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FM Panel Antennas
- Thread starter reelyreal
- Start date
reelyreal said:
The backplanes of the panel elements largely isolate the radiators from the mounting structure, which mean that:
- the net radiation pattern from the array is much more predictable and well behaved (no deep nulls for omni patterns in/near the horizontal plane, even on towers with large cross-sections)
- it has better c-pol axial ratios at all rotation angles (not just H & V)
- the impedance at the antenna input connector is much less affected by the mounting environment.
These advantages over a typical non-panel FM sidemount have to be balanced against the cost of the panel approach (at least three times the hardware per bay level), and the effect of its weight/windload on the cost of the tower structure needed to support it.
But when an FM station needs/wants to radiate either an omni or a directional signal with known and well-controlled pattern characteristics, then a panel design is preferable to a typical non-panel design.
Single-layer FM panels (Harris CBRs) were installed by each of the FM stations using Sears Tower, Chicago.
Short answer - potential for a better pattern. Better bandwidth. For more money, more complexity, and greater windload.
Good answer.
They 'bay' antenna will never go away. It is often used (via mounting) to legally push (via pattern distortion) more signal to the 'money' DMA.
They 'bay' antenna will never go away. It is often used (via mounting) to legally push (via pattern distortion) more signal to the 'money' DMA.
Just a personal observation: I have observed about a dozen panel installations that are, or were, operating in close proximity to standard antennas. I have never observed a panel antenna that outperforms a proper standard antenna. I have seen the reverse to be true on several occasions where a standard antenna performs better than the panel. This is just my personal observation and has no scientific basis or proof behind it. My gut feeling is that panels are great for multi-station installations where a broadband solution is required, but probably not such a great idea for the single station. Maybe there's a small tradeoff between bandwidth and gain. If the elements aren't quite lined up for a particular channel, the gain may be affected.
I've had good luck with ERI's panels and their cavity backed. The Shiveley panel close to me is chitty, I don't know if it's the antenna or transmitter problems though. With a G-5, you can 'optimize' the pattern, and as long as where you stop is rounder than whre you started, it can be considered optimized. If that process puts 6-8dB in a direction to your liking, then cool beans for you.
Just as a reference point, the particular station I was thinking about is 96.5 WTIC-FM Hartford. They're using an ERI 1083-2CP two-bay panel antenna, and with a pretty killer signal. The top of their tower is UHF TV (WEDH) and the tower has a moderately sized face, so it makes sense that they'd use a panel antenna for a more uniform signal.
littlejohn said:
However "optimizing" can't add more than 6 dB to the r.m.s. gain of the h-plane azimuth pattern, even in theory. More than likely the maximum gain improvement over r.m.s. will be on the order of 3 dB.
But optimizing can add 8 dB and even more to a pattern minima, to avoid radiating relatively low ERP toward that sector compared to the value expected for the r.m.s. of the pattern. Sometimes these nulls can be 10-15 dB deep, depending on the tower construction, antenna mounting location, frequency etc etc.
Avoiding deep nulls in an "omni" pattern may be more important to an FM station than having a few dB gain in a major lobe. This is a very good reason to buy pattern testing even for non-panel, omni antennas.
Compare WTIC, which was excellent last time I was around it - many years ago - to WEZN in Bridgeport, assuming WEZN is still using the ERI cavity backed antenna. That toy worked just real well when it went up.
Mr Fry misses the point I was making - we are in agrement though. As a forinstance, the first run on a G-5 we put up in North Georgia on a ten foot face tower showed a circularity of roughly + - 18dB, and a correlation of H and V which was sort of non-existant. The end result showed a circularity of about + - 6DB, with the H and V components very closel together... and of course we pointed the lobe at the population center of interest. The point being, if you just bolt one on the tower, it probably won't work worth a damn. A fee of ten - 15% of the cost to have it properly optimized to the tower is a bargain.
Mr Fry misses the point I was making - we are in agrement though. As a forinstance, the first run on a G-5 we put up in North Georgia on a ten foot face tower showed a circularity of roughly + - 18dB, and a correlation of H and V which was sort of non-existant. The end result showed a circularity of about + - 6DB, with the H and V components very closel together... and of course we pointed the lobe at the population center of interest. The point being, if you just bolt one on the tower, it probably won't work worth a damn. A fee of ten - 15% of the cost to have it properly optimized to the tower is a bargain.
It sort of depends on where the stick is too in the grand scheme of things. If it's right in town the panel makes a lot of sense. If it's out of town, pushing more signal towards town via side-mounting makes sense. Like most things, one size doesn't fit all. I'm about ready to take a class A up to slightly over double the height and go from a 3-bay side-mount to a baby panel 4-round to even out our signal. As R. Fry mentioned previously, our problem is that the side-mount on a large-faced tower creates a bunch of almost nulls for us right in the metro. Our signal sucks really. We shall see how much of an improvement the new panel provides. Hopefully it will be noticeable as it certainly hasn't been a cheap option at all compared to the normal side-mount antenna!
A panel sure fixed a lot of problems for us. Before, the 6 stations were on individual side-mount 4 or 6 bay antennas clustered on the corners or FACE of a tower that was 40 to 10 ft wide! One station couldn't be heard 60 miles to the SE but was solid 150 miles away to the west. The others had less drastic nulls. Two other MAJOR issues that were fixed with the panel & combiner was the intermod and the RFR were drastically reduced.
littlejohn said:
Paper 6 at http://rfry.org explores this situation using a NEC-2D analysis of a 2-bay Rototiller® type array when mounted on a 24" and then a 42" face tower section.
The elevation patterns of these assemblies may be surprising, as they are quite different than the theoretical, free space patterns published for them.
Elevation patterns are seldom measured on the test ranges of most FM antenna OEMs, even for "optimized" omni and FCC-required directionals -- and couldn't be, unless the antenna under test included all the elements that will be used in the final array.
One case where this was done for a 7-bay, full-wave spaced FM antenna mounted on a length of large cross-section tower is shown at the link below. Here the complete assembly was rotated on its horizontal axis to measure its h-pol/v-pol azimuth patterns, and on its vertical axis to measure its h/v patterns for +90 to -90 degrees elevation, repeated at 45-degree intervals of the azimuth pattern.
This procedure is quite a bit more elaborate (and expensive) than just measuring the h&v azimuth patterns for several bays of a large antenna, only in the horizontal plane.
http://i62.photobucket.com/albums/h85/rfry-100/AntennaToweronHarrisTT.jpg
I did a set of measurements on a 6-bay G5 here, including vertical, using a helicopter. I found the vertical pattern to be very close to what the manufacturer (E.R.I.) said it should be. (And found out that, while they don't mind ascending over a point at a constant rate, helicoprer jocks are allergic to descending over a point at a constant rate.)
littlejohn said:
Such measurements can provide a lot of data.
However scientific confidence in the accuracy of that data as relates to the true ERP from the transmit antenna toward the point of measurement depends on the real, c-pol, polar pattern of the receive antenna for that frequency when mounted on the helicopter at the time of the measurement, the height AMSL of that rx antenna w.r.t. the height AMSL of the radiation center of the transmit array, the distance between the rx and tx antennas, terrain profiles, reflections from structures in the physical environment, and the degree to which such multipath reflections arriving at the rx antenna affect the field intensity values recorded in the measurement.
The effects of such variables either are negligible or well-known, for pattern measurements taken on a qualified test range -- such as used to measure the patterns of the 7-bay antenna + tower section shown in the link I posted earlier in this thread.
My experience is entirely anecdotal, and I admit ours was a VERY special case, but that being said, ERI's panel antennas rock!
As the CE of Z100 I was part of the committee of Chief Engineers who specified the panel antenna/diplexing system that replaced the 1960's era Alford antenna/diplexers on the Empire State Building. Many of the decisions and hard work had been done before I entered the picture in 1988, but when the new (two levels; four bays/level) antenna went online (IIRC, 1992) we could not have been happier with the outcome of our choice. Part of it was due to having greater height which, with the concurrent ERP reductions, reduced the blanketing and receiver overload at street level that had been a huge problem. But also, the amount of multipath interference and consistency in fringe reception was generally improved. This was undoubtedly due in part to the much smoother, near-roundess of the pattern, as opposed to the scalloped daisy pattern of the Alford (IIRC, two levels of 16 bays surrounding the upper observation deck).
Over the years, I developed a great relationship with the ERI folks, most especially Robert Rose and Tom Silliman. They may not be the only antenna designers in the world, but they certainly are at the top.
Kind Regards,
David
As the CE of Z100 I was part of the committee of Chief Engineers who specified the panel antenna/diplexing system that replaced the 1960's era Alford antenna/diplexers on the Empire State Building. Many of the decisions and hard work had been done before I entered the picture in 1988, but when the new (two levels; four bays/level) antenna went online (IIRC, 1992) we could not have been happier with the outcome of our choice. Part of it was due to having greater height which, with the concurrent ERP reductions, reduced the blanketing and receiver overload at street level that had been a huge problem. But also, the amount of multipath interference and consistency in fringe reception was generally improved. This was undoubtedly due in part to the much smoother, near-roundess of the pattern, as opposed to the scalloped daisy pattern of the Alford (IIRC, two levels of 16 bays surrounding the upper observation deck).
Over the years, I developed a great relationship with the ERI folks, most especially Robert Rose and Tom Silliman. They may not be the only antenna designers in the world, but they certainly are at the top.
Kind Regards,
David
They've always built good products; and they stand behind them. As they have grown and added products, bureaucracy has to a degree crept into the mix, but you can still get the gurus on the phone when there's a need. I miss the old houses in Tussletown and the HP putting dotted circles on the screen to print and fax - but I doubt they do.
I certainly agree on the red-tape anymore at ERI. They put out a fine product, but they aren't very coordinated at times. I've had issues building out stations because they failed to send all the parts. A tower company owner friend of mine joked that ERI aught to buy lots of stock in FEDEX. They are constantly forgetting to send crap needed to get stuff hung on the tower in his experience. With patents expiring where others can copy their design exactly and their lack of shipping organization, they have their issues. Hopefully they can get a handle on it soon. As companies become bigger sometimes they need to make some adjustments to keep their overall experience the best it can be. I think ERI is certainly a company that could re-assess some of their "ways". Will a buy another ERI antenna? Absolutely! Will I buy some other things they sell for towers? Probably not. Will I demand the antenna show up at least a week before I need it? You BET.
That way I can put all the little parts together and make sure I have everything I need...
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