[BC] lpfm question

[Richard Fry]

RichardBJohnson wrote:
> It has always been that power is everything.  I would
> much rather use a high power into a single-bay (designed
> for high power),than a lower power into a multiple-bay
> antenna if energy costs and hardware costs were not
> factored in.
_______________

RADIATED power in every direction is important, but for a given ERP in any 
of those directions, the means used to produce that radiated power is not.

Consider omni UHF TV transmit antennas, which can have an elevation gain of 
24 X (13.98 dB) and more, and a very narrow main lobe in the elevation plane 
compared to FM transmit antennas.  But when these antennas are top mounted, 
they radiate the fields expected for that frequency, ERP and HAAT, including 
those in the elevation plane.  The reason for this is that these installed 
antennas actually have at least close to the azimuth and elevation patterns 
given in their design and final test data.

The effects of the mounting environment mean that most side-mounted FM 
transmit antennas do not have the radiation patterns assumed for them, 
especially in the elevation plane.  Even the test range measurements made by 
most FM antenna OEMs do not include measuring the elevation pattern at any 
azimuth, let alone at enough azimuths to adequately characterize it.

Side mounted, multi-bay FM antennas can develop fairly narrow lobes at some 
azimuths in the elevation plane sector serving most of the coverage area. 
So a range test that shows high relative field in the horizontal plane at 
some azimuths can have significantly less relative field at lower elevation 
angles than is shown in the elevation pattern plots provided by the OEM --  
which are essentially based on the theoretical array pattern of perfect 
elements in free space (no tower, antenna feed system, and mounting 
effects).

The affect of the tower on the pattern of a single, side-mounted FM element 
generally doesn't produce an important difference in the relative field it 
radiates over the elevation sector that serves most of its market.  Probably 
this accounts for the success some report for this configuration.

Getting back to the OP about LPFM systems, if the 250 ft tower is located 
about in the center of the population it might be advisable to ask the 
antenna OEM for a pattern study for that structure and mounting location on 
it, and maybe even to order/install v-pol parasitics to smooth out the v-pol 
field produced by this combination of antenna and tower.  Otherwise there 
could be noticeable differences in the amount of v-pol radiation toward some 
parts of the coverage area.  And with LPFM systems every bit of radiated 
power can be important.

A very good antenna for this LPFM application would be a 1-bay, top-mounted 
Lindenblad (link below).  It has an excellent c-pol axial ratio in all 
rotation planes, and is almost perfectly omnidirectional.

http://i62.photobucket.com/albums/h85/rfry-100/LindenbladRendered.gif,

RF