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QST Article: An Easy to Build, Dual-Band Collinear Antenna
Dear OMs,
Via this newsgroup I learned that last September the following article appeared in QST: p.28: An Easy to Build, Dual-Band Collinear Antenna by Dan Friedrichs, K0IPG I am particularly interested in this article, because at this very moment I am designing myself a 70cm colinear. Unfortunately, I do not have access here to this article. Before I contact the ARRL's reprint service, I was wondering if somebody of this newsgroup who has read this article, could brievely describe the basis principle of this antenna (coils?, phased line?, electrical length?). Many thanks! 73 de Serge ON4BAA / HB9DWU http://salsawaves.com/propagation/ |
Well, Serge,
It consists of 5 major parts. A piece of 300 ohm twinlead, a coil, some wire, a copper disk plus a length of PVC pipe and cap on top. The twinlead matches 50 ohm input to the radiator, which consists of two half waves on 2M separated by a phasing coil. On 70 CM, the radiator will be two full wavelength radiators in phase. Matching section: Start with a piece of 300 ohm twinlead about 50 cm long. Strip and short one end. 3.8 cm from the short, remove a few mm of insulation from both conductors, and connect a 50 ohm coax to it, center conductor to one side (call it A) and shield to the other side. Now, trim the twinlead to exactly 42.4 CM long, leaving this end open. Remove 1.3 cm of insulation from the conductor on the A side. This is where the radiator will attach. Phasing coil: Dry wood dowel (or Plexiglas or polystyrene, but not PVC) 1.3 cm diameter (actually 1/2") and 3.8 cm (1.5") long. Drill a 2.5 or 3 mm hole through each end to hold the ends of a coil. You need about 55 cm of solid 20 gage (0.889 mm dia.) insulated wire. Wind 13 equally spaced turns of this 20 gage wire on the dowel and pass the ends through the holes. Cut the ends leaving a couple of cm on each end. Strip a cm of insulation off each end. These dimensions are likely to be fairly critical. Radiator: Made from straight pieces of 12 gage (2.053 mm) insulated solid wire. (Typical house wiring material in the US.) Cut one piece 97.8 cm long, strip a few mm of insulation from each end, and connect (solder) from A of the twin lead to one end of the coil. Cut a second piece the same (97.8 cm) length, and solder to the other end of the coil. The wire diameter is not critical, but the lengths are. Pipe and top disk: The author used a 10 foot (3.1 M) piece of 3/4" (nominal OD ~ 27 cm) PVC pipe to support this assembly. He cut a piece of copper clad material just larger than the ID of the pipe, drilled a hole in the center and passed the end of the second piece of 12 gage wire through it and soldered it. Then he dropped the antenna in the pipe, and put a PVC pipe cap over the end. The author recommends adjusting the lengths of the twin lead and the radiator wires to achieve best SWR. -- Crazy George Remove NO and SPAM from return address "Serge Stroobandt, ON4BAA" wrote in message ... Dear OMs, Via this newsgroup I learned that last September the following article appeared in QST: p.28: An Easy to Build, Dual-Band Collinear Antenna by Dan Friedrichs, K0IPG I am particularly interested in this article, because at this very moment I am designing myself a 70cm colinear. Unfortunately, I do not have access here to this article. Before I contact the ARRL's reprint service, I was wondering if somebody of this newsgroup who has read this article, could brievely describe the basis principle of this antenna (coils?, phased line?, electrical length?). Many thanks! 73 de Serge ON4BAA / HB9DWU http://salsawaves.com/propagation/ |
On Sat, 11 Oct 2003 22:20:19 -0500, "Crazy George"
wrote: On 70 CM, the radiator will be two full wavelength radiators in phase. Hi George, Hardly sounds very useful except for working satellites or mountain-top repeaters if you live in a valley. 73's Richard Clark, KB7QHC |
Dear George,
Thank you for having taken the time to answer me in such detail. This is very kind of you. 73 de Serge ON4BAA / HB9DWU http://salsawaves.com/propagation/ "Crazy George" wrote in message ... Well, Serge, It consists of 5 major parts. A piece of 300 ohm twinlead, a coil, some wire, a copper disk plus a length of PVC pipe and cap on top. The twinlead matches 50 ohm input to the radiator, which consists of two half waves on 2M separated by a phasing coil. On 70 CM, the radiator will be two full wavelength radiators in phase. Matching section: Start with a piece of 300 ohm twinlead about 50 cm long. Strip and short one end. 3.8 cm from the short, remove a few mm of insulation from both conductors, and connect a 50 ohm coax to it, center conductor to one side (call it A) and shield to the other side. Now, trim the twinlead to exactly 42.4 CM long, leaving this end open. Remove 1.3 cm of insulation from the conductor on the A side. This is where the radiator will attach. Phasing coil: Dry wood dowel (or Plexiglas or polystyrene, but not PVC) 1.3 cm diameter (actually 1/2") and 3.8 cm (1.5") long. Drill a 2.5 or 3 mm hole through each end to hold the ends of a coil. You need about 55 cm of solid 20 gage (0.889 mm dia.) insulated wire. Wind 13 equally spaced turns of this 20 gage wire on the dowel and pass the ends through the holes. Cut the ends leaving a couple of cm on each end. Strip a cm of insulation off each end. These dimensions are likely to be fairly critical. Radiator: Made from straight pieces of 12 gage (2.053 mm) insulated solid wire. (Typical house wiring material in the US.) Cut one piece 97.8 cm long, strip a few mm of insulation from each end, and connect (solder) from A of the twin lead to one end of the coil. Cut a second piece the same (97.8 cm) length, and solder to the other end of the coil. The wire diameter is not critical, but the lengths are. Pipe and top disk: The author used a 10 foot (3.1 M) piece of 3/4" (nominal OD ~ 27 cm) PVC pipe to support this assembly. He cut a piece of copper clad material just larger than the ID of the pipe, drilled a hole in the center and passed the end of the second piece of 12 gage wire through it and soldered it. Then he dropped the antenna in the pipe, and put a PVC pipe cap over the end. The author recommends adjusting the lengths of the twin lead and the radiator wires to achieve best SWR. -- Crazy George Remove NO and SPAM from return address "Serge Stroobandt, ON4BAA" wrote in message ... Dear OMs, Via this newsgroup I learned that last September the following article appeared in QST: p.28: An Easy to Build, Dual-Band Collinear Antenna by Dan Friedrichs, K0IPG I am particularly interested in this article, because at this very moment I am designing myself a 70cm colinear. Unfortunately, I do not have access here to this article. Before I contact the ARRL's reprint service, I was wondering if somebody of this newsgroup who has read this article, could brievely describe the basis principle of this antenna (coils?, phased line?, electrical length?). Many thanks! 73 de Serge ON4BAA / HB9DWU http://salsawaves.com/propagation/ |
I agree. Which is why I mentioned it. You should discuss it with the
author of the article. I was just trying to do a favor for someone "over there". -- Crazy George Remove NO and SPAM from return address "Richard Clark" wrote in message ... On Sat, 11 Oct 2003 22:20:19 -0500, "Crazy George" wrote: On 70 CM, the radiator will be two full wavelength radiators in phase. Hi George, Hardly sounds very useful except for working satellites or mountain-top repeaters if you live in a valley. 73's Richard Clark, KB7QHC |
Crazy George wrote:
I agree. Which is why I mentioned it. You should discuss it with the author of the article. I was just trying to do a favor for someone "over there". EZNEC sez that two vertical dipoles stacked end to end have a gain of about 3dBi with their sources in phase at their centers. I'm sure you have seen the UHF folded dipole vertical stacked arrays. -- 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! =----- |
That is absolutely correct, Cecil.
I am currently designing a 70cm colinear with two 5/8 elements, fed in the middle over a stub with balun and made entirely out of standard (in Europe) 12mm diameter copper tubing. AO (Antenna Optimizer) predicts a gain of 3.3dBi in free space. Due to the stub the pattern is not entirely uniform and therefore in one direction I would have a bit more gain upto 5dBi according to AO. In the coming weeks (sorry, but I have a demanding job and a HF antenna project that takes presidence) I will model the antenna also in EZNEC to double-check its matching to 50ohm and than build it. Of course I will share the results and final measure in this newsgroup. I had asked for the QST design in this group, simply to see if I could improve on my design but it does not seem so. Thanks guys for your help and Cecil, intersting stuff on your website! 73 de Serge ON4BAA / HB9DWU http://salsawaves.com/propagation/ "Cecil Moore" wrote in message ... Crazy George wrote: I agree. Which is why I mentioned it. You should discuss it with the author of the article. I was just trying to do a favor for someone "over there". EZNEC sez that two vertical dipoles stacked end to end have a gain of about 3dBi with their sources in phase at their centers. I'm sure you have seen the UHF folded dipole vertical stacked arrays. -- 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! =----- |
Cecil Moore wrote in message ...
Crazy George wrote: I agree. Which is why I mentioned it. You should discuss it with the author of the article. I was just trying to do a favor for someone "over there". EZNEC sez that two vertical dipoles stacked end to end have a gain of about 3dBi with their sources in phase at their centers. I'm sure you have seen the UHF folded dipole vertical stacked arrays. Cecil I used stacked vert dipoles and 12 watts for two meters when my son was in hospital a 100 miles away and we used a repeater nearby so he could use a H.T. I don.t remember seeing a MOUNTAIN here in Central Illinois and other antennas couldn't reach it I do like the dipole overlay on the vertical pattern on your page.Actualy I enjoyed the whole page Makes you wonder why all people don't use horizontal polarisation to get away from ground losses. The pattern looks just like the one I generated for a dipole over a beverage for the top band using AO Pro and just goes to show that max radiation at a high angle in not always bad. ( That comment will attract a few people to your page! ) Regards Art Regards Art |
"Art Unwin KB9MZ" wrote in message m... Cecil Moore wrote in message ... Cecil I do like the dipole overlay on the vertical pattern on your page.Actualy I enjoyed the whole page Makes you wonder why all people don't use horizontal polarisation to get away from ground losses. This conversation is starting to get interesting :-) INVERTED GROUNDPLANES: What about using an inverted 1/4wave groundplane in order to avoid ground losses? Then you have the high impedance point of the antenna close to the lossy (low impedance) earth. Due to the severe mismatch less power will couple into the earth. The low impedance point (feed point) of the antenna is safely high up in the (high impedance) air then. -- 73 de Serge ON4BAA - HB9DWU http://salsawaves.com/propagation/ |
That question has been answered several times in this newsgroup. For one
of the most recent answers, go to groups.google.com and find my posting on July 21, 2003 in the thread " efficiency of horizontal vs vertical antennas". Roy Lewallen, W7EL Serge Stroobandt, ON4BAA wrote: This conversation is starting to get interesting :-) INVERTED GROUNDPLANES: What about using an inverted 1/4wave groundplane in order to avoid ground losses? Then you have the high impedance point of the antenna close to the lossy (low impedance) earth. Due to the severe mismatch less power will couple into the earth. The low impedance point (feed point) of the antenna is safely high up in the (high impedance) air then. |
Serge
It is nice to read what other people have to say but it should never overide your own inclinations to the point that you stop experimenting Patterns are very informative but ground losses and efficiency are just as important factors, as Cecil's page shows with his overlapping patterns of a dipole over a vertical. Using the program that you have ( AO ) you can verify these things for yourself. I use the same program and it allowed me to create a rotatable dipole for the top band so that I was not encombered by ground losses. True the low dipoles tend to bea cloud warmer, but because of its higher efficiency it can equal the best of verticals as Cecil's page shows. After all we are only interested in a very small portion of the energy that we radiate and that is the portion used for communication, if the rest that is not used is used for cloud warming then so be it You have the best program around, go with your instincts with respect to ground losses but be sure that you have enough pulses available for acceptable confirmation. Enjoy the experimenting Regards Art Roy Lewallen wrote in message ... That question has been answered several times in this newsgroup. For one of the most recent answers, go to groups.google.com and find my posting on July 21, 2003 in the thread " efficiency of horizontal vs vertical antennas". Roy Lewallen, W7EL Serge Stroobandt, ON4BAA wrote: This conversation is starting to get interesting :-) INVERTED GROUNDPLANES: What about using an inverted 1/4wave groundplane in order to avoid ground losses? Then you have the high impedance point of the antenna close to the lossy (low impedance) earth. Due to the severe mismatch less power will couple into the earth. The low impedance point (feed point) of the antenna is safely high up in the (high impedance) air then. |
Serge, ON4BAA wrote:
"What about using an inverted 1/4wave groundplane in order to avoid ground losses?" How high above the earth can the inverted ground plane be placed? Brown, of the famous RCA trio of Brown, Lewis. and Epstein, invented the groundplane antenna. B,L,& E did most of the work on broadcast ground systems when broadcasting was new. Brown described the purpose of elevated radials as capture of the antenna current which otherwise might include the earth which could impose exorbitant loss. The energy fed to a groundplane produces capacitive (displacement) current between the vertical radiator and its return paths, be they radials, other antenna element(s), or the earth. If you place the groundplane upside down, you lose the screen between the radiator and the earth, which it hides from the radiator. If the groundplane is very high, capacitance between the radiator and the earth will be very small. Likewise, current between the radiator and the earth will be very small. So, the question of how high can the groundplane antenna be elevated is not entirely facetious. Loss is likely lower in the rightside up groundplane Best regards, Richard Harrison, KB5WZI |
Dear Roy, Richard, Art and other readers,
I had a look at that thread (for other readers' convenience included at the bottom of this message). Although it is not immediately answering my question whether inverted groundplane verticals present fewer losses than normal verticals, it contains some interesting observations. Again for the sake of the other readers, I took the liberty to rephrase and summarise these in terms that are more common in antenna literatu The electromagnetic field around an antenna exists out of two parts: a near field and a far field. Due to maths, the near field rapidly becomes negligible at larger distances, whereas the far field is the field we use normally to communicate. Losses in the near and far field can be reduced by reducing ground losses in the immediate vecinity of the antenna. This can be done through installing the antenna above "good RF ground", empoying a low impedance radial system (burried or elevated) for verticals or = 5/8 lambda or choosing a (double so long!) vertical antenna with a built-in return-path (half square, bobtail curtain, and half wavelength vertical). NOTE: Both the half square and the bobtail curtain can be interpreted as an array of 2, respectively 3 inverted ground plane verticals! (For a brief description see John Devolder, ON4UN, "Low-Band Dxing," chapter 12) Losses in the far field are in addition also caused by the ground properties at larger distances from the antenna, and are therefore less controlable (apart from choosing a coastal or salt-lake QTH). Now, my assumption is that by inverting a vertical ground plane, some of the near field ground losses could be prevented by presenting the high impedance end of the radiator to the low impedance of the earth immediately underneath. This tremendous mismatching would cause less coupling of the antenna's near field into the lossy earth. Assumptions of the mind are one thing, but eventually the proof of the pudding is in the eating (and this goes especially to Richard!). (Art, you see, you don't need to convince me anymore about antenna modelling ;-) However, I do will post a message about a possible bug in AO!) So I used version 1.0 of Roy's software, called EZNEC to model a couple of swiftly designed (i.e. non-resonant) antennas. (Roy, we need to talk about upgrading my version - I will send you a seperate email for this.) Here are the results: Normal 80m (3.650MHz) ground plane vertical with 4 elevated radials 4.5m above "good" ground (according to Christman KB8I, see also John Devolder, ON4UN, "Low-Band Dxing," chapter 9) radiator and radial lengths: 20m maximum height 24.5m azimuth 0°: 0.23dBi @ 22° elevation azimuth 45°: 0.21dBi @ 22° elevation Inverted 80m ground plane vertical with 4 elevated radials 24.5m above "good" ground radiator and radial lengths: 20m maximum height 24.5m azimuth 0°: 0.08dBi @ 22° elevation azimuth 45°: 0.12dBi @ 22° elevation Result: 0.09-0.15dB in favour of the normal ground plane. The radials up in the air of the inverted ground plane probably cause some shielding. The same difference remains when we lower both antennas to 1m above ground: Normal 80m ground plane vertical with 4 elevated radials 1m above "good" ground radiator and radial lengths: 20m maximum height 21m azimuth 0°: 0.00dBi @ 24° elevation azimuth 45°: 0.00dBi @ 24° elevation Inverted 80m ground plane vertical with 4 elevated radials 21m above "good" ground radiator and radial lengths: 20m maximum height 21m azimuth 0°: -0.16dBi @ 25° elevation azimuth 45°: -0.11dBi @ 25° elevation Being an appartment dweller myself, I once dreamed of putting up an inverted ground plane antenna using the building as a support for the radiator and the roof as support and hideawy for the two elevated radials. Normal 80m ground plane vertical with 2 elevated radials 1m above "good" ground radiator and radial lengths: 20m maximum height 21m azimuth 0°: -0.86dBi @ 24° elevation azimuth 45°: -0.66dBi @ 24° elevation azimuth 90°: -0.45dBi @ 24° elevation Inverted 80m ground plane vertical with 2 elevated radials 21m above "good" ground radiator and radial lengths: 20m maximum height 21m azimuth 0°: 0.55dBi @ 29° elevation azimuth 45°: 0.05dBi @ 26° elevation azimuth 90°: -0.96dBi @ 21° elevation Result: Now, the balance turned in favour of the inverted groundplane vertical in 3 of the 2 azimuth headings, probably because fewer radials for the inverted GP means less shielding. Differences for 0°, 45° and 90° azimuth are 1.41dB, 0.71dB and -0.51dB, respectively. FIRST TAKE-AWAY MESSAGE: Do not rely on accounts of particular antenna installation, always model your own antenna installation before jumping to conclusions. Ok, but now taking into account Roy's previous thread. How does a low dipole at the same height of 21m stands out of this? azimuth 0°: -5.77dBi @ 30° elevation azimuth 90°: 4.23dBi @ 30° elevation Result: In its preferred azimuthal direction, a low dipole, even at a height of about a quarter wavelength, still outperforms any ground plane vertical antenna. This is an eye-opener, taking into account that low dipoles (a.k.a. cloud burners) send most of their energy radially away from earth, without any ionospheric diffraction right into space. Most of the gain of the low dipole is pattern gain, however. This means that performance in the perpendicular azimuthal direction is extremely bad with -5.77dBi. SECOND TAKE-AWAY MESSAGE: Appartment dwellers might find an effective radiating system in installing a dipole on the roof of their building (may droop off at the ends) and making it (behind the current balun) remotely switchable into an inverted ground plane vertical antenna for working the directions that lie in the dip of the azimuthal dipole pattern. That is, the current balun should also be employed to make the vertical radiator current equal to the sum of the currents in the radials. PS1: I have posted also an exact copy of this message under the subject "Inverted ground plane antenna: compared with normal GP and low dipole." in order to attract more readers with this more appropriate title. PS2: Please, send your comments to the newsgroup under above-mentioned header. As of tomorrow I will be on a business trip for the next two weeks and will only be able to respond occasionally. (Not that I am afraid of you guys!!! :-P) -- 73 de Serge ON4BAA - HB9DWU propagation & ham info at: http://salsawaves.com/propagation/ "Roy Lewallen" wrote in message ... That question has been answered several times in this newsgroup. For one of the most recent answers, go to groups.google.com and find my posting on July 21, 2003 in the thread " efficiency of horizontal vs vertical antennas". Roy Lewallen, W7EL Serge Stroobandt, ON4BAA wrote: This conversation is starting to get interesting :-) INVERTED GROUNDPLANES: What about using an inverted 1/4wave groundplane in order to avoid ground losses? Then you have the high impedance point of the antenna close to the lossy (low impedance) earth. Due to the severe mismatch less power will couple into the earth. The low impedance point (feed point) of the antenna is safely high up in the (high impedance) air then. All messages from thread Message 1 in thread From: Ron ) Subject: efficiency of horizontal vs vertical antennas View this article only Newsgroups: rec.radio.amateur.antenna Date: 2003-07-15 18:38:29 PST There doesn't appear to be any ground loss resistance in horizontal antennas (e.g., dipoles) such as there is in verticals. Does this mean that verticals are virtually always less efficient than horizontals due to their ground loss? RonMessage 2 in thread From: Roy Lewallen ) Subject: efficiency of horizontal vs vertical antennas View this article only Newsgroups: rec.radio.amateur.antenna Date: 2003-07-15 19:01:01 PST A vertical dipole, or a base fed half wave vertical, has good efficiency and no significant ground loss. That is, there's no appreciable loss from current flowing through the ground to a feedline terminal. For other verticals, such as a base fed quarter wavelength, ground loss due to return currents can be reduced to an arbitrarily low value by using a good enough system of radials. However, after the wave is launched from the antenna, vertically polarized signals react differently than horizontally polarized ones when they strike the ground. Horizontally polarized waves are reflected with little loss, except ones at high angles. With vertically polarized waves, it's the low angle ones that take the greatest beating. And since most vertical antennas tend to concentrate radiation at low angles, a very sizable fraction of the total radiated power is often lost in the ground reflection. This mostly takes place well beyond a reasonably sized radial field, so radials don't help. The only way to reduce this loss is to see that the reflection takes place from highly conductive ground, like sea water(*). Otherwise, you lose most of the power that you want the most. (*) The effective conductivity of the ground improves as frequency decreases. So verticals tend to do better at lower frequencies, and often outperform horizontal dipoles on 160 and 80 meters, even over average ground. Roy Lewallen, W7EL Ron wrote: There doesn't appear to be any ground loss resistance in horizontal antennas (e.g., dipoles) such as there is in verticals. Does this mean that verticals are virtually always less efficient than horizontals due to their ground loss? Ron Message 3 in thread From: Bob Colenso ) Subject: efficiency of horizontal vs vertical antennas View this article only Newsgroups: rec.radio.amateur.antenna Date: 2003-07-21 15:16:10 PST But aren't there ground independant verticals, like the Half Square or a Bob-Tail-Curtain? U of M GO BLUE!!! God, Guns, and Guts Protect America!!!Message 4 in thread From: Yuri Blanarovich ) Subject: efficiency of horizontal vs vertical antennas View this article only Newsgroups: rec.radio.amateur.antenna Date: 2003-07-21 15:30:11 PST But aren't there ground independant verticals, like the Half Square or a Bob-Tail-Curtain? U of M GO BLUE!!! God, Guns, and Guts Protect America!!! Every vertical "needs" the ground for its efficient performance especially at the low angles. Vertical dipoles and their elevated cousins are still "looking" out at the ground, farther waway and with a bit less of participation. Wanna see dramatic display of salt water "ground" performance/contribution? Take your any verticaly polarized antenna and compare its performace between ground ground and salt water ground. You would see somewhere between 10 - 15 dB difference. YuriMessage 5 in thread From: Roy Lewallen ) Subject: efficiency of horizontal vs vertical antennas View this article only Newsgroups: rec.radio.amateur.antenna Date: 2003-07-21 16:51:50 PST There are two quite distinct sources of ground loss involved with vertical antennas. You're talking about one, and Yuri is talking about the other. Makes for a lively discussion, but it's a lot like the blind men describing the elephant. One type of loss is caused when it's necessary to connect one conductor of the feedline to the Earth. Current returning to this feedline conductor is equal in magnitude to the current flowing into the antenna from the other conductor, and it creates a simple I^2 * R loss flowing through the ground. This loss can be minimized by using a system of ground radials to decrease the loss resistance R near the base of the antenna, where the current density is highest. Another solution is to use a vertical antenna with a high feedpoint resistance. Examples are the half square, bobtail curtain, and half wavelength vertical. These antennas require very little feedpoint current, and consequently very little ground current. They can be very efficient with only a very simple ground system. But there's another source of loss, encountered after the signal is radiated. When a vertically polarized signal strikes the ground, a lot of its energy is lost to heating of the ground. This is particularly true at low angles of incidence. The end result is severe attenuation of low angle radiation. Particularly for low angles, this occurs farther away from the antenna than a reasonable radial system extends. So you're stuck with this loss, unless you can physically move your antenna to a swamp or similar high-conductivity environment. Horizontally polarized waves react differently. The demo version of EZNEC will show this quite dramatically. If you choose the MININEC type ground model, it acts like you have a perfect radial system. That is, the first source of loss I mentioned is zero. But the second is still there. You can simulate the effect of ground system loss simply by adding a resistive "load" at the antenna base. Compare the patterns of a vertical and horizontal, *to the same scale*, by superimposing them on a 2D plot, using different qualities of ground. You'll find it quite educational. Roy Lewallen, W7EL Yuri Blanarovich wrote: But aren't there ground independant verticals, like the Half Square or a Bob-Tail-Curtain? U of M GO BLUE!!! God, Guns, and Guts Protect America!!! Every vertical "needs" the ground for its efficient performance especially at the low angles. Vertical dipoles and their elevated cousins are still "looking" out at the ground, farther waway and with a bit less of participation. Wanna see dramatic display of salt water "ground" performance/contribution? Take your any verticaly polarized antenna and compare its performace between ground ground and salt water ground. You would see somewhere between 10 - 15 dB difference. Yuri Message 6 in thread From: luke ) Subject: efficiency of horizontal vs vertical antennas View this article only Newsgroups: rec.radio.amateur.antenna Date: 2003-07-22 07:12:04 PST Hi, Good answers to the question ! Always walk away a little smarter after reading this group ! 73 luke Roy wrote: There are two quite distinct sources of ground loss involved with vertical antennas. You're talking about one, and Yuri is talking about the other. Makes for a lively discussion, but it's a lot like the blind men describing the elephant. One type of loss is caused when it's necessary to connect one conductor of the feedline to the Earth. Current returning to this feedline conductor is equal in magnitude to the current flowing into the antenna from the other conductor, and it creates a simple I^2 * R loss flowing through the ground. This loss can be minimized by using a system of ground radials to decrease the loss resistance R near the base of the antenna, where the current density is highest. Another solution is to use a vertical antenna with a high feedpoint resistance. Examples are the half square, bobtail curtain, and half wavelength vertical. These antennas require very little feedpoint current, and consequently very little ground current. They can be very efficient with only a very simple ground system. But there's another source of loss, encountered after the signal is radiated. When a vertically polarized signal strikes the ground, a lot of its energy is lost to heating of the ground. This is particularly true at low angles of incidence. The end result is severe attenuation of low angle radiation. Particularly for low angles, this occurs farther away from the antenna than a reasonable radial system extends. So you're stuck with this loss, unless you can physically move your antenna to a swamp or similar high-conductivity environment. Horizontally polarized waves react differently. The demo version of EZNEC will show this quite dramatically. If you choose the MININEC type ground model, it acts like you have a perfect radial system. That is, the first source of loss I mentioned is zero. But the second is still there. You can simulate the effect of ground system loss simply by adding a resistive "load" at the antenna base. Compare the patterns of a vertical and horizontal, *to the same scale*, by superimposing them on a 2D plot, using different qualities of ground. You'll find it quite educational. Roy Lewallen, W7EL Read the rest of this message... (24 more lines) Message 7 in thread From: sideband ) Subject: efficiency of horizontal vs vertical antennas View this article only Newsgroups: rec.radio.amateur.antenna Date: 2003-07-22 19:01:10 PST Really? Even a vertical dipole? de AI8W, Chris Yuri Blanarovich wrote: But aren't there ground independant verticals, like the Half Square or a Bob-Tail-Curtain? U of M GO BLUE!!! God, Guns, and Guts Protect America!!! Every vertical "needs" the ground for its efficient performance especially at the low angles. Vertical dipoles and their elevated cousins are still "looking" out at the ground, farther waway and with a bit less of participation. Wanna see dramatic display of salt water "ground" performance/contribution? Take your any verticaly polarized antenna and compare its performace between ground ground and salt water ground. You would see somewhere between 10 - 15 dB difference. Yuri Message 8 in thread From: Yuri Blanarovich ) Subject: efficiency of horizontal vs vertical antennas View this article only Newsgroups: rec.radio.amateur.antenna Date: 2003-07-22 19:23:32 PST Really? Even a vertical dipole? de AI8W, Chris Yesereee! K2KW and "team vertical" (Force 12) did some tests and measurements across the San Francisco Bay and found that using half wave vertical dipole and moving it from the salt water/beach boundary (0 dB reference) they would get 3 dB gain being 1/4 wave away from the edge, - 2dB for 1/2 wave and + 2 dB for 3/4 wave. Anything that is vertically polarized gets help from better ground, especially at the low angles in the pattern. Claims of advertisers that their wundervertical needs no radials or ground are full of SWR! I am trying to find some practical results of verticals being operated on the side of a hill. Anyone out there with experience? Yuri da vertical fan BUm |
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