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Mounting Structure Affects on Non DA FM Antennas

W

wcozBoston

When FM multi-bay omni-directional antennas are installed on a mounting structure, whether that is a pole or on the side of a tower, can the mounting structure cause such an antenna system to become directional? Or said differently, how much of an affect does the mounting structure cause on the performance of the antenna? See some examples and photos below:

WKAF, does the tower that antenna is mounted create gain in the direction of where the antenna is mounted while a null are in the other direction, (signal passing through the tower is nulled)?

https://necrat.us/biz/wp/?tag=wkaf

Same for WODS, see below for their back-up facility using this eight-section antenna on the side of a tower:

https://www.necrat.us/candelabra.html (Scroll down to the "The master backup FM"

Does ice build up have any affect on the signal on WHOM non DA facility?

http://blog.nhstateparks.org/wp-content/uploads/2012/01/12-10-2010-0371.jpg

Is the cog-type antenna mounted on either a pole or tower eliminate the affects of tower by mounting multiple elements comprising of one bay around the structure (stacked elements form an array). See the next tow photos of the FM-128 master, WKLB and the WCRB antennas:

https://www.necrat.us/wjmn_pro.html (two-bay)

https://www.necrat.us/1025boston.html (four bay)

https://www.necrat.us/995boston.html (five bay)

Is WXLO's facility more of an omni-directional facility mounted on a pole (elements are still on one side of the pole however):

https://www.necrat.us/1045fitchburg.html

Thanks Necrat for the great photos as part of this post!
 
wcozBoston said:
When FM multi-bay omni-directional antennas are installed on a mounting structure, whether that is a pole or on the side of a tower, can the mounting structure cause such an antenna system to become directional? Or said differently, how much of an affect does the mounting structure cause on the performance of the antenna? See some examples and photos below:
Click to expand...

To varying degrees, you bet. The antenna radiation pattern can be distorted either by blockage by large members of a wide-faced tower, or through what is known as "re-radiation", where portions of the tower or mounting structure can actually radiate like a parasitic element. Most antenna manufacturers offer either computer modeling, or range testing, where they place an antenna on a similar structure as the proposed mount, and measure the signal around the structure for distortion. Many times the antenna can be designed to compensate for structure re-radiation, assuming the structure isn't so large that it blocks the field completely.
 
I was involved in developing a plan to move WWBN Tuscola, MI from 101.7 to 101.5 back in the 1990s. They wanted a maximum in the DA pattern toward Flint, with just four commercial allotments, and to protect the the then vacant 101.5 allotment in Sarnia, ON, to go from 3 kW nondirectional on 101.5 to 6 kW DA from the tower near Millington, before moving to a recently abandoned 500 foot tower near Otisville a few years later. The late Bill Sanderson kept pushing the antenna manufacturer to improve the pattern. But the best they could do on the range was 5.4 kW V Pol ERP toward Flint and a much lower H Pol ERP. It was essentially 6 kW Circular Polarization toward Saginaw, which was already flush with many allotments, and four new ones in the area as a result of Docket 80-90. The signal was very good in Saginaw, but disappointing in Flint. So if the path to where you want to serve is iffy, distance wise or terrain and Fresnel Zone wise, either side mounted nondirectional or DA, you don't want to depend on the higher V Pol ERP, which shows as the ERP on FCC applications and contour maps. Remember also that the actual pattern ERP of a DA is LESS than the stylized FCC APP pattern in directions except for one or two maximum directions. At least with nondirectional, the maximum ERP exceeds the nondirectional average ERP in some directions.
 
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