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Inverted L..Directional?..




 
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ve6pg
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« on: April 03, 2012, 08:02:51 PM »

  Ok,,I have 2 inverted Ls, one fer 160, the other fer 75...vertical portions are not very high, 30ft or so...they do have vertical properties..pic attached is the basic idea...the main vertical portion is a tv tower, insulated above ground. one coax feed...swr is great...lots of radials beneath...ok, so, do inverted Ls have any directional properties?..

..tim..

..sk..


* 160-6 VERT.jpg (93.22 KB, 850x1100 - viewed 482 times.)
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...Yes, my name is Tim Smith...sk..
kb3ouk
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« Reply #1 on: April 03, 2012, 08:07:42 PM »

 If you use a single elevated radial and place it underneath the horizontal part and run it the radial in the same direction as the horizontal part of the radiating element, then it will be directional in the direction of the radial
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R. Fry SWL
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« Reply #2 on: April 04, 2012, 07:46:35 AM »

If you use a single elevated radial ... then it will be directional in the direction of the radial

This seems intuitive, but actually the radiation pattern of that configuration still is essentially omnidirectional in the horizontal plane (see attached NEC-2D result).  The conductor lengths in the model are 30 ft vertical and 90 ft horizontal for the L and 130 ft for the elevated radial; freq = 1.8 MHz.

However driving the inverted L against one or more elevated radials might result in higher radiation efficiency from the antenna system than when driving it against a collection of randomly-placed, random length, buried conductors.


* Inv L, 1 Elev Radial.jpg (64.38 KB, 541x242 - viewed 411 times.)
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flintstone mop
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« Reply #3 on: April 04, 2012, 09:33:23 AM »

There are no short cuts around the math. Antennas get directional when 1/2 half wave length above ground. The "beginning" of an effective L antenna is 60 feet vert and the remainder horiz..for 160M...80M starts to shine, but just a little. 40M might be really happy.

Last Summer I tried an "L" 30 foot vert and it took almost 200 horiz to just barley get to 160M without a lot of magic tuner action at the feed point.
The vertical section is for the low angle and Horiz wire is the high angle. And 30 feet high is a cloud burner.......good for local work, 4-500 miles.
The radials are a big factor. Raised Radials are very effective when they are tuned.
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« Reply #4 on: April 04, 2012, 09:57:24 AM »

From what I have read it is slightly directional showing a small amount of gain out in the direction 180 degrees opposite from the direction the horizontal part of the radiator travels.

If you can make the vertical  part for 160 m. 50 feet or better yet, 65 feet that would be really good.  Use lots of radials and tune the antenna at the feedpoint (something I need to do).  Don't use cheap coax--many hams think because the frequency is low they can get by with something like 8X.  Not true--hardline is physically robust and more reliable; it has much lower loss which is more important if the run is over 50 feet and you are not matching the Z at the feedpoint. 

AMers need to think of antennas as systems--for AM it is important to reduce all losses everywhere:  feedline, matching network, ground loss   and so on.  Then there is a cumulative positive effect.   

You should see something like 14 ohms at the feedpoint when you are on the resonant frequency (little or no reactance) with a lot of radials and a 1/4 w. driven wire.

Rob
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W4NEQ
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« Reply #5 on: April 04, 2012, 11:26:45 AM »

If you use a single elevated radial ... then it will be directional in the direction of the radial

This seems intuitive, but actually the radiation pattern of that configuration still is essentially omnidirectional in the horizontal plane (see attached NEC-2D result).  The conductor lengths in the model are 30 ft vertical and 90 ft horizontal for the L and 130 ft for the elevated radial; freq = 1.8 MHz.

However driving the inverted L against one or more elevated radials might result in higher radiation efficiency from the antenna system than when driving it against a collection of randomly-placed, random length, buried conductors.

Richard,

I assume you considered vertical polarization only?   If not, how can the inverted L remain omni with significant current in the horizontal portion of the wire?  If it was a symmetrical tee, it would be easier to see ...
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« Reply #6 on: April 04, 2012, 11:44:08 AM »


However driving the inverted L against one or more elevated radials might result in higher radiation efficiency

The fly in the ointment here is that on 1800 kc the elevated ground system has to be high up in order to be sufficiently isolated from ground and work.  The height is a function of fraction of wavelength at operating frequency, nominally 1/10 to 1/15.   in other words, it is too high to be practical for most hams.  5 feet high is essentially no different from laying them on the ground.   
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R. Fry SWL
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« Reply #7 on: April 04, 2012, 02:23:33 PM »

I assume you considered vertical polarization only?

The posted NEC pattern shows all radiation, h-pol and v-pol.  The elevation of the lower tip of the inverted L and the horizontal radial wire connected there is 3 feet.  The radiation pattern was modeled over a perfect ground plane.

The fly in the ointment here is that on 1800 kc the elevated ground system has to be high up in order to be sufficiently isolated from ground and work.  The height is a function of fraction of wavelength at operating frequency, nominally 1/10 to 1/15.   in other words, it is too high to be practical for most hams.  5 feet high is essentially no different from laying them on the ground.

This appears to be a common belief among amateurs, however the investigation and experience of broadcast consultants have shown differently.  The horizontal, elevated radials in such systems can be as little as 0.024 wavelengths above the earth, as shown in the clip below from a paper titled

   NEW AM BROADCAST ANTENNA DESIGNS
   HAVING FIELD VALIDATED PERFORMANCE
                  Clarence M. Beverage
          Communications Technologies, Inc.
                          Marlton, NJ

The first permanent use of an elevated radial ground system appears to be at WPCI, 1490 kHz in Greenville, South Carolina. This installation, designed by William A. Culpepper, involved replacing a standard buried system with a four wire elevated system consisting of #10 solid copper wire, one quarter wave in length, and supported on treated wooden posts which keep the radials 4.9 meters above ground.

The antenna radiation efficiency, based on field strength readings on the eight cardinal radials, was 302 mV/m at 1 kilometer versus the predicted FCC value of 307 mV/m.
(R. Fry notation: the applied power was 1 kW.  The "FCC value of 307 mV/m" is that for a 1/4-wave monopole driven against an r-f ground system of 120 x 1/4-wave buried radials.)

The WPCI installation was unique in that the tower was base insulated but the radials came right up to the tower, 4.9 meters above ground and terminated in insulators. The tower was fed from the tuning unit, through a piece of coax to the 5 meter point on the tower where the center conductor of the coax was attached to the tower and the shield to the elevated radials. This feed system resulted in a higher feed resistance than would normally be expected. Data on this facility was taken from the FCC files.
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ke7trp
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« Reply #8 on: April 04, 2012, 02:53:57 PM »

I have had two L's.  One Coax fed at 60 ft or so.   The latest one is fed with Half inch hardline and a DC ground coil to tap for match.   This one is about 55ft.  I have used this L on various bands using a Dentron 3kw tuner.  The antenna is resonant on 160 and 15 meters with no tuner.  I have never noticed any directionality with either of the Ls. 

I have three 57ft wires layed out in the yard in a star pattern where I had a vertical in the past. I ran a wire from the tower to one leg of the Star pattern ground radials.  The tower has a hallo of 4 ground rods and that round is tied to it.  The entire tower, L, coil, shield and center of coax is all grounded. 

C
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W7TFO
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« Reply #9 on: April 04, 2012, 03:08:39 PM »

Back in the late 70's, I had a contract to move an AM/FM combo station in Yuma, AZ. 

We shut the FM down for a week, and before the tower was dismantled and moved I had the utility company come and plant two rented poles that left the tops some 50' up.

I secured the station an STA from the FCC and away we went.

I ran #10 THHW solid up one pole and across to the other.  I also ran that type of wire on the ground from pole to pole.  The feedpoint of those two wires needed only a .01 uF in series to give me 50Z @ 1400 kHz.  Really simple, robust, & cheap.  Kinda' like an AK-47...

It ran that way for three weeks at 1kW input, and the local area service was such that the listeners didn't know the difference. 

The kicker was from the skywave launching...the engineer got a lot of QSL requests from the pacific rim, Australia, and Asia.  People on other side of the world were listening to entire hours of country music from that class IV station.

Note to self, do inverted "L"s at home.

73DG
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« Reply #10 on: April 04, 2012, 04:03:03 PM »

Richard,

I assume you considered vertical polarization only?   If not, how can the inverted L remain omni with significant current in the horizontal portion of the wire?  If it was a symmetrical tee, it would be easier to see ...

Answer to self:

Must not be much current is the horizontal portion, otherwise the that portion would have significant radiation.
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R. Fry SWL
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« Reply #11 on: April 04, 2012, 04:25:13 PM »

Must not be much current is the horizontal portion, otherwise the that portion would have significant radiation.

Yes, significant current flows in the horizontal portion of an inverted L.

That is the reason why the radiation pattern I posted does not show ~zero relative field toward the zenith, as would be the case if little/no current flowed in the horizontal part of the L.
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K6IC
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« Reply #12 on: April 04, 2012, 06:12:26 PM »

Elevated Radial verticals have been discussed here in some detail.  As noted,  then,  Rudy Severns,  N6LF has done extensive testing and documentation on this subject.

Any interested in some more detail may want to look at Rudy's site:
http://www.antennasbyn6lf.com/

Have two verticals here,  one is a 160 M Inverted U,  with three elevated radials,   at a low height. These work well enough for me.   Vic
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« Reply #13 on: April 04, 2012, 06:23:23 PM »

The pattern of the inverted L depends on the current distribution and the relative vertical / horizontal portions.  Here is an EZnec analysis where the quarter-wave wire has 3/4 of its length horizontal, and starts to become noticeably directional.   I haven't modeled it, but it gets more interesting when longer wires are used, with different vertical / horizontal ratios ...

I recall talking to the Chief Engineer of WSM (50 KW AM) - he had an auxiliary antenna which was a short inverted-L - he couldn't keep end insulators on the thing because the extreme voltage caused really bad corona.


* invL_2-views.jpg (169.43 KB, 1344x616 - viewed 465 times.)
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R. Fry SWL
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« Reply #14 on: April 04, 2012, 07:08:10 PM »

... Here is an EZnec analysis where the quarter-wave wire has 3/4 of its length horizontal, and starts to become noticeably directional.   I haven't modeled it, but it gets more interesting when longer wires are used, with different vertical / horizontal ratios ...

This model was patterned in the NEC "far field" over real earth of some undefined conductivity.  This is the radiation pattern from that structure that survives at an infinite distance over a flat ground plane of infinite extent (not the real world).

However the radiation pattern that is actually launched from an antenna of these configurations does not show such directional effects.

A NEC "near field" analysis showing the azimuth pattern from such structures for various low elevation angles, and distances closer to the antenna will lead to a different conclusion.

For example, your pattern shows essentially zero field in/near the horizontal plane.  Yet that does not support the fact that for the same applied power, an inverted L can radiate ~the same, omnidirectional, groundwave field in the horizontal plane as an unloaded, series-fed monopole -- as shown by the real-world example posted in this thread by W7TFO.
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kb3ouk
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« Reply #15 on: April 04, 2012, 07:23:08 PM »

Back in the late 70's, I had a contract to move an AM/FM combo station in Yuma, AZ. 

We shut the FM down for a week, and before the tower was dismantled and moved I had the utility company come and plant two rented poles that left the tops some 50' up.

I secured the station an STA from the FCC and away we went.

I ran #10 THHW solid up one pole and across to the other.  I also ran that type of wire on the ground from pole to pole.  The feedpoint of those two wires needed only a .01 uF in series to give me 50Z @ 1400 kHz.  Really simple, robust, & cheap.  Kinda' like an AK-47...

It ran that way for three weeks at 1kW input, and the local area service was such that the listeners didn't know the difference. 

The kicker was from the skywave launching...the engineer got a lot of QSL requests from the pacific rim, Australia, and Asia.  People on other side of the world were listening to entire hours of country music from that class IV station.

Note to self, do inverted "L"s at home.

73DG

How far apart were the two poles?
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« Reply #16 on: April 04, 2012, 08:05:17 PM »

I recall talking to the Chief Engineer of WSM (50 KW AM) - he had an auxiliary antenna which was a short inverted-L - he couldn't keep end insulators on the thing because the extreme voltage caused really bad corona.

Did he try flairing the wire at the end with some sort of mushroomed surface, something like a big corona ball?

They have some kind of aux tower but I don't know much about it.

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« Reply #17 on: April 04, 2012, 08:05:36 PM »

IMHO it was a quarter wavelength at 1.4 MHz.

1kW is several orders of magnitude removed from 50kW...

For insulators I just used those big, thread in type found on electrical services. Of course, wooden poles don't need a whole lot of insulation in that dry climate:  

Rainfall <4"/yr.

73DG
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kb3ouk
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« Reply #18 on: April 04, 2012, 08:17:28 PM »

I might try something like that on 160 meters then. have some old poles that would be perfect for it.
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flintstone mop
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« Reply #19 on: April 05, 2012, 07:43:40 AM »

Amazing story about the station getting into the Pacific Rim. Antenna modeling is getting very popular and gets a lot of grey matter moving.
The members here really benefit from the broadcast engineers that deal with this stuff all the time. There are some tricks out there but no short cuts to an effective antenna in the MF region.
Fred
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« Reply #20 on: April 05, 2012, 08:08:58 AM »

As for NEC far-field plots over real earth showing zero radiation in the horizontal plane...

Attached is a NEC near-field plot of field intensity vs distance for a groundwave path ranging from 500 meters to 5,000 meters from the radiator.  The NEC model is the same configuration that I used earlier: 30 ft vertical section, 90 ft horizontal section for the inverted L, and a single 1/4-wave elevated radial running from the bottom of the L in the same direction as the horizontal conductor of the L.  The entire assembly is elevated 3 feet above the real earth.  Earth conductivity is 5 mS/m, d.c. = 13.

The groundwave fields shown in this study and in real-world applications could not be generated if this radiator had zero relative field in the horizontal plane, as shown in its far-field pattern.

This illustrates the analytic error that occurs when considering the far-field pattern to accurately show the radiation launched by an antenna near the earth.


* Inv L Gndwv Field.jpg (43.28 KB, 436x437 - viewed 417 times.)
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