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Author Topic: Loop antenna height?  (Read 2989 times)

Posts: 8

« on: November 17, 2013, 07:36:25 PM »

I am using a horiz. loop antenna that is cut for 75 meters and it works fine. I do have a question that I am hoping someone can answer.

There are five corners to the loop and the feed line- a 450 ohm ladder line - is off-center on one side. The antenna, because of differing tree heights and the topography in the yard, varies irregularly in height above ground from about 30 to 60 feet.

I am just curious as to how the differing heights affect the over all performance of the antenna. Does the antenna  react to the earth as if is 30 feet, 60 feet or an average of all the heights? If the antenna reacts as if it is 30 feet over all then there would be no point in trying for greater height I guess.
I like the antenna, just wondering about this for a future project.

Posts: 17484

« Reply #1 on: November 17, 2013, 07:44:20 PM »

I've used such irregular loops before with good results.  (One of mine had 7 or 8 corners
to it as it wrapped around the roof, chimney, apple tree, fence, etc.)

Determining the pattern and the relative effect of varying height isn't a trivial task.
Basically you need to analyze the current distribution around the loop to find the
points of maximum current, because that accounts for maximum radiation.

For example, say you have a 1 wavelength loop with two opposite corners higher
than the two lower corners.  If you feed it at one of the upper corners, the maximum
current parts will be at the highest parts of the antenna.  If you feed it at one of
the low corners, the effective height will be less even though the shape of the
antenna hasn't changed.

How can you tell?  Well, the point opposite the feedpoint on a loop is always a
point of maximum current.  You can then work your way around the wire from
there knowing that the current maxima are spaced 1/2 wavelength apart.

Posts: 2276

« Reply #2 on: November 18, 2013, 06:42:20 AM »

The horizontal loop and the related current maxima's as noted are 1/2 wave apart relative to the opposite side of the feed point.

The current maxima's are indeed the relevant radiating points about the perimeter of the loop.

They are located as a function of frequency applied and the resulting position is set about the loop.

So the height of the wire above the earth surface below measured from the current maxima's as in your case, varies by percentage of wave length and frequency applied does effect the power manifest toa to some extent since your" height above earth is from 30 ft to 60 ft that is a 50% swing large enough to make some difference worthy of knowing where and what height the current maxima's are resting relative to the loop wire height above earth surface.


Posts: 624

« Reply #3 on: November 18, 2013, 08:25:52 AM »

Hi Don

I am in line with the last post.
But even then you have to consider all influences of the trees and all other conductors in about a wavelength of the loop to.

It would help to raise the antenna to 60 feet overall height, but you are still having the problem of influences by surounding conductors as trees etc.

But even making average height 60 feet will still result in high angle take of.
And this antenna still will be extremely good for local NVIS contacts.
And the higher it gets the more suited it will be for DX contacts.

But if you use 450 ohm ladderline to feed it this antenna also can be used on a lot of other bands.
And height on 40 mtrs could mean that local contacts can become more difficult and dx becomes more easy.

For all other and higher bands more height will only mean improvement.

Though after all your effort it might even not be seen in your signal strength at all, due to all influences of conductors in the vincinety of your antenna.

This loop used with high quality 600 ohm line (AWG 13 or better) and a good tuner can improve your antenna in use on 160 mtrs a lot too. 

A loop antenna doesn't have to be a full wave loop to work on 160 mtrs and we even tried full wave loops for 20 mtrs on 160 mtrs with reasonable succes.
Even a 2 x 10 mtrs dipole proved to be worse on 160 as a loop of 20 mtrs total because 600 ohm ladderline losses and tuner losses were much higher with the 2 x 10 mtr dipole  in compare with the 20 mtr loop.

If it takes a big effort and money to raise overall height , it just might be an expencive gamble Don that doesn't pay out.

73' Jos
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