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Counterpoise pointers

The technical side of broadcasting. Think IBOC is a sham? Talk about it here! How about HDTV? Post DX reports here as well.
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SolarMax
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Counterpoise pointers

Post by SolarMax » Wed Jan 11, 2017 9:44 pm

Mike Vanhooser in RadioWorld discusses the merits of elevated counterpoise vs. buried radials, cites installation at WDTW (AM).



k8jd
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Re: Counterpoise pointers

Post by k8jd » Fri Jan 13, 2017 5:54 pm

I had no Idea that WDTW was rebuilt with an elevated radial antenna system ! GoogleEarth had insufficient detail to see it.
I had read about a few other stations using that, but not in our area.
Any other AM towers have elevated radials in S.E. MI , .....anybody....?



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Fingerboard Corners
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Re: Counterpoise pointers

Post by Fingerboard Corners » Fri Jan 13, 2017 6:08 pm

I'm curious about the measured Night pattern on this. There are few radials and many very deep nulls in the pattern. Day pattern not so much. Two very shallow nulls, equivalent to 1 kW. Third minimum even more shallow.



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Plate Cap
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Re: Counterpoise pointers

Post by Plate Cap » Sat Jan 14, 2017 8:06 pm

At 1310 is was all about money. The existing ground radio system for the old system was in very bad shape, and digging it up and removing it to add a new one would have been cost prohibitive. You can't reasonably hope to reuse the old, connect to it evenly, etc. Some was removed when Clear shut down, a lot of it was still there. It would have been a 'ohmic' nightmare.

Elevated systems work, but there are a lot of drawbacks. Falling ice from the towers and guys plays hell with the counterpoise elements and supports, wind and weather have an effect, and grounds maintenance is an issue. Because it's cheap you will see more and more of it, but it's not something people are standing in line to do unless there are ground conductivity issues or a small bank account.

1310 had nearly perfect ground conductivity. Numerous field mowing contractors became stuck in the mud and muck trying to keep the 28 acres mowed.....sometimes many days after the last rain. Crayfish abounded all over the property. An elevated ground system there makes sense to the guy paying the construction bills, but will be cursed ad infinitum by any consultant called in or any engineer hired.


The box that many broadcasters won’t look outside of was made in 1969 and hasn’t changed significantly since.

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rst599
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Re: Counterpoise pointers

Post by rst599 » Mon Jan 16, 2017 11:55 am

Any ham with a vertical antenna will tell you that a few elevated radials work as well as dozens of buried ones. Bill, W4BZ, was one of the first to try elevated radials with a broadcast antenna and found that eight radials elevated above the ground, level with the base of the tower, worked as well as 120 bare copper radials buried a foot underground. (More on the field trial and MOM analysis from a NAB paper presented by Clarence Beverage. (pdf))

Of course, one of the big problems with elevated radials is that they get in the way unless you put them up high enough so you can walk underneath them to mow the field or get to the towers (and so deer don't snag their antlers). But if one does break, it's a relatively simple matter to grab a roll of THHN and string yourself a new one.

My vertical ham antennas are counterpoised with insulated copper radials just at the surface, not buried more than an inch or so (under the thatch that keeps them in place). They are not grounded at any point including at the end. I get good performance out of 20-30 radials (of varying lengths) installed this way. With in-ground radials, the length is less important than the amount of copper you can string along, whereas elevated radials should be of a somewhat resonant length.

My understanding of the phenomenon is that buried bare copper essentially shorts itself to earth continuously. When you think of the radial system as a counterpoise, it's actually trying to carry equal and opposite currents as the vertical element. Elevated radials just float there; they aren't shorted together by whatever ground resistance is present in the earth, and as such they can do so much better. Imagine having a dipole with one side in the trees and the other side lying flat on the ground (like you might see after an ice storm or a squirrel attack on your support ropes) and you can visualize what that does to efficiency.

The reasons for 120 buried bare copper radials becoming the broadcast standard are not entirely clear. Perhaps when vertical antennas became the standard in the early thirties it was thought that such a system was needed to "couple to the earth", and the number 120 came about because that was how much wire was on the spool when one of the early antennas was installed. Burying them six to twelve inches also puts them farther from harm's way - either by lawn mowers or other vehicles, or copper thieves (who will usually stop at nothing - just search 'copper thief electrocuted' on YouTube).

In the case of WDTW, elevated radials were employed along with top-loaded towers shorter than the originals, so as to keep from needing to be lighted.

Elevated radials are appearing at more broadcast facilities, as are cage-loaded short verticals (the Kinstar antenna), essentially a four-legged inverted-L, but these are mostly special cases where such techniques facilitate installation of an otherwise-unbuildable antenna.



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Fingerboard Corners
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Re: Counterpoise pointers

Post by Fingerboard Corners » Mon Jan 16, 2017 12:29 pm

My educational background is extensively Applied Physics related. In Physics, they talk about "symmetry" a lot, particularly in regard to Electricity, Magnetism, and Electromagnetism and Waves. 120 radials per tower has a lot more "symmetry" than four radials per tower in a X arrangement. I don't doubt that you could use 12 or 36 or 72 above ground radials like this for AM BC DAs depending on the amount of "symmetry" you need, for critical AM BC DAs like WDTW's Night pattern. Ham monopole antennas, even monopole arrays, do not need sharp nulls. AM BC DAs often need sharp nulls. I don't see much of a problem with the WDTW Day pattern, as I said. The Night pattern is already extensively augmented, if you look at the most recent application, and I suspect more problems will crop up with deterioration. Notice that the Kintronics antenna was not approved for AM DAs, at least the last I knew.

https://en.m.wikipedia.org/wiki/Symmetry_(physics)



jry
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Re: Counterpoise pointers

Post by jry » Mon Jan 16, 2017 1:28 pm

At WLQV, we had the standard ground radials and those goofy, above ground radials. About 6' off of the ground and 15 feet out. I was always having to duck. I doubt that there was any benefit to those things. I'm pretty sure that Salem took them all down.



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rst599
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Re: Counterpoise pointers

Post by rst599 » Mon Jan 16, 2017 1:56 pm

I can see how a small number (say, 8) of elevated radials would work in a directional array - even one with sharp or deep nulls. Certainly in the near field you'll get varying field strengths depending on how close you are to the radials, but directional antennas are evaluated in the far field - typically ten times the tower spacing and beyond. Once you get out far enough for the directional null to be meaningful, the site appears more and more like a point source and performs similarly regardless of the configuration and placement of the counterpoise system. Still, I don't think you could get away with four radials and maintain a theoretically circular radiation pattern within less than 2 or 3 dB. Eight would seem to be the fewest. The proof of the pudding is always the proof (if you even need to run a physical proof instead of doing a moment-method one.)

The Kinstar antenna was indeed approved only for non-directional sites, though there's no reason you couldn't also use it in a directional configuration. The Commission's concerns with the antenna are mainly with efficiency and stability.



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rst599
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Re: Counterpoise pointers

Post by rst599 » Mon Jan 16, 2017 1:57 pm

jry wrote:At WLQV, we had the standard ground radials and those goofy, above ground radials. About 6' off of the ground and 15 feet out. I was always having to duck. I doubt that there was any benefit to those things. I'm pretty sure that Salem took them all down.
I was out there a couple years ago and didn't see any elevated radials.



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Fingerboard Corners
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Re: Counterpoise pointers

Post by Fingerboard Corners » Mon Jan 16, 2017 3:25 pm

If a ground system has a certain number of mirror image planes, the way it behaves has that same behavior. The four radial X has two mirror planes, and a rounded rectangle behavior, eight at 45 degree angles would have four mirror planes, and a rounded octagonal behavior. In free space with "spherical chickens", as they say. 120 radials would have 60 mirror planes. Part of the advantage of 120 radials would be redundancy. A few breaking off doesn't change it like one of four breaking off.



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Fingerboard Corners
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Re: Counterpoise pointers

Post by Fingerboard Corners » Mon Jan 16, 2017 6:07 pm

It seems like I've heard of people adding screens near the tower base in areas where the conductivity is really bad, like in Western and Northern Michigan (not shown on M-3 Maps) but these are at ground level or buried like radials. I guess the idea, Rich or others can correct me if I'm wrong, is that a large amount of loss occurs within a few meters of the tower. I suspect that around the WJBK...WLQV array, the conductivity is quite good, because historically, WJBK...WLQV has a good signal for being that far up the dial, and the area closest to the Detroit River is demonstrably the best in that area. It seems like somewhere I read that the above ground radials were added because of extreme differences in conductivity during different temperature and soil wetness conditions, and the associated problems of large differences in monitoring point field strength readings.



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Fingerboard Corners
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Re: Counterpoise pointers

Post by Fingerboard Corners » Tue Jan 17, 2017 1:21 pm

There is Augmentation to both the Day and Night Patterns. To the South, in the Day Pattern, it's quite a bit. Not sure if this has anything to do with the limited number of radials. There's considerable variation in the Nondirectional proofs in various directions also. Equivalent to some directions being 50 percent more power in the maximum direction compared to the minimum direction. So that is about 2 dB. That could be reradiation from cell towers or high tension towers. With that much variation and augmentation, they might want to find any nearby hot tower(s) and detune them.

https://licensing.fcc.gov/cdbs/CDBS_Att ... &exhcnum=1



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rst599
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Re: Counterpoise pointers

Post by rst599 » Wed Jan 18, 2017 1:58 pm

There's nothing unusual about augmentations on a directional standard pattern. It's just a way to represent what the array is actually measured as doing, versus what the standard pattern predicts. For a pattern like WDTW's night pattern, with very deep theoretical nulls, augmentations describe the actual measured radiation in those directions.

Back before we had standard patterns, the proof would specify the theoretical pattern with MEOV (maximum expected operating value). This was usually the consultant's estimate of the actual radiation, and would be represented by a line drawn on the graphical pattern plot that was filed with the Commission. The MEOV represented the maximum radiation for interference protection purposes and was used in allocation studies. The FCC saw computers coming, so standard patterns (theoretical pattern with a multiplying constant to approximate "real world" performance) with augmentations were introduced in 1971 to provide a way to tabulate a pattern's data for computer analysis.

Ground screens around towers are fairly common, since that is where most of the current flows and it helps to reduce loop loss. A typical screen consists of expanded copper mesh or concentric wires around the base, going out as much as 25 feet, with the radial wires bonded wherever they intersect.



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Fingerboard Corners
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Re: Counterpoise pointers

Post by Fingerboard Corners » Wed Jan 18, 2017 2:33 pm

In the 178 degree Day direction, the Augmentation is 1420.50 mV/m @ 1 km IDF with a span on 40 degrees. The Standard Pattern is 1140 mV/m @ 1 km in that direction. 1420.50/1140=1.24605. The power necessary if it were at the Standard Pattern value would be 5000*(1.24605^2)=7763.23 watts to get this augmented IDF. Back to Theoretical Electricity and Magnetism, the power has to go somewhere, and there is only 5000 watts to be radiated. This would fall under the First Law Of Thermodynamics. It would stand to reason that there are directions where the measured pattern is substantially less than the Standard Pattern. In other words, whatever causes more power to be radiate more in that near South direction, probably reradiation, reinforces in that direction, and cancels in another. If a complete proof were run and published, I would suspect you would see where the power is not going. Like you say, this is not unusual. "Nondirectional" AMs with nearby FM or other towers often have a real world directional pattern attributable to the other tower if it is not detuned. Sometimes, this is advantageous and nothing is done. With a Class IV/Class C, there's only 1000 watts to radiate, so you want it to be in the advantageous direction. Since one of the engineers who reads this section worked at such a Class IV, I won't say which one, but when the new FM tower went up, radiation to the South went down, to an area outside the market. The much maligned (on another thread, and technically undeservedly) Cris has had articles on how to estimate radiation and phase from these nearby towers and structures and detune them, as do ancient NAB Engineering Handbooks available online.

X Ray Crystallography uses rereadiating atoms and molecules (mainly H20) to match the crystal structure and to compensate for these stray molecules. Phase determination uses a probability method of a range of phases to model the interference pattern produced.



k8jd
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Re: Counterpoise pointers

Post by k8jd » Wed Jan 18, 2017 4:00 pm

I have heard from several sources that the idea of 120 radials was to obtain stable results of performance over the seasonal extremes in temperature, rain, snow, ice, etc...



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