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Author Topic: 2 dipoles-1 feedpoint  (Read 553 times)
WD8DKB
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Posts: 181




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« on: January 12, 2004, 12:19:52 AM »

Hello. I would like to feed two 40 mtr dipoles from the same feedpoint. They would be of equal lengths but would be at right angles to each other hopefully resulting in an omnidirectional pattern and horizontal polarization. Would the feedpoint still present the same or close to the same impedance as a single dipole? Would the length of the dipole legs be different than with a single dipole due to the added wire from the other dipole? Would the system still resonate on its 3rd harmonic of 15mtrs? Thanks for any help.  Max  
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NA4IT
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« Reply #1 on: January 12, 2004, 12:46:55 AM »

I use a 40M / 160M choke dipole, a full size 80M dipole, and a full size 60M dipole all fed from the same center insulator with no problems at all.

It used to be set up for 10/15/20/40/80/ & 160. Go to http://www.qsl.net/na4it/farm.html to see more with diagrams.
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VA3TTN
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« Reply #2 on: January 12, 2004, 02:02:55 AM »

First of all, simply connecting two right-angled dipoles in does NOT provide omnidirectional coverage. Radiation will still be directional along the 45 degree line.

In order to get [nearly] omnidirectional coverage, currents must differ in phase by 90 degrees. Such a system is called "turnstile".

Some information is available on http://www.cebik.com


73,

Sinisa  YT1NT, VA3TTN
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WD8DKB
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Posts: 181




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« Reply #3 on: January 12, 2004, 07:34:39 AM »

Thanks for the replies gentlemen. Even if this setup does not afford omni' coverage, wouldn't it still give me more coverage than a single dipole? Wouldn't I have 2 figure 8 lobe patterns? Or does the phasing issue cause that not to be true? Will the feed point impedance still be the same or close? Will I have to use another feedline to use the other dipole? I just figured it could be done like a fan dipole with multiple elements for different bands only have the elements cut for the same band. Seems as though I would at least have a clover leaf pattern. From what you say here, the issue is that the dipoles are for the same band. If one dipole was cut for say 80mtr and the other cut for 40 mtr their would be no problem. That's the theory I was going on. Staighten me out on this if you would. But keep in mind that a pure omni' pattern is not necessary. Just covering more area than a single dipole and using one feedline is my goal here. Thanks again for your help. Max
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AC4FD
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« Reply #4 on: January 12, 2004, 09:23:52 AM »

Hi Max,

I don't believe that you will achieve any Gain from using the antenna's in this configuration although you may become more omni directional.

Have you thought about using an Inverted V?  This will give you a practially Omnidirectional pattern.  The gain will be slightly less than that of an actual Flat Top dipole, but I believe that you would achive about the same thing by the configuration that you are talking about.  Also, the impedance more closely approximates 50 ohms in the inverted V configuration.

I have heard that you can get 15meters from this but I have always opted to create another dipole for 15.  In my current configuration I have 10, 15, 17 and 40 meter dipoles all connected to a single feedpoint. and it is in a cage configuration.

I hope this gives you some ideas.

73

Randy
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G4IJE
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Posts: 245




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« Reply #5 on: January 12, 2004, 11:00:13 AM »

The feedpoint impedance of two identical dipoles in parallel would be half that of a single dipole (25-30 Ohms approx). I wonder what you are trying to achieve? Remember, you get vertical polarisation off the ends of a horizontal dipole so there is no deep "null" that needs to be filled in. If you are sure you really need an omnidirectional horizontally polarised antenna then you will need to use a phasing line to feed the two dipoles, as mentioned above.

73, Paul G4IJE.
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AA4PB
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« Reply #6 on: January 12, 2004, 01:22:54 PM »

I would suggest that on 40M unless your dipole is very high it is going to be mostly omni-directional anyway.

The transmit power will divide between the two dipoles so if they are very directional then you will essentially be subtracting energy from one direction in order to distribute it to the other. You might be better off with a switch to select one dipole or the other to change directions rather than trying to make it omni-directional.
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KT8K
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« Reply #7 on: January 12, 2004, 01:45:54 PM »

I'd try the switch, too.  Then throw in a 40M vertical and you can switch 3 ways to find which antenna works the best for a particular station and condition (or set up a diversity receiving setup that receives using two antennas at once for decreased QSB - search on eHam.net).

The turnstile, with 1/4 wavelength of feedline between the two dipole feedpoints, would work fine if you can get the whole thing up 65 feet or so.
For the same amount of wire you could make two inverted vees and phase them for directional gain!

You can't have too many antennas (unless they collapse of their own weight and fall on you ... but that would indicate a different problem ...)
73 es best dx de kt8k - Tim
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WB6BYU
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Posts: 13244




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« Reply #8 on: January 12, 2004, 05:30:01 PM »

The two dipoles are not independent, which is why
you don't get more omnidirectional radiation.  (If the
antennas were fed 90 degrees out of phase it would work
that way - this is the "turnstyle".)

Imagine standing broadside to a standard half wave
dipole.  You would see each wire connected to a different
side of the feedline.  Now imagine standing on the
45 degree line (between the wires) of two crossed
dipoles and look at the antenna.  In one quadrant you
would see the nearest wires connected to opposite sides
of the coax, but in the other quadrant you would see
the two nearest wires connected to the SAME side of the
coax, basically cancelling out the radiation in this
direction.  So all you have done to the pattern by
adding the second identical dipole is so shift the
nulls around by 45 degrees.

But, as has been pointed out, a dipole at a low height
(Say, under 3/8 wavelengths or so) has little directivity
in the pattern.  The theoretical null in a dipole
pattern only appears directly off the ends at zero
degrees elevation.  The higher the angle of radiation,
the more omnidirectional the dipole pattern.  And for
distances out to a few hundred miles (500km) or so,
the radiation angle is usually over 45 degrees.

You can see how this works by holding a pencil horizontally
in your fingers and rotating it around in front of your
face:  when the end of the pencil is pointing at your
eye, it virtually disappears.  But lay the pencil on
the flow and look down on it - no matter what direction
you look at it from, it never appears as a zero length
point.  The same thing happens with a dipole.
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WD8DKB
Member

Posts: 181




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« Reply #9 on: January 12, 2004, 05:56:30 PM »

I can't thank you folks enough. The responses have really been informative and most helpful. I must have had a senior moment because I totally forgot about the fact that at low heights the antenna has no real pattern to speak of and is pretty much omni-directional. I appreciate the explaination of phasing, that cleared up alot of questions right there. Heading out to hang some more wires,with seperate feedpoints, before the temp' drops again!  73 and thanks to all. Max
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W0ORW
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Posts: 45




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« Reply #10 on: January 13, 2004, 12:00:30 PM »

If you really want to build a turnstile, there is a way to do it without a phasing line. If one antenna is adjusted to yield 50 + j50 and the other is adjusted to 50 -j50 (or any other impedance - just so the reactance = impedance), they may be fed in parallel. ("Antennas" by John D, Kraus PhD, McGraw-Hill, 1950)
If you don't understand what that means, you may wish to reevaluate whether this is the antenna for you.

73,
Nick
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