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Author Topic: Top Band Loop  (Read 743 times)
EI5GUB
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Posts: 60




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« on: February 01, 2013, 02:50:17 PM »

Hi Im thinking of putting up a loop antenna for 160mtrs,I have 2 trees that are near 70 ft high and will have the longest part between the trees with the apex at the bottom,I was thinking of putting the feed point in one of the top corners and using an electric length quarter wave 75 ohm to match it,can anyone please jig this on a computer and tell me what it looks like plot wise,thanks EI5GUB
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K3VAT
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Posts: 730




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« Reply #1 on: February 01, 2013, 03:21:52 PM »

Hi Im thinking of putting up a loop antenna for 160mtrs,I have 2 trees that are near 70 ft high and will have the longest part between the trees with the apex at the bottom,I was thinking of putting the feed point in one of the top corners and using an electric length quarter wave 75 ohm to match it,can anyone please jig this on a computer and tell me what it looks like plot wise,thanks EI5GUB

That's a nice project.  When you say, "the apex at the bottom" do you mean that this antenna will be in the form of a delta loop (3 sided with the upper part being horizontal between those 70' trees?)  Before we can model we need to know the length of the wire ... is this a fullwave version (~ 534') or something different?  How far above ground is the "apex"?   GL, Rich, K3VAT
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WB6BYU
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Posts: 13335




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« Reply #2 on: February 01, 2013, 04:43:10 PM »

With such a long and narrow loop, I think you'd be better off putting it up as
a rectangle.  Given some space to walk under the lower wire, that would make
it about 60 feet high and 210 feet long.  If fed in the middle of one end wire
it would be vertically polarized, but the input impedance will be below 50 ohms.

My guess is that the rectangle will work better than the delta loop in this
case, but it would be good to check both.  I'll try it the next time I have some
time, if somebody else doesn't do it first.
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WB6BYU
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« Reply #3 on: February 02, 2013, 08:36:07 AM »

OK, I ran a couple models based on a top height of 70' and a bottom height of 10'.

The delta loop was 266 feet long with a feedpoint impedance about 38 ohms.

The rectangle was 210 feet long with a feedpoint impedance around 64 ohms.

The rectangle is just under 1dB better than the delta loop.

Both were run with "poor" soil ((0.002, 11).  With better soil the efficiency
improves and the feedpoint impedance drops accordingly.  Both are most
suitable for direct coax feed, though the delta loop may need a shunt matching
coil (or capacitor) across the feedpoint if your earth has better conductivity.

While these will give you an idea of performance, you'll have to see how they
do in your specific installation, particularly soil type.  Also, running the coax
away from the loop in such as way as to keep it out of the pattern, and using
an effective balun to decouple it, will help to maintain the pattern.
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EI5GUB
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Posts: 60




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« Reply #4 on: February 02, 2013, 12:02:55 PM »

Hi guys thanks for replying,yes it is a delta loop with the horizontal part at top,havnt decided yet what length any part will be,yes a rectangle would be excellent as it ia a larger opening than a delta loop,however it is agricultual land and i need to keep everything out of animal and tractor reach,I guess vertical pol is best as it is so low for a top band antenna,im nat an antenna expert so I need your ideas,I built my own version of the Radio Works Super Loop and it works great,thanks again Richard EI5GUB
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