Minimum distance above roof
 

Is there a minimum distance an anteena needs to be above the roof? I am
runnign into trouble thinking of mounting locations.

Bill. W6WRT wrote:
"If the roof is made of non-conductor material such as wood or
composition shingle, the antenna will not "see" it and the height above
ground is the important thing."

Yes, at least one of the important things. Antenna elevation is
important to lower the elevation angle of propagation. It`s also
possible there are noise generators within the house which make
reception more difficult when a conductive roof isn`t present to shield
the roof mounted antenna from noise. A small separation can make a large
difference in noise source "signal" strength. A one wavelength
separation can lower noise source received level by 22 dB. Then 6 dB
more every time the distance doubles after the initial 22 dB loss. It`s
best to have receiving antennas located well away from noise sources.

Best regards, Richard Harrison, KB5WZI

How do you get 22dB down for one wavelength and 6 more for each doubling?
tnx

--

73
Hank WD5JFR
"Richard Harrison" wrote in message
...
Bill. W6WRT wrote:
"If the roof is made of non-conductor material such as wood or
composition shingle, the antenna will not "see" it and the height above
ground is the important thing."

Yes, at least one of the important things. Antenna elevation is
important to lower the elevation angle of propagation. It`s also
possible there are noise generators within the house which make
reception more difficult when a conductive roof isn`t present to shield
the roof mounted antenna from noise. A small separation can make a large
difference in noise source "signal" strength. A one wavelength
separation can lower noise source received level by 22 dB. Then 6 dB
more every time the distance doubles after the initial 22 dB loss. It`s
best to have receiving antennas located well away from noise sources.

Best regards, Richard Harrison, KB5WZI

Its called the "Inverse Square Law", - you double the distance, you get
1/4 th the power - Double again, get 1/16 the power- ad nausium -Jim NN7K

Henry Kolesnik wrote:
How do you get 22dB down for one wavelength and 6 more for each doubling?
tnx

Henry Kolesnik wrote:
"How do you get 22 dB down for one wavelength and 6 more for each
doubling?"

Measure it. That relationship is convenient to remember.

Best regards, Richard Harrison, KB5WZI

Richard,

It should be a fun experiment.

The ideal power transfer for isotropic radiators in free space at one
wavelength separation is given by 1 / ((4 * pi)^2) = 0.00633 or -22 dB.

The specific number, 22 dB, comes with some specific conditions.

Such as:

* Isotropic radiators, i.e., no gain.

* Free space, no modification of fields by the earth or a roof.

* The antennas are in the far-field regions of each other. This usually
means the antennas are small compared to the wavelength. (Automatic for
the isotropic radiator.)


22 dB may be an convenient number to remember, but it is essentially
meaningless for the question at hand.


73,
Gene
W4SZ




Richard Harrison wrote:
Henry Kolesnik wrote:
"How do you get 22 dB down for one wavelength and 6 more for each
doubling?"

Measure it. That relationship is convenient to remember.

Best regards, Richard Harrison, KB5WZI

Gene Fuller wrote:
"The specific number, 22 dB comes with some specific conditions."

Yes, but when you are calculating the decline in field due to distance
from the source, 22 dB is convenient as the reduction within the first
wavelength. Every time the distance doubles after the first wavelength,
the volts per meter are halved. The power per square meter is thus
quartered (decreases by 6 dB).

Best regards, Richard Harrison, KB5WZI