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Author Topic: 40 Meter Antenna / Vertical or dipole(Only at 45 FT)  (Read 2285 times)
NN2X
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Posts: 159




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« on: June 25, 2011, 07:50:15 PM »

Hi Ham brothers!

I am seeking a 40 Meter Antenna, I am restricted to 45 FT tower, The questioned is for DX, am I am better off using vertical, or a rotable dipole at 45 ft...?

By the way, open to any other suggestions, like vertical Moxon...

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W8JX
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« Reply #1 on: June 25, 2011, 08:30:55 PM »

well a dipole kinda needs to be at least 1/2 wave above ground to start showing good directivity but it would be worth a try for a 40m dipole 
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N4JTE
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Posts: 1158




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« Reply #2 on: June 25, 2011, 09:41:41 PM »

I think you will be MUCH happier with a rotatable dipole at that height even not optimum height. Verticals are great when a whole bunch of ground conditions are in your favor, not easy.
Regards,
Bob
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KB3LIX
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Posts: 1125




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« Reply #3 on: June 26, 2011, 12:54:48 AM »

Verticals have a lower angle of radiation, they SHOULD
be better for DX.

I use a Hustler 4BTV, mounted about 12' agl,
seems to work just find.

YMMY
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G3TXQ
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« Reply #4 on: June 26, 2011, 01:47:21 AM »

45ft is around the height that a dipole begins to better a vertical at low take-off angles on 40m. Over average ground, and at 10 degrees take-off angle, EZNEC puts the dipole about 1dB stronger than a quarter-wave vertical mounted over an extensive ground radial system.

Don't be misled by looking at the *shape* of the relative radiation patterns - it's not the shape that matters, it's how much signal is being radiated at the angles of interest to you. For example, the dipole at 45ft peaks at 45 degrees elevation whilst the vertical peaks at 25 degrees; but the dipole still puts out the stronger signal at 10 degrees elevation. Of course you would need to be able to rotate the dipole to see that advantage at all azimuths.

73,
Steve G3TXQ
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STAYVERTICAL
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Posts: 875




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« Reply #5 on: June 26, 2011, 03:27:32 PM »

If I could put up a rotary dipole at that height, I would certainly do it.
There is not really much advantage to putting a vertical on the top of that tower, except for height above noise sources and getting a good angle on the elevated radials.

I use both verticals (groundplane) and a rotary dipole for 20m.
The magic factor for me is that I use a remote antenna tuner at the base of the vertical and at the feedpoint of the rotary dipole.
That way, I can use the rotary dipole on 20m through 6m !
I know the radiation pattern varies band by band, but who cares.

Moxons, hexbeams, yagis etc are fantastic antennas, but they are stuck on a particular band, whereas a rotary dipole with a remote ATU works multiband and exhibits useful bi-directivity (beaming short AND long path).

My ATU is a FC40 remote unbalanced ATU, and I dont seem to have problems with common mode currents or other nasties, but in principle a balun would be dictacted. The choice is yours - this is an experimental hobby.

Good luck, and with your 45 ft tower you have a universe of choices.

73s.
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G8YMW
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« Reply #6 on: June 26, 2011, 04:01:37 PM »

You could always put up a delta loop with the tower supporting  the top corner and feed it at one of the bottom corners .
Someone I knew had one. He showed me a comparison between the delta and a dipole. As he switched between the 2 some came up while others went down.  A GM in Shetland told him he was the strongest G that he had worked!
With wire there is no reason you cant have both (Cheap enough)
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73 de Tony
Sent by WW2 Royal Navy signal lamp
VE3FMC
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Posts: 1001


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« Reply #7 on: June 26, 2011, 06:25:09 PM »

You could always put up a delta loop with the tower supporting  the top corner and feed it at one of the bottom corners .
Someone I knew had one. He showed me a comparison between the delta and a dipole. As he switched between the 2 some came up while others went down.  A GM in Shetland told him he was the strongest G that he had worked!
With wire there is no reason you cant have both (Cheap enough)

I once had a delta loop up with the apex at 45 feet. Fed it with 450 ohm ladder line and it kicked some serious butt on the bands back in the 90's. Until one winter evening when we got an ice storm and the antenna and mast loaded up with 1/4 inch of ice and it all came tumbling down  Sad
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N1UK
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Posts: 1572




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« Reply #8 on: June 27, 2011, 02:05:54 PM »

Take a look at the Alpha Delta Slopers. A lot of people swear by them. I have good results with mine and I can compare it against several different wire antennas. One wire is never always the best.


Mark N1UK
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WB2WIK
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Posts: 20636




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« Reply #9 on: June 27, 2011, 03:27:47 PM »

Rotary dipole, assuming it's "near full sized" and not horribly shortened.

The Cushcraft D40 and similar products from others are close enough to full sized that they work very well.

It will absolutely "blow away" a 40m vertical almost all the time, I'd bet the bank on that.

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W8JX
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« Reply #10 on: June 27, 2011, 08:35:13 PM »


Don't be misled by looking at the *shape* of the relative radiation patterns - it's not the shape that matters, it's how much signal is being radiated at the angles of interest to you. For example, the dipole at 45ft peaks at 45 degrees elevation whilst the vertical peaks at 25 degrees; but the dipole still puts out the stronger signal at 10 degrees elevation.



I would not bet the farm on this.  A dipole has edge on short to medium range skip angles but I would not rate it about a vertical on low angle long hauls.  A vertical polarized dipole would have a lower angle of radiation than a horizontal one.
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W8JI
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« Reply #11 on: June 28, 2011, 04:06:07 AM »

Unless you live in a salt water marsh, a dipole would by far be your best bet.

Avoid a loop, too. If you install any type of loop and it is horizontally polarized, you will simply lower the effective height and raise the wave angle. If the loop is vertically polarized you will have about the same performance as a good vertical, with a very slight directional pattern.

The last thing Hams should be looking at is take off angle or "angle of radiation", but that is what they do. The only thing that matters is absolute field strength at the desired angle or range of angles for transmitting.

73 Tom
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WB6BYU
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Posts: 13488




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« Reply #12 on: June 28, 2011, 09:23:05 AM »

Here are some sample pattern plots to demonstrate the difference between the angle of maximum
radiation and the magnitude of radiation at a specific vertical angle:

http://www.chem.hawaii.edu/uham/portant.html
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KQ6Q
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Posts: 991




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« Reply #13 on: June 28, 2011, 02:30:59 PM »

Put up a dipole oriented to the position you'll use it most, and also set up a way to feed your tower as a vertical (gamma match). then have a relay box at the tower or a coax switch in the station so you can have EITHER horizontal or vertical polarization, and can switch between them to see which is better on a given signal at a given moment. Often you will see a difference of several S-units. You really don't even need a rotator, and a 40 rotatable dipole will be quite a load. You could even just side-mount it near the top of the tower.
The switchable feedpoint setup does work - the military HF Discones we used to use let us select the feed at the top for 6-30MHz, or at the bottom for 3-6MHz.
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