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Author Topic: What happens when antenna is close to the ground?  (Read 3231 times)
N8TI
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Posts: 115




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« on: January 30, 2012, 03:30:18 PM »

Hello.  Whenever I read antenna books and look at radiation patterns, the assumption always is that the antenna is a wavelength or 1/2 wavelength up in the air.  For example, the ARRL Antenna book shows a pattern for a 20 meter dipole, with the assumption that it is 70 feet in the air, about one wavelength. Then it goes and compares that antenna to a long wire antenna.

But my antennas have always been lower than 70 feet.   I have a 40 foot high tower, so my antenna might be 50 feet up, max. I am presently fooling around with a long wire and it's maximum height is about 20 or 25 feet up tapering to about ten or twelve feet down the line. Of course, I know that "higher is better," but of course, beverage antennas are normally low and apparently they are supposed to work that way as receiving antennas only. In addition, I have used "bumblebee" antennas that worked fine and other antennas that should have worked but ended up as big dummy loads. Of course, some antennas have worked just as they should.

So, the question is, as a practical matter, can patterns really be predicted if the antenna is close to the ground, or does everything that is ten or twenty feet up just end up acting like a vertical?  And, how do you know what kind of "ground" your soil makes. Stick an ohm meter into the ground? What about when the ground is covered with snow or when it rains? 

Thanks,

Joe

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WA4FNG
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« Reply #1 on: January 30, 2012, 04:52:35 PM »

I think the simple answer in most cases, at least with horizontal wires, is below 1/2~ the take off angle increases. That's why you read many times that a dipole makes a good DX antenna when it's at the proper height. Lower antennas are going to radiate higher angles.
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K0ZN
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« Reply #2 on: January 30, 2012, 08:33:00 PM »

Hi.

First, yes, patterns can be predicted for low antennas. One can model them and some of the older ARRL Antenna Books have pictures of plots down to 1/8th wavelength.
Short version is that at low heights, a dipole gets pretty non-directional and the amount of low angle energy decreases dramatically.  i.e. they become "cloud warmers".
Most "average" ham dipoles on 80 and 160 and a lot of them on 40 are "low to very low" in terms of electrical height. That doesn't mean they don't work; they do....
but they are not highly effective DX antennas.

There are two other downsides to a very low antenna. (1.) ground losses go up; less of the power put into the antenna is radiated. The lower the antenna, the more of the radiated power is absorbed in the earth. Ordinary dirt is NOT a good conductor of RF. (2.) The feedpoint impedance of a very low dipole can get quite low. This creates matching problems (to the transmission line) and the ratio of Radiation Resistance to Ohmic resistance goes down....meaning resistance losses in the wire go up.

Cold reality is that most of us can't or won't put up an antenna for 80 and 160 that is truly 'high'. A few big guns do, and the signals show it. What an 80 M dipole will do
at 150 ft. vs. one at 35 ft. is amazing....but few of us will put up TWO 150 ft. towers to find out.  BUT....we still work a LOT of stuff, have a good time and even work some
DX with our "low antennas".

The frequency at which the antenna is "very low" will have LOT to do with how well it gets out. An 80 M dipole that is "only" 1/8th wave high (about 33 ft. up) will work very well because at 3.5 Mhz, high angle energy is useful and reflected.... you will get good reports and work about anything you hear. However, a 10 Meter dipole that is 1/8th wave high (8 ft.) is a very poor performer because it has little low angle radiation and the high angle radiation is not reflected back down. Fortunately, a 10 M antenna is easy to raise to a half wave or a full wave high...and will work very well at those heights.  It is kind of relative.........

The whole subject of RF and ground conductivity is too much to put on here. There are maps of this available in some books.

Spend a little time with the ARRL Antenna Book.... All of this (and your questions) is really a more complex subject than a couple of a paragraphs on a website can
possibly explain and the graphics will help a lot. An older copy (1960's or '70's) off of Ebay is sometimes even better because they show more printed "plots" of this subject.
Several hours digging into the Antenna Book will be both interesting and helpful. Antennas are not super complex, but neither are they intuitive. An organized
text like the ARRL Antenna Book puts it all into perspective.  (No!! I don't work for the ARRL or have any vested interest....other than seeing people learn
and understand more about antennas.)

73,  K0ZN
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WB6BYU
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« Reply #3 on: January 30, 2012, 09:11:51 PM »

Quote from: K0ZN

There are two other downsides to a very low antenna. (1.) ground losses go up; less of the power put into the antenna is radiated. The lower the antenna, the more of the radiated power is absorbed in the earth. Ordinary dirt is NOT a good conductor of RF. (2.) The feedpoint impedance of a very low dipole can get quite low. This creates matching problems (to the transmission line) and the ratio of Radiation Resistance to Ohmic resistance goes down....meaning resistance losses in the wire go up.



Actually these two effects work against each other:  the higher the ground losses, the higher the
impedance and the lower the SWR.  Over some soils the SWR impedance actually stays pretty close
to 50 ohms even when the antenna is close to the ground.

But otherwise a good summary.  Radiation patterns can be predicted easily for low heights, though
the magnitude of the losses may be a bit uncertain for antennas close to ground.  (That was one of
the faults with some of the early studies of NVIS propagation - they relied on the ground model in
MININEC, which was quite inaccurate for such things and greatly underestimated the losses at
low heights.  The ground models in NEC2 or NEC4 are much more accurate for such purposes.)  For
a dipole the maximum radiation is straight up, tapering off at the lower angles to nearly nothing at
low angles.  Except for losses, it really doesn't change much for heights below around 1/8 wavelength.

Such low antennas aren't nearly as good for DX due to the reduced radiation at low angles.  They
are useful, however, on the lower bands (160m, 80m, and sometimes 40m) for local work when the
ionosphere reflects such high angle signals back down to Earth.


At no height does a horizontal antenna start acting like a vertical.


One way to measure your ground characteristics is to install a dipole at a low height and
measure the impedance of it.  Otherwise there are descriptions of different soil types and
the measured characteristics thereof - from those you can get a pretty good guess of
the parameters for your soil.
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W5DXP
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« Reply #4 on: January 31, 2012, 04:52:23 AM »

The free demo version of EZNEC is a good way to study radiation patterns. It is available at:

http://www.eznec.com
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W1JKA
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« Reply #5 on: January 31, 2012, 10:14:10 AM »

 
If its a horizontal wire it can turn into a good NVIS antenna.

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NH7O
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« Reply #6 on: January 31, 2012, 10:32:01 AM »

The increased ground loss and high angle radiation of a low horizontal antenna was summarized by Cebik: "Small and tall is better than long and low."
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N8TI
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Posts: 115




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« Reply #7 on: January 31, 2012, 03:31:28 PM »

I will have to look at the older ARRL antenna books, because the current one does not seem to address the issue of a low antenna. The plots and graphics do look nice, but only tell us what is supposed to happen when the antenna is high. As was pointed out, such information is more worthwhile when you are dealing with the higher bands, but gets rather depressing when dealing with 40 meters, for example.

Perhaps that is why, if us "small guns" want to work DX, spending time to put in a good system of ground radials under a vertical antenna and not worry about the height.  Just a thought.

Joe

Of course, we do have a lot of fun with "low" antennas and apparently even if most of the radiation is going straight up, enough must go out at low angles to allow DX to hear us. So, the best thing to do is get something up there as high as possible, not worry about it and just play radio.
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