[TowerTalk] DB-304 Antennas and 3 degrees down tilt

[Bob Nielsen]

On Wed, Jun 13, 2001 at 01:44:05PM -0500, Paul Finch wrote:
> Hello,
> 
> A Ham friend and I have a couple of tall towers that we are getting
> complaints of getting into the repeaters while fairly close to the tower.
> Both of these towers use receive multicoupler systems off the top of the
> tower and transmit combiners off lower antennas.  We both run Ham repeaters.
> We both have commercial systems on these towers to support our toys (500
> foot towers).
> 
> Has anyone out there played with the Decibel Products DB-304 antenna phasing
> harness to achieve 3 degrees of down tilt at the 3 dB half power point?  I
> have talked to one of the design engineers that originally designed the 304
> antenna around 30 years ago.  He did some initial testing on down tilting
> this antenna but can't remember the cable lengths to accomplish 3 degrees of
> down tilt.  Unfortunately, he no longer works for DB and DB will not make
> the harnesses.  Is there an antenna simulator out there that would help?

Paul,

The half-power beamwidth of a DB-304 is 16 degrees according to the
data sheet.  If I understand you correctly, this means you wish to
achieve (16/2) + 3 = 11 degrees of downtilt.  The data sheet gives the
overall length as 260 in. but does not say what the vertical spacing
is between dipoles.

To achieve 11 degrees of downtilt, you need to successively retard the
phase of each dipole below the top dipole by

(vertical spacing in wavelengths) X tan (11 deg)

Assuming that this spacing is 72 in (I'm guessing from a photograph),
and further assuming a frequency of 150 MHz, this would be 0.915
wavelengths X 0.1944 = 0.1779 wavelength.  For a coaxial cable with
polyethylene dielectric, Vp = 0.659, so you would need to increase the
length of the feed line to the second dipole from the top by 9.22 in.,
the length to the third by 18.45 inches and the length to the bottom
dipole by 27.67 in.  

This should give you enough of an idea to adjust for the actual
frequency, dielectrric and element spacing.  I'm assuming that the feed
uses simple power dividers, not having actually seen one of these
antennas.

I haven't done this calculation for about 20 years, so someone please
check my math (I did catch myself a few times!)

73, 

Bob, N7XY

-- 
Bob Nielsen, N7XY                          nielsen@oz.net
Bainbridge Island, WA                      http://www.oz.net/~nielsen
IOTA NA-065, USI WA-028S 

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