The roof will hardly be noticed by the signal.
I have beams and windoms 20 feet from my roof 50 feet above ground. Also
similar antennas only above ground.
Negligible difference. When new they follow prediction curves and deliver DX.
You will have neglegible roof effect.
73,
John, K2SFS
___________________________________________________
---- K7LXC--- via TowerTalk <towertalk@contesting.com> wrote:
=============
> I would appreciate some suggestions and guidance. I have a 40 foot
Rohn 55
tower mounted on top of a building. The top of the tower is 100 feet
above the street.
> At present there is a Force 12 XR-5 just above the tower, which plays
very well.
I am planning to place a Force 12 Delta 230/240 above the XR-5 on the
same mast.
They will be 8 feet apart.
Question:
The two elements for 40 meters will only be about 48 feet above the roof.
Since this is less than a full-wave length in height above the roof, low
angle radiation would be affected, does it make sense to put up ? Would
a
> slightly longer mast fill the gap ?
Go with what you've got - I wouldn't do anything. While the antenna
will be 48 feet above the roof, it really doesn't electrically see the roof
much. Methinks the near-field is pretty complicated what with other
structures and metal lumps of HVAC, etc. so it probably doesn't matter much
difference where it's mounted.
Here in the Pacific NW, the question about whether to make a change or
not comes down to "Will the JA's tell the difference?" Or in your case,
"Can the EU's tell the difference?" In many cases, the answer is "probably
not". Also for example, a 10' height difference is just a fraction of a
wavelength so the change is only very incremental.
Cheers,
Steve K7LXC
TOWER TECH
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