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Author Topic: How about a loaded-loop for 80M?  (Read 2084 times)
KE7FD
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« on: September 13, 2010, 10:14:00 AM »

I've used a full size 80m loop for years with very good results.  Recently however, one of my neighbors who has allowed me to use one of his trees as a support has decided to move and I cannot rely on new neighbors to allow me the same privilege.  Also, the wire is quite worn in places and needs to be replaced so now is the time to redesign and adapt.  Is there anyone out there in the either who has successfully used a "loaded loop" on 80m; one which takes up less real estate than a full size loop? given there will be some losses in a loading coil(s) but at least it's still a loop.
 
Any loaded-loop Elmers out there?

Glen  - KE7FD
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K0BG
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« Reply #1 on: September 13, 2010, 11:05:33 AM »

What's wrong with a loaded dipole? I suspect you'd do better with it, than a loop.
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KE3WD
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« Reply #2 on: September 13, 2010, 11:25:13 AM »

Linearly loaded dipole, like the Cobra, comes to mind here, as well as the coax-loaded Bazooka design, too. 

Linear loading on the 75 meter band, I have found to work quite well, actually.  Considering that loading of any kind is a compromise of sorts. 
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WA3SKN
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« Reply #3 on: September 13, 2010, 11:33:53 AM »

It's been done. Now, how small?
However, please remember that small loops do not radiate all that well... bigger is better!
So go ahead and try it... if it works, GREAT!  If not, try something else.
73s.

-Mike.
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K2DC
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« Reply #4 on: September 13, 2010, 11:35:12 AM »

Glenn,

I'm no lawyer and property laws vary from state to state.  But if the previous neighbor owned the property and you've been using it for years, you may have and easment for use that transferred with the property.  Having said that, it may not be worth the fight if the current residents put one up.  But it still might be worth asking, explaining that the previous residents had no problem and that it never interfered with any of their use of the air space.

Still, if you had room for a full wave loop with only one support off your property, I'm sure there are other wire designs that would fit and do just as well as the loop.  There are a number of things you can do with doublets to fit a particular area.  Linear loading will reduce the length and gain a bit, but without losses from loading coils.  Bending the ends vertically or horizontally will also work well.  An inverted vee sloping in both planes will work, although it may skew the pattern in a particular direction.

You might want to download one of the free versions of EZNEC, or one of the other free antenna analysis software packages.  That way you can sketch out the area and check out the performance of several candidates against the loop.

GL &73,
Don, K2DC

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KE7FD
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« Reply #5 on: September 13, 2010, 02:53:04 PM »

Thanks for the replies.  As far as dipoles go, been there, done that ad nauseum; as I once heard some guy on the air say, "Life's too short for dipoles".  I'm a little short of space for the fullsize 80m loop so ezNec might be the thing to try next to see how far off I'll be.  My wife's an attorney but she's not about to step in and help me on this one; besides, I'd rather play nice with others cause you never know when that will come in handy down the road.  I like experimenting with antennas anyway but have resisted eznec up until now. Guess it's time to think abstractly.

g
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WB6BYU
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« Reply #6 on: September 27, 2010, 11:27:01 AM »

Yes, it is possible to load a loop using coils or folded stubs.  You can also use a short loop with a tuner.  These
all will work.

But the reason that a loop has gain over a dipole is that the two sides with high current are spaced some
distance apart:  the closer they are to each other, the more the antenna acts like a folded dipole (which
is just like a regular dipole.)  As you shorten a loop, you always get maximum current at the point opposite
the feedpoint, but the current at the feedpoint decreases, which also reduces the overall gain of a loop
compared to a dipole.  (Which is perhaps 1dB anyway - hardly enough to notice.)

Let's assume for the moment that you have a loop 60' square, feed in the middle of one side.  Maximum
current (and hence radiation) is at the feedpoint and in the middle of the opposite side.

A common place to put the loading coils would be in the centers of the sides, though this requires more
inductance than other locations.  You can also fold up the excess wire into a stub at this point, though
that requires more wire length for the same resonant frequency.  (Do NOT assume that you can just
wind the extra wire into a coil - that doesn't work at all.)  Perhaps a better approach, if you are going
to keep the loop as a square, is to put the loading coils in the 4 corners, which is more like adding loading
coils in the middle of each side of a dipole instead of at the ends.

My suggestion, if the trees permit, would be to try to maintain the same spacing between the feedpoint
and the center of the wire, but make the loop less wide.  For example, you might make the feed segment
and the opposite one 40' long and keep  the sides at 60', making a rectangle.  Add the loading coils or
wire stubs in the sides, but not in the short ends which are doing most of the radiating.

This can be made to work on 80m, but it becomes much more of a problem when you try to operate
on harmonics, since the loading coils have a different relative impact.  If you look at the current
distribution around the loop on 40m you will see that there are points of minimum current on 40m
that aren't minimums on 80m:  this is the place to put the coils to affect the 80m resonance without
shifting 40m much.

You'll need to consider the shapes that can be made using the available supports first to see just what
is practical, of course.  Then consider what bands you want to cover besides 80m and you can take it
from there.  Certainly a good first step if you are feeding it with twinlead to a tuner is just to put up as
much wire as you can manage and see if it works well enough for you - that may be sufficient, even if the
loop is somewhat shorter than a full wavelength.  If not, there are various options for folding the wire to
make it fit and/or using loading coils.
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LA4RT
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« Reply #7 on: September 27, 2010, 11:55:31 AM »

I've used a full size 80m loop for years with very good results.  Recently however, one of my neighbors who has allowed me to use one of his trees as a support has decided to move and I cannot rely on new neighbors to allow me the same privilege. 
True.

But there's no reason to take the loop down until the old or new owner asks you, is there?
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W8JI
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« Reply #8 on: September 27, 2010, 09:20:35 PM »

Thanks for the replies.  As far as dipoles go, been there, done that ad nauseum; as I once heard some guy on the air say, "Life's too short for dipoles".  I'm a little short of space for the fullsize 80m loop so ezNec might be the thing to try next to see how far off I'll be.  My wife's an attorney but she's not about to step in and help me on this one; besides, I'd rather play nice with others cause you never know when that will come in handy down the road.  I like experimenting with antennas anyway but have resisted eznec up until now. Guess it's time to think abstractly.

g

There is barely any advantage to a loop when it is optimum height and size and shape, other than feed impedance on harmonics.

Why go through all kinds of work to use one with loading???

By the way, just to keep the record accurate, linear loading is not especially good loading for efficiency. The Cobra antenna, given as an example, has pretty poor efficiency.

73 Tom
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NE5C
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« Reply #9 on: September 28, 2010, 01:32:36 AM »

Hi Glen
If you have to change...I go along with reconfiguring the shape of the loop if you can.
example; if it's a square, then make it a Delta or 3 sided shape. I have one 80 meter, shaped like a house and it works great! God Bless and 73
Jerry N5JFJ
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KE3WD
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« Reply #10 on: September 28, 2010, 05:59:44 AM »


True.

But there's no reason to take the loop down until the old or new owner asks you, is there?


Now there's a plan...

73
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AD4U
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« Reply #11 on: September 28, 2010, 08:28:25 AM »

ANY antenna that is "loaded" will not be as efficient as a corresponding full size antenna.  Hence a "loaded" antenna will not work as well as its full size counterpart.  How much difference and whether you can detect the difference between the two depends on many different factors and each situation is different.


Dick  AD4U
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KB2CPW
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« Reply #12 on: September 28, 2010, 10:58:35 AM »


True.

But there's no reason to take the loop down until the old or new owner asks you, is there?


Now there's a plan...

73


  Better yet, repair your old one now and leave it. And remember...

  Its better to beg for forgiveness than to ask for permission.

  Greet your new neighbors with something for their home or your wife's best dish and they may have a hard time asking you to remove one little wire. Kindness goes a long way sometimes. Regards, Richy N2ZD
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