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Author Topic: antenna height  (Read 210 times)
K6GC
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« on: March 20, 2007, 12:10:02 PM »

I have a pitched roof on my house.  If I mount a 6 meter dipole over the crest, and in line with it, AND if I lay some copper screening on the roof, WHAT IS AN OPTIMUM HEIGHT?
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N3BIF
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« Reply #1 on: March 20, 2007, 12:16:50 PM »

 What is the purpose of the copper? A dipole is a balanced antenna and needs no additional counterpoise.
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KX8N
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Posts: 543




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« Reply #2 on: March 20, 2007, 12:24:58 PM »

Yeah, you don't need that copper, or anything else.  It would just act as a groundplane, which a dipole does not need.

Optimum height?  As high as you can get it.
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KB9CRY
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« Reply #3 on: March 20, 2007, 12:41:36 PM »

For local line of sight type of work, higher is best.  For weak signal, domestic DXing, 75-120 ft. works well for most guys!!!

Higher is best.  
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WB6BYU
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Posts: 13038




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« Reply #4 on: March 20, 2007, 04:46:04 PM »

Are you trying to use the copper screening for some sort of reflector?
Depending on the slope of the roof it may be possible to get some gain
at the horizon this way.  For example, if the slope of the roof was
45 degrees and you mounted the antenna over the sloping part about
half a wave up, the signal reflected off of the roof and that from the
antenna would reach the horizon in phase.  With a flatter roof you
could mount it in the center and have a bidirectional pattern.  You'll
have to do a bit of geometry knowing your slope so that the total
distance travelled by a wave reflected off of the roof at the desired
vertical angle is a half wave (or odd multiple thereof) further than the
direct wave from the antenna at the same vertical angle.

The problem with this approach, however, is that your power is being
radiated from two effective heights - that of the antenna and that of
the reflection.  A good rule of thumb is that two vertically stacked
antennas (which is effectively  what you have in this case) won't give
any significant improvement unless the lower one is at least half the
height of the upper one.  If the center point of the reflection was, say,
12 feet off the ground and you could get the dipole up at 24 feet,  the
dipole alone would be the better choice, as the ground screen would
contribute little to the overall pattern.  But if you were very limited to
the height above the roof that you could accomplish, you had a second
story roof that sloped in the desired direction, you might be able to get
some useful gain out of the system with the antenna only 5 to 10 feet
above the roofline (depending on the slope).

While there may be specific cases when such an antenna would be
useful, the general answer is "as high as possible" and don't worry
about the copper screen.
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W8JI
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« Reply #5 on: March 20, 2007, 05:37:54 PM »

The optimum height depends on what you are trying to do.

The screen might make a big improvement or no change at all or it might even hurt.

It depends on what you are trying to do.

73 Tom



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