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Author Topic: One - Do- Tree... Tests with the 2x2x2 40M Yagi stack  (Read 202766 times)
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WA1GFZ
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« Reply #150 on: September 22, 2009, 12:23:23 PM »

the VSWR is flat at 7.4 MHz. so it should work fine once I reinstall the wire I lopped off. My simulation says force fed better FB and loops even better.
I let the program loose on top fed loops and it configured it self as folded dipoles with the bottom a foot below the feed point. I need to play with that one since it gives loop FB performance with higher average height. VSWR wasn't great though
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K1JJ
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« Reply #151 on: September 22, 2009, 12:45:26 PM »

Regardless of what the computer and swr says, in the end, expect to spend time up on the tower tuning for best front to back. Put a beacon out in the woods a few wavelengths away or use a receiver tuned into some W4's to null it out. Pull that sweet spot f-b into the 7150 area.

I used my cell phone up on the tower to tune the 75M delta loops. I listened to my inside house telephone that sat by the receiver speaker tuned to a beacon I placed out in the woods 1/4 mile away to the SW.  My stubs were off and I quickly pruned them to get the beacon into the noise. Since the beacon is at  zero degrees on the horizon, (actually negative elevation)  it wasn't a perfect representation of the real whirl at 10-30 degrees on the air. But still, my actual f-b reports came in at 20-30db and I could tell by tuning the receiver around and switching on the W4's where the sweet spot was.   Fine tuning needs to be done for max performance unless the system is very broadbanded.  Parasitic elements are usually tuned sharply for a narrow range and need this extra effort.  Mud ducks need not apply... Grin

T
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Steve - WB3HUZ
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« Reply #152 on: September 22, 2009, 01:27:39 PM »

Don't get too wound up about F/B numbers at any one elevation angle. You can play with things to make a big null at one angle and have crap elsewhere. Look at the entire back side of the pattern "optimize" it. Also look at the high angle respone on both the front side and the back side of the pattern. Minimizing the response in these areas will pay far more dividends in receive improvement than a big null at some single lower elevation angle. In other words, unless you have a lot of local noise, trade off lower angle F/B for better higher angle F/B.

F/B at a zero elevation angle is worthless, unless you are trying to remove local groundwave propagated noise or doing BC band DXing. Select an angle like 30 degree to work with. You can still do this on the ground. Just set your signal source 30 degrees off the back of the array (e.g instead at 180 degrees, put the source at 210 or 150 degrees azimuth). Tuning for a null this way will also give you a null at 30 degrees elevation.

Quote
I let the program loose on top fed loops and it configured it self as folded dipoles with the bottom a foot below the feed point. I need to play with that one since it gives loop FB performance with higher average height.

I think you may have a modeling problem there. Folded dipoles will act no differently than a single wire dipole in an array when considering F/B or gain (other than maybe some loss reduction in a low-Z situation). Real loops will have greater coupling than dipoles, thus the currents are more nearly equal. This is what yields the higher F/B.
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K1JJ
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« Reply #153 on: September 22, 2009, 01:51:49 PM »

Just set your signal source 30 degrees off the back of the array (e.g instead at 180 degrees, put the source at 210 or 150 degrees azimuth). Tuning for a null this way will also give you a null at 30 degrees elevation.


The beacon in the woods antenna is limited to about 50' high in the trees. So, it will always be at zero degrees or less cuz the main antenna is much higher on the tower.  I COULD float a balloon up to 1000' to simulate a higher incoming angle... Wink

I've found tuning the reflector at zero degrees elv to be a FAIRLY good representation of the real whirl angles at 30 degrees elv, etc.  I use zero degrees elv beacon for getting in the ballpark cuz a long, stable tone is needed without fading - to fiddle around. The fine tuning is listening to the W4's off the back on the receiver  as the array is switched.


BTW, I don't follow how when a source (beacon)  is at zero degrees elevation, how moving it to one side or the other will represent a higher elevation angle to the antenna.  A source at zero degrees is still a source at zero degrees elevation, just off to the side, right?  Please explain the geometry, OM... Grin

T

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Steve - WB3HUZ
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« Reply #154 on: September 22, 2009, 03:28:06 PM »

I won't go into all the trig here but the short answer is that with most simple arrays, if there is a null at a particular elevation angle, there is a corresponding one at zero degrees elevation and azimuthally (is that a word) at 180 degrees + the null elevation angle. Don't believe me?

Fire up your modeling program. Take a look at a two-element array. Play with the phasing. You'll see the elevation null angle change. You'll also see two nulls move around on the azimith pattern when looking at zero degrees elevation.

Just set your signal source 30 degrees off the back of the array (e.g instead at 180 degrees, put the source at 210 or 150 degrees azimuth). Tuning for a null this way will also give you a null at 30 degrees elevation.


The beacon in the woods antenna is limited to about 50' high in the trees. So, it will always be at zero degrees or less cuz the main antenna is much higher on the tower.  I COULD float a balloon up to 1000' to simulate a higher incoming angle... Wink

I've found tuning the reflector at zero degrees elv to be a FAIRLY good representation of the real whirl angles at 30 degrees elv, etc.  I use zero degrees elv beacon for getting in the ballpark cuz a long, stable tone is needed without fading - to fiddle around. The fine tuning is listening to the W4's off the back on the receiver  as the array is switched.


BTW, I don't follow how when a source (beacon)  is at zero degrees elevation, how moving it to one side or the other will represent a higher elevation angle to the antenna.  A source at zero degrees is still a source at zero degrees elevation, just off to the side, right?  Please explain the geometry, OM... Grin

T


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flintstone mop
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« Reply #155 on: September 22, 2009, 03:48:33 PM »

The delta loops are working FB, Tom.  It's almost 8AM here in SV and yer still S-9.  Before sunrise you were stronger than the Italians - and that's quite a message!  Now I can't hear most of the EURs at all.

I thought maybe 75M was dying out cuz of 40M activity, but later in the night (11:30 PM local)  the Euro pileup grew so thick and deep I had trouble pulling out a letter sometimes. I'm still recovering after holding court continuosly for about 4 hours last night. Gawd, it was fun! There was latent demand built up like a sewer pipe I guess. I took out my screwdriver and unplugged that little hole in the bowl and all hell broke loose... Grin


It's was prolly like 3 months of no sex. When the opportunity arose it was a great release.

FRED
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K1JJ
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« Reply #156 on: September 22, 2009, 05:07:47 PM »

Fire up your modeling program. Take a look at a two-element array. Play with the phasing. You'll see the elevation null angle change. You'll also see two nulls move around on the azimith pattern when looking at zero degrees elevation.

Interesting.  I've seen the nulls move around, but didn't realize there's a direct numerical correlation between  az and alt that would be useful for beacon tuning.

Thanks for the hot tip, OM!

T


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K1JJ
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« Reply #157 on: September 22, 2009, 06:54:09 PM »

I was out hiking today with Yaz and spotted the 190' towers from one mile away. This is a telephoto shot. (2nd picture)  Look closely to the right side and you will actually see the 40M Yagi elements.  The bottom Yagi at 62' is below the treeline - the other two are clearly above. We are standing Northeast of the towers - the Yagis are beaming towards us, towards Europe.    Also, to the left, see the boom of the 75M delta loops.

The first picture is from the top of the same 40M Yagi tower looking towards Europe from 190'. Today's picture was taken somewhere in the middle of the 1st picture frame in the woods at 500' elevation. The tower bases sit at 805' elv on top of a hilltop. The ground stays flat for about 600' then rolls off into the valley in our direction - clear shot towards Europe.  There's an airplane in the first picture... Grin

Expand the 2nd pic for a better view, if your software allows it. You will see the two elements on both Yagis.

T


* K1JJ-View Northeast To Europe.jpg (45.07 KB, 640x480 - viewed 493 times.)

* K1JJ-Antennas-One Mile Away.jpg (376.24 KB, 1251x929 - viewed 493 times.)
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There's nothing like an old dog.
Ralph W3GL
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« Reply #158 on: September 22, 2009, 07:45:03 PM »



   That's an airplane?  What's that to the left of it, a fly spec on my screen?
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73,  Ralph  W3GL 

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« Reply #159 on: September 22, 2009, 08:01:55 PM »



   That's an airplane?  What's that to the left of it, a fly spec on my screen?


Yeah, I see the spec too - there's actually FOUR of them buzzing the airplane. After the picture, the airplane disappeared into thin air. This is solid scientific proof that UFOs exist and we have been harrassed by ET's for thousands of years.

(There's also one hovering above the right tower... taking an RF batt charge, I  assume.)

T
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Steve - WB3HUZ
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« Reply #160 on: September 22, 2009, 08:02:52 PM »

Great shots OM. Gives a good perspective on why you strap into EU.
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Steve - WB3HUZ
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« Reply #161 on: September 22, 2009, 09:25:30 PM »

They say a pix is worth a thousand word, so I made some pix.

As discussed earlier, when adjusting your array for best F/B, you have to pick an elevation angle for that best F/B. Then you measure the F/B not off the geometrical back of the array (180 degrees), but offset in degrees equal to the desired optimum F/B elevation angle.

Take a look at the two plots below. These are of two 40 meter dipoles at 60 feet about average ground, spaced one-quarter wavelength and fed 125 degrees out of phase. As you can see in the elevation plot, there is a null at 55 degrees elevation on the back side of the pattern. Now look at the azimuth pattern taken at 1 degree elevation (essentially ground level). Notice the two nulls in the pattern at 55 degrees either side of 180 (125 and 235 degrees). This is where you would want to measure if you were optimizing the phasing to have the best F/B at an elevation angle of 55 degrees. Also notice how poor the F/B is at 180 degrees or directly off the back of the array - only 10 dB. What would happen if you took a measurement there and "optimized" the F/B? See the next post of the answer......


* el55.gif (37.95 KB, 604x602 - viewed 458 times.)

* az55.gif (42.93 KB, 604x603 - viewed 493 times.)
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Steve - WB3HUZ
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« Reply #162 on: September 22, 2009, 09:28:26 PM »

If we optimized the F/B directly off the back of the array, we would get an awesome F/B, more the 30 dB. (Look at the first plot). But take a look at the elevation pattern! (Second plot). It's kinda lazy looking and lots of high angle response on the back side. These patterns happen to coincide with 90 degree phasing.

If we change the phasing just a little bit, to 110 degrees, look how much better the pattern looks on the back side. (Look at the third plot. Here I compared the 90 degree phased system - dashed line - to the 110 degree phased system - solid red line).

The worst case lobe still produces a F/B of 20 dB, and you have a nice null at about 30 degrees elevation. And notice that the response at the higher angles on the back side is also down. Finally, there's a bit more gain.

There you go. This is why you usually don't want to measure your F/B directly off the back of your array.


* 90az.gif (42.25 KB, 604x603 - viewed 508 times.)

* 90el.gif (40.49 KB, 604x603 - viewed 461 times.)

* 90comp110el.gif (42.21 KB, 604x603 - viewed 401 times.)
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K1JJ
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« Reply #163 on: September 22, 2009, 09:51:33 PM »

Vely vely good info, OM!  You get AMfone 2009 Nobel Prize for Antennas.


I assume these phase changes on a driven array are the equivalent to changing the lengths of the reflector and/or director on a [parasitic] Yagi?


A real-whirl way to simulate this excercise would be to tune in some W4's and null them out on the fly.  It wud require climbing the tower to be up where the stubs or adjustments were.

There was a time when I brought the openwire stubs for a 3el 75M wire Yagi into the shack for remote tuning. About 250' of openwire for each element. I had roller inductors across both the dir and ref. I could null out stations off the back like you wudn't believe. With a set of switches I wud null a station,  then null the other direction - and the array was all set for that freq. 

BUT, even though I used #5 copper wire for the openwire stubs into the shack, I was always down about 5db compared to a conventional wire Yagi with fixed wires.   I later removed them.  I figgered there was loss from using 30 ohm dipole els into 800 ohm openwire... more than you wud normally expect. I couldn't think of any other reason since the gain came back when I eliminated the stubs.  I was using Chuck's 3el rotary Yagi as a baseline into Eu.  It was quite clear.

T
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There's nothing like an old dog.
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« Reply #164 on: September 23, 2009, 01:32:58 PM »

It all a matter of current distribution between the two elements, both ampluitude and phase. You can move the lobes and nulls around by tuning the reflector/director. The tuning can be actual length changes or by changing the amount of inductive or capacitive reactance at the center of the element.



I assume these phase changes on a driven array are the equivalent to changing the lengths of the reflector and/or director on a [parasitic] Yagi?


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WA1GFZ
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« Reply #165 on: September 23, 2009, 04:01:08 PM »

I'm seeing the same type responses in a force fed system when you play with feed line lengths.
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Steve - WB3HUZ
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« Reply #166 on: September 23, 2009, 04:05:04 PM »

That's what I modeled below - a phased system. Of course I modeled it with perfect phasing and zero loss feedlines. The pattern won't be quite as pretty in the real world.


I'm seeing the same type responses in a force fed system when you play with feed line lengths.
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« Reply #167 on: September 23, 2009, 04:18:25 PM »

I did a pair of top fed loops and let the program go with bottom height and end lengths as variables. It ended with the bottom of the loops a foot below the feed point. HMMM folded dipoles at 135 degrees.
Reading further landed the real name of that antenna "ZL special"
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Steve - WB3HUZ
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« Reply #168 on: September 23, 2009, 08:50:57 PM »

Or HB9CV, depending on the variation.

I did a pair of top fed loops and let the program go with bottom height and end lengths as variables. It ended with the bottom of the loops a foot below the feed point. HMMM folded dipoles at 135 degrees.
Reading further landed the real name of that antenna "ZL special"

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« Reply #169 on: September 23, 2009, 09:21:39 PM »

Well just finished some testing in RX. FB is really good tonight around 20 db on the stingy TR7A. Checked in the West direction and it is also good. Then did a sweep and have good VSWR 7.250 to over 7.450. I think I'm pretty close just have to add wire back on the elements I lopped off last week when it was closer to the ground.
Maybe the simulation looked better with a flat  loop only a foot tall because the average height was 20 feet higher?? FB was 30.
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K1JJ
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« Reply #170 on: September 23, 2009, 09:33:36 PM »

Well just finished some testing in RX. FB is really good tonight around 20 db on the stingy TR7A. Checked in the West direction and it is also good. Then did a sweep and have good VSWR 7.250 to over 7.450. I think I'm pretty close just have to add wire back on the elements I lopped off last week when it was closer to the ground.
Maybe the simulation looked better with a flat  loop only a foot tall because the average height was 20 feet higher?? FB was 30.


20db is vely acceptable. Hopefully you will see 25-30db f-b on certain nights on certain stations.

Just to axe the obvious: Where are you measuring the swr on the line?  You're using a 1/2 wavelength coax multiple, right?   Otherwise, as you know, the readings will be something different than what is really at the feedpoints.

T
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« Reply #171 on: September 23, 2009, 09:49:09 PM »

I'm only looking at the SWR at the rig. I'm sure it is different at different places along the feed line. Feed line lengths not balanced. actually 135 degrees off to favor west. I just series a bunch of coax jumpers to get the right delay to flip it East. No football Thursday so I will work on it some more. More luck than science at this point.
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K1JJ
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« Reply #172 on: September 23, 2009, 10:15:33 PM »

I'm only looking at the SWR at the rig. I'm sure it is different at different places along the feed line. Feed line lengths not balanced. actually 135 degrees off to favor west. I just series a bunch of coax jumpers to get the right delay to flip it East. No football Thursday so I will work on it some more. More luck than science at this point.

"Then did a sweep and have good VSWR 7.250 to over 7.450."



Just to be sure...

Your swr may NOT be good from 7.250 - 7.450 at the dipole feedpoint, where it matters.

Add whatever jumpers are needed to give you a multiple of 1/2 wavelength to get an accurate swr reading of the dipole feedpoint. (with velocity factor)  THEN adjust your dipole lengths.  Otherwise, if you really have an swr problem with either or both dipoles (and don't know it) your 135 degree phasing line may be something far removed from 135 degrees due to the swr.   IE, your total coax length should be 45.4'  or 90.8', etc to see a real feedpoint reading for each dipole. Once they are tuned independently and the swr is really 1:1, then play with phasing lengths.

I usually tune my delta loops up on the tower with the feedpoint temporarily pulled to the tower.

[Maybe I'm preaching to the choir, if so, then maybe this will help some others...]

T
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There's nothing like an old dog.
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« Reply #173 on: September 24, 2009, 09:04:42 AM »

The SWR is never going to be good at the feedpoints with a phased array. And the feedpoint impedances will be quite different with the 135 degree phase feed. You can only be concerned with matching/proper SWR AFTER the two feedlines have been joined.
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K1JJ
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« Reply #174 on: September 24, 2009, 10:26:04 AM »

Yep, understood.  I'm talking about getting the dipoles set up equally before playing with the phasing lines - and to get the two dipoles resonant on 7150 to begin with. What if one is way off due to an error in cutting?  I'm not sure if Frank did that using 1/2 wavelength feedlines first. This error would show up when switching directions.  But they're probably close enuff, knowing Frank's attention to detail.

I can understand the 135 degree phasing creating a reactive condition at the feedpoints.    Though not all phased "arrays" have swr problems at the feedpoints. For instance, my 40M stack has a theoretical 50 ohms  J0 at each feedpoint (when joined into three components) and measure out pretty close in practice. (They are all fed in phase, however, not 135 degrees out - and are complete units, not individual elements within an antenna requiring specific phase treatments)

As discussed in the past, I've never liked the complexity and potential matching problems of driven arrays vs: the simplicity of parasitc arrays. They both can be gotten to work, however, and a driven array is even a better choice for a low antenna below 3/8 wave above ground.

T
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There's nothing like an old dog.
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