Home |
Search |
Today's Posts |
|
#1
|
|||
|
|||
End Effect on folded dipoles/monopoles?
To achieve resonance, non-folded dipoles/monopoles must be cut slightly less
than 1/2 or 1/4 wave due to "end effect", so I've read. EZNEC agrees. Is this true for the folded dipole/monopole? Thanks, John |
#2
|
|||
|
|||
"John" wrote in message ... To achieve resonance, non-folded dipoles/monopoles must be cut slightly less than 1/2 or 1/4 wave due to "end effect", so I've read. EZNEC agrees. Is this true for the folded dipole/monopole? Thanks, John After I posted this, I learned that EZNEC would handle the loop formed by a 1/4-wave monopole. So I modeled it and found that the answer to my question is yes. John |
#3
|
|||
|
|||
John wrote:
"To achieve resonance, non-folded dipoles/monopoles must be cut slightly less than 1/2 or 1/4 wave due to "end effect", so I`ve read. EZNEC agrees. Is this true for the folded dipole/monopole?" My ARRL Antenna Book (19th edition, page 8) says: "Since the antenna section (of 300-ohm twin-lead) does not operate as a transmission line, but simply as two wires in parallel, the velocity factor of twin-lead can be ignored in computing the antenna length." I wish the author had said: "---the transmission line velocity factor of twin-lead can be ignored---." Wave velocity on the antenna wire is very important. End effect is sometimes defined as the capacitive effect at the ends of a 1/2-wave antenna. Length in free-space wavelengths is reduced in an antenna by a "fat" radiator. The 1/2-wave resonance point (first resonance in an ordinary open-circuit dipole), for a thin radiator, produces a drivepoint resistance of 72 ohms. This can be cut in half by using conductors with a very large periphery. Radiators of large periphery slow the wave velocity along the surface. This reduces the physical length required for electrical resonance. Feedpoint resistance of the center-fed 1/2-wave dipole results from far-end reflections.In the open-circuit dipole, the high impedance at the ends of the radiators is transformed by the 1/4-wave return to the feedpoint into a low impedance. The short-circuit at the far ends of a 1/2-wave folded dirole is reflected to the drivepoint as a high impedance. Constructed of the same size wire, a folded dipole has 2 wires effectively in parallel as radiators and their size is enhanced by spacing so that wave velocity is reduced more than in the open-circuit dipole. The slower the antenna wave velocity, the shorter the length to produce resonance. I would expect more "end effect" in the usual folded dipole / monopole. Best regards, Richard Harrison, KB5WZI |
#4
|
|||
|
|||
Richard Harrison wrote:
My ARRL Antenna Book (19th edition, page 8) says: "Since the antenna section (of 300-ohm twin-lead) does not operate as a transmission line, but simply as two wires in parallel, the velocity factor of twin-lead can be ignored in computing the antenna length." I wish the author had said: "---the transmission line velocity factor of twin-lead can be ignored---." The phase of the currents in the adjacent sections of twinlead is what is important. If the phase of the adjacent currents is 180 degrees, the twinlead is acting like a transmission line and T-line VF must be taken into account. If the phase of the adjacent currents is zero degrees, the twinlead is acting like an antenna and the VF is considerably higher, essentially equal to insulated wire. If the phase of the adjacent currents is zero degrees, all the current is "common-mode current", something not desirable for transmission lines but something most desirable for antennas since common-mode currents do not inhibit radiation. Bottom line: The currents flowing in a folded dipole antenna are common- mode currents which radiate, not transmission line currents which do not radiate (much), and that's a very good thing for an antenna. -- 73, Cecil http://www.qsl.net/w5dxp -----= Posted via Newsfeeds.Com, Uncensored Usenet News =----- http://www.newsfeeds.com - The #1 Newsgroup Service in the World! -----== Over 100,000 Newsgroups - 19 Different Servers! =----- |
#5
|
|||
|
|||
Cecil, W6DXP wrote:
"Bottom line: The currents flowing in a folded dipole are common-mode currents which radiate, not transmission line currents which do not radiate (much), and that`s a very good thing for an antenna." Nicely said! Agreed that it is the common-mode currents which radiate, but differential-mode currents exist too. At the tips of the folded dipole, some current turns the corner and flows in the opposite direction after its U-turn. This creates a situation much like the reflection from the open circuit ends of the common dipole. At the center of the continuous wire which ties the tips of the driven side of the folded dipole together, the colliding currents have traveled the same distance at the same velocity so they are still 180-degrees out-of-phase. This amounts to a short-circuit, and in fact this amounts to a zero voltage point which may be grounded without consequences in most cases. In the folded monopole (unipole), the currents into the input terminals flow nearly as they would flow into any 1/4-wave short-circuit stub. The difference seems to be that the grounded side of the transmission line feeds both its side of the folded unipole and the ground plane, creating an opportunity for imbalance and radiation. And, radiate it does with very nearly the same characteristics as an open-circuit ground plane antenna. Best regards, Richard Harrison, KB5WZI |
#6
|
|||
|
|||
Now that it`s posted, I see I moved Cecil back to 6-land. That would be
unfortunate and I apologize. Happily, Cecil is now back in Texas where he can comingle with the Sidewalk Cattlemen, I believe. Cecil`s call: W5DXP Best regards, Richard Harrison, KB5WZI |
#7
|
|||
|
|||
I have absolutely no experience with analyzing a folded dipole. I would like to take advantage of this thread to ask for some information. I had thought of a folded dipole as an antenna with a pair of 1/4 wave shorted stubs across its feed point. I thought the antenna length for a 1/2 wave dipole would be the same if it was fed as a 72 ohm dipole or fed as a 300 ohm folded dipole. But, if there was some dielectric loading in the twin lead it would make it necessary to account for the VP of the twin lead used for the *folded dipole" so the impedance of the stubs would be accounted for. Specifically -- If a 1/2 wave folded dipole an antenna is constructed with a twin lead with a VP of 0.9, and the total antenna length is close to 1/2 wave, the shorted stubs will impose a capacitive reactance shunting the feed point impedance. But, when I take my thinking to extreme configurations, my ideas seem to fail. I'd sure like to get some "real life" data. I'm sure some of you guys know if the real life folded dipole gets measurably effected by VPs of 0.8 or 0.9. Jerry "Cecil Moore" wrote in message ... Richard Harrison wrote: My ARRL Antenna Book (19th edition, page 8) says: "Since the antenna section (of 300-ohm twin-lead) does not operate as a transmission line, but simply as two wires in parallel, the velocity factor of twin-lead can be ignored in computing the antenna length." I wish the author had said: "---the transmission line velocity factor of twin-lead can be ignored---." The phase of the currents in the adjacent sections of twinlead is what is important. If the phase of the adjacent currents is 180 degrees, the twinlead is acting like a transmission line and T-line VF must be taken into account. If the phase of the adjacent currents is zero degrees, the twinlead is acting like an antenna and the VF is considerably higher, essentially equal to insulated wire. If the phase of the adjacent currents is zero degrees, all the current is "common-mode current", something not desirable for transmission lines but something most desirable for antennas since common-mode currents do not inhibit radiation. Bottom line: The currents flowing in a folded dipole antenna are common- mode currents which radiate, not transmission line currents which do not radiate (much), and that's a very good thing for an antenna. -- 73, Cecil http://www.qsl.net/w5dxp -----= Posted via Newsfeeds.Com, Uncensored Usenet News =----- http://www.newsfeeds.com - The #1 Newsgroup Service in the World! -----== Over 100,000 Newsgroups - 19 Different Servers! =----- |
#8
|
|||
|
|||
Jerry Martes wrote:
"I`d sure like to get some "real life" data." Personal experience testimonials are often unreliable. Modeling data are often more complete, skillfully designed, well executed, and less colored by the operator`s opinion, by the right modeler. One highly educated, experienced, and competent source is L.B. Cebik, W4RNL. The plain folded dipole is just one of the many antenna types he has modeled and enriched his web pages with. He compares it with the common open-circuit single wire dipole in his analysis. Just search on "folded dipole". Cebik`s web pages will appear near the top of your list of options. Click on the most likely of your options and you are there. Best regards, Richard Harrison, KB5WZI |
#9
|
|||
|
|||
Richard I have a severly limited capacity for understanding alot of theoritical presentations. I did read some of Cebik's information, and I still wonder if the velocity of propagation of the "twin lead" used for the folded dipole can be ignored. I wonder if the VP of the twin lead is an important consideration when designing a folded dipole. If the resonant frequency of a folded dipole is identified as that frequency where the input (feed point) impedance is R+/-j0, it seems that the 1/4 wave stubs that shunt the feed point might strongly effect the input impedance. I recognize that there is a good chance that my reasoning is wrong. It may be that the "stubs" that I consider to be shunting the fed point, are not acting the same as a 'non disipative' stub. But, this is where my mind could benefit from having some "lab data" which is what I refer to as "real life" data. Jerry "Richard Harrison" wrote in message ... Jerry Martes wrote: "I`d sure like to get some "real life" data." Personal experience testimonials are often unreliable. Modeling data are often more complete, skillfully designed, well executed, and less colored by the operator`s opinion, by the right modeler. One highly educated, experienced, and competent source is L.B. Cebik, W4RNL. The plain folded dipole is just one of the many antenna types he has modeled and enriched his web pages with. He compares it with the common open-circuit single wire dipole in his analysis. Just search on "folded dipole". Cebik`s web pages will appear near the top of your list of options. Click on the most likely of your options and you are there. Best regards, Richard Harrison, KB5WZI |
#10
|
|||
|
|||
Jerry Martes wrote:
I had thought of a folded dipole as an antenna with a pair of 1/4 wave shorted stubs across its feed point. "Stub" has more than one meaning and therefore, more than one response. Your definition of "stub" seems to assume the currents are 180 degrees out of phase. Therefore, you should not use the word "stub" on a folded dipole antenna where the currents are in phase. It's a semantics problem. If you revise your definition of "stub" to include stubs with in phase currents, you must give up on your present definition of stubs with only out of phase currents. Most of us have a feel for the difference between a parallel stub fed from a line with balanced currents and a series stub where the currents can have any phase relationship. "Series stubs" is a very confusing topic and could support a technical article of some kind. I don't remember it ever being explained in detail before, at least in the amateur literature. -- 73, Cecil http://www.qsl.net/w5dxp -----= Posted via Newsfeeds.Com, Uncensored Usenet News =----- http://www.newsfeeds.com - The #1 Newsgroup Service in the World! -----== Over 100,000 Newsgroups - 19 Different Servers! =----- |
Reply |
|
Thread Tools | Search this Thread |
Display Modes | |
|
|
Similar Threads | ||||
Thread | Forum | |||
How does rain effect antennas | Antenna | |||
Folded monopole dilemma | Antenna | |||
Folded monopole dilemma | Antenna | |||
Folded Dipole | Antenna | |||
Bricks effect in dipole resonance? Help! | Antenna |