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Author Topic: 20 meter loop  (Read 3935 times)
EI5GUB
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Posts: 66




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« on: April 11, 2012, 11:46:10 AM »

Hi everyone,Richard EI5GUB here,Ive just put a 20 mtre loop up and fed it into a 4:1 balun,it shows 50 ohms at 14.185 but has an SWR of 3 to 1,how can I get this down,also when I removed the Balun and fed direct to the Anaylser it shows an impedeance of around 75 to 80 ohms which probably puts a 4:1 balun out of the picture,the antenna was 4 feet off the ground when tested and the cable from balun to anaylser was 1 foot longim planning a complete circular loop if I can get this delta working right,any advice please,cheers Richard Gorman.
« Last Edit: April 11, 2012, 12:03:15 PM by EI5GUB » Logged
AD4U
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Posts: 2179




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« Reply #1 on: April 11, 2012, 12:01:15 PM »

Depending on several different factors the impedance of a full wave loop is 80 - 100 ohms.  You would do better by feeding it directly with 50 ohm coax instead of the 4:1 balun.  Better yet feed it thorugh a 1/4 wave coax matching section.

Dick  AD4U
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EI5GUB
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Posts: 66




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« Reply #2 on: April 11, 2012, 12:05:18 PM »

high Dick thanks,can you tell me more about this matching section,am I wasteing my time altogether with a 4:1 balun,cheers Richard
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K0ZN
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Posts: 1560




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« Reply #3 on: April 11, 2012, 12:11:46 PM »


 The 1/4 wave (electrical length, accounting for velocity factor) per AD4U's suggestion is easiest but does not provide the decoupling a balun would.

 The other way is to feed it through a 2:1 balun. They are available (at least here in the States) from a couple of commercial sources.

 2:1 baluns are quite handy....you can feed dipoles at electrical elevations where the Z is near 90 ohms and they work well on Loops which are about 100 ohms Z.

 If you want low SWR, your coax must see something CLOSE to its 50 ohm impedance. A 4:1 balun would theoretically multiply the 50 ohm coax impedance to about 200 ohms, so
 you then have a 200 to 100 mismatch..... so it is not what you want.

73,  K0ZN
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EI5GUB
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Posts: 66




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« Reply #4 on: April 11, 2012, 12:34:11 PM »

Hi K0ZN,thanks for replying,yes I was thinking a 2 to 1 would do the job but they are impossible to get here in Ireland and I would build one no problem if I could get some instructions,9 6 and 4 to 1 seem quiet common but 3 to 1 or 2 to 1` seem scarce,im putting this loop up at 70 feet pointing north west /south east out of central Ireland so will be good for USA and down into Asia
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WB6BYU
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Posts: 13486




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« Reply #5 on: April 11, 2012, 02:14:42 PM »

Quote from: EI5GUB

...it shows 50 ohms at 14.185 but has an SWR of 3 to 1...



How are you measuring the antenna?  A 50 ohm antenna can't have an SWR of 3 : 1 unless
there is significant reactance.  If that is the case, then adjusting the wire length should
bring the SWR to a reasonable value.  (Or inserting a coil or capacitor in series with the
wire at the feedpoint.)  But this may not be the case if you are measuring the antenna
through any significant length of coax.

Loops can have a wide range of input impedances depending on the shape.  If you make
it rectangular and about twice as tall as it is wide, then feeding it in the middle of the
bottom wire gives very close to 50 ohms.  If you spread the sides out so it is wider than
it is tall then the impedance will be closer to 200 ohms (with slightly lower gain and wider
SWR bandwidth than the 50 ohm version.)  The same approaches apply to delta or diamond
shapes.



Quote

...the antenna was 4 feet off the ground...



Antenna impedance varies with height above ground.  I would guess that the resistance will
increase and the resonant frequency will shift up the band by perhaps 100kHz when the
antenna is raised into the air.  Getting the bottom wire 5m off the ground will give you a
better sense of what it will be at the final height.  (This also depends on whether you are
using a single loop or a quad, as the parasitic elements of the latter will increase the
variation.)

I suspect that the elongated rectangle will end up giving you more gain than a circular
loop and a more convenient feedpoint impedance (besides being easier to construct.)
Here are some references:

http://www.cebik.com/content/a10/quad/qloop.html
http://www.cebik.com/content/a10/vhf/cs.html

(Yes, they require a free registration, but this is one of the best antenna resources
on the internet.)  The circular loop can give about 0.3dB improvement over a square,
while the elongated loop is about 1dB better.

Generally I find that I can get close enough to match any full-wave loop using either
direct feed (50 or 75 ohms), a quarter wave of 75 ohm transmission line, or a 4 : 1 balun,
depending on the shape of the loop.
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W4OP
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« Reply #6 on: April 11, 2012, 02:23:56 PM »

I am guessing this is a vertically oriented 20M loop? A horizontal full wavelength  loop at any height has the majority of its energy going straight up- not what one would generally want for 20M DX. At low angles  a high full wave loop would have a pattern about like a dipole. So if this loop is horizontal, put up a dipole, you'll be much better off for DX.

Dale W4OP
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EI5GUB
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Posts: 66




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« Reply #7 on: April 11, 2012, 02:30:14 PM »

Hi WB6BYU thanks for alot of info,I was using 1 foot of coax to analyser,the loop is triangular and is equal sided,7 mtrs a side and was tested horizontal,I will have to find a 1/4 wave piece of 75 ohm coax and try it.
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EI5GUB
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Posts: 66




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« Reply #8 on: April 11, 2012, 02:33:45 PM »

yes Dale it is vertical and will be 70 feet high between 2 big fir trees,I know you can feed them at different points so they are either vetical or horizontal,but will stick with bottom corner,Iwill stick with a triangle loop rather than a circular one as it seems they have not much gain
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WB6BYU
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« Reply #9 on: April 11, 2012, 03:12:39 PM »

Quote from: EI5GUB
...the loop is triangular and is equal sided,7 mtrs a side and was tested horizontal...


That will be quite different than what you will see when it is up in the air due to the
coupling to the ground.  You really can't take good measurements for either impedance
or resonant frequency unless it is vertical and some distance above the ground.


I've never figured out what that "resistance" scale reads on some analyzers when the
load has reactance, as it must in this case (otherwise a 50 ohm load would have a 1 : 1
SWR.)  Even 1' of coax can shift reactance measurements, though it shouldn't affect
the SWR curve.  If you can find the point of minimum SWR that will give you a good idea
of the resistive component of impedance.

Since you have the 4 : 1 balun on hand you can still use it.  Find the SWR at minimum and
from that determine the input impedance (let's say the minimum SWR with the 4 : 1 balun
is 2 : 1, so the input impedance of the loop is 100 ohms.)  Then you can use a beta match
(shorted stub or shunt coil) across the balun to raise 100 ohms to 200 ohms, along with a
bit of adjustment to the loop length.  W4RNL's matching spreadsheet suggests that about
1m of shorted 600 ohm ladder line or a 2.25uH inductor would give a good match on 20m.
That's probably easier to find than a fractional ratio balun (which I find suspect anyway.)

But it's probably easier simply to make the delta loop wider so it provides a 200 ohm match.
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K0ZN
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Posts: 1560




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« Reply #10 on: April 11, 2012, 09:12:36 PM »


Another thought:  I saw in a very old Antenna book of some sort where the full wave loop was TWO turns and fed with balanced line.  I don't have data on this
and/or what the feedpoint impedance would be.  You may want to research this some. I have not looked into the math to see what a second turn would
do to the input impedance....or even if this is a viable idea.

73,  K0ZN
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EI5GUB
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Posts: 66




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« Reply #11 on: April 12, 2012, 12:10:13 PM »

hi i got some 75 ohm coax and am making a 1/4 wave matching section,when I have it connected to the antenna and go analysing will I see 50 ohms at the end of the 1/4 wave or does it need to fed into 50 ohm cable also and then then checked
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WB6BYU
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Posts: 13486




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« Reply #12 on: April 12, 2012, 12:15:42 PM »

If it is 50 ohms at the end of the matching section it should still be 50 ohms even with
a length of 50 ohm coax attached.  Assuming you mean 50 + j0 ohms, that is.

Pay attention to the SWR rather than the R meter reading.  When the SWR is high
then the R value will vary significantly with line length.  It's the frequency of minimum
SWR, and the SWR at that point, that you want to work with.  If the frequency of
the SWR dip is outside the band, adjust the wire length to put it where you want it.
If the SWR at the minimum is too high, adjust the loop shape to improve it.
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EI5GUB
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Posts: 66




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« Reply #13 on: April 12, 2012, 02:10:01 PM »

ok wb6 thanks again,you seem well up in feed lines,will let you know when its finished and might sked a contact with you next week,cheers Richard
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WB6BYU
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Posts: 13486




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« Reply #14 on: April 14, 2012, 07:26:28 AM »

OK, I finally got a chance to model it.

Using 2mm wire it looks like 7m on a side is a bit short.  About 22 to 22.5m total length looks
better, but that will require some adjustment if you are using insulated wire.  If you make the
top section 7.5m long then you can adjust the lengths of the other two sides at the feedpoint.

Feed it with 4m of 75 ohm coax (velocity factor = 0.8, typical of foam dielectric.  Use 3.3m
if it has a solid dielectric.  +/- 25cm will still give good results.)  SWR bandwidth is about
500kHz at 1.5 : 1, so no problem covering the whole band.




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