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Author Topic: end fed vs center fed dipole?  (Read 12902 times)
KD8QOI
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« on: March 23, 2014, 06:38:33 PM »

Hello all!
 Sorry for asking a ton of questions but what type of dipole would perform better? A end fed or center fed one.. and why if you can explain?

 Thanks a bunch!
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W2AEW
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« Reply #1 on: March 23, 2014, 07:02:42 PM »

The performance of either will depend on how/where they are mounted. Assuming their similar, then for their designed band of operation, they will be similar.

However, a half wave end fed dipole will require a matching network at the feed point to match the 3000-4000 ohm input impedance of the antenna to the transmission line impedance.  This impedance matching network effectively restricts this antenna to single band operation.

A half wave center fed dipole will have a low input impedance at resonance, and will not require a matching network. This type of antenna can often be used on other bands with the use of a decent transmatch.
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AC5UP
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« Reply #2 on: March 23, 2014, 07:48:48 PM »

...what type of dipole would perform better? A end fed or center fed one.. and why if you can explain?

It's not as much a matter of performance as it is of practical limitations. A half wave of wire radiating at resonance will have the same pattern and efficiency regardless of how or where it's coupled to the feed line. As mentioned in the previous response, the feed point impedance is highest at either end and lowest at the center of the wire. What that exact impedance value is will be influenced by height over ground, proximity to other objects, and the properties of the wire itself. Any specific value can be found somewhere between the center and end of the wire but in practical terms the common feed line impedances of 50 through 600 Ohms are likely the values we're most interested in. Of those values the easiest to find is the lowest... Dead center on the wire... And the most natural match to the SO-239 on the back panel of a typical transceiver.

Which explains the popularity of center fed wire antennas in general and dipoles in particular.   They work.   And often on the first try.
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WA3SKN
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« Reply #3 on: March 24, 2014, 05:16:19 AM »

Most hams do not realize that there should be no difference in pattern in a half wave dipole fed at the end, the middle, or anywhere in between.
However, it is easy to use a center fed one on multiple bands, while end feed is usually a single band set up.  Patterns are affected by feed line radiation, etc in the local area, so multiband operation makes for a difficult feed.
A dipole, made one half wavelength (or more) at the lowest frequency, mounted as high as practical, and fed with balanced line and a tuner, is an easy multiband setup... do consider it!
73s.

-Mike.
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W5DXP
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« Reply #4 on: March 24, 2014, 05:18:18 AM »

A horizontal end fed wire depends on common-mode feedline radiation for the counterpoise (the "other half") of the antenna. Common-mode feedline radiation is generally considered to be undesirable an can result in RFI and RF-in-the-shack. If at all possible, go with the center-fed dipole with a 1:1 choke balun at the BALanced to UNbalanced junction(s).
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My antenna says, "What makes me happy is when the tuner is adjusted for maximum available current through my radiation resistance!" 73, Cecil, w5dxp.com
W4VR
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« Reply #5 on: March 24, 2014, 02:03:05 PM »

end feeding a horizontal antenna is not a good idea.  If at all possible center feed it.
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K2YO
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« Reply #6 on: March 24, 2014, 03:44:33 PM »

One of the big advantages of end feeding a dipole is for people who live in apartment buildings. I know a couple of hams in Queens who drop these out their window or crank them up a cloths line when they want to operate and reel them back in when they are done.

In a field operation, you only have to throw a line over one tree.

So there are practical advantages, along with the electrical issues of high impedance and high voltage.

I don't agree with W5DXP's explanation of the coax being the other half if the antenna. This is true if you cut the dipole to quarter wave length, but at half wave length and end feed, both halves of the circuit are in the wire.

Bernie
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WB6BYU
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« Reply #7 on: March 24, 2014, 04:12:33 PM »

Quote from: K2YO

I don't agree with W5DXP's explanation of the coax being the other half if the antenna. This is true if you cut the dipole to quarter wave length, but at half wave length and end feed, both halves of the circuit are in the wire.



I think you are talking about different halves of the antenna.

With an end-fed wire there are essentially 4 conductors at the feedpoint:  the antenna wire, the
coax center conductor, the inside of the coax shield, and the outside of the coax shield.  The currents
must be equal and opposite on the coax inner and on the inside of the shield, as these are operating
in a transmission line mode.

This means that, if there is any current flowing into the antenna wire, the same amount of
current must be flowing onto the outside of the coax shield
causing common mode current.  This
is a basic fact of physics, and holds true even when there is an impedance matching network at
the feedpoint.

Yes, you can add other conductors to give it somewhere else to flow - that's adding radials.

That doesn't mean that the antenna won't make contacts - it just means that, depending on the
length of your feedline and what else is connected to it, you can have as much radiation from it as
from the antenna.  Or you can have much less, especially if you can arrange for the current to flow
into a low impedance rather than a high one.

Of course it is also possible to have common mode current with a center-fed dipole.  It's not the
end of the world.


But that "half of your antenna", the current that flows onto the outside of the coax in the absence
of an alternate path (rather than the standing wave pattern along the antenna) cannot be prevented
on an end-fed antenna without the addition of some sort of additional radials or ground connection.
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K2YO
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Posts: 436




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« Reply #8 on: March 24, 2014, 04:30:40 PM »

With an end-fed wire there are essentially 4 conductors at the feedpoint:  the antenna wire, the
coax center conductor, the inside of the coax shield, and the outside of the coax shield.  The currents
must be equal and opposite on the coax inner and on the inside of the shield, as these are operating
in a transmission line mode.

This means that, if there is any current flowing into the antenna wire, the same amount of
current must be flowing onto the outside of the coax shield
causing common mode current.  This
is a basic fact of physics, and holds true even when there is an impedance matching network at
the feedpoint.

Using this definition, it would apply exactly the same to a center feed dipole. There are four wires at the connection. Any current flowing into the dipole, must return to the coax. I'm working at a disadvantage of years having since studied this so I'm using the term belief rather than know. Doesn't the resonate state of the length of the wire determine if it's behaving as a half wave dipole or a quarter wave dipole. A quarter wave needs a ground plan to effectively push against to make of for the missing part. A half wave segment will resonate within it's self.

Basing on the face that a half wave length piece of wire will resonate at a particular frequency, then it doesn't really matter where you inject the signal to start the resignation. The only significance of the injection point are the impedance and voltage issues at different portions of the wire, caused by the functions of the wave.

All of the end feed dipole I have seen or used are one half wave length in length. So the full resignation pattern of a half wave dipole applies. The only issue is make sure the feed point is properly engineered to transmit all of the power into the antenna rather than leaking it back down the coax.

I may be completely wrong here and if I am, please point out the specific areas and explain why.

Bernie
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W5DXP
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« Reply #9 on: March 24, 2014, 05:01:03 PM »

Basing on the face that a half wave length piece of wire will resonate at a particular frequency, then it doesn't really matter where you inject the signal to start the resignation.

What you are missing is there has to be two separate directions of current flow at the injection point. That's because transmission line currents exist as two equal out-of-phase currents. If the injection point is at the end of an antenna wire, there is only one direction possible in that wire. The other direction has to exist somewhere and that somewhere is down the outside braid of the coax (if no radials or counterpoise exists).

If it were possible to prevent any and all current from flowing down the outside of the coax, it would be impossible to get any current at all into the 1/2WL wire because the transmission line current waves would see an infinite antenna feedpoint impedance with all of the power being reflected and none accepted by the antenna wire.

Differential transmission line currents have to have two separate paths. In an end-fed, coax-fed antenna, one of those paths must be back down the outside braid of the coax. Here is a model of the currents in a Zepp's 1/4WL tuned feeder for a 1/2WL end fed wire. As one can see, there is exactly the same magnitude of common-mode current on the transmission line as the antenna wire feedpoint current. Note that this happens with ladder-line as well as with coax.

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My antenna says, "What makes me happy is when the tuner is adjusted for maximum available current through my radiation resistance!" 73, Cecil, w5dxp.com
WB6BYU
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Posts: 18135




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« Reply #10 on: March 25, 2014, 09:24:08 AM »

Quote from: K2YO

Using this definition, it would apply exactly the same to a center feed dipole. There are four wires at the connection. Any current flowing into the dipole, must return to the coax...




Actually in that case there are 5 wires:  the two antenna wires, the coax inner conductor, the inside
of the coax shield, and the outside of the coax shield.

On one side of the feedpoint the coax inner connects to one wire.  On the other side of the feedpoint
the other antenna wire and the outside of the shield are connected to the inside of the shield.
Since the current in the center conductor and the inside of the shield must be the same (transmission
line currents) then the currents in the two antenna wires are balanced only if no current flows on
the outside of the shield.  That's why we use a balun (though the same practical effect can be
obtained by other means, such as grounding the shield 1/4 wave from the feedpoint so it looks like
a high impedance.)

The difference between this and the end-fed case is the second antenna wire, which provides an
alternate path for the current other than the outside of the coax.


A good description of the various currents can be found in W7EL's paper on baluns here:
http://www.eznec.com/Amateur/Articles/Baluns.pdf
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W5WSS
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« Reply #11 on: March 25, 2014, 10:08:22 AM »

Well stated Gentlemen these things happen simultaneously and really fast!
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