background like many of you, so please forgive my newbiness. I'm going to give this antenna a try, but I've got a few questions.
Some of you speak of "bays". 4-bay, 8-bay, etc. Is the antenna on the uhfhdtvantenna.blogspot.com site a 1-bay?
Getting onto the roof of my house is a scary proposition, so I like the idea of putting it in the attic. I live almost 30 miles from the transmitting tower. The signal has to go through a few hills and trees to reach me. Assuming I'm not going to gain much height by placing the antenna outside, does the signal lose much by having to pass through the wall of my house to get into the attic?
The uhfhdtvantenna.blogspot.com antenna uses tin foil. I see others using chicken wire and other light fencing. I've got all of them lying around. Is one material better than the others?
At the point where the coax cable must connect to your antenna, there is a coax-to-dual-wires gizmo. What is this called? (It's the only thing I don't have, and I'll have to pick one up at Radio Shack.)
The cable going from the antenna to my tv will be approximately 70' long. Is there some rule of thumb about when I'll need an amplifier?
Thanks for any help.
You brought this thread up to the top; you deserve a response, albeit from someone of limited RF background as well.
A "bay" is a single dipole antenna, the bow-tie like thing that clips to rabbit ears. Some have straight elements, others a pair of V-shaped elements, the principle of all dipoles is the same. What you need to know is that they are "300 ohm" elements and that the width of the dipole determines the frequency it receives best.
The antenna at UHFHDTVAntenna is a 4-bay: 4 pairs of dipole wiskers connected in a special "series/parallel" arrangement that gives you a 300 ohm antenna. 2-bays and 8-bays don't retain a 300 ohm impedance, but the 2-bay is smaller and the 8-bay has higher gain. But all require that the elements of each bay be in the same plane, so all signals add constructively.
Yes, the signal is greatly degraded by mounting under the roof compared with on top. Here's some data:http://www.avsforum.com/avs-vb/showt...32#post5410432
By way of calibration, each 3dB is a factor of 2 (-6 dB = 4x less). The chart at the bottom shows the CM-4221 antenna had a -20 db to -30 dB loss that's
- 1/2^7 to 1/2^10, or
- 1/128 to 1/1024 as much signal, or
- 99.22% to 99.91% signal loss in his attic.
That said, mine's in the attic and it's fine for my weakest station, a -77.8 dBm aerial signal strength. You need to find out how much signal strength you have. That's at:www.tvfool.com
This will show you all the stations you might possibly receive, along with signal strength, distance and direction to each. They're sorted by signal at the antenna height you specify, best reception first.
The size of the holes in the reflector should be small compared with the radiation it should reflect, and it must be metal. The shortest wavelength is Channel 83, 887 MHz, a 338mm wavelength, so holes smaller than 34mm look solid to the wavefront. I used 1x1" cage wire, others have used solid (Al foil) or chicken wire; Channel Master uses 1x2".
The more important requirement is that it be flat like the bow tie arrays, or possibly bent to a slight curve like the actual 4221.
That gizmo where the antenna connects to the feed line is a matching transformer called a "balun" for "balanced/unbalanced" line interface. It connects the "balanced" 300 ohm antenna with the "unbalanced" 75 ohm coaxial cable. Without it, most of the signal stays in the antenna. Get a good one; Radio shack is a crap shoot, or make one.
about half way down, right before Baluns ... a mast amp will more than accommodate 70 ft. of coax.
In the situation you describe, I'd proceed in steps
- buy or make an antenna and try it in the attic
(This is where I declared victory and stopped.)
If you don't like what you see ...
- add a mast amplifier to reduce noise, primarily
If you don't like what you see ...
- mount it on the roof.
But first, see what your TVFool profile looks like (post it here, if you'd like) so we have some idea what you need.