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Windom antennas - down to earth
According to my ARRL Antenna Handbook, the Windom Antenna was described by Loren Windom in QST in 1929. The design is a horizontal half wave (ie single band) fed by a vertical single wire feedline attached just off centre (~14%). Explanations go that this approximately matches the feedline Zo (which is quite high) with the horizontal wire. It is single wire (ie ultimately unbalanced) feedline and therefore radiates. The Antenna is fed between the source end of the feedline and ground, and the load impedance should be somewhere in the many hundreds of ohms. The feedline carries an appreciable net current. More recently, the Off Centre Fed (OCF) Dipole design emerged, principally as a multi-band antenna. The OCF Dipole is a horizontal wire with a coaxial feed and coupling transformer (often called a balun) attached offset from the centre of the dipole. The feedpoint excursions at a half wave length and harmonic frequencies are much lower than centre feeding, and may be operated as a multiband antenna with reasonable efficiency, though it probably really needs an ATU at the tx end of the coax. The OCF dipole feedline does have current flowing on the outer of the outer conductor, at least as a result of the assymetric coupling to the dipole legs, and to some extent because the ineffectiveness of practical coupling transformers to isolate the feedline ends from the differing voltages on each dipole leg. The feedline carries an appreciable net current. Then along came the Carolina Windom, which appears to be a OCF dipole with a proprietary (ie secret, undescribed) coupling transformer, a vertical coax section (feedline and radiator) and a proprietary (remember the meaning) "isolator" located at a given distance along the coax to prevent the current flowing on the outer of the outer of the coax from flowing further towards the transmitter. The isolater would appear to be a ferrite choke and it would introduce a series impedance (reactance and resistance) to current on the coax, so influencing the establishment of the standing wave pattern on the outer of the outer of the coax. You might naively think that this isolator prevents current flowing into the shack, but that is unlikely. In all these cases, there is an expectation that the feedline carries a net radiating current, and it seems to me, that if you don't want to bring that into the shack, you need to design an appropriate solution. In the case of the true Windom, it seems the easiest solution is to end the single wire feedline outside the shack and place a matching unit connecting to ground and the single wire feedline at that point, and transforming the load to something suitable to coax or balanced feedline to the shack. In the case of the OCF Dipole and the Carolina Windom, shunting the current on the outer of the outer to ground outside the shack is a potential solution. Series chokes might help, but the magnitude of the choking impedance is limited, and their effectiveness could be improved greatly by a low impedance shunt to ground. Comments? Owen PS: In todays paranoid world where rules in many jurisdictions restrict the maximum permitted exposure to electromagnetic radiation, antennas such as these with radiating elements that are close to areas accessible by people are a safety challenge. |
Windom antennas - down to earth
Owen Duffy wrote:
According to my ARRL Antenna Handbook, the Windom Antenna was described by Loren Windom in QST in 1929. The design is a horizontal half wave (ie single band) fed by a vertical single wire feedline attached just off centre (~14%). Explanations go that this approximately matches the feedline Zo (which is quite high) with the horizontal wire. It is single wire (ie ultimately unbalanced) feedline and therefore radiates. The Antenna is fed between the source end of the feedline and ground, and the load impedance should be somewhere in the many hundreds of ohms. The feedline carries an appreciable net current. More recently, the Off Centre Fed (OCF) Dipole design emerged, ... EXCELLENT page on OCF construction: http://www.radioelectronicschool.net.../ocfdipole.pdf Will take a bit to load up in adobe reader (is a BIG file) on slow 56K dialup connection. JS -- http://assemblywizard.tekcities.com |
Windom antennas - down to earth
John Smith I wrote in news:esfki4$k29$1
@nnrp.linuxfan.it: Owen Duffy wrote: EXCELLENT page on OCF construction: http://www.radioelectronicschool.net.../ocfdipole.pdf Yes, a good article. But Ron doesn't really address the common mode current issue, and routing common mode current into the shack. Seems like an excuse to give him a ring, we haven't talked in a while. Owen |
Windom antennas - down to earth
Owen Duffy wrote:
... Yes, a good article. But Ron doesn't really address the common mode current issue, and routing common mode current into the shack. Seems like an excuse to give him a ring, we haven't talked in a while. Owen Yes, for that any good page on voltage\current baluns or hybrid baluns will bring one up to speed. For example, a page which provides more than you want to know: http://www.dxzone.com/cgi-bin/dir/jump2.cgi?ID=12661 -- http://assemblywizard.tekcities.com |
Windom antennas - down to earth
John Smith I wrote in
: http://www.dxzone.com/cgi-bin/dir/jump2.cgi?ID=12661 Hmmm, content hijackers. |
Windom antennas - down to earth
I tried a real Windom on 20 meters. Used the ARRL antenna book
formula. Ran a single wire feedline to the random wire terminal of an old Murch transmatch. Had a 1/4 wave elevated counterpoise attached to the ground terminal of the tuner. I couldn't match the thing to my little QRP rig to save my soul. Lots of RF and buzzing sounds. Ended up turning it into a standard dipole. bob k5qwg On Sun, 04 Mar 2007 22:07:30 GMT, Owen Duffy wrote: According to my ARRL Antenna Handbook, the Windom Antenna was described by Loren Windom in QST in 1929. The design is a horizontal half wave (ie single band) fed by a vertical single wire feedline attached just off centre (~14%). Explanations go that this approximately matches the feedline Zo (which is quite high) with the horizontal wire. It is single wire (ie ultimately unbalanced) feedline and therefore radiates. The Antenna is fed between the source end of the feedline and ground, and the load impedance should be somewhere in the many hundreds of ohms. The feedline carries an appreciable net current. More recently, the Off Centre Fed (OCF) Dipole design emerged, principally as a multi-band antenna. The OCF Dipole is a horizontal wire with a coaxial feed and coupling transformer (often called a balun) attached offset from the centre of the dipole. The feedpoint excursions at a half wave length and harmonic frequencies are much lower than centre feeding, and may be operated as a multiband antenna with reasonable efficiency, though it probably really needs an ATU at the tx end of the coax. The OCF dipole feedline does have current flowing on the outer of the outer conductor, at least as a result of the assymetric coupling to the dipole legs, and to some extent because the ineffectiveness of practical coupling transformers to isolate the feedline ends from the differing voltages on each dipole leg. The feedline carries an appreciable net current. Then along came the Carolina Windom, which appears to be a OCF dipole with a proprietary (ie secret, undescribed) coupling transformer, a vertical coax section (feedline and radiator) and a proprietary (remember the meaning) "isolator" located at a given distance along the coax to prevent the current flowing on the outer of the outer of the coax from flowing further towards the transmitter. The isolater would appear to be a ferrite choke and it would introduce a series impedance (reactance and resistance) to current on the coax, so influencing the establishment of the standing wave pattern on the outer of the outer of the coax. You might naively think that this isolator prevents current flowing into the shack, but that is unlikely. In all these cases, there is an expectation that the feedline carries a net radiating current, and it seems to me, that if you don't want to bring that into the shack, you need to design an appropriate solution. In the case of the true Windom, it seems the easiest solution is to end the single wire feedline outside the shack and place a matching unit connecting to ground and the single wire feedline at that point, and transforming the load to something suitable to coax or balanced feedline to the shack. In the case of the OCF Dipole and the Carolina Windom, shunting the current on the outer of the outer to ground outside the shack is a potential solution. Series chokes might help, but the magnitude of the choking impedance is limited, and their effectiveness could be improved greatly by a low impedance shunt to ground. Comments? Owen PS: In todays paranoid world where rules in many jurisdictions restrict the maximum permitted exposure to electromagnetic radiation, antennas such as these with radiating elements that are close to areas accessible by people are a safety challenge. |
Windom antennas - down to earth
John Smith I wrote:
Owen Duffy wrote: ... Yes, a good article. But Ron doesn't really address the common mode current issue, and routing common mode current into the shack. Seems like an excuse to give him a ring, we haven't talked in a while. Owen Yes, for that any good page on voltage\current baluns or hybrid baluns will bring one up to speed. For example, a page which provides more than you want to know: http://www.dxzone.com/cgi-bin/dir/jump2.cgi?ID=12661 Practical example of winding a guanella type balun ... (good detail in picture depicting green/white wires) http://www.n0ss.net/qrp_4-1_guanella-type_balun.pdf JS -- http://assemblywizard.tekcities.com |
Windom antennas - down to earth
On Mar 4, 4:07 pm, Owen Duffy wrote:
According to my ARRL Antenna Handbook, the Windom Antenna was described by Loren Windom in QST in 1929. I can't stand windoms myself... A poor design overall.. Bout par for 1929 technology.. I compared a dipole to one of those "carolina windoms" one time using a A/B switch.. It was ugly.. The dipole thrashed it. The windom setup had way too much loss. Mostly in the perverted mess of a feedline/tuner I suspect. But.... It doesn't bother me if anyone else wants to use them. IE: field day, etc. I like having an unfair advantage... :) All those windoms strung across the country on field day just help me get a better score... Ughhhmm, maybe I should keep my mouth shut... Yea.. Forget what I said. The windoms are great antennas. A very efficient antenna system, and everyone should use one. If you don't use a windom at field day, you ain't really living.. Those funky fed tuner/choke G5RV's also have a special place in my heart. Greatest antenna I've ever used.. :/ Grrrrr... I still remember losing about 3 mm of tooth length due to excessive grinding when using a "funky fed" G5RV on 80m at field day a few years ago.. MK |
Windom antennas - down to earth
John Smith I wrote in news:esg31b$n0a$1
@nnrp.linuxfan.it: John Smith I wrote: Practical example of winding a guanella type balun ... (good detail in picture depicting green/white wires) http://www.n0ss.net/qrp_4-1_guanella-type_balun.pdf Notwithstanding all the focus on current baluns, a current balun does not (by itself) prevent common mode current on an OCF Dipole feedline. The feedline has assymetric mutual coupling to the dipole and can not be expected in any configuration of significant length to be balanced wrt the dipole over a wide frequency range. Owen |
Windom antennas - down to earth
Owen Duffy wrote:
John Smith I wrote in news:esg31b$n0a$1 @nnrp.linuxfan.it: John Smith I wrote: Practical example of winding a guanella type balun ... (good detail in picture depicting green/white wires) http://www.n0ss.net/qrp_4-1_guanella-type_balun.pdf Notwithstanding all the focus on current baluns, a current balun does not (by itself) prevent common mode current on an OCF Dipole feedline. The feedline has assymetric mutual coupling to the dipole and can not be expected in any configuration of significant length to be balanced wrt the dipole over a wide frequency range. Owen Wide freq range? Well, 40-50uh should provide 500 ohms impedance to common mode currents on the outside of the braid of 50 ohm coax at ~2mhz, this is about the minimum usable (in my humble opinion), although I have seen designs only providing a 5:1 "common mode impedance to coax impedance", at lowest freq, in common use (and especially when spanning wide bandwidth. And, all figures used here are approx. and taken from my guess/memory/past experiments--although I am sure they are in the ballpark) At ~10 meters, the same inductance (same current balun) will present an impedance of 7,000+ ohms to common mode currents (impedance increasing with freq though the HF bands) It is true, coupling to the coax, past the current balun can occur (and probably most noticeable at low freqs/high power.) I seen a post by Richard addressing that very problem, I believe, and he is using ferrite beads along a section of coax leading away from the antenna, and apparently in addition to some other current balun at the antenna. I have seen others using an additional 1:1 current balun 1/4 wave away from the antenna on the coax, or near their rig. However, you will recognize that "magical" point where you realize minimum/reduced/tolerable common mode currents as the coax can be moved about without wild swings in SWR performance and you have a good match over the freqs/bands which the antenna is designed for (given that you don't have one end of the antenna close to a large metal building or some other structure/object providing an object to couple to and set up a LARGE imbalance.) Now, the above is only VERY general. As even the bifilar/trifilar/quadfiler windings and their spacings are of importance to a properly designed and constructed balun and will affect its' performance. I make only VERY GENERAL statements in the construction of these baluns and I don't wish to accept ANY responsibility in the results of someone using them. There are plentiful designs, many-many tried and true, available on the net and I leave finding those as an exercise to anyone wishing to use them. There is also abundant formulas/equations and examples given, on the net, on how to design your own (material to use for rod/toroid, wire, spacing, etc.) And someday we will talk about the "rest of the story", those guys who can hook a coathanger to their rig with a wet thread and realize good characteristics!!! (well, almost grin) Personally, I use the current balun whether it appears I need it or not, I find the "insertion loss" tolerable. But, I agree, in a perfect world there would be a better solution ... JS -- http://assemblywizard.tekcities.com |
Windom antennas - down to earth
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Windom antennas - down to earth
On Mar 5, 9:18 am, John Passaneau wrote:
We have used OFC dipoles here for a few years at field day. They fill a very specific set of needs for us. 1) They are simple to put up 2) The feed line coming from one end of the dipole is shorter than from center feed dipoles in our setup. 3) OFC dipoles offer an impedance at the end of the coax that is within the range of the tuners built into our radios on the bands that are important to field day. This simplifies our setup and operation. 4) They work as well as an antenna of that physical length on any one frequency would no mater how it's feed. The radiation pattern from an OFC is set by the length of the wire not where RF is feed in/out of the antenna. In our setup open wire line and tuners would be a pain in the butt, and an operational inconvenience that gains us nothing. Fan dipoles or separate dipoles are hard to setup and or tune and would perform no better for us. The antennas we use were built by myself and use a 4:1 current balun which minimize feed line radiation. On 80/40/20m we can easily match the antenna with the built in tuners so the SWR must be under 3:1. OFC dipoles don't work well on 15m but with the current sun spot cycle not a problem. We see no indication of common mode current problems, so we don't worry about it, we just operate and have fun. John W3JXP Sounds fine... Just as long as I don't have to use it.. :) Myself, I prefer either separate, or fan dipoles on the low bands.. 20-10, a tribander.. "A4S" I never use a tuner. All coax fed too... To each his own I say... MK |
Windom antennas - down to earth
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Windom antennas - down to earth
On Wed, 07 Mar 2007 19:16:57 -0600, Mike Coslo
wrote: OCF dipoles are obviously a compromise. Hi Mike, What happens to be the compromise? As a multiband antenna it better not outperform a specific band dipole. Seems unlikely to perform any different than any equal length of wire (the length of the fundamental band, that is). Only the feed Z changes is all (or so modeling would lead me to believe). 73's Richard Clark, KB7QHC |
Windom antennas - down to earth
On Wed, 07 Mar 2007 19:16:57 -0600, in rec.radio.amateur.antenna you
wrote: OCF dipoles are obviously a compromise. As a multiband antenna it better not outperform a specific band dipole. That's a general statement, but for a correctly fed OCF, your statement is generally false. If an OCF is shorter than 1/2 wave for the frequency of operation, it will not perform as well as the 1/2 wave dipole. However, as the length of the OCF increases over the 1/2 wave, it generates lobes of gain in various directions. How many lobes and to where they point changes according to the relative length to the band, but there is actually gain. You can find more information in the ARRL Antenna book or by modeling the antenna on one of the antenna model ling programs. -- 73 for now Buck, N4PGW www.lumpuckeroo.com N4PGW |
Windom antennas - down to earth
On Thu, 08 Mar 2007 05:24:52 -0500, Buck
wrote: If an OCF is shorter than 1/2 wave for the frequency of operation, it will not perform as well as the 1/2 wave dipole. Hi Buck, Modeling would suggest otherwise. The difference between a 1/4 wave center fed and off-center fed (at 10% from the end) is about a 12% increase in real resistance, and ..25dB gain improvement, both favoring the off-center fed. Are you comparing short antennas to long antennas? If so, it wouldn't be a surprise, would it? Coming in late, did I miss someone's extravagant claim that a OCF could do better? 73's Richard Clark, KB7QHC |
Windom antennas - down to earth
On Thu, 08 Mar 2007 08:03:56 -0800, Richard Clark
wrote: On Thu, 08 Mar 2007 05:24:52 -0500, Buck wrote: If an OCF is shorter than 1/2 wave for the frequency of operation, it will not perform as well as the 1/2 wave dipole. Hi Buck, Modeling would suggest otherwise. The difference between a 1/4 wave center fed and off-center fed (at 10% from the end) is about a 12% increase in real resistance, and .25dB gain improvement, both favoring the off-center fed. Are you comparing short antennas to long antennas? If so, it wouldn't be a surprise, would it? Coming in late, did I miss someone's extravagant claim that a OCF could do better? 73's Richard Clark, KB7QHC The original comment was that a multiband OCF antenna is a 'compromise' antenna and wouldn't be as good as dedicated 1/2 wave dipole cut to frequency (or so I understood it.) If a 135 foot OCF were compared to a 1/2 wave 160 meter dipole, the OCF would lose, but if it were compared to a 20 meter dipole, it would have gain in the direction of various lobes. Yes, the comparison is between different length antennas, and you are right, generally speaking, the longer, the better. (no doubt someone can find an exception to the rule, but that isn't the point of this discussion.) The reason for my statement of 'properly fed' is that I know that the impedance changes radically from band to band at the feed point. Buxcomm is selling a popular OCF with a 6:1 balun to coax. I don't think that is good as the impedance will be pretty low at the coax at times. However, to each his own. Feeding an OCF directly (no balun) with coax probably isn't a great idea either. I used one that way for a long time, and with great results (75 ohm indoor cable-tv coax) and switched to Radio Shack low-loss 300 ohm tv twin-lead which appeared to have better results. The best results seemed to come from a friend's setup which used 600 ohm twin lead (the good wire-man stuff) all the way to the tuner. My favorite OCF design is still the clothesline antenna... a loop of wire to make a 40 meter dipole at 300 ohms, a 4:1 balun with coax attached and a motor at the pulley on one end. Tune the antenna by moving the balun towards or away from the center of the dipole. No tuner needed and it worked adequately in the man's attic, according to the article I read. It is an old design, but interesting still today. ;) -- 73 for now Buck, N4PGW www.lumpuckeroo.com N4PGW |
Windom antennas - down to earth
On Mar 7, 7:16 pm, Mike Coslo wrote:
If you have found such a *drastic* difference however, perhaps there was something wrong with your particular antenna or setup? - 73 de Mike KB3EIA - Wasn't mine..It was one they were using at a field day. As far as I could tell, it was a regular ole carolina windom, fed with their feedline. I can't remember if he had a tuner inline. It was a dud though compared to a standard coax fed dipole. A good 2 s units down on *everything*. "40m" Noise, desired signals, the whole ball of wax. Obvious feeder loss... Sure, you can make contacts with such a device, but it's not for me.. Two S units difference is about the equal of adding an average amplifier to a 100 watt radio. I'm used to coax fed dipoles where the appx system efficiency is in the mid/upper 90's % range.. So almost any other compromise antenna is going to be inferior as far as total system loss. The main problem with the carolina windom I tried was the goofy feeder system with coax, choke, twin lead, etc... What a cluster%$#@ of engineering that is... : If I'm going to use a compromise one wire/all band antenna, it's going to be fed with ladder line the whole way to a tuner which will be carefully tuned using the least inductance possible. It will also be center fed. Even that will be inferior to my usual coax fed... I've compared.. But usually my preferred multi band antenna will be paralled dipoles, with the legs spread apart as far as possible.. Fed with a single coax feed. Thats what I use here at home. No loss in system efficiency compared to many other multi band designs. MK |
Windom antennas - down to earth
In rec.radio.amateur.antenna you write:
Wasn't mine..It was one they were using at a field day. As far as I could ....[snip].... But usually my preferred multi band antenna will be paralled dipoles, with the legs spread apart as far as possible.. Fed with a single coax feed. Thats what I use here at home. No loss in system efficiency compared to many other multi band designs. Maybe you can suggest why our formula-cut paralleled-dipole didn't work all that well at our last "Kids Day at the Mall" effort: Using some 4-foot-high decorative pillars about 15 feet outside the mall doors as base mounts, we were able to erect and guy two 25-foot-high metal masts about 40 feet apart. We then pulleyed-up a 10/15/20 meter coax-fed parallel-dipole (20m on top, 15m on bottom, 15m between) with the ropes to each dipole spread about 2 feet at the masts. VSWR was something like 6:1, although a "tuner" brought it down to where the radio worked OK. We made lots of contacts, but still haven't determined why the "raw" VSWR was so high. Any ideas? Sure. Coupling between the elements being they are so close together, and inline with each other. Thats why I spread mine apart as far as possible. The wider apart, the less coupling in general. If two dipoles are at right angles, they is almost zero coupling. In such a case, I've had a leg of one of the dipoles fall down, with little change in SWR for the other band being used. Also the wider apart, the less skewing of the pattern from a normal dipole pattern. You can use the type antenna you describe, but tuning is tricky. You have to start with the lowest band, get it tuned, and then add the next higher band, get it tuned, and so on. Thats about the only way you will get one tuned for each band. And I've seen weird things happen with close coupling. I've had cases where I would have to increase the length of the legs on a higher band to go *up* the band. Pretty weird.. With them spaced as far apart as possible, you don't have to go through all that tuning torture, and the antenna will act more like a normal dipole for each band used. I've used that system here at home for years. I change it up quite a bit depending on season, etc.. Right now, it's a turnstile on 80m, and a dipole on 40. At times, I'll have it 160,80,40 , and others 80/40/20.. Just depends what I'm doing at the time. MK |
Windom antennas - down to earth
Buck wrote:
On Thu, 08 Mar 2007 08:03:56 -0800, Richard Clark wrote: On Thu, 08 Mar 2007 05:24:52 -0500, Buck wrote: If an OCF is shorter than 1/2 wave for the frequency of operation, it will not perform as well as the 1/2 wave dipole. Hi Buck, Modeling would suggest otherwise. The difference between a 1/4 wave center fed and off-center fed (at 10% from the end) is about a 12% increase in real resistance, and .25dB gain improvement, both favoring the off-center fed. Are you comparing short antennas to long antennas? If so, it wouldn't be a surprise, would it? Coming in late, did I miss someone's extravagant claim that a OCF could do better? 73's Richard Clark, KB7QHC Sorry, I haven't been able to find replies until now- musta been a news server burp or something.. The original comment was that a multiband OCF antenna is a 'compromise' antenna and wouldn't be as good as dedicated 1/2 wave dipole cut to frequency (or so I understood it.) If a 135 foot OCF were compared to a 1/2 wave 160 meter dipole, the OCF would lose, but if it were compared to a 20 meter dipole, it would have gain in the direction of various lobes. And I'm not sure I would define those lobes as something other than a compromise. If the lobe is in a good place for you, fine. If not, not so fine. Yes, the comparison is between different length antennas, and you are right, generally speaking, the longer, the better. (no doubt someone can find an exception to the rule, but that isn't the point of this discussion.) I'm a little dense here. 8^) Is a antenna cut for a half wavelength at 80 meters a better antenna at 10 meters than an antenna specifically cut for 10 meters? I'd also have to go back and look, but isn't the SWR on some bands on the Capacitive reactance end, even though it may be 50 ohms? I know my old Icom did not like capacitive reactance very much. The idea that an OCF is superior to a dipole, and certainly Richard's statements would indicate that; makes me wonder why everyone isn't using them! - 73 de Mike KB3EIA - |
Windom antennas - down to earth
On Thu, 15 Mar 2007 15:57:50 -0400, Michael Coslo
wrote: On Thu, 08 Mar 2007 08:03:56 -0800, Richard Clark wrote: Coming in late, did I miss someone's extravagant claim that a OCF could do better? The idea that an OCF is superior to a dipole, and certainly Richard's statements would indicate that; makes me wonder why everyone isn't using them! Hi Mike, With all the intervening comments removed, want to reconsider that again? An OCF is simply resonant at exactly (by modeling) the same points as the dipole of the same length. Resonance resides in the wire, not the drive point. Of Course, F***! (expletive deleted to explain the meaning of OCF) as the drivepoint is moved through the length, its fundamental Z (still real) varies from low (at midpoint) to high (near endpoint) in much the same manner as we would expect for the difference between a conventional halfwave dipole and conventional halfwave end-fed. The harmonic drivepoint Zs follow their own sinusoidal roller coaster through the shift in feed point. 73's Richard Clark, KB7QHC |
Windom antennas - down to earth
On Mar 15, 10:40 pm, Richard Clark wrote:
On Thu, 15 Mar 2007 15:57:50 -0400, Michael Coslo wrote: On Thu, 08 Mar 2007 08:03:56 -0800, Richard Clark wrote: Coming in late, did I miss someone's extravagant claim that a OCF could do better? The idea that an OCF is superior to a dipole, and certainly Richard's statements would indicate that; makes me wonder why everyone isn't using them! Hi Mike, With all the intervening comments removed, want to reconsider that again? An OCF is simply resonant at exactly (by modeling) the same points as the dipole of the same length. Resonance resides in the wire, not the drive point. Of Course, F***! (expletive deleted to explain the meaning of OCF) as the drivepoint is moved through the length, its fundamental Z (still real) varies from low (at midpoint) to high (near endpoint) in much the same manner as we would expect for the difference between a conventional halfwave dipole and conventional halfwave end-fed. The harmonic drivepoint Zs follow their own sinusoidal roller coaster through the shift in feed point. 73's Richard Clark, KB7QHC Fine Guys except for one point WHY does a caroliner Windom have 2 Balums? I suggest for one reason to heat the garden I have known several catch fire. also the performance is not that great. mike M0DMD |
Windom antennas - down to earth
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Windom antennas - down to earth
Richard Clark wrote:
On Thu, 15 Mar 2007 15:57:50 -0400, Michael Coslo wrote: On Thu, 08 Mar 2007 08:03:56 -0800, Richard Clark wrote: Coming in late, did I miss someone's extravagant claim that a OCF could do better? The idea that an OCF is superior to a dipole, and certainly Richard's statements would indicate that; makes me wonder why everyone isn't using them! Hi Mike, With all the intervening comments removed, want to reconsider that again? An OCF is simply resonant at exactly (by modeling) the same points as the dipole of the same length. Resonance resides in the wire, not the drive point. Of Course, F***! (expletive deleted to explain the meaning of OCF) as the drivepoint is moved through the length, its fundamental Z (still real) varies from low (at midpoint) to high (near endpoint) in much the same manner as we would expect for the difference between a conventional halfwave dipole and conventional halfwave end-fed. The harmonic drivepoint Zs follow their own sinusoidal roller coaster through the shift in feed point. Sorry for the delay in response Richard, but as a confessed not-so-wise guy, I've gone back to modeling to see exactly where I've erred and to discover the source of my density. And darned if I can't figure it out! I've modeled Both OCF and frequency cut dipoles, and darned if the frequency cut dipoles don't look better. Your argument makes it sound as if the OCF has identical performance at those same points as a frequency cut dipole. Resonance or not, there are bands for which I still need a tuner, which makes the whole purpose a little moot. The SWR curve of the OCF really doesn't look all that hot, sometimes it is just passable at the frequencies of interest,and looks better off frequency. and it looks like something a radio with a *good* autotuner could take care of. That has been my experience with them. Sure seems like a compromise to me. YMMV. - 73 de Mike KB3EIA - |
Windom antennas - down to earth
Mike Coslo wrote in
: .... Sorry for the delay in response Richard, but as a confessed not-so-wise guy, I've gone back to modeling to see exactly where I've erred and to discover the source of my density. And darned if I can't figure it out! I've modeled Both OCF and frequency cut dipoles, and darned if the frequency cut dipoles don't look better. Your argument makes it sound as if the OCF has identical performance at those same points as a frequency cut dipole. Resonance or not, there are bands for which I still need a tuner, which makes the whole purpose a little moot. The SWR curve of the OCF really doesn't look all that hot, sometimes it is just passable at the frequencies of interest,and looks better off frequency. and it looks like something a radio with a *good* autotuner could take care of. That has been my experience with them. Sure seems like a compromise to me. YMMV. Mike, You seem to be considering just the flat-top of the OCF and that is not the only conductor of an OCF dipole antenna system carrying current, the other is the feedline. If you offset the source in a halfwave dipole (zero length feedline), I expect you will just see an increase in feed point R, and no significant change in loss. So on that basis you could argue they are equivalent... but you haven't compared an OCF dipole antenna system with a centre fed dipole antenna system. Then you talk about the SWR curve and ATU. Aren't you trying to compare the entire system? Is there much point in comparing the flat-top of an OCF with a centre fed, it is only part of the picture. Of course, the system performance will depend on assumptions that you make about the ground, feedline route, length, type, ATU etc... but having chosen a scenario, you can get to an overall performance figure that properly deals with the complex interaction between components. Owen |
Windom antennas - down to earth
On Sat, 17 Mar 2007 21:17:00 -0500, Mike Coslo
wrote: Your argument makes it sound as if the OCF has identical performance at those same points as a frequency cut dipole. Hi Mike, Well, there are two "performances" to consider (and not just the matinee and the evening show). Resonance or not, there are bands for which I still need a tuner, which makes the whole purpose a little moot. For that, the tuner will be called to perform different chores for different bands for different offsets. However, the resonances will fall principally at the same frequencies. The SWR curve of the OCF really doesn't look all that hot, sometimes it is just passable at the frequencies of interest,and looks better off frequency. Much the same could be said for any garden variety dipole. and it looks like something a radio with a *good* autotuner could take care of. Much the same could be said for any garden variety dipole. That has been my experience with them. Sure seems like a compromise to me. YMMV. Much the same could be said for any garden variety dipole. The two performances would be tune-up and launch characteristics. If modeling is any indication, the offset affects the magnitude of the R at resonance (again, no different an experiance comparing a normally fed half-wave dipole to an end-fed half-wave dipole). However, the gain, number of nulls (or lobes) does vary at the higher frequencies when offset is added to the variables. Higher gains for the near end-fed (albeit 1dB). 73's Richard Clark, KB7QHC |
Windom antennas - down to earth
In article ,
Richard Clark wrote: The harmonic drivepoint Zs follow their own sinusoidal roller coaster through the shift in feed point. Richard- Someone asked that if the OCF Dipole was so good, why didn't everyone use one? When I got my start back in the 50s, everyone did use one. I used my "Full Windom" for several years on 80/75/40/10 CW and AM. In today's world, the G5RV antenna appears to have taken over as the popular antenna of choice, and is probably equally as bad as the OCF Dipole. As a teenager I knew little about SWR. I used a balanced tuner to match the 300 Ohm feed-line, tuning for maximum brightness of a pilot lamp connected to a loop of wire taped to the feed-line. I understood that the feed-point was chosen so impedance was reasonably close to 300 Ohms on all bands except 15 Meters. Your reference to a roller coaster suggests that it might not be reasonably close. Using the modeling software, is there a feed-point where impedance is close to an available balanced feed-line on multiple bands? As close, I would accept a 2:1 SWR. Fred K4DII |
Windom antennas - down to earth
On Sun, 18 Mar 2007 14:31:38 -0400, Fred McKenzie
wrote: Using the modeling software, is there a feed-point where impedance is close to an available balanced feed-line on multiple bands? As close, I would accept a 2:1 SWR. Hi Fred, The usual designs include a BalUn that transforms from a higher drive Z to the 50 Ohms of a line. In that sense, the Off Center Dipole introduces accessible resonances at every harmonic instead of at odd harmonics. Depending upon the offset, some come into play, some go out and for a variety of transformations. Some suggest 2:1, others 4:1, and yet others higher. And you would still need to decouple the line (if the BalUn design doesn't already answer that). Given the field imbalance, it may require an aggressive decoupling (a second choke, or a distributed choking). I have a large document available to those whose mail box can stand the load. 73's Richard Clark, KB7QHC |
Windom antennas - down to earth
Fred McKenzie wrote in news:fmmck-9C2AC4.14313718032007
@nntp.aioe.org: In article , Richard Clark wrote: .... Someone asked that if the OCF Dipole was so good, why didn't everyone use one? When I got my start back in the 50s, everyone did use one. I used my "Full Windom" for several years on 80/75/40/10 CW and AM. In Fred, I think the term "OCF Dipole" is usually used today to mean a dipole fed with coax and balun (often 4:1, usually not 1:1) fed offset from the centre and often operated at half wave resonance or harmonic multiples. .... Using the modeling software, is there a feed-point where impedance is close to an available balanced feed-line on multiple bands? As close, I would accept a 2:1 SWR. If you are going to use an ATU and open wire line (as distinct from balanced line) why are you restricting the max VSWR to 2. Practical open wire lines can operate at much higher VSWR with acceptable losses. Once you have addressed that question, then ask yourself why you wouldn't just feed such a dipole in the centre and reduce the common mode current problem caused by the asymmetric feed. A dipole of more than about 35% wavelength at its lowest operating frequency, centre fed with practical open wire line and a good ATU will allow multiband operation with efficiency should be acceptable as part of the multiband compromise. For an example, look at Fig 10 in the article http://www.vk1od.net/G5RV/index.htm . Although the article is about the G5RV, Fig 10 is just a 100' dipole, centre fed with classic tuned feeder and ATU. Owen |
Windom antennas - down to earth
In article ,
Owen Duffy wrote: Fred, I think the term "OCF Dipole" is usually used today to mean a dipole fed with coax and balun (often 4:1, usually not 1:1) fed offset from the centre and often operated at half wave resonance or harmonic multiples. ... Using the modeling software, is there a feed-point where impedance is close to an available balanced feed-line on multiple bands? As close, I would accept a 2:1 SWR. If you are going to use an ATU and open wire line (as distinct from balanced line) why are you restricting the max VSWR to 2. Practical open wire lines can operate at much higher VSWR with acceptable losses. Once you have addressed that question, then ask yourself why you wouldn't just feed such a dipole in the centre and reduce the common mode current problem caused by the asymmetric feed. Owen- My friends with money used a 4-to-1 BalUn coil with their Windoms and drove them with rigs such as the DX-100 and Viking II. I think their Pi-network output stages matched a wider range of impedances than the modern solid state rigs can match, but I didn't know about that at the time. I thought the 300 Ohm TV feed-line was a close match to the antenna, and the BalUn transformed it to a nearly perfect 75 Ohms. My current interest is two-fold. First, I was curious to know just how good the match might have been on the old antenna. Second, it would be handy to have a multi-band antenna that could be fed off-center so the feed-line didn't have to run parallel to the antenna wire before entering the shack. I can afford a BalUn now. A little vertical radiation from the feed-line would be OK unless there was a problem with RF burns! Fred K4DII |
Windom antennas - down to earth
Owen Duffy wrote:
Fred McKenzie wrote in news:fmmck-9C2AC4.14313718032007 @nntp.aioe.org: In article , Richard Clark wrote: ... Someone asked that if the OCF Dipole was so good, why didn't everyone use one? When I got my start back in the 50s, everyone did use one. I used my "Full Windom" for several years on 80/75/40/10 CW and AM. In Fred, I think the term "OCF Dipole" is usually used today to mean a dipole fed with coax and balun (often 4:1, usually not 1:1) fed offset from the centre and often operated at half wave resonance or harmonic multiples. That would be the type that I'm talking about. I've used them and like them. For some reason I see them as a nice compromise antenna, and Richard doesn't. Or maybe I'm talking about this type, and Richard is speaking of the other, "classic" Windom. But then again, I'm way out of me league here! ... Using the modeling software, is there a feed-point where impedance is close to an available balanced feed-line on multiple bands? As close, I would accept a 2:1 SWR. If you are going to use an ATU and open wire line (as distinct from balanced line) why are you restricting the max VSWR to 2. Practical open wire lines can operate at much higher VSWR with acceptable losses. Once you have addressed that question, then ask yourself why you wouldn't just feed such a dipole in the centre and reduce the common mode current problem caused by the asymmetric feed. A dipole of more than about 35% wavelength at its lowest operating frequency, centre fed with practical open wire line and a good ATU will allow multiband operation with efficiency should be acceptable as part of the multiband compromise. For an example, look at Fig 10 in the article http://www.vk1od.net/G5RV/index.htm . Although the article is about the G5RV, Fig 10 is just a 100' dipole, centre fed with classic tuned feeder and ATU. That is pretty much exactly what I am using now, and it works a charm. It's a fine antenna, if a little tight to match on 75/80 meters. - 73 de Mike KB3EIA - |
Windom antennas - down to earth
"Owen Duffy" wrote in message ... According to my ARRL Antenna Handbook, the Windom Antenna was described by Loren Windom in QST in 1929. The design is a horizontal half wave (ie single band) fed by a vertical single wire feedline attached just off centre (~14%). Explanations go that this approximately matches the feedline Zo (which is quite high) with the horizontal wire. It is single wire (ie ultimately unbalanced) feedline and therefore radiates. The Antenna is fed between the source end of the feedline and ground, and the load impedance should be somewhere in the many hundreds of ohms. The feedline carries an appreciable net current. More recently, the Off Centre Fed (OCF) Dipole design emerged, principally as a multi-band antenna. The OCF Dipole is a horizontal wire with a coaxial feed and coupling transformer (often called a balun) attached offset from the centre of the dipole. The feedpoint excursions at a half wave length and harmonic frequencies are much lower than centre feeding, and may be operated as a multiband antenna with reasonable efficiency, though it probably really needs an ATU at the tx end of the coax. The OCF dipole feedline does have current flowing on the outer of the outer conductor, at least as a result of the assymetric coupling to the dipole legs, and to some extent because the ineffectiveness of practical coupling transformers to isolate the feedline ends from the differing voltages on each dipole leg. The feedline carries an appreciable net current. Then along came the Carolina Windom, which appears to be a OCF dipole with a proprietary (ie secret, undescribed) coupling transformer, a vertical coax section (feedline and radiator) and a proprietary (remember the meaning) "isolator" located at a given distance along the coax to prevent the current flowing on the outer of the outer of the coax from flowing further towards the transmitter. The isolater would appear to be a ferrite choke and it would introduce a series impedance (reactance and resistance) to current on the coax, so influencing the establishment of the standing wave pattern on the outer of the outer of the coax. You might naively think that this isolator prevents current flowing into the shack, but that is unlikely. In all these cases, there is an expectation that the feedline carries a net radiating current, and it seems to me, that if you don't want to bring that into the shack, you need to design an appropriate solution. In the case of the true Windom, it seems the easiest solution is to end the single wire feedline outside the shack and place a matching unit connecting to ground and the single wire feedline at that point, and transforming the load to something suitable to coax or balanced feedline to the shack. In the case of the OCF Dipole and the Carolina Windom, shunting the current on the outer of the outer to ground outside the shack is a potential solution. Series chokes might help, but the magnitude of the choking impedance is limited, and their effectiveness could be improved greatly by a low impedance shunt to ground. Comments? Owen PS: In todays paranoid world where rules in many jurisdictions restrict the maximum permitted exposure to electromagnetic radiation, antennas such as these with radiating elements that are close to areas accessible by people are a safety challenge. |
Windom antennas - down to earth
Back in 1974 I used to visit an amateur radio repair shop in Jacksonville
FL, it was located on Pearl, ave or st, cant remeber which near 8th street. I remember that quite a few hams would come into the place and often conversations about different aspects of ham radio would get started. One day they were discussing the Windom antenna with both praise and damnation. At the end(3hrs or so) at least I had deciced the the antennas greatest claim to fame was that it would load up on all the HF bands with at least mediocre performance, Probably not a bad antenna if you could only have one antenna. I do remember that someone working out current distribution in the antenna and claimed that on some bands the current was distrubted primarily in the vertical and the short section of the horizontal and in othe current was in the vertical and the long section of the horizontal section for other freqencies. I look back on this with a certain amount of aw(assuming he was reasoably correct in his computations) that this was performed with pencil and paper bag and not with aid of a computer. Jimmie "Owen Duffy" wrote in message ... According to my ARRL Antenna Handbook, the Windom Antenna was described by Loren Windom in QST in 1929. The design is a horizontal half wave (ie single band) fed by a vertical single wire feedline attached just off centre (~14%). Explanations go that this approximately matches the feedline Zo (which is quite high) with the horizontal wire. It is single wire (ie ultimately unbalanced) feedline and therefore radiates. The Antenna is fed between the source end of the feedline and ground, and the load impedance should be somewhere in the many hundreds of ohms. The feedline carries an appreciable net current. More recently, the Off Centre Fed (OCF) Dipole design emerged, principally as a multi-band antenna. The OCF Dipole is a horizontal wire with a coaxial feed and coupling transformer (often called a balun) attached offset from the centre of the dipole. The feedpoint excursions at a half wave length and harmonic frequencies are much lower than centre feeding, and may be operated as a multiband antenna with reasonable efficiency, though it probably really needs an ATU at the tx end of the coax. The OCF dipole feedline does have current flowing on the outer of the outer conductor, at least as a result of the assymetric coupling to the dipole legs, and to some extent because the ineffectiveness of practical coupling transformers to isolate the feedline ends from the differing voltages on each dipole leg. The feedline carries an appreciable net current. Then along came the Carolina Windom, which appears to be a OCF dipole with a proprietary (ie secret, undescribed) coupling transformer, a vertical coax section (feedline and radiator) and a proprietary (remember the meaning) "isolator" located at a given distance along the coax to prevent the current flowing on the outer of the outer of the coax from flowing further towards the transmitter. The isolater would appear to be a ferrite choke and it would introduce a series impedance (reactance and resistance) to current on the coax, so influencing the establishment of the standing wave pattern on the outer of the outer of the coax. You might naively think that this isolator prevents current flowing into the shack, but that is unlikely. In all these cases, there is an expectation that the feedline carries a net radiating current, and it seems to me, that if you don't want to bring that into the shack, you need to design an appropriate solution. In the case of the true Windom, it seems the easiest solution is to end the single wire feedline outside the shack and place a matching unit connecting to ground and the single wire feedline at that point, and transforming the load to something suitable to coax or balanced feedline to the shack. In the case of the OCF Dipole and the Carolina Windom, shunting the current on the outer of the outer to ground outside the shack is a potential solution. Series chokes might help, but the magnitude of the choking impedance is limited, and their effectiveness could be improved greatly by a low impedance shunt to ground. Comments? Owen PS: In todays paranoid world where rules in many jurisdictions restrict the maximum permitted exposure to electromagnetic radiation, antennas such as these with radiating elements that are close to areas accessible by people are a safety challenge. |
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