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Author Topic: Ground Radials for an Inverted-L Question. . .  (Read 5648 times)
N4NOO
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« on: September 11, 2011, 07:24:55 PM »

I have an inverted-L antenna with the vertical section next to the back of the house.  It is 23 feet tall with about 90 feet of wire running from the vertical part to a tree at the back of the yard.  I started with three ground rods driven in the ground at 9, Noon and 3 o'clock forming a semicircle around the base.  I clamped a piece of #4 bare solid copper wire around the three rods.  To this wire I clamped 8 #10 copper wires running parallel to each other running on the ground under the horizontal antenna wire to the back of the yard.  I have also added several more wires running around the side of the house and then all the way the the street.  These wires run 180 degrees from the horizontal portion of the antenna.
And now the question:
Does an inverted-L benefit from having radials running out 360 degrees from the base of the vertical section much as a vertical 1/4 wave antenna does?  

Thanks,

Rick - N4NOO
« Last Edit: September 11, 2011, 08:16:21 PM by N4NOO » Logged
N4JTE
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« Reply #1 on: September 11, 2011, 08:35:51 PM »

What band??
bob
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N4NOO
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« Reply #2 on: September 11, 2011, 09:28:03 PM »

I use this antenna on all bands 160 - 6.
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W5CPT
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Posts: 556




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« Reply #3 on: September 11, 2011, 10:03:37 PM »

Yes - An inverted L is a vertical - all verticals benefit from having the best counterpoise you can install.  The ONLY question is at what point is the point of diminishing returns.  That is: what you add costs more (time & $) than the improvement you get is worth.  I add radials till the feedpoint impedance stops going down. Somewhere in the 35 to 37 ohm range. 

Clint - W5CPT -
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N3WAK
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« Reply #4 on: September 12, 2011, 03:51:17 AM »

Rick--Since my 160 meter inverted L is not too high--initially the vertical portion was 29' but I was recently able to extend it to 40'--I put in as many crooked radials as I could.  I'm up to 59, very crooked, radials which average 65' in length, going off in the most uncluttered directions from my flagpole in the back yard which supports the antenna.  I've installed these radials over a couple of seasons, adding them when I got the itch.  I found installing radials kind of an addiction--perhaps that one radial more will allow me to reach 160 meter nirvana.  Recommend installing them on top of the grass and allowing the grass to cover them, if at all possible.  However, I think installing radials is still a tiring, tough, dirty job.  I buried about half of mine in slits I made with a power lawn edger.  Then you get on your hands and knees, shoving the #14 (or whatever) radials into the slit.  I get my #14 THNN radial wire (along with copious amounts of landscaping cloth wire staples) at Lowe's.  Good luck!

73, Tony N3WAK
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AA4PB
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« Reply #5 on: September 12, 2011, 05:03:43 AM »

A lot depends on the total length of the inverted-L in terms of wavelength. At one extreme if the length is 1/4 wavelength then the feed impedance is low and the radial requirement is pretty much the same as any 1/4 wave vertical. The highest currents flow in the vertical section so the radiation pattern is similar to a vertical.

At the other extreme if the length is 1/2 wavelength then the feed impedance is high and you can get by with less in terms of radials. Assuming the typical short vertical section and long horizontal section, the highest currents now move up into the horizontal section and the radiation pattern becomes more like a horizontal antenna.

If you are using the antenna on all bands (presumably with a tuner at the feed point) then you get all of the above, depending on which band you are operating on.
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K1ZJH
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« Reply #6 on: September 12, 2011, 06:48:16 AM »

Nirvana would be running as many radials as you can, and as long as possible, but keeping a minimum
of .025 wavelengths distance between the wire ends.  I'm not an expert, but I wouldn't go under
1/8 wavelength for the lowest band, 160 meters.  Sometimes a lot of short radials work better than
a few long ones. If have the room for long radials, you might consider using them, with the goal
of adding more until the magic .025 wavelength seperation between the wire ends is reached.
There are one or two very good webpages regarding radial length and the number required.

Severn's webpages are a credible resource:   http://rudys.typepad.com/

I'm in the same boat, since we just shot the support lines for my 160 meter inverted L last
weekend, and I am facing the same questions you have regarding radials. I'll be running THHN
16 gauge wires out about 70 feet, with a goal of 12 to 14 radials to start for this year.

Pete
« Last Edit: September 12, 2011, 07:08:33 AM by K1ZJH » Logged
N3OX
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« Reply #7 on: September 12, 2011, 07:43:20 AM »

Nirvana would be running as many radials as you can, and as long as possible, but keeping a minimum
of .025 wavelengths distance between the wire ends.

I dunno, "radial Nirvana" for me would be a good ability to predict the minimum number of radials and the minimum length that would get me within 0.5dB of "perfect."

Right now I only have a 50 foot square backyard so I just put 27 radials down out to the edges.

If I was trying to really optimize performance, like get within 0.5dB of perfect and I had an arbitrary amount of space I would set up a simple signal measuring test (http://n3ox.net/projects/n3oxflex/testset_lg.jpg) and I would look to some of N6LF's and other tests for a starting point and add radials until things flatten out to my satisfaction.

My 27 radials in about a 50 foot square seem to be within 1dB of perfect with a 6 ohm short vertical on 40m, so I do think it's important to not go insane with radials.  If you have a "radial addiction" you like to feed and it makes you feel stronger on the air to do that, go ahead, it's just wire. 

I could sure use a bunch more for 160m but I would have to annex the neighbors' yards.  I do want to test if some dense mesh on TB could make it better.  I kind of doubt it will help but it's not that hard to test it.
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73,
Dan
http://www.n3ox.net

Monkey/silicon cyborg, beeping at rocks since 1995.
W4VR
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« Reply #8 on: September 12, 2011, 08:58:12 AM »

The good rule of thumb to follow is to put down as many as you can and make them as long as your backyard will accommodate.  That is what have done here with my 160 meter inverted-L.
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K4SAV
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« Reply #9 on: September 12, 2011, 09:46:11 AM »

The good rule of thumb to follow is to put down as many as you can and make them as long as your backyard will accommodate.

My back yard will accommodate 400 to 1000 ft radials and I have a tractor mounted radial plow.  In one day I could put in about 600 radials.  Would that be your recommendation?
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RFRY
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« Reply #10 on: September 12, 2011, 11:14:18 AM »

Does an inverted-L benefit from having radials running out 360 degrees from the base of the vertical section much as a vertical 1/4 wave antenna does?

All forms of vertical, end-fed radiators such as used by stations in the AM broadcast band and by some amateur operators need a good "r-f ground" in order to radiate as efficiently as possible.

The transmitter provides a source for the r-f current, and the r-f ground provides the completion of the path caused by radiation from the monopole needed for those currents to return to the ground side of the transmitter circuits.

A monopole antenna system can be thought of as a series circuit where energy circulates back and forth at an r-f rate between the radiator and the r-f ground connection. The path between a monopole and an r-f ground is produced by the capacitance of the monopole to the r-f ground, across which path displacement currents flow.

Those displacement currents become conducted currents at, and just below the surface of the earth out to about 1/2-wavelength from a monopole, regardless of the monopole height in wavelengths. Once those currents enter the earth they need to be conducted back to the ground terminal of the transmitter in order to complete the path needed for r-f current to flow in the antenna system.

Soil is a rather poor (lossy) conductor of radio waves, so a system of buried radial wires often is used to provide a low-resistance path back to the transmit system in the area of the earth where those currents are highest -- within 1/2-wavelength radius of the monopole.

Carefully done physical experiments back in the 1930s determined that an r-f ground consisting of about 120 buried radials spaced 3 degrees apart around the tower base, and each radial about 0.4 wavelengths long (in free space) would produce an antenna system that radiated about 95% of the power applied to it for monopole heights of about 45 degrees or more. These results are produced regardless of the actual earth conductivity at, and within 1/2-wavelength of the base of monopole.

Such an r-f ground is the norm for licensed AM broadcast stations, but probably would be judged as too expensive, or even unnecessary by most amateur operators.

It should be noted that an r-f ground consisting of one or more "ground rods" buried vertically at or near the base of a monopole, or a few buried radial wires of any length do not constitute a good r-f ground. The r-f resistance of such paths is very high to the r-f earth currents surrounding the monopole, because they are forced to travel long paths through the lossy earth from up to 1/2-wavelength away to reach those conductors.

For reference, the r-f resistance of a broadcast type buried radial system is 2 ohms or less, while the r-f resistance of a few buried ground rods or wires may be 50 ohms or more. This added loss makes a big difference in the percentage of available r-f energy that will be radiated by a monopole, especially if the monopole is electrically short (ie, has low radiation resistance).

Note that a functional r-f ground as discussed here does not exist along the vertical length in space of any wire or other conductor such as a tower, billboard, water tower etc that is connected to a true r-f ground buried in the earth. All of those conductors will radiate into free space as a result of the r-f current flowing along them. And by definition, an r-f ground does not and cannot radiate.

Instead those exposed, vertical "ground" conductors become a radiating part of the antenna system. In some cases they can radiate more than the vertical conductor considered to be "the antenna."

A NEC-2D evaluation of this has been done for Part 15 MW AM installations (link below), and the same principles apply to HF systems in the amateur radio service.

http://i62.photobucket.com/albums/h85/rfry-100/AM_System_Comparison.gif

 R. Fry
 http://rfry.org
« Last Edit: September 14, 2011, 03:21:45 PM by RFRY » Logged
LU2DFM
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Posts: 91




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« Reply #11 on: September 12, 2011, 02:43:20 PM »


Right now I only have a 50 foot square backyard so I just put 27 radials down out to the edges.


Sorry Dan, 50 sq. foot or 50 square yards? Aren't 50 sq. foot ~ 5 m2 (sq. meters)?
73 de Fer

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N3OX
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« Reply #12 on: September 12, 2011, 02:54:54 PM »

Sorry Fer...

I said "in a 50 foot square", and meant a square with 50 foot sides!  

2500 sq. feet.

Certainly not 50 square feet... that would NOT be a very good ground system.  



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73,
Dan
http://www.n3ox.net

Monkey/silicon cyborg, beeping at rocks since 1995.
LU2DFM
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Posts: 91




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« Reply #13 on: September 12, 2011, 03:04:43 PM »

I said "in a 50 foot square", and meant a square with 50 foot sides!  
2500 sq. feet.
Certainly not 50 square feet... that would NOT be a very good ground system.  

My fault..., sorry. Thanks for the clarification.
Well, that would be ~ 15 * 15 meters, so my own backyard ground system is growing to similar dimensions, and I'm getting good results so far for such a minuscule ground for a vertical.

73 de Fer
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N4JTE
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« Reply #14 on: September 12, 2011, 06:21:00 PM »

160 to 6 inverted L at 23 ft high is a sure dissapointer even if you figure out the feed and extensive ground system that it will take to work anywhere. If as you say, you have 90 ft horizontal  space available then spend more thought on a horizontal version of a ladderline fed system, you will never get 7 bands from either one but at least someone might hear you this way.
Bob
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