[BC] Directional FM Antennas

[Phil Alexander]

Determination of directional patterns may be done either
by direct modeling or by scale modeling. 

For example, one major manufacturer certifies patterns of
both their own antennas and those of others (imported) 
using 1/4 scale working models at 4x design frequency 
(i.e. 352-432 MHz).

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Some other comments about VHF DA's
==================================

While use of parasitic elements is probably the most 
common method, antennas using both passive and active
elements are in common use.

One of the common Italian imports used for the (vertical
only) NCE reserved band applications is the Aldena which
is based on a log periodic array (LPA) where all elements
are driven. Alternately, a Yagi with similar performance
may be used (e.g., Kathrein GMBH). One common configuration
employs a pair of LPA's or Yagis mounted 180 degrees apart
forming a rather narrow uni-directional, high gain array.

Other active DA's employ multi-bay panels in configurations
other than the conventional quadrilateral (omni) arrangement.
The slotted coax radiator (from ERI) represents yet another
type of VHF DA radiator.

The Commission has always required mechanical configuration
to control directionality, and full scale or reduced scale
model, range certification combined with certified orientation
(by a registered Civil Eng. or Land Surveyor) to control DA
radiation.

There obviously will be slight seasonal DA variations, but
the Rules do not permit the extreme null depth common at MW
frequencies, so these variations are minor and may be ignored.
Secondly, the FM band developed before reliable phase and 
power control equipment and instrumentation was readily 
available at 100 MHz, thus electronic control and measurement 
such as is the method in the MW band is not used for broadcast
VHF.

Phil Alexander, CSRE, AMD

-----Original Message-----
>From: RichardBJohnson at comcast.net
>
>At Jampro, the antenna to be characterized was mounted on 
>a tower section of the exact kind that the customer would 
>use. 
> <snip>
>When I was in Sacramento, they always used the customer's
>antenna, never a model.