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Author Topic: 80 meter Full loop Calculations  (Read 3531 times)
K8POS
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« on: October 11, 2012, 08:47:42 AM »

I currently have an 80m full loop up with a 4:1 current balun, and a 200' run of RG213 (under ground in conduit for 30') In a delta config at 65'on one tree 60' on the other and 30' third tree.
It performs great on just about any band (in the phone areas) with the built in tuner on my TS-2000.  Does not like the lower end of 6M but no surprise there.
But I can not tune below about 3.7 on 80m.
Did some calculations and I am short about 25' in antenna length.

My question is:
I know I need 285' of wire to get me in the ball park of 3.525 (digital end) on 80m.
Is there a way to calculate how well it might do on the other bands?
I know there is some give and take here.  May have to leave it alone and be happy it does as well as it does across all bands.

Bob
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KU3X
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« Reply #1 on: October 11, 2012, 09:02:11 AM »

Rather than screw up what you already have that works, do what I did.

Place a coil about 89 feet away from the feed point, on the base of the antenna and use it to lower
the resonat frequency to the CW end of 80 meters. I use a forked crimp terminal and move the jumper
when needed. I can do this in about 3 minutes going from the shack and back. I just walk out to the
back yard with a nut runner that fits an 8-32 machine screw and move the jumper. If it screws up
the other bands, just put the coil into play when you want to go on the CW end of 80 meters.
Here's my site and you'll be able to see the coil.

https://sites.google.com/site/ku3xmobile/primary-station-antennas

Barry, KU3X
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WB6BYU
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« Reply #2 on: October 11, 2012, 09:19:40 AM »

Or you could figure out how to match the impedance at the end of the coax and
not have to change the antenna at all.

Modeling the antenna and feedline will give you a good idea of why the impedance
is out of the range of your tuner, as well as how lengthening the loop will affect the
other bands.  Of course, you also need to know the matching range of your tuner:
not just as SWR, but in actual impedance.

It may be that a simple change like adding some coax, or a shunt coil across the
end of the coax, will allow you to cover the low end of 80m.


However, my experience has been that tuning a full wave loop for around 3.55 to 3.6 MHz
usually gives the lowest SWR on 40 / 20 / 15m.   17m and 12m have lower SWR with a
slightly different length.  Whether or not you can get a match on any particular frequency
depends on the matching range of your tuner.
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WX7G
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« Reply #3 on: October 11, 2012, 11:50:00 AM »

I will run simulations and post the results here. How long is the antenna now and where is the feedpoint?
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K8POS
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« Reply #4 on: October 11, 2012, 12:01:35 PM »

Current antenna is 230 ft.
Fed at the high corner 65' with a 4-1 balun at the antenna connection point.

I also have an ldg 100 pro tuner I can use but have not tried.
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VE3FMC
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« Reply #5 on: October 11, 2012, 12:45:48 PM »

Current antenna is 230 ft.
Fed at the high corner 65' with a 4-1 balun at the antenna connection point.

I also have an ldg 100 pro tuner I can use but have not tried.

Most internal tuners in radios can not match SWR's of 3:1 or higher. I am surprised your radio's tuner is able to tune that antenna on all the other bands.

Try the LDG and I bet it will tune the bottom of 80 meters with that antenna.
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AJ4WC
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« Reply #6 on: October 12, 2012, 10:01:05 AM »

When I built my 80M Skyloop, I used a 1:1 balun to match wide ranging impedances in multiband operation.  It works well on 80M thru 6M, including the very bottom of 80M, although it's a bit longer at 295 ft.  At 80M, I wonder if the 4:1 balun isn't making the antenna input impedance too low and more difficult for the tuner to match.  Just out of curosity, I would by-pass the balun and see what happens on 80M.
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KB4QAA
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« Reply #7 on: October 12, 2012, 02:24:01 PM »

I too suspect that your 4:1 balun is not appropriate near 80m.   Many of the autotuners don't like low impedances.   A 1:1 balun may give you better matches. 

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WB6BYU
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« Reply #8 on: October 12, 2012, 03:06:53 PM »

Quote from: KB4QAA

I too suspect that your 4:1 balun is not appropriate near 80m.   Many of the autotuners don't like low impedances.   A 1:1 balun may give you better matches. 



For a given impedance at the antenna, whether it is low or high at the tuner depends
on the feedline length.  If the problem is a low impedance at the tuner, then adding
50' of coax should fix it.

The 4 : 1 balun actually gives a better match on most bands.

The loop is rather short for the low end of 80m, however, so the SWR may be higher
than it can handle.  (Tuner range is often more limited on 80m and 160m and sometimes
10m and 12m than on the rest of the HF bands.)  Lengthening the loop will help, but
if that isn't practical than a bit of impedance matching in the shack should be all that
is needed.
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K8POS
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« Reply #9 on: October 12, 2012, 08:36:21 PM »

My antenna analyzer (MJF269) showed up today.
Will post some numbers in the morning.
Based on what everyone is saying and what I know I am beginning to suspect I am fighting a wire length
issue as well as a balun issue.
Just playing around with the 269 (still reading the manual) I am not finding a less than 2:1 swr ANYWHERE
on the HF bands.

Thanks for the input will post findings later.  May help others as well.

Bob
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WB6BYU
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« Reply #10 on: October 13, 2012, 09:43:13 AM »

Quote from: K8POS

Just playing around with the 269 (still reading the manual) I am not finding a less than 2:1 swr ANYWHERE
on the HF bands.



That's not unexpected.  For that loop length (well, somewhere close) the SWR at the antenna
dips below 2 : 1 around the following frequencies:

4.5 MHz
8.5 MHz
12.75 MHz
17 MHz
21.25 MHz
25.5 MHz
29.75 MHz

So on 15m and the very high end of 10m, you might see a low SWR.

On the ham bands, the SWR looks something like this:

80m: 30 : 1
75m:  10 : 1
40m:  12 : 1
30m:  20 : 1
20m:  20 : 1
17m:  7 : 1
15m:  1.3 : 1
12m:  4 : 1
10m:  12 : 1

That's all before adjusting for the loss in the coax, which will lower the SWR
at the shack end.

Your loop might not exactly match these numbers, as they depend on some
assumptions about the exact shape and the local ground conditions.


If we assume a perfect 4 : 1 balun (which isn't necessarily the case) and
put the impedance and feedline information into VK1OD's transmission line
loss calculator here:

http://vk1od.net/calc/tl/tllc.php

Then we find that we have about 6dB of loss on the low end of 80m and
the feepoint impedance at the shack is 9 - j45 ohms.

A 1uH coil across the feedline in the shack may move this to an impedance
where your tuner can match it, but you still have the 6dB of loss in the
feedline.
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K8POS
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« Reply #11 on: October 13, 2012, 12:03:20 PM »

Quote
On the ham bands, the SWR looks something like this:

80m: 30 : 1
75m:  10 : 1
40m:  12 : 1
30m:  20 : 1
20m:  20 : 1
17m:  7 : 1
15m:  1.3 : 1
12m:  4 : 1
10m:  12 : 1

Measured with the MFJ 269

80M       SWR      Xs     Rs
3.5         5.9      150   106
3.8         8.3       44     10
4.0         6.6       15       8
   
40M   
7.025      6.7       83      27
7.175      6.6       47      13     
7.300      6.3       28      10

20M
14.025    5.2       30       16
14.225    5.2        8        11
14.350    5.2        0        10

17M
18.068    5.2       39       17
18.110    5.2       46       20
18.165    5.2       56       24

15M
21.025    5.2         2       10
21.275    5.1        21      10
21.450    5.2        44      15

12M
24.890    4.9        58      20
24.930    4.9        66      24
24.990    4.9        82      34

10M
28.000    4.3        18      15
28.300    4.2        60      37
29.700    4.4        24      11

6M
50.100    2.9        44      37
51.500    3.0        18      19
52.500    3.2        52      33
53.500    2.7        49      43
54.000    3.0        73      74

Is rainy and windy today, but the numbers put me in the ball park.
Will have to take it down and put one up with the correct length of wire.
I am also suspecting a bad if not incorrect balun.  The Balun I used 4:1 (current) is rated for 300 Watts
but I only run 100 from the TS-2000 (30 watts in digital modes).  I know that still does not mean that it is good.

Will actually be easier to change the wire out in another week when the leaves are gone.
There is much conflicting data out there, one place implies using a 4:1 balun, some a 1:1, and a few I stumbled across that
said to direct feed it.  I know there is not one perfect antenna for all bands, but the loop is the first one I have put up that would
work (tune up) on most of them and seems to perform pretty well.
My yard layout makes putting up a fan dipole rather impossible with out a tall bucket truck to get it thru the trees.
I have run an 80M dipole, and a G5RV in the past with ok results on some of the bands and good on fewer.  But the loop has been the best so far.
And for those who say it is quieter, Can't prove it by me, just as noisy as the dipoles I have tried.

Thanks for the help everyone

And to Dale, looking at the link you gave me now.

Bob


     
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WX7G
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« Reply #12 on: October 14, 2012, 12:13:25 PM »

Bob, here are simulated results for a 282' loop up 45' above average ground. the length is set so that in the 20 meter band it's resonant at 14.150 MHz. It's fed thru a 4:1 balun.

3.65 MHz   1.8:1
7.05 MHz   1.3:1
10.65 MHz 1.9:1
14.15 MHz 1.1:1
17.55 MHz 1.3:1
21.10 MHz 1.3:1
24.70 MHz 1.3:1
28.05 MHz 1.4:1


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WX7G
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« Reply #13 on: October 14, 2012, 01:47:22 PM »

K8POS, your VSWR numbers do not match the R and X numbers. Something is wrong with your MFJ-269.
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WB6BYU
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« Reply #14 on: October 14, 2012, 04:01:58 PM »

Quote from: K8POS

There is much conflicting data out there, one place implies using a 4:1 balun, some a 1:1, and a few I stumbled across that said to direct feed it.  I know there is not one perfect antenna for all bands, but the loop is the first one I have put up that would work (tune up) on most of them and seems to perform pretty well.



The plural of "anecdote" is not "data".

There are examples of people feeding loops many different ways, and I've probably tried
all of them (and a few more) over the years, but most of those are not accompanied with
actual data for the SWR curves,  feedline losses, etc.

Typically a full wave horizontal is about 100 - 120 ohms or so on the fundamental, rising
on the higher resonances.  While the 4 : 1 balun doesn't give a perfect match on 80m (a
quarter wave transformer of 75 ohm coax is better for single band use) it does give a very
usable SWR on multiple HF bands, much better than you will get with direct feed.  SWR
may be high on 60m, 30m, and the upper half of 10m, but otherwise the losses should be
reasonable even on the bands where you still need a tuner.



Quote

And for those who say it is quieter, Can't prove it by me, just as noisy as the dipoles I have tried.



Noise is an electromagnetic radiation, just like desired signals, and any antenna picks up both.

But if someone is comparing it to an antenna without an effective balun that is picking
up RF noise as the feedline passes though the house, they might notice a difference.
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