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Author Topic: 40m inverted delta loop  (Read 1130 times)
K1DBO
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Posts: 20




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« on: December 04, 2017, 01:30:38 PM »

I got sick of reading about what I should do and just went out to the backyard and did something.  Now, I'm trying to make sense of what I've done.  That's where you kind folks come in.

The goal was to get a better take off angle for 40m DX.  My "reference" for the experiment is a multi-band parallel dipole (DX-CC) configured as an inverted V with the apex around 45ft.  I dont have a lot of space in my lot and didnt want to take the DX-CC down for testing the loop. So, I hung the loop about 10ft under the dipole.  The loop is nearly flat across the top and about 45ft wide but does have an apex centered about 5ft above the top corners.  The sides drape down and slope away a bit but never get closer than 7ft to the ground.  Total length of the loop is about 140ft give or take.  It's fed at one of the top corners with 40ft of 450ohm window line that heads back towards the center of the loop and enters an attic window where's it's connected to a balun, then 50ft of LMR400 to a KAT-500 autotuner.

I've tried a few baluns: 1:1, 4:1, and 6:1.  The 6:1 seems to give the lowest SWR across the most bands.  The KAT-500 will match it on 40m, 20m, 15m, 10m and even 80m with either the 4:1 or the 6:1.  My use of the ladder line was to avoid putting the weight of the balun out at the end of the spreader and also to make it easier to experiment with.  It wasnt until after I cut the ladder line and started testing/tuning that I realized the ladder line was part of the system and that I probably should have thought about its length. 

I've done some RBN tests to compare the loop to the dipole.  In many cases, especially on 40m and 80m, it's heard by more and farther stations than the dipole.  But, there seem to be very few stations that hear both antennas.  I'm sure I'll get around to modeling this but thought I would see if I could get a discussion going to help guide me through this journey.  At this point I dont have any specific questions, except perhaps, why does it work at all on 80m?
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WB6BYU
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Posts: 17195




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« Reply #1 on: December 04, 2017, 04:57:20 PM »

Quote from: K1DBO

I got sick of reading about what I should do and just went out to the backyard and did something...



Congratulations!  An excellent way to get started.




Quote

Now, I'm trying to make sense of what I've done...



I built an EZNEC model based on your description, though there is no guarantee that mine really
looks much like yours.  But the width of the top and the approximate heights of the top and bottom
should be close.

First, the optimum feed location for vertical polarization is half the distance between the top center
and the bottom center, regardless of the shape of the loop.  With my assumptions of the dimensions,
your feedpoint is only about 23' from the top.  If the loop is 140', then it should be another 12' down
the wire to maximize the vertical polarized radiation.

The radiation off the edges of the loop will be primarily vertically polarized, and will be weaker in
the direction away from the feedpoint.  (This is because the loop is acting like a sloping dipole due
to the location of the feedpoint.)  For my model, there is about a 10 dB difference between the two
sides at a 45 degree elevation angle, and about 5 dB at 15 degrees.

Comparing the plot of this loop with that of an inverted vee at 45', in the broadside direction the
loop is slightly better below 15 degrees, and perhaps 6 dB better at 5 degrees, but that value is
greatly dependent on your local ground conditions.  For high angle signals the inverted vee is
5 - dB stronger.

Of the sides of the loop and the dipole (assuming they are oriented in the same direction) the
loop is better from about 35 degrees down to 5 degrees on the side with the feedpoint, with
a maximum difference of about 8 dB at 10 degrees (though it is getting hard to read the plot
at that point.)  On the other side, due to the dimple in the pattern, the inverted vee is better
down to about 17 degrees, and the loop is no more than about 3 dB better.

One thing you might try doing is to print out an Azimuthal Map centered on your location
and plot the relative locations of the stations and which antenna they can hear best to see
if that gives you an idea of the relative patterns of the two antennas.

But there is another possibility as well:  propagated waves are subject to random polarization
in the ionosphere.  Changing the polarization of your transmitted signal will change the
polarization at the other end, and one or the other may better match the polarization of the
receive antenna.  Over time these tend to level out, but that is one of the causes of random
fades:  the ionosphere is changing the polarization shift over time, and at any moment one
or the other polarization may happen to be better for the path.  So if you go back and compare
the two antennas the next night, or even 5 minutes later, you may find the opposite result for
the same station.



Quote

At this point I dont have any specific questions, except perhaps, why does it work at all on 80m?



Probably due to losses in the LMR400.

The antenna still radiates on 80m, of course, and primarily vertical polarization, but it has a low
radiation resistance (high maximum current to radiate much power) and a high feedpoint impedance
(tens of thousands of ohms) because it is a half wave loop.  The 400 ohm line acts as a transformer
and steps this down to an impedance of about 3 - j200 ohms at the balun.  (That's assuming no losses
in the ladder line, but the SWR is high enough that the losses should be taken into account.)

Now I have no idea what the balun is going to do with an impedance of that sort - it probably don't
provide the impedance transformation ratio marked on the case (since any step-down is going to make
the impedance even harder to match.)  However, a poor balun may provide enough shunt reactance to
actually step the impedance up rather than down, which would lower the SWR on the coax.  We
can only guess about that without measuring the SWR right at the balun connection.

Then there is a further impedance transformation in the LMR400, as well as losses due to high SWR.

It's not surprising that a tuner can match it on 80m, as many tuners can match a shorted piece of
coax if it is lossy enough.

And, of course, the dipole is comparatively lower to the ground (in wavelengths) on 80m, so it won't
do as well at low angles, though you may also have the polarization factor as discussed above.


Does that give you a start to the discussion?
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K1DBO
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Posts: 20




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« Reply #2 on: December 04, 2017, 09:34:10 PM »

Does that give you a start to the discussion?

It sure does!  Thanks.

When I started playing with the idea of trying to get better low angle radiation than what my inverted v could do, my first thought was to try an elevated vertical and to use the DX-CC as radials.  This would have put the radials at about 35 feet and the top of the vertical 30ft or so above them (wire inside a 12m spiderbeam pole).  I didnt go that route because the modeling I did didnt show any real advantage.  Instead, the idea of a loop stuck in my head.  I built some models (4nec2) for that next but mostly as a replacement for the inverted V.  This version of the loop was more "house shaped" (classic stick figure house).  I tried feed points in a few different places and mostly the model responded as expected.  But, when compared to the inverted V, the results again were not very promising.  When the far field was overlaid with the same from the inverted V, the inverted V plot would surround anything else I tried. 

Eventually I realized that if I inverted the loop and hung it under the inverted V I could keep both in play for experiments.  (yes, both have the same orientation) I didnt even bother to model it for fear of talking myself out of the project.  So, as noted in my previous post, I went and built it anyways.  Your response convinced me to build the model.  Again, however, the inverted V dominates the loop except for the side without the dimple where the loop starts to win at about 15 degrees and down, but not by much.  By the time it hits 30 degrees the inverted V is up by almost 6db.  Our numbers differ enough that I'm inclined to believe yours.  So all of this makes me question my modeling skills. 

(Of course now, I just realized why there are so few pictures posted here.  I have one ready to go but nowhere to host it.)

Your comments on the 80m question give me a lot to think about.  The SWR numbers I'm seeing at the tuner are 10.3:1 @3.5mhz, 19.9:1@3.9mhz.   The antenna analyzer 1ft from the balun shows 35:1 and 30:1 at the same frequencies.  So yes, the loss in the LRM400 certainty is in play.  My RBN tests usually show the loop doing better than the inverted V. This, I guess, should be expected because the DX-CC is not full length on 80m so has very narrow bandwidth.  For most of the band, the losses are likely similar with the vertical polarization of the loop giving it the advantage.

It sounds like I have a lot of work to do.  I'd like to get my modeling skills to the point where I can reason about the model and the results I'm seeing on the antenna analyzer.   

Thanks again for your help.
 

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WB6BYU
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Posts: 17195




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« Reply #3 on: December 04, 2017, 10:50:13 PM »

I had to make a lot of assumptions about dimensions, and such things as the ground conductivity
can make a significant difference, too, especially for the difference between vertical and horizontal
polarization.

However, we also haven't considered the interactions between the two antennas, since they are
so close together, and the feedlines running through the middle of the antennas.  Certainly if the
antennas aren't fed through effective baluns, that can change the patterns due to radiation from
the shields.  That may be in phase or out of phase with the radiation from the antenna when
received at a specific station.  I certainly know of cases where it greatly improved the DX performance
of a low dipole, and others where it raised the angle of radiation of a vertical.

If you have a more accurate model of the loop dimensions I'll be happy to try it out and see how it
compares with what I ran.
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K1DBO
Member

Posts: 20




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« Reply #4 on: December 05, 2017, 06:35:50 PM »

I had to make a lot of assumptions about dimensions, and such things as the ground conductivity
can make a significant difference, too, especially for the difference between vertical and horizontal
polarization.

The assumptions you made seemed pretty close to me.  I dont know what to say about ground conductivity.  For modeling I've been using the defaults.  I tend to ignore as many variables as I can right up to the point where I realize I cant any more.  The trouble there of course is reaching the realization. 

However, we also haven't considered the interactions between the two antennas, since they are
so close together, and the feedlines running through the middle of the antennas.  Certainly if the
antennas aren't fed through effective baluns, that can change the patterns due to radiation from
the shields.  That may be in phase or out of phase with the radiation from the antenna when
received at a specific station.  I certainly know of cases where it greatly improved the DX performance
of a low dipole, and others where it raised the angle of radiation of a vertical.

I did put a small amount of thought into the potential for interactions.  Prior to putting up the loop I ran a SWR sweep of most bands from 80m to 10m on the inverted V.  After I put up the loop I did the same thing and noticed a bit of change but nothing crazy.  How that change shows up in the radiation pattern is still a question. 

The inverted V has 1:1 DXE current balun at the feedpoint.  I hope it's worth at least half what I paid for it Wink  I think I already mentioned that the balun for the loop is in the attic at the end of 40ft of ladder line.  While making my own is an option, it's back to that problem of eliminating variables.  At this point in the game I'm willing to trust their engineering.  The other baluns I mentioned are theirs as well.  RFI in the house is minimal to non-existant, so I suspect they are helping.

If you have a more accurate model of the loop dimensions I'll be happy to try it out and see how it
compares with what I ran.

I appreciate the offer.  Below you'll find the model I built for 4nec2.   I think you mentioned EZNEC.  You would know better than me if EZNEC can import it.  If not, the geometry information should be pretty easy to extract.  I've also included the model I am using for my inverted V.  The DX-CC also covers 80m with a loading coil at the end of the 40m wire.  Modeling that is beyond my skill set.  Feed points are still a bit of a mystery too, but I think I've worked out a coping strategy.

Both models have been parameterized to work within my constraints.  I have a very small lot with the house taking up most of it.  The antennas are strung from three jackite poles, two horizontal and one vertical.  The horizonal poles extend about 25ft from the center of the house nearly to the property line.  Currently the vertical pole extends about 15ft above the horizontals.  The three poles come together about 5ft below the peak of my roof which is about 35ft above ground. With a stiffer pole or a lighter balun, the vertical pole could go a bit higher.  The model for the loop include a bend part way down to help approximate the curve of the drooping wire.  I didnt bother to check if it changed the pattern at all, but to my eye it more closely approximates the look of the antenna and consumes a few more feet of wire.




CM mostly vertical delta-ish loop for 40m
CM 
CM K1DBO -- 12/2017
CE

SY freq=7.2

SY top_z=40ft         'height of top vertex

SY spreader_len=23ft      'spreaders extend spreader_len to each side from center
SY spreader_dz=10ft      'spreaders are spreader_dz feet below the top vertex

SY bottom_z=7ft         'height of bottom wire (both bottom vertices)
SY bottom_x=20ft      'distance along X between bottom vertex and center support (determines slope)
SY bottom_len=10ft      'length of bottom wire (use this parameter for tuning the length of the loop)

SY FP=.25ft         'feedpoint wire length

SY x1=0
SY y1=0
SY z1=top_z

SY x2=x1
SY y2=y1-spreader_len
SY z2=z1-spreader_dz

SY x4=bottom_x
SY y4=y1-(bottom_len/2)
SY z4=bottom_z

SY x5=bottom_x
SY y5=y1+(bottom_len/2)
SY z5=bottom_z

SY x7=x1
SY y7=y1+spreader_len
SY z7=z1-spreader_dz

SY x3=x2-(x2-x4)*.25      'fudge a point between bottom and spreaders to approximate the wire slope
SY y3=y2-(y2-y4)*.25
SY z3=z2-(z2-z4)*.75

SY x6=x7-(x7-x5)*.25      'fudge a second point between bottom and spreaders
SY y6=y7-(y7-y5)*.25
SY z6=z7-(z7-z5)*.75

GW 0 55   x1  y1  z1     x2  y2  z2+FP   0.0012941   ' wire 1
GW 1 3    x2  y2  z2+FP  x2  y2  z2-FP   0.0012941   ' short wire for feedpoint
GW 0 55   x2  y2  z2-FP    x3  y3  z3      0.0012941   ' wire 2
GW 0 55   x3  y3  z3     x4  y4  z4      0.0012941   ' wire 3
GW 0 55   x4  y4  z4     x5  y5  z5      0.0012941   ' wire 4
GW 0 55   x5  y5  z5     x6  y6  z6      0.0012941   ' wire 5
GW 0 55   x6  y6  z6     x7  y7  z7      0.0012941   ' wire 6
GW 0 55   x7  y7  z7     x1  y1  z1      0.0012941   ' wire 7

GE 0

EX 0   1   2   0   1   0
GN 2   0   0   0   13   5.e-3
FR 0   1   0   0   freq
EN





CM three band inverted V with fixed length spreaders
CM excess length of longest band spills off the ends and goes vertical
CM
CM K1DBO -- 12/2017
CE

SY freq=7.1
SY FP=.33ft

SY height=45ft      ' feedpoint height

SY radl_40=33.5ft   ' element lengths
SY radl_20=17.1ft
SY radl_10=8.25ft

SY spreader_dx=0
SY spreader_len=25ft      'spreaders extend spreader_len feet to each side of feedpoint
SY spreader_dz=-10ft      'vertical distance between spreaders and feedpoint

SY space=-0.5ft      'spacing between elements

SY vm=(spreader_dx^2+spreader_len^2+spreader_dz^2)^0.5   'compute unit vector to wire ends
SY vx=spreader_dx/vm
SY vy=spreader_len/vm
SY vz=spreader_dz/vm

SY rmax=vm
SY s1_len=min(rmax,radl_40)
SY s2_len=max(radl_40-s1_len,FP)

SY r1_x1=0+FP*vx      'start of first 40m wire
SY r1_y1=0+FP*vy
SY r1_z1=height

SY r1_x2=r1_x1+s1_len*vx      'end of first 40m wire
SY r1_y2=r1_y1+s1_len*vy
SY r1_z2=r1_z1+s1_len*vz

SY r2_x1=0-FP*vx      'start of 40m second wire
SY r2_y1=0-FP*vy
SY r2_z1=height

SY r2_x2=-s1_len*vx      'end of second 40m wire
SY r2_y2=-s1_len*vy
SY r2_z2=r2_z1+s1_len*vz

SY r3_x2=r1_x2         'end of first 40m dangler
SY r3_y2=r1_y2
SY r3_z2=r1_z2-s2_len

SY r4_x2=r2_x2         'end of second 40m dangler
SY r4_y2=r2_y2
SY r4_z2=r2_z2-s2_len


SY r5_x1=r1_x1      'start of first 20m wire
SY r5_y1=r1_y1
SY r5_z1=r1_z1+space

SY r5_x2=r5_x1+radl_20*vx      'end of first 20m wire
SY r5_y2=r5_y1+radl_20*vy
SY r5_z2=r5_z1+radl_20*vz

SY r6_x1=r2_x1      'start of 20m second wire
SY r6_y1=r2_y1
SY r6_z1=r2_z1+space

SY r6_x2=-radl_20*vx      'end of second 20m wire
SY r6_y2=-radl_20*vy
SY r6_z2=r6_z1+radl_20*vz

SY r7_x1=r5_x1      'start of first 10m wire
SY r7_y1=r5_y1
SY r7_z1=r5_z1+space

SY r7_x2=radl_10*vx      'end of first 10m wire
SY r7_y2=radl_10*vy
SY r7_z2=r7_z1+radl_10*vz

SY r8_x1=r6_x1      'start of 10m second wire
SY r8_y1=r6_y1
SY r8_z1=r6_z1+space

SY r8_x2=-radl_10*vx      'end of second 10m wire
SY r8_y2=-radl_10*vy
SY r8_z2=r8_z1+radl_10*vz


GW 1 3 r1_x1  r1_y1  r1_z1     r2_x1  r2_y1  r2_z1   0.0012941   ' feed point

GW 0 1 r1_x1  r1_y1  r1_z1     r1_x2  r1_y2  r1_z2        0.0012941   ' first 40m wire
GW 0 1 r1_x2  r1_y2  r1_z2     r3_x2  r3_y2  r3_z2   0.0012941   ' first 40m dangler

GW 0 1 r2_x1  r2_y1  r2_z1     r2_x2  r2_y2  r2_z2        0.0012941   ' second 40m wire
GW 0 1 r2_x2  r2_y2  r2_z2     r4_x2  r4_y2  r4_z2   0.0012941   ' second 40m dangler

GW 0 1 r5_x1  r5_y1  r5_z1     r5_x2  r5_y2  r5_z2        0.0012941   ' first 20m wire
GW 0 1 r6_x1  r6_y1  r6_z1     r6_x2  r6_y2  r6_z2        0.0012941   ' second 20m wire

GW 0 3 r1_x1  r1_y1  r1_z1     r5_x1  r5_y1  r5_z1    0.0012941
GW 0 3 r2_x1  r2_y1  r2_z1     r6_x1  r6_y1  r6_z1    0.0012941

GW 0 1 r7_x1  r7_y1  r7_z1     r7_x2  r7_y2  r7_z2        0.0012941   ' first 10m wire
GW 0 1 r8_x1  r8_y1  r8_z1     r8_x2  r8_y2  r8_z2        0.0012941   ' second 10m wire

GW 0 3 r5_x1  r5_y1  r5_z1     r7_x1  r7_y1  r7_z1    0.0012941
GW 0 3 r6_x1  r6_y1  r6_z1     r8_x1  r8_y1  r8_z1    0.0012941

GE 0

EX 0   1   2   0   1   0
GN 2   0   0   0   13   5.e-3
FR 0   1   0   0   freq
EN




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WB6BYU
Member

Posts: 17195




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« Reply #5 on: December 05, 2017, 10:03:22 PM »

Pretty close, actually.  My intermediate point was at 0, 18, 12 instead of 5, 20, 13.
But I didn't have the bottom pulled out in X, and that affects the pattern somewhat.

I still see some asymmetry due to the feedpoint position, but it isn't too strong.
I'm using Norton-Sommerfield ground at ( 0.005, 13 ) or "average", though that is
better than what we often run into around here.

As expected, the antenna is a mixture of vertical and horizontal polarization, a bit
more so than my first model due to the X offset at the base.

Minimum current is about your point 4, which is 37' from the feedpoint (should be
equidistant between top center and bottom center for maximum vertical polarization.)
Moving the feedpoint 10' down towards point 3 is much easier to do on my model than
outside at the spreaders.  That might give you an extra 1 dB in the peak direction,
and much less horizontally polarized radiation in the plane of the loop.  The "sloping
wire dimple" is still there, but is now broadside to the loop due to the pull-out at
the base.

Probably worthwhile running further tests on the current antenna before you move
the feedpoint.
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K6UJ
Member

Posts: 1138




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« Reply #6 on: December 06, 2017, 08:04:24 AM »

Very interesting study guys.   Like was mentioned earlier I am curious about any interaction between the DX-CC and the loop.  The SWR reading didn't show significant impact but there may be an effect on the radiation pattern of the loop.  Would be great to see the radiation pattern with both of them together compared to just the loop alone.   If possible can you do a modeling study with both antennas together ?
I am a work in progress with EZNEC and would try it myself but too much for me to try  Cheesy

Bob
K6UJ
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WB6BYU
Member

Posts: 17195




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« Reply #7 on: December 06, 2017, 10:53:31 AM »

When optimally fed for vertical polarization, there is minimum current at the top of the loop.
That should reduce the coupling (though because it is a high voltage point there is more
opportunity for a change in tuning.)

I suspect the most significant interaction will be the (vertical) feedline for the dipole hanging
down through the (vertically polarized) loop, but that will depend on the length and alignment
of the cable.  I could make some guesses...


4NEC2 does have the advantage that it allows for the use of variables, a great boon when
dealing with something like a square quad so you don't have to change as many places
as you adjust the size.  (It also allows for an optimizer to adjust the variables automatically.)
I haven't used it much - I had it on my work computer for a while.  (EZNEC is running on our
only Windows machine, which has no internet connection - that's the only thing I use the
computer for at this point, and it makes it a bit awkward to import or export files.)

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K1DBO
Member

Posts: 20




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« Reply #8 on: December 06, 2017, 11:45:48 AM »

My plan for the evening is to try and combine my two models and see how the radiation patterns compare to the individual models.


I suspect the most significant interaction will be the (vertical) feedline for the dipole hanging
down through the (vertically polarized) loop, but that will depend on the length and alignment
of the cable.  I could make some guesses...

...and if you think the interaction with the feedline from the dipole will be significant, I should probably mention the 25ft run of 1-5/8" strut channel that stands vertically in the center of the loop.  It's the mast that supports the three kite poles.  I should be able to put it into the model too.  But that begs the question of where to stop?  My attic which is also near the center of loop has A/C ducting and then there are the aluminum gutters on the side of house that are in the near field too.

4NEC2 does have the advantage that it allows for the use of variables, a great boon when
dealing with something like a square quad so you don't have to change as many places
as you adjust the size.  (It also allows for an optimizer to adjust the variables automatically.)
I haven't used it much - I had it on my work computer for a while.  (EZNEC is running on our
only Windows machine, which has no internet connection - that's the only thing I use the
computer for at this point, and it makes it a bit awkward to import or export files.)

Based on it's popularity I gave EZNEC a couple of tries and found it anything but easy.  Playing with models that I found was pretty straight forward, but changing them was far too tedious.  I gave up on modeling and planned to build my own parametric geometry editor to create EZNEC input files.  Luckily, before getting around to that, I stumbled across 4nec2.  The optimizer, variables, and decent enough math library are just what I thought was missing from EZNEC.  But, as a software developer, these are tools of my trade.  I might still write my own editor but at least now I feel like I can do some modeling without it.
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AC6LA
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Posts: 134




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« Reply #9 on: December 06, 2017, 01:27:09 PM »

... a great boon when dealing with something like a square quad so you don't have to change as many places as you adjust the size.

Dale, are you aware of EZNEC's "Change Loop Size" feature?  From the Wires window select Wires > Change Loop Size.  That will get you this dialog window.



From there you can enter the start and end wire numbers for an existing loop.  The loop can be part of a larger collection of wires and the loop can have any number of sides.  As long as the entered wire numbers form a continuous loop you will next get this dialog.



You can then choose to modify any of three different metrics and you can apply any of three different operations to the chosen metric.  If you have the View Ant window showing you can see the results of your changes as you make them.  If the change was not what you intended you can use Edit > Undo from either the Wires window or the View Ant window.

Dan, AC6LA
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WB6BYU
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Posts: 17195




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« Reply #10 on: December 06, 2017, 03:11:54 PM »

Thanks, Dan.

Maybe one of these days I'll upgrade to a more modern version...
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K1DBO
Member

Posts: 20




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« Reply #11 on: December 06, 2017, 07:26:26 PM »

I combined the two models and made some comparisons to the uncombined models.  It was easier than I would have guessed.  The only "trick" was to disable the feed point of the antenna not being fed.  The first thing that struck me was that the dipole pattern was distorted in about the same way as the loop's pattern is distorted due to it's feed point location (that dimple mentioned in previous posts). This confuses me a bit and made me guess that leaving the unused feed point in place as a short would give different results than if it was left open.  I tried both.  While results did differ, the basic nature of the interaction didnt change much.  Now, there is a small asymmetry in the model due to the way I modeled the feed point.  It's in there as a small 6" wire that doesnt exist on the other side of the antenna.  But it's hard to believe that a difference that small could matter so much, so until I model it, I'll continue to doubt that it does.

I wish I could post a picture because I dont think I can describe the distortion any better than to say that for 40m with the loop under it, the dipole is slightly squished towards the dimple and between 2db and 2.5db down all around.  20m has similar loss and looks similar.  15m was nearly unchanged while 10m actually picked up 2db below 15 degrees.  Going the other way and feeding the loop but leaving the dipole above it, isnt so bad.  The dimple is exaggerated for the worst by about 4db but below 30 degrees, there is almost not change at all. 

I'm really not happy with way the dipole is affected by the loop on 20m. Switching to the loop for 20m doesnt add anything.  The gains on 40m are minimal as well.  While I'm not in a big hurry to take the loop down, I really dont see much of a reason to keep it in place either.

Dale made the suggestion to move the feed point around.  I'll put some effort into modeling that and if I find anything promising I'll give it a try out in the antenna garden.

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W5DXP
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« Reply #12 on: December 06, 2017, 08:17:57 PM »

But it's hard to believe that a difference that small could matter so much...

Don't know if I'm understanding what you are saying but one antenna's effect on another is a lot like the effect of guy wires on a vertical antenna. One of the worst length for guy wires is 1/2WL, i.e. resonant. That's why long guy wires need insulators installed in them - to avoid any resonance effects. Not everyone realizes that a 1/4WL wire open at both ends is not resonant.
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WB6BYU
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« Reply #13 on: December 06, 2017, 09:23:42 PM »

Quote from: W5DXP

...Not everyone realizes that a 1/4WL wire open at both ends is not resonant.



And a full wave loop is still resonant (or close to it) when the feedpoint is either
open or shorted.


If you want to experiment with detuning the loop, try putting a coil or capacitor
across the feedpoint rather than a short or open.   You can do this in the shack (or
at the tuner) as well, though the length of the feedline to that point will affect what
load works best.
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K1DBO
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« Reply #14 on: December 07, 2017, 07:35:44 AM »

But it's hard to believe that a difference that small could matter so much...

Don't know if I'm understanding what you are saying but one antenna's effect on another is a lot like the effect of guy wires on a vertical antenna. One of the worst length for guy wires is 1/2WL, i.e. resonant. That's why long guy wires need insulators installed in them - to avoid any resonance effects. Not everyone realizes that a 1/4WL wire open at both ends is not resonant.

The small difference I was referring to was the small (6") wire segment I added to implement the feed point on the loop.  The loop is symmetric otherwise.  What surprised me was not that the loop interacted with the dipole but the way in which it did.  The distortion in the dipole's pattern is similar to the pattern the loop creates when feed.  The feed point of the loop seems to determine where that dimple is in pattern.  Intuitively, that makes sense to me.  That is, the place where the current is applied should look different than the place where it isnt.  But, in the combined model, only the dipole was driven.  The feed point of the loop was simply another short wire segment, no different than the rest of the loop.  Given that the loop is symmetric, I expected it's affect on the dipole to be symmetric... it wasnt.
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