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Author Topic: Which is a better 40m limited space DX antenna?  (Read 6090 times)
KT0DD
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« on: November 05, 2012, 12:43:18 PM »

Hello, if I had to choose between one antenna, a rotating 40m dipole or a 40m vertical, which one would be more consistent for 40m DX? I am in a small lot limited space situation. Also maximum height will be about 25Ft.

Also, any suggestions for a more directional rotatable 40m DX antenna, that doesn't take up as much room as a full size 40m 2 element beam? I heard Optibeam makes some but aren't they pretty expensive?

Thanks for any feedback. 73.
Todd - KT0DD
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VE3FMC
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« Reply #1 on: November 05, 2012, 12:51:48 PM »

Hello, if I had to choose between one antenna, a rotating 40m dipole or a 40m vertical, which one would be more consistent for 40m DX? I am in a small lot limited space situation. Also maximum height will be about 25Ft.

Also, any suggestions for a more directional rotatable 40m DX antenna, that doesn't take up as much room as a full size 40m 2 element beam? I heard Optibeam makes some but aren't they pretty expensive?

Thanks for any feedback. 73.
Todd - KT0DD

Todd my opinion would be to go with the vertical, if you can lay down enough radials. A rotating dipole at 25 feet would not be that great of a dx antenna on 40. It would be ok for local communications.
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WB6BYU
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« Reply #2 on: November 05, 2012, 01:28:42 PM »

That will partly depend on the ground conductivity in your area (well beyond
the range where it is practical to install radials.)  If you live in a salt marsh or
other highly-conductive ground, a vertical can work very well.  Over lossy
ground the dipole may be better.

Here is one evaluation:

http://www.chem.hawaii.edu/uham/portant.html
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K3VAT
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Posts: 701




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« Reply #3 on: November 05, 2012, 02:38:16 PM »

... I heard Optibeam makes some but aren't they pretty expensive?
Thanks for any feedback. 73. Todd - KT0DD

Optibeam (German Technology at its Finest) is rather expensive due to the sad exchange between the Euro and the Dollar.  If you want the top in antennas (IMHO) then it is Optibeam, all the way from the Dipoles, thru the wire beams and mainstream yagis all the way up to the big boys (3 elements on 80M) ... and they don't make end-fed stuff  Grin

GL, 73, Rich, K3VAT
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N4JTE
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« Reply #4 on: November 05, 2012, 03:24:09 PM »

I aggree that a rotatable dipole at 25 ft is not worth the effort or expense, spend the money on the best near field radial system you can install, as said, the farfied reinforcing element is beyong most of our control.
Bob
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W0BTU
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« Reply #5 on: November 05, 2012, 05:01:41 PM »

A 40 meter dipole 25' high has higher gain than a 25' high vertical at low angles, as follows:
.14 dBi @ 18 degrees elev.
.95 dBi @ 20 deg elevation
2.87 dBi @ 26 degrees elev.
And this dipole's maximum  gain is 7.78 dBi! (straight up)

In contrast:
A 40m vertical with elevated radials (ground plane) with its base 25' high only has a max gain at any elevation angle of of only -.04 dBi (max @ 18 degrees elev.)
A ground-mounted 40m vertical only has a max gain in any dir of -.03 dBi (max @ 26 degrees elev.)

At 25', a rotatable dipole is the clear winner at those low angles. Not only is it better for DX, but it will blow the vertical out of the water for stateside contacts.

The vertical at 25' would only be better for DX than the dipole at elev. angles below 17 degrees or so.
« Last Edit: November 05, 2012, 05:18:53 PM by W0BTU » Logged

G4AON
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« Reply #6 on: November 06, 2012, 12:24:15 AM »

At 25', a rotatable dipole is the clear winner at those low angles

Is that based on practical experience? I have a 40m dipole at 30 feet and a ground mounted quarter wave (wire up a fishing pole), the ground mounted antenna has 16 buried radials...

On DX the vertical is hands down the better performer. I have done A/B comparisons many times and it's not a fluke that the vertical is better for DX.

73 Dave
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K0OD
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« Reply #7 on: November 06, 2012, 02:09:08 AM »

Quote
At 25', a rotatable dipole is the clear winner at those low angles. Not only is it better for DX, but it will blow the vertical out of the water for stateside contacts.

I have to qustion that statement too. Typo? I have virtually every country in the world worked with verticals on 40 and won many DX contests for my call area. True, the dipole is better within about 1,000 miles. I too have done A/B tests. A dipole under about 40 feet high isn't much of a DX antenna on 40 meters.  At about 50' up, the dipole begins to equal the vertical.
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W1JKA
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« Reply #8 on: November 06, 2012, 02:19:04 AM »

     Actual experience here also.As Dave stated above try a simple wire ground mounted vertical.Mine also does better than my 30 ft. high 40m dipole on dx(fixed direction considered),but as also stated above a lot will depend on your qth and ground conditions,as for me I have only 4 ground surface radials with solid ledge under about 5 ft. of top soil.You will only find out by experimenting.   Jim
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W0BTU
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« Reply #9 on: November 06, 2012, 04:04:22 AM »

I really don't know how low of an angle is consistently useful for DX on 40. Below a certain angle, the vertical has more gain than a dipole, as I mentioned.

What's important here is that the dipole is rotatable, so it can be aimed broadside (maximum gain) at the DX stations. I have heard many distant stations (well after dark) using rotatable dipoles on 40, and was surprised how well they worked.
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WX7G
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« Reply #10 on: November 06, 2012, 05:55:34 AM »

For DXing the dipole you mention suffers two limitations: low height and short length.

The vertical, full size or top loaded (like the Hustler 4BTV) and has a decent ground system (twenty or more 20' radials ground mounted) is the clear winner.
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NK7Z
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« Reply #11 on: November 06, 2012, 06:36:21 AM »

For 40, try a Gap Challenger...  I have had VERY GOOD LUCK with that antenna on 40, and 20, but not so much on 80, 15, and 10...
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Thanks,
Dave
For reviews and setups see: http://www.nk7z.net
W8JX
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« Reply #12 on: November 06, 2012, 06:47:39 AM »

For DXing the dipole you mention suffers two limitations: low height and short length.

The vertical, full size or top loaded (like the Hustler 4BTV) and has a decent ground system (twenty or more 20' radials ground mounted) is the clear winner.

I have used a 5BTV for about 20 years and it is a solid performer on 40.
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N3OX
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« Reply #13 on: November 06, 2012, 07:20:55 AM »

Something to keep in mind is that there's a good chance that the elevation pattern of a vertical will be better for RECEIVING to that of a 25 foot high dipole for all DXing angles, whether they're higher angles in the 20s or really low angles.  Even if the vertical ends up tied or even a little worse (not likely) than the dipole for low angle *gain*, the dipole has maximum response straight up, and there's plenty of signals coming in from high angles on 40m... both other hams' QRM and natural noise propagated in.  

Late at night on a cold winter night with no thunderstorms, it's probably hard to tell the difference.  But if the band is busy or it's storming nearby and it's around grayline, the strong reception of natural noise and QRM at high angles makes the dipole a less-good receiving antenna.

If you look at EZNEC predictions over "Average" ground and put it in terms of "receiving directivity factor"  (http://www.w8ji.com/receiving.htm) calculated for a takeoff angle of 10 degrees, I get +2.6dB RDF for a vertical and -3.7dB (!) for a 25 foot high dipole.

If noise is coming in uniformly from all azimuth and elevation, then, the vertical should be about 6dB better than the vertical for reception at 10 degrees elevation.  This includes the fact that the dipole has some azimuthal directivity.  The noise distribution changes on 40 and will make this different at different times, but I suspect the vertical is probably a far better receiving antenna most of the time, and never a worse one.

I further suspect that the strong reception advantage is why people are really enamored with verticals for DXing even if they don't have that much more low angle GAIN than a dipole because of the ground reflection losses that verticals suffer. 
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73,
Dan
http://www.n3ox.net

Monkey/silicon cyborg, beeping at rocks since 1995.
W0BTU
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« Reply #14 on: November 06, 2012, 08:12:25 AM »

...  Even if the vertical ends up tied or even a little worse (not likely) than the dipole for low angle *gain*, the dipole has maximum response straight up, and there's plenty of signals coming in from high angles on 40m... both other hams' QRM and natural noise propagated in.  

Late at night on a cold winter night with no thunderstorms, it's probably hard to tell the difference.  But if the band is busy or it's storming nearby and it's around grayline, the strong reception of natural noise and QRM at high angles makes the dipole a less-good receiving antenna.

Yeah, I didn't think of that, Dan. :-)

I was basing some of what I said on Tom's article The Myth of Takeoff Angle at http://www.eham.net/articles/23758 and your own comments on that page.
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