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Author Topic: Phasing 40m Inverted Vees - Troubleshooting  (Read 1324 times)
KC7BEY
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Posts: 15




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« on: June 10, 2013, 08:32:59 AM »

I'm building a phased array for 40m for field day and was curious if anyone can think of any troubles that might arise, given this setup:

- Envision two 40' conduit masts, both supporting the center of 40m inverted vees. 

- There is a two-foot plastic PVC pipe on top of each mast to space the vees from the conduit. 

- The 71 and two 84 degree phasing lines are measured using an online christman phasing calculator and tuned using an MFJ antenna analyzer (http://www.va7st.ca/home.html/2010/08/phased-40m-wire-verticals/).

- The vees are 66' long (33' per side), the coax is soldered directly to each vee (without a balun) and a relay is not being used.

- The angle of each vee is about 35 degrees (not 45)

- Using 50ohm RG8-X coax for the entire antenna system, 100w, feedline will be about 60'.

I don't think I can get the group together to hoist up the masts before the big day, so am trying to prepare for anything that could go wrong.  Any help would be appreciated.  Thanks.

73,

Garrett
KC7BEY
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WB6BYU
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Posts: 13239




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« Reply #1 on: June 10, 2013, 09:19:19 AM »

Quote from: KC7BEY

- The angle of each vee is about 35 degrees (not 45)



How are you measuring that angle?  Is it the angle between the wire and the mast, or
the amount the wire is tilted down from horizontal?

While a 90 degree angle between the wires sometimes gives a better match to 50 ohms,
I find performance is often better with a wider angles (wires closer to horizontal).


Quote

I don't think I can get the group together to hoist up the masts before the big day, so am trying to prepare for anything that could go wrong.  Any help would be appreciated.  Thanks.



Have you put up the conduit masts that high before?  How are you planning to install
them?  EMT conduit is pretty heavy.  I use the sectional military masts, and have put
up 40' of the fiberglass all by myself by stacking it vertically rather than trying to tilt
it up.

Also, put pulleys and halyards on your masts so you can lower the antenna to make adjustments
or repairs without having to lower the masts.


What is the purpose of phasing the two elements?  Better F/B ratio?  I've modeled
several such variations and a simple 2- or 3-element wire yagi using a rope between
the two masts as the boom looks like it would do about as well as the phased array
but much simpler to install.  I've used 2- and 3-element wire quads before, hanging
either from a wood spreader or a rope between two trees, and that is another simple
option (though at 40' the yagi would be better:  the quad needs more height.)
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KC7BEY
Member

Posts: 15




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« Reply #2 on: June 10, 2013, 09:40:52 AM »

I miscalculated the angle.  It's greater than 90 degrees between the vee elements, not less.  I'm basically trying to get the elements as parallel to the ground as I can.

I chose a phased array because they work really well at low heights, the gain is good, and I need a low takeoff angle (I'm all the way up in Seattle, hi hi).  I also don't trust my reflector measurement abilities enough to pursue a parasitic array with vee or quad elements, but I trust my ability to tune phasing cables (the time I have to tune these things is limited because I can't get a group of guys together to put up the masts before field day).

I've been able to put up masts like that before with others' help, and I will definitely place pulleys at the tops of each to raise/lower the vees for necessary adjustments.

Great thoughts.  Thank you.  Keep 'em coming!
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WB6BYU
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Posts: 13239




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« Reply #3 on: June 10, 2013, 01:03:04 PM »

Quote from: KC7BEY

...I'm basically trying to get the elements as parallel to the ground as I can.




A good idea.



Quote

I chose a phased array because they work really well at low heights, the gain is good, and I need a low takeoff angle. 



From my modeling so far I haven't seen any significant difference between the gain at
low angles for a phased array compared to other options at the same height.

We have the same issues here in NW Oregon:  the bulk of our contacts are at distances of
1000 miles or more.  Closer stations are strong enough that we don't have to worry about
working them:  instead the major focus is improving gain at angles below about 30 degrees.
Height is the most important factor, and having some douglas fir trees can come in handy.
Ground slope in the desired direction also improves your low angle signals - operating from the
East side of a hill can make several dB of improvement. 

We just point our beams East:  40 to 60 degree half power beamwidth is about ideal to cover
most of the Field Day stations from here, and I expect it is from WWA also.

Unfortunately I don't yet know how the stations will be arranged on our site this year - in
one corner there are trees behind us to string up a 3-element 40m beam, but otherwise
we may have to get by with an 80m doublet strung up at 80'+.  (Which, with some ground
slope, should work pretty good on 80m as well.)



Quote

I also don't trust my reflector measurement abilities enough to pursue a parasitic array with vee or quad elements,


Are you using a wire measuring board?  It really makes life easier.  My first one was on the
back of my Dad's garage door:  I pounded two nails in 10' apart and marked off the feet and
halves between them.  Hook the wire on one nail and wrap it back and forth:  even at 144' I
can get it with in a few inches (depending on the wire thickness and how sharply it bends
around the nails:  for #12 solid wire I'd use a square or octagon.)  I now keep 5' and 2.5'
measuring boards built on 1x2 furring strips handy for measuring antennas:  sure beats trying
to lay the wire out on the driveway!



Quote

I've been able to put up masts like that before with others' help...



A push-up mast isn't too difficult if you have a step ladder so you can guy the bottom
section upright, then extend it.  Tilting a heavy mast upright can generate a lot of
forces on the hinge:  the best approach is the "falling derrick" method where you use
a shorter mast (perhaps 15' or so) at a right angle to the main mast.  Tie the guys
from the tall mast to the short one with both laying on the ground, then pivot the
short mast vertical and pull it down to raise the main one.  Some Russian friends put
up a 160m quarter wave vertical this way using 3 separate steps (and only dropped
it 3 times in the process.)  There's a good description in one of the RSGB antenna
books on how to do this - they used aluminum scaffold poles to go to 60'.

By stacking the sections vertically I never have to deal with the weight and loads
of tilting up the mast.  I tie off two of the guy ropes about where I think they will
need to be, put together the top 2 sections of the mast with whatever goes on top,
and walk out until both guy ropes are tight.  I should be able to set down the mast
and lean it against both ropes so it stays upright.  Then I pick up the mast, slip the
next section in the bottom, and put it back down, all the while moving slightly to keep
it leaning just enough against the ropes.  The 4' sections are about as long as I'd
want to use without having a truck bed or ladder to stand on.  You can make your own
sections out of other materials as long as the total mast isn't too heavy for one or two
people to lift.  If your material comes in 10' pieces, cutting it in thirds is probably the
best length to use.  We regularly put up a tribander at 32' for Field Day, and I think I
could do it myself.  (I was going to try last year, but someone insisted on helping.)
In that case we put up the mast, get all the guy ropes set, then take it down, put
the antenna on, and put it back up.
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WX7G
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Posts: 6035




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« Reply #4 on: June 10, 2013, 01:39:04 PM »

In 1972 two of us built a 40 meter inverted-vee 2-element Yagi-Uda for field day. We were able to raise and lower the reflector during testing and stations reported one S-unit difference.
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N4JTE
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Posts: 1155




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« Reply #5 on: June 10, 2013, 02:51:11 PM »

If you connect the two 84 degree phase lines to each dipole yuo will have a bidirectional beam, if you connect an 84 to the rear dipole and a 155 degree phase line to the favored direction, it will be an end fired beam with substantial rear signal rejection and 3dbd gain to the front element direction.
Bob
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N4JTE
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Posts: 1155




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« Reply #6 on: June 10, 2013, 02:59:10 PM »

Assuming 1/4wl spacing.
Bob
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KC7BEY
Member

Posts: 15




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« Reply #7 on: June 12, 2013, 09:49:10 AM »

From my modeling so far I haven't seen any significant difference between the gain at
low angles for a phased array compared to other options at the same height.

Here's where I got the idea a phased array would work better at low heights (around 1/4 WL high) compared to a parasitic array: http://vss.pl/lf/12.pdf  Converting Figure 12-1 to 40m, a phased array appears to lower the radiation angle from 45 to 37 degrees.  The gain also increases somewhat and better rejects noisy high-angle radiation, allowing for a quieter receiving antenna.

I've used a wire measuring board with success.  I guess I'm just more concerned with optimizing a parasitic reflector.  I know the general 5% rule, but with my limited time putting up the antenna, I believe I'll get better results with a few phasing lines and two 66' inverted vees.  If I had no trouble with masts, it would be a different story, hi hi.  But I will look into the derrick method a little more for future contests; it sounds really cool.

- Garrett
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KC7BEY
Member

Posts: 15




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« Reply #8 on: June 12, 2013, 09:50:51 AM »

PS thanks for everyone's help!  If anyone can think of any other problems you've encountered with phased arrays and how you mitigated them, that would be great!
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WB6BYU
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Posts: 13239




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« Reply #9 on: June 12, 2013, 10:44:32 AM »

Quote from: KC7BEY

Here's where I got the idea a phased array would work better at low heights (around 1/4 WL high) compared to a parasitic array: http://vss.pl/lf/12.pdf  Converting Figure 12-1 to 40m, a phased array appears to lower the radiation angle from 45 to 37 degrees.  The gain also increases somewhat and better rejects noisy high-angle radiation, allowing for a quieter receiving antenna.



You can't just look at the angle of maximum radiation - you have to look at the actual
signal strength at each angle.  In this case Figure 12-1 compares the angle of maximum
radiation between a bi-directional phased array and a unidirectional yagi:  it is
quite possible for the phased array to have a lower angle of maximum radiation, but the
yagi to have more gain at lower angles.

If I get a chance I'll have to model the phased antenna and compare it to some of the
other options I have in my portfolio.  From the perspective of simplicity of installing it
in the field (especially in among trees) the parasitic arrays have an advantage.

Modeling is also a good approach to getting the right parasitic lengths.

At this point I don't know if I'll be in a position to put up a wire beam or not - our
site is along the edge of an open field, with tall trees to the East.  I think I'll get
more improvement from putting an inverted vee high in one convenient tree than
from putting it lower where I have two supports available, but with somewhat
narrow spacing I might be able to hang it from branches on either side of the
tree.  Or I may put up an 80m doublet fed with ladder line, which gives me a couple
dB gain on 40m as well.  Unfortunately I won't know until the day before where the
data station will be placed, then I get to figure out antennas for CW and SSB.
(I put the best antennas on the SSB station, as they need all the help they can get
when running QRP.)
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WB6BYU
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Posts: 13239




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« Reply #10 on: June 13, 2013, 09:03:08 AM »

I ran some models last night:  for the same element spacing, center height of 40'
and end heights of 25', a version with 150 degree phasing had about half a dB gain
over my 2-element wire yagi (which had been designed at a different height, so
might not be quite optimum.)  Below about 20 degrees the patterns overlayed so
closely I couldn't distinguish the traces.  A 3-element yagi was a little bit better,
but not more than 1dB over the phased array.  There was more variation at higher
angles (for the 40m stations from those densely populated areas between you and
the Rockies) but overall performance is set more by the height above ground
than anything else.

Both yagis were designed for direct 50 ohm feed rather than maximum gain.
I didn't model the matching and phasing system for the phased array - instead I
just programmed the phase shift and equal magnitudes into the feedpoints.  Further
modeling will be required to see what happens with coax phasing lines, and the
impact of imperfect amplitudes.  (I tried various phase shifts between 120 and
160 degrees - the smaller numbers had somewhat lower gain, and the F/B ratio
varies somewhat, but not enough to make the antenna unusable.)  The changes
in antenna impedance due to variations in height and wire angle probably suggest
that you won't get perfect phasing with pre-cut lines, but likely still good enough.

From what I've seen so far, the 3-element yagi (about 50' long) would be my
choice, since it is more tolerant of variations in spacing (not having to deal
with a phasing line connecting the two elements.)  One issue then is the weight
of the feedline hanging from the center of the boom, and you might run into a
similar problem with the coax and phasing stubs, but at least in the 2-element
version you can put each feedpoint on top of a mast.  (In my case I would run a
rope boom between two trees and suspend the wires from that.  But first priority
is to get the antenna up to 60' or so, then worry about adding parasitic elements.)
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KC7BEY
Member

Posts: 15




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« Reply #11 on: June 14, 2013, 12:43:46 PM »

You can't just look at the angle of maximum radiation - you have to look at the actual signal strength at each angle.

Very true.  I haven't modeled a two-element parasitic yagi versus a two-element driven phased array, but that makes sense; the signal strength of each antenna at 20 degrees, for example, could be the same even though it's not the maximum angel of radiation for one versus another.

What was also attractive about the phased array is the prminent null of radiation overhead.  From what I've read, this decreases atmospheric noise.

From what I've seen so far, the 3-element yagi (about 50' long) would be my choice, since it is more tolerant of variations in spacing (not having to deal with a phasing line connecting the two elements.)

I imagine the radiation area is narrower with three-elements, as opposed to a two-element phased array.  I would worry the signal isn't being transmitted as broadly with a three-element parasitic yagi.  Also, the max. height I can achieve with my setup is about 1/4 wavelength; if I had the ability to further raise the antenna, maybe a three-element parasitic would work better.

Good stuff.  Last night, I visited our group's clubhousen and used an MFJ analyzer to measure the phasing lines.  The Christman calculator available online was just about perfect compared to the tuned lines.  Pretty cool.

73,

Garrett
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WB6BYU
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Posts: 13239




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« Reply #12 on: June 14, 2013, 01:36:31 PM »

Quote from: KC7BEY

What was also attractive about the phased array is the prminent null of radiation overhead.  From what I've read, this decreases atmospheric noise.



Well, maybe...   It might reduce the strength of nearby stations in Oregon, BC or Idaho when
40m is open for short distances, but there are comparatively few stations in that range, and
atmospheric noise is rarely the limiting factor on Field Day.

My guess is that you could put up any two of the antennas side-by-side and not be able to
tell the difference among them easily.



Quote

Quote from: WB6BYU
From what I've seen so far, the 3-element yagi (about 50' long) would be my choice, since it is more tolerant of variations in spacing (not having to deal with a phasing line connecting the two elements.)

I imagine the radiation area is narrower with three-elements, as opposed to a two-element phased array.  I would worry the signal isn't being transmitted as broadly with a three-element parasitic yagi. 



A half-power beamwidth of +/- 30 degrees gets most of the active stations in the US and
Canada.  It does miss the SW corner, primarily CA, NV, NM and AZ, along with VE6 and VE7,
but those stations are generally close enough that you don't need full power to reach them.
Narrowing the beamwidth to +/- 20 degrees reduces coverage of Canada, UT, CO and TX,
though again, except perhaps for parts of Ontario and the Maritimes, this probably won't hurt
your ability to work stations.

The 3-element yagi has a half power beamwidth around 70 degrees so, as we do on 20m, we
just point the antenna East and don't bother rotating it.

I actually went through this analysis for a club presentation:  I pulled up an Azimuth map from
NS6T's web site based on our Field Day location
and compared it to some pattern plots from EZNEC.  In most cases it isn't until you get up to
5 or 6 elements before you run into problems with the pattern being too narrow:  we used to
run a 4-element 20m yagi from Vandenberg AFB in California and never bothered rotating it.

(Drop me an email and I can send you a copy of the presentation if you are interested.)



Quote

Also, the max. height I can achieve with my setup is about 1/4 wavelength; if I had the ability to further raise the antenna, maybe a three-element parasitic would work better.



Height makes a big difference in signal strength, but not necessarily in the relative gain of two
antennas.  At 30 degrees vertical angle you gain about 4dB increasing your height from 1/4
wavelength to 1/2 wavelength, and closer to 6dB at 15 degrees.  That's about the same gain
improvement that you would get going from a dipole to a typical 2- or 3-element array.  If I
can get a rope over a tree branch at 90' then I'll string up an 80m doublet and, on 40m, it will
nearly make my QRP signal equivalent to a 100W station with a typical mediocre antenna. 
We'll see...

Otherwise I may be threading elements through tree branches to build an array, which will
make it less convenient to have a phasing line connected between them.

Looks like both 40m and 80m may be open all night.  Could be interesting...
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