FM antenna coverage

HadYourPhil said:

I'm currently using a Shively 6810-4 for a station on 92.5. 25kw at 620'. Don't think the coverage is what it could be. Any comments/suggestions/recommendations??

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Do you know if a pattern study was done on the current antenna / tower setup? I see it is at 620' on a TV tower, so I'm guessing the tower face is 60" or more. Side mounting FM antennas on TV towers will almost always cause nulls in one direction or another. I've ran into a situation where a LPTV antenna was installed at the same level on an opposite tower face from a 12 bay FM antenna at 960'. It was done despite a clause in the FM lease that said no other antennas at that level. Yes... it screwed up the coverage on the FM, but stayed there for several years. The LPTV finally went dark and the antenna was removed - FM coverage returned to what the pattern study indicated.

RFB

One word: Shively

I'll take an ERI any day

Well, the ERI is probably more omnidirectional than the Shively, but on a big tower you are still going to get nulls in inconvenient directions even if you had an ERI. If there are real concerns about coverage, I would suggest a new antenna--and have the new antenna modeled based on the type of tower, guy levels, etc. now in use. (Same procedure used to create directional FM antennas--except reflectors, other do-hickeys added to make the antenna more NON-directional).

We have a ERI too. Best investment made.

regardless of what antenna you use a side mounted one will always have nulls

tpt said:

Well, the ERI is probably more omnidirectional than the Shively, but on a big tower you are still going to get nulls in inconvenient directions even if you had an ERI. If there are real concerns about coverage, I would suggest a new antenna--and have the new antenna modeled based on the type of tower, guy levels, etc. now in use. (Same procedure used to create directional FM antennas--except reflectors, other do-hickeys added to make the antenna more NON-directional).

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Shivleys are horrible in my opinion in radiation...I had a main and aux at same heights and same TPOs...This was done before I was the CE....The ERI AUX would blow away the main any day, etc....in ice, the Shivley would show high reflected even wit the deicers on two days before anything happened...yet the ERI with no deicers or radomes??? Kept on rocking..IF I could have, I would have had the ERI moved to the main (which is where it came from!! The former VP/DOE was a Shivley fan....hence thats why it got changed but the proof is in the pudding and the AUX would SLAM the main all the time...I might add about 1/2 mile between towers and same HAAT/ERP.....Same xmtr too!! CEC 816R5 at 33.5 KW)...On a new install, its an ERI rotortiller for me...(OR Dielectric CBR if multistation)...

Shivleys are bad news ! I use swr-rf http://swr-rf.com/fm3.html

One should focus on the "type", not the brand. Most Shively antennas are "ring-stub" type, while most other brands offer various versions of a "slant-v" design. Ring-stubs are inherently narrow band, the slant-v antennas are more broadband. Both types have their advantages, depending on the situation. ERI rototillers (and knockoffs) are larger, heavy-duty slant-v with an added twist.

Partially due to the bandwidth, different designs will act differently on large cross-section towers. Also, the proximity to the tower will change the effect, as will leg-mount vs side-mount.

As I have found from personal experience, the statements made about mounting on large cross-section towers are absolutely true. I guess my point is that Shively makes great antennas, as do several other manufacturers, but you need to understand that the antenna is not the only radiating element. You must consider the supporting structure close to it.