On Thu, 02 Sep 2010 07:56:08 -0400, Jeff Thies
wrote:
On 9/2/2010 2:59 AM, mm wrote:
On Wed, 1 Sep 2010 20:05:02 -0700 (PDT),
wrote:
I have an old antenna in the attic. When i moved into my house i
hooked it up to the tv. It works ok. Some channels dont come in
perfect some of the time. I've tried adjusting it w/ no luck. I see
these new style antennas for sale on line.
Are they better then the old style?
Has anyone tried both?
I just noticed the dimensions.
Dimensions: 22.8" x 17.7" x 25.8"
I'm not sure which dimension refers to what, but all but one of the
elements are smaller than the maximum in the same direction. They
are folded over, but I'm not sure that's good. Let's assume it's not
bad. It still leaves those elements at about 3/2 the dimension, ad
most 38", and 5 of the 6 of them are the same size. (or 7. One or
two things are reflectors)
The antennas they have been selling for 60 years have elements of
different lengths because there are channels o
f different wavelengths.
That's a LPDA (log periodic dipole array).
A popular choice for UHF are bow tie.
This antenna has large diameter closed loop elements which leads to a
wide frequency range.
That won't work well on VHF as the elements, particularly in the low
range are small compared with the wavelength. Ditto on the reflector
which becomes smaller than a wavelength.
http://www.hdtvprimer.com/ANTENNAS/types.html
I have to take time to read this. It looks complicated.
It is, however, the first thing I've read that at all addresses what I
asked about elsewhere, about waves received by elements that bend and
go in the oppoosite directino, about the waves cancelling each other
out.
Well, actually they refer to this in a much different context, iiuc,
but I'm please that it says anything at all. I only have a little
bit of theory and a tiny bit of practice, and when I get an idea, I'm
glad to see it's not crazy.
Nonessential reading:
Question: How does the energy collected by the directors get to the
cable?
Answer: It is re-radiated to the driven element as normal radio
waves.
Question: Why don’t the re-radiated waves go backward or laterally?
Answer: Because all the directors cancel each other in those
directions.
Question: Why don’t these re-radiated waves prevent the diffraction
of incident waves inward toward the boom?
Answer: Because the phase of the re-radiated waves has been changed
by about 90 degrees, so they neither subtract nor add to the incident
waves.
Question: How did the director currents get changed by 90 degrees?
Short answer: The element lengths control this. The director
currents are shifted -90 degrees while the reflector current is
shifted +90 degrees.
Long answer: This graph shows how the current induced in a rod is
affected by the length of the rod. The phase changes quickly with a
small change in element
.....
Ah, but it probably has an amplifier. It's much better to have a
strong signal from the antenna, than a weak signal that is amplified.
Amplifiers are recommended when there is a long distance from the
antenna to the tv. Of course maybe that used to be more true, because
the antenna would amplify the "noise" too. Now most noise is filtered
out in the process of digital detection. Maybe. I'm no techie.
That is my take also.
Note if you take the number of elements and do a rough gain
calculation, the numbers don't add up to all antenna gain. My rough
guess is about 8 dB or so.
Not sure what you mean here. Are you talking about the antenna the OP
brought up. It does have an amp after all.
If you're not talking about that, pelase explain a little.