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Author Topic: Which is a better 40m limited space DX antenna?  (Read 6340 times)
NO9E
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Posts: 391




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« Reply #15 on: November 06, 2012, 08:59:24 AM »

The performance of a vertical depends on soil conductivity and on obstacles around. Poor conductivity means poor performance even with zillions of radials. When portable from a beach, the verticals do very well for me. At home in GA, even full size verticals do 5-15 db below dipoles even for DX. The best is to try at your QTH.
Ignacy, NO9E
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W0BTU
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Posts: 1620


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« Reply #16 on: November 06, 2012, 09:18:29 AM »

I fully agree. A vertical near the ocean is a killer DX antenna.

Inland, a lot of power fed to a vertical warms the earthworms a little. :-)
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W8JX
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Posts: 5591




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« Reply #17 on: November 06, 2012, 12:34:05 PM »

I fully agree. A vertical near the ocean is a killer DX antenna.

Inland, a lot of power fed to a vertical warms the earthworms a little. :-)

Well you are not warming too many worms when I can get S9+20 and more at times state side on 40 and S9
+ in Europe and down under. Dipole is better on shorter hauls usually but not long on low bands. 
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STAYVERTICAL
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Posts: 854




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« Reply #18 on: November 06, 2012, 12:48:12 PM »

I have done it many times - tried both dipoles and verticals.

No contest - for DX my vertical is better.
The dipole is great for local ragchewing and nets, where the vertical is lousy due to its overhead null.
But as the distance increases, the vertical shines and blows away the dipole.

It's nothing strange - unless you can get a dipole at its optimum height it is almost omnidirectional and has a high angle of radiation maximum.
The vertical, being a type of side mounted dipole (I know its not strictly true), has its radiation maximum at low angles to the earth.
So, fewer skips, means more signal - but if the ionosphere height changes the skip zone moves a great deal - hence the worse qsb on the vertical.

With the dipole, more skips, less signal - but more stable due to the higher angle not moving the skip zone as much when ionospheric height changes.

Its pretty simple really, but many people interpolate local signal strength to DX work.
Sure on the local nets, the dipole is going to be much better, but that very reason is why the dipole is worse on DX.
This does not apply to people who have dipoles up at optimum height, but we are talking limited space here.

In my DX work ( my primary interest and activity), I use a vertical because it kicks butt, not because I love verticals.
But I have my butt kicked on the local nets every time by the dipole guys, it's just physics.

73 - Rob
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WB6BYU
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Posts: 13114




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« Reply #19 on: November 06, 2012, 12:52:22 PM »

Again, it depends to a large degree on soil conditions.

I remember one ham who worked a lot of DX on his vertical when he lived in the coastal
plain, then moved about 20 miles inland.  With the same antenna he could hear his friends
working DX, but no longer could do so himself.

When comparing the vertical patterns of vertical and horizontal antennas you really
should specify the soil conditions used in the model, otherwise there is no guarantee
that those results will be applicable to anyone else.
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STAYVERTICAL
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Posts: 854




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« Reply #20 on: November 06, 2012, 01:08:36 PM »

BYU,

Agreed, you cannot get good results with just half an antenna.
A good radial system is essential with a vertical - not optional but mandatory.

I am on a rocky hilltop, sand and rock - soil conductivity zero.
I tried 10 ground mounted radials - they worked but results were ordinary.
Then I changed to 4 elevated radials (elevated 5 feet at the antenna, sloping to 3 feet at the tips) - now your talking!

For a poor soil area, I would recommend elevated radials very strongly.

Believe me, I am no masochist, I would not use an antenna which did not perform.
The reason I tried so many dipoles is the generally espoused popular wisdom of dipoles being such great antennas.
I kept thinking I must be missing something, or making a basic mistake somewhere.

But the DX results did not lie, and in the end, I realised that since I could not get a dipole high enough, they would always underperform on DX.
On the other hand for local nets, the dipole was far superior - no contest there.
With the vertical is was like I had a geographic filter out to 100 miles - almost no local signals were very strong.
The overhead null was obviously very effective for local stations.

One antenna I did find very effective however was the classic single wire fed windom.
With an  elevated counterpoise, and a remote ATU feeding it, it worked well on both DX and local stations.
I suppose it was due to it having both vertical and horizontal sections, and so this old antenna was a winner as well.

In the end, you are right, local conditions are paramount.
But the popular fiction about having to have a salt marsh underfoot to get good results with a vertical is simply not true in my experience.
The trick is to use elevated radials - something not everyone can do, as they are also handy trip wires or garrotting devices!

Keep experimenting,

73 - Rob
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WB6BYU
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Posts: 13114




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« Reply #21 on: November 06, 2012, 01:24:54 PM »

It goes beyond the radial system - it is the ground conditions out to 100 wavelengths
or so from the antenna, far beyond where most folks are going to string radials.

This is because of the pseudo-Brewster angle and all that.

It isn't just a matter of conductivity:  a friend lives on solid basalt and his vertical
works fairly well, because the rock acts more like an insulator than a resistive ground.
(Ground slope also has some effect.)  But between "insulator" and "perfect
conductor" are a range of much more lossy media that limit the possible low angle
radiation from a vertical.

That's why I used the example I did:  same ham, same antenna, same radial system,
but totally different results after moving 20 miles due to the ground conditions.
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KB1GMX
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Posts: 740




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« Reply #22 on: November 06, 2012, 01:53:20 PM »

There is one vertical that is height limited but not loaded.  Consider the inverted L with as many ground mounted radios as possible.

I have one here for 40M and compared to a dipole at the top of the L is 28ft the dipole is 29ft
and the L wins for DX and far better for general local use (under 60miles) with better than
average soil and many radials.   

However, If your soil is really poor (lousy conductivity) then a low dipole can work ok, but
25ft (0.17 wave) is very low and most of its radiation is up!  Dipole start to perform well at
more than twice that height (about 66ft).  However if the MUF is good then a low dipole
works as NVIS and useful out to 500-700 miles but needs better than average conditions
for more than that.


Allison
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WA8JNM
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Posts: 170




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« Reply #23 on: November 06, 2012, 02:32:52 PM »

Would someone care to chime in on the same considerations for a vertical wire half wave 40 meter dipole, essentially a few feet above ground at the lower end?
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WB6BYU
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Posts: 13114




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« Reply #24 on: November 06, 2012, 04:28:37 PM »

If that means you have a support for the top of the vertical dipole that is
over half a wavelength high, hanging an inverted vee from that support
probably gives the best performance.

Unless, of course, you live in a salt marsh.
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WA8JNM
Member

Posts: 170




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« Reply #25 on: November 06, 2012, 06:40:39 PM »

Yes, the top is up in a tree, almost against the trunk. The bottom is about 5 foot laterally from the trunk, because of branches pushing it out a bit. So there is a slight slope to it. The wire is insulated.i can't do an inverted vee, because I need to hide the antenna near the tree, and don't really have two places to tie off the ends without being visible. I posed this question because I don't see a lot of discussion on vertical wire half wave antennas. No intention of hijacking the thread here, but thought it would be interesting to compare to comments relating to a vertical with radials and the horizontal dipole.
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N8CMQ
Member

Posts: 353




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« Reply #26 on: November 06, 2012, 07:08:13 PM »

I also favor the ground mounted vertical for DX and stateside contacts.
My radial field is 125 30+ feet radials in a 60 foot square.
My next radial field will be 130 or more 60+ feet radials in a 120 foot square.
My antenna is a 4BTV with an 80M resonator in the phone portion of the band.
The soil conductivity here is not good, as it is mostly sand.
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N4JTE
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Posts: 1155




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« Reply #27 on: November 07, 2012, 03:34:11 PM »

The original question asked if a 25 ft dipole, rotatable would be better for dx than, insert by me; A Well Designed Vertical with substantial near field ground loss reductions and nice far field reinforcement.
Answer still is, and will be the vertical.
That low of a dipole is not directional except for non dx signals or might be a recipiant of a nice 4 element mono bander up 100 ft over in Italy doing all the heavy lifting, not my definition of dx.
Get the dipole up beyond a 1/4 wl high and then it might exceed the vertical on most days.
Bob
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K7KBN
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Posts: 2782




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« Reply #28 on: November 07, 2012, 05:21:33 PM »

I fully agree. A vertical near the ocean is a killer DX antenna.

Inland, a lot of power fed to a vertical warms the earthworms a little. :-)

Only vertical better than one near the ocean is one a few thousand miles from land ON the ocean!  Wanna try it?  See your Navy recruiter...
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73
Pat K7KBN
CWO4 USNR Ret.
WB6BYU
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Posts: 13114




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« Reply #29 on: November 07, 2012, 05:44:54 PM »

Quote from: WA8JNM
... thought it would be interesting to compare to comments relating to a vertical with radials and the horizontal dipole.


There isn't a lot of difference between a half wave vertical (whether fed in the
middle as a dipole or a ground plane with multiple radials, or at the bottom) and
a quarter wave vertical wire.  The same principle applies:  when the ground
conditions (both conductivity and permeativity are important) are good then
it can have low angle radiation, but it won't over poor ground.

The biggest problem with a vertical dipole usually is running the coax to the
feedpoint without having it run too close to either of the legs:  ideally it
should run off horizontally from the feedpoint for some distance (a quarter
wave or so).  If the coax is running down the tree then it will couple to the
lower wire, and you'll have to deal with common mode currents that may
affect your radiation pattern.
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