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Author Topic: Bottom fed dipole, not Center?  (Read 4878 times)
KA5N
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Posts: 4380




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« Reply #15 on: December 26, 2012, 07:30:54 AM »

A 160 foot steel pipe!!!!   Since you include few details  (diameter of pipe, guying if
any, freestanding?Huh  how mounted at lower end  etc. etc.)   I wonder at the safety
of such a pole.  Since servicing the antenna(s) will be nigh impossible after installed,
how do you propose to keep it going and has anyone calculated the effect of high winds
on such a tall thin structure?  It would seem a better idea to use a climbable wooden mast of 50 to 60 feet and install center fed slopping dipoles in three or four directions to be able to "steer" the array.   There are dozens of antenna setups you can use that
are more likely to result in a usable, safe antenna arrangement than what you have
described.
Good Luck Allen   KA5N
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WY9C
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Posts: 6




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« Reply #16 on: December 27, 2012, 08:00:13 PM »

  I was planning on using 6 inch irrigation piping which comes in 40 foot sections that are flanged and bolted together. I was also going to weld the joints together and guy the pole.  The pole was going to be set into a pit that would be filled with cement.  My idea was to run a 125 foot vertical wire  that would be attached at both ends several feet out from the pole and feed it in the middle.  There would be about 35 feet between the bottom of the dipole and the ground. Now I am not sure what to do. Maybe a sloper/ inverted V/ flattop dipole? WY9C
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WB6BYU
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Posts: 13039




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« Reply #17 on: December 27, 2012, 09:18:41 PM »

Quote from: WY9C
  I was planning on using 6 inch irrigation piping which comes in 40 foot sections that are flanged and bolted together. I was also going to weld the joints together and guy the pole.  The pole was going to be set into a pit that would be filled with cement.  My idea was to run a 125 foot vertical wire  that would be attached at both ends several feet out from the pole and feed it in the middle.  There would be about 35 feet between the bottom of the dipole and the ground. Now I am not sure what to do. Maybe a sloper/ inverted V/ flattop dipole? WY9C


I do know a club that put up a similar mast as a 160m quarter wave vertical, though
I think it was more like 8" to 12" in diameter.  Getting it upright was NOT a trivial task -
they dropped it 3 times before finally getting it into position.  But they have one of the
strongest signals on 160m from their zone.


However, a number of comments on your plan:

1) most of the irrigation pipe I've seen is aluminum rather than steel.  It is NOT
self-supporting at such heights.  At a minimum it should be guyed at each joint.

2) the end clamps are probably not secure enough for attaching the sections if
there is any lateral stress.  a more secure way to join them is to cut up another
section, slit it open, and overlap the joints.  I think a 10' length (so it overlaps
5' on each section) secured with multiple sheet metal screws is a good start.
But it still might not withstand the bending forces involved in trying to get it
upright without a lot of support ropes to hold it straight in the process.

3) A dipole folded into a "U" shape as you propose is NOT an effective antenna.
The wires need to be more colinear (both in a straight line, fed in the middle.)
With the legs parallel as you describe most of the radiation from one side will
be cancelled by out-of-phase from the other half.

4)  If you have a tall tower to work with, string up an inverted vee, or use the
tower itself as a vertical antenna by insulating the base (you may be able to
use a length of PVC or ABS pipe as a base insulator.)

5)  Don't under-estimate the effort required to install such a mast.

6)  If you do put it up, find a length of pipe that is a slip fit over the
mast and set that in concrete in the ground instead.  Then you can drop
the main mast inside it, and still be able to lift it out later to take it down
if you need to.  This works with smaller push-up masts also - if the pipe
doesn't extend above ground level you can take the mast out and mow
over it.

7) since it will be very difficult to do any maintenance on the mast itself
(or antennas mounted on it) once the mast is in place, I'd add at least one
(and preferably two) pulleys and halyards at the top of the mast, perhaps
one on each end of a 4' crossbar.  This allows you to hoist various types
of wire antennas up the mast to try them out without needing to lower
the mast to change antennas.
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WY9C
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Posts: 6




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« Reply #18 on: December 28, 2012, 04:17:33 AM »

Thank you for your advice. I really appreciate it.  The pipe that I was going to use is  six inch diameter medium thickness steel with flanges welded on that
have  multiple bolt holes in them for bolting together each section. I was also going to weld the sections together.  I was considering taking an eight foot section with it's flange pointing up and setting it about eight feet into a large hole and filling it with cement.  Then bolting the rest of the pipe onto it. Then guying it up at several places.  I was planning on having a straight up/down 125 ft wire dipole fed in the middle with ladder line. The dipole would be about 4-6 ft from the pole. The bottom end of the dipole would be about 35 ft from the ground unless I taper it out like a sloper and put up another shorter pole to attach the end to. I like the idea of pulleys so that I could take the dipole down for maintenance.  An inverted V or Sloper is sounding better all the time. WY9C
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WY9C
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Posts: 6




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« Reply #19 on: December 28, 2012, 04:46:12 AM »

  I had originally planned on using thick wall 4 inch pipe((1/4 inch wall thickness) and yesterday located some 6 inch diameter (described in my earlier post). The four inch pipe was extremely heavy and the whole assembly would have weighed over a ton. This six inch pipe that I found yesterday is 1/8 inch wall thickness  and should be strong enough with guying to take strong winds. I have family that owns the equipment necessary to lift and set it in place. I had originally considered using two sections of 125 ft long wire  spaced 4 ft apart or a U shaped dipole but upon the kind hams advice that I have received on this forum I dropped those ideas faster than a steaming hot potato.  I love to experiment but I don't want to wind up with a setup that is only good for a sundial.
WY9C
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VU2NAN
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Posts: 239




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« Reply #20 on: December 28, 2012, 05:50:30 AM »

Hi OM Aaron,

Why not get 'maximum performance' instead from an open-stub J-Pole antenna?

https://picasaweb.google.com/lh/photo/jIqbs6WNsH9_Y1c2CIJCjmy_Sn2_u52_9m6iHxMPp2w?feat=directlink

It's a  'bottom-fed' half-wave antenna.

73

Nandu
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WB6BYU
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Posts: 13039




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« Reply #21 on: December 28, 2012, 08:13:35 AM »

There really are two separate discussions here:

1) the mast, which may be practical, but does need some engineering to make sure
it is going to hold up.

2) the antenna(s) that goes on the mast.  The choice of antenna will depend on what
bands you want to operate, who you want to talk to, and the space you have available.


Getting an antenna up over 100' will help a lot for DX, though it won't necessarily be
optimum for distances between about 50 to 500 miles.  The simplest approach is an
inverted vee:  the ends can tie off down to ground level if you have enough space.
(Ideally this would be be about 3 times the height of the mast on each side.)  The
flatter the antenna (which means the further apart the ends are tied off) the better
it will work, especially if you are going to use it on multiple bands.  For multiband use,
feeding it with open wire line or twinlead is the best approach (unless you are using
traps or some other method to provide a good match on multiple bands), otherwise
for a single band antenna you can use coax feed.

But once you have such a mast in place, you may decide you want to put a beam
of some sort on top of it.  That puts different stresses on the mast, and runs into
the further problem of access to the top to mount such antennas.  If you have
access to a crane and a bucket truck that goes high enough that does make life easier.

I'm actually working with a friend on a similar project - he has a 200' tower (with
the antenna for the local repeater on top) and is considering adding a directional wire
antenna for the lower bands.  However, any vertical antenna running near a tower or
mast is going to cause a lot of coupling and interaction between them.  (This is true
of any two conductors running parallel to each other, and spaced less than about
1/4 wave apart - sometimes more.)

There are lots of designs available for wire antennas.  The first step is to decide
  •      what bands you want to cover
  •      what stations you want to work
  •      how big you can make the antenna
  •      what resources you have available to work with
  •      what weather conditions the antenna (and mast) have to survive

Once you have thought these through, you can look at various designs that will
meet your needs.
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W5WSS
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Posts: 1654




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« Reply #22 on: December 28, 2012, 09:28:18 AM »

Dale, Excellent advice.

73
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WY9C
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Posts: 6




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« Reply #23 on: December 30, 2012, 04:24:05 PM »

 Would it be possible to use the pole itself as a vertical antenna?  I could paint it with conductive paint or wrap it with aluminum to increase conductivity over the steel pipe and pour the base in cement to eliminate grounding.  Would I be better to feed it at the top or bottom? Ground radials needed? How tall would it need to be to work 160- maybe 130FT? I know that I have a lot of questions but I don't want my project to wind up as just a giant sundial. Thanks for all input?  Robert WY9C
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WB6BYU
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Posts: 13039




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« Reply #24 on: December 31, 2012, 10:59:05 AM »

Quote from: WY9C

 Would it be possible to use the pole itself as a vertical antenna? 



Yes, it is possible.

I wouldn't worry about conductivity - on the lower bands (80m and 160m) it will
be OK.  Most AM broadcast stations use steel tower sections as verticals.

Yes, you'll need a good set of ground radials.  How many and how long may be the
topic of further discussion:  while "more" and "longer" are usually better, there are
points of diminishing returns.  Something like 30 radials each as long as the antenna
is tall is a reasonable starting point.

130' would be a quarter wavelength on 160m, and can be a very effective antenna
(especially in areas with good ground conductivity).  Look for the YouTube video of
the 130' vertical antenna going up at RW0CWA.  This would also be a half wave
on 80m, but somewhat too long for best results on 40m and higher frequencies.
To use it on 40m ideally you'd keep it below 5/8 wavelength (about 75') though it
can work OK at 3/4 wavelength (about 90').  But with that much height, putting
a horizontal antenna on the top is probably better for 40m rather than using the
mast as an antenna.

You can ground the base of the antenna and shunt feed it, which will effectively
give you a single band antenna.  Or you can insulate the base from the ground
and feed it with some sort of matching network:  this might be trivial on 160m,
but would need to match a high impedance on 80m.  If you want to get more
elaborate you could build a 160m version of the HyGain 18HT vertical with stubs
to provide a match on other bands.

Concrete by itself isn't a particularly good insulator - though it isn't a good conductor,
either.  However, if the mast is guyed (as it would need to be to remain upright) then
the base really only needs to keep the bottom from sinking into the ground, and you
can use various sorts of ceramic or plastic insulators that will take the weight of
the mast + the downward force due to the guys when the wind blows against it.
(I've seen shorter masts simply set on a beer bottle as a base insulator.)


One major problem with your proposed approach is that you can't conveniently climb
the mast to add antennas once it is up.  One way around that is to weld steps on
to the mast before you put it up.  Once you get such a mast in place, you'll probably
want to mount antennas for various bands on it (assuming you've done the engineering
analysis to determine what the safe windload is for it.)  Of course, if you have a 100'+
bucket truck handy, that isn't as much of a limitation.  You'll also want to secure the
coax feedlines to the tower for strain relief, otherwise the weight of that much coax
may stress the line.

So while such a mast can be a great vertical antenna on 80m and 160m, on the rest
of the HF bands you're best results will be using it to mount other antennas on, which
will require more planning, especially if you are going to add and change antennas
over time.
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W8NSI
Member

Posts: 21


WWW

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« Reply #25 on: January 01, 2013, 11:56:03 AM »

I attached a picture that may help.

I made a 1/2 wave dipole to use vertically for 2 meters.

It consists of two 19" pieces of 1/2" copper pipe joined with a 1/2" PVC coupler.

At the very bottom, my 1/2" hardline terminates to RG-8X coaxial cable running INSIDE the bottom 19" section of the dipole.
THIS is wear I made the ground connection, at the bottom, not at the center of the antenna.
I then ran RG-8X INSIDE the bottom 1/2" pipe 19" section to the center of the dipole and soldered the RG-8X center conductor to the bottom of the top element.

Is this equivalent to a 1/2 wave dipole or is there any type of cancellation or anything going on?

I have never seen another dipole where the ground connection was made on the far side from the center and wasn't sure if it made any difference or not.


http://i50.tinypic.com/2rmvztj.jpg


What you have pictured in your image is simply using the bottom copper tube as a poor impedance transformation to an end fed 1/4 wave with no radials if I interpret the drawing correctly. It may improve if you add 4 drooping 20 inch radials attached at the top of the bottom tube where they meet at the PVC.

73 de w8nsi/nnn0uzw jim
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73 de w8nsi/nnn0uzw jim
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