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Radials
"John S" wrote in message ... : 0 deg 1.09 dBi @ 12.5 deg 30 deg 1.37 dBi @ 12.5 deg 45 deg 1.66 dBi @ 12.5 deg I would appreciate a definition of gain as used in this thread. I have a hard time understanding how a passive device can supply gain. Thanks, John If you look above you will see dBi. That is dB over an isotropic antenna. That type of antenna is not possiable to make, but a math modle. It is single point where the power is radiated from equally in all directions. To get "gain" you take some of the power in some directions and put in another direction. For sake of discussion, look at a simple dipole. There will not be much radiation off the ends of the antenna so there will be more at right angles to the wire. The differance is the 'gain'. As I mentioned, gain by its self does not help and can actually hirt the signal if it is not in the right direction. The gain of the verticals , unless designed for a certain distance, does not mean much unless you can tilt it so the maximum gain lobe is heading in the direction you want. At field day one fellow was always wanting to put up an extended double Zep for the low bands. He talked about the gain. While it has gain in some directioins, it has 'loss' or 'negative gain' in others, but he was not thinking about that, just raw gain numbers. I have up a 3 element beam for 20, 15,10 meters and also an off center fed antenna that is about 125 feet long near the same height. On 20 meters in some directions the OCF and beam are at almost the same strength. At others, there is around 20 dB of differance.. It is just not practical to rotate that OCF. If it was, I would just use it. --- This email is free from viruses and malware because avast! Antivirus protection is active. http://www.avast.com |
Radials
Wimpie wrote:
El 04-04-14 2:35, escribió: snip Yep, but it isn't a GP antenna which by definition has a radiator about 1/4 lambda. I understand your reasoning, but do a google (image) search on "GPA antenna" and you we see that many people don't follow your convention. 5/8 and 1/2 WL are also included. It is not the name, but the operating princple that is of importance. Sorry, I follow what engineering text books say, not what some Google search algorithm comes up with. snip You will have common mode currents of some magnitude with ANY GP type antenna. Clear, but by using a half wave radiator (end-fed) CM current is about 13 dB lower to start with, and that saves me a lot of aluminum that doesn't contribute to the radiation. Again, I know design/construction is more elaborate. Did you include the coax shield in your simulation? All antennas connected with coax will have a long conductor consisting of the coax shield running from the radials to about ground. You will be hard pressed to notice 1 dB difference in a typical amateur system. that was the reason I mentioned: "The effect of sloping angle on zero elevation gain is small, and you get hardly measurable more gain when they are almost vertical. Sloping radials have some other advantage: less birds." The biggest advantage to sloping radials is they move the impedance from 20 Ohms to much closer to 50 Ohms. I still can't reproduce, or find a reliable reference for your 3.67 dbi for a quarter wave with quarter wave 85 degr sloping radials. This will make the third time I will say that number is probably the result of small angles and closely spaced wires, i.e. garbage. We drifted away somewhat from Irv's posting.... -- Jim Pennino |
Radials
John S wrote:
On 4/3/2014 7:37 PM, wrote: Ralph Mowery wrote: wrote in message ... Note than because we are now over real ground vertical lobes are formed. Again I will leave it as an exercise for the reader to get the demo EZNEC and view the graphs. droop impedance max gain length SWR 0 deg 22.6 Ohms 2.48 dBi @ 35 deg .245373 lambda 2.12 30 deg 43.4 Ohms 2.24 dBi @ 40 deg .236269 lambda 1.15 45 deg 51.1 Ohms 1.94 dBi @ 45 deg .231667 lambda 1.022 It should be noted that there is a large second lobe: 0 deg 1.09 dBi @ 12.5 deg 30 deg 1.37 dBi @ 12.5 deg 45 deg 1.66 dBi @ 12.5 deg So which antenna is "best" in the real world? I would go for 5% longer radials drooping at 45 degress. Now we are getting somewhere in the discussion. For simple antennas that can not be rotated unless one wants to talk to a certain distance, the antenna does not make much if any differance. You get 'gain' in one direction and 'loss' in another. Just match it to the coax and take what you get. There is no real gain in an antenna, just redirecting the power that is supplied to it. And there is no such thing as cold, just the absense of heat... Hi, Jim - I would appreciate a definition of gain as used in this thread. I have a hard time understanding how a passive device can supply gain. Thanks, John By definition. Antenna gain is defined as the ratio of the maximum field strength in the far field compared to the field strength of an isotropic radiator expressed in decibels. That is dBi. Some times the comparison is to a dipole, in which case it is dBd. FYI the gain of a lossless dipole is 2.15 dBi. An isotropic radiator is a theoretical antenna that produces a perfectly spherical pattern. -- Jim Pennino |
Radials
On Friday, April 4, 2014 12:26:30 PM UTC-5, wrote:
A 5/8 system mounted at 13 feet (top of an average house) will have a squarish pattern with a maximum of 4.4 dBi and a minimum of 2.2 dBi. A 1/4 system at the same height has a circular pattern of 4.9 dBi. In both cases the major elevation lobe is at 20 degrees. Free space patterns can look wonderfull, but unless you live on the ISS your antenna is going to be mounted over ground. True, but all mine have been at 36-40 feet up, so the numbers change a bit. When I modeled them all, I used a height of 40 ft, and "avg" ground if I remember right. At 40 ft, the 1/4 GP with sloping radials gave a max 3.003 dbi at 9 degrees. The 1/2 wave gave a max 3.831 dbi at 8 degrees. The 5/8 with straight 1/4 radials, 4.293 dbi at 39 degrees.. Not good. :( The 5/8 with 45 degree sloping 5/8 radials, 4.941 dbi at 8 degrees. The same with even steeper sloping radials, 5.334 dbi at 9 degrees. |
Radials
On Friday, April 4, 2014 11:58:29 AM UTC-5, Wimpie wrote:
Other thing with the half wave end-fed can be absorption of the plastic insulation. To make UV-resistant black they frequently use carbon black. When the concentration is too high the dissipation factor of the plastic increases signficantly. Recently I had really weird results with a half wave monopole design. Everything went wrong. It was the black UV-stabilized HMPE plastic containing around 2.5% carbon black. When changing to green HMPE everything was according to my calculations. All my 1/2 waves have been made from aluminum tubing, and fed with the gamma loop type matching scheme. They worked very well, and my final version had a decoupling section with a lower set of radials. But all the variations of 5/8 antennas did even better on 10m local paths. |
Radials
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Radials
El 04-04-14 19:35, escribió:
wrote: El 04-04-14 2:35, escribió: snip Yep, but it isn't a GP antenna which by definition has a radiator about 1/4 lambda. I understand your reasoning, but do a google (image) search on "GPA antenna" and you we see that many people don't follow your convention. 5/8 and 1/2 WL are also included. It is not the name, but the operating princple that is of importance. Sorry, I follow what engineering text books say, not what some Google search algorithm comes up with. I know the "classical" definition, but we live in 2014, and when the scope of some definitions changed over the last 20..30 years, it is good to know how they changed to avoid misunderstandings. Visit some antenna manufacturer's websites and see what changed over the years (that doesn't mean that I agree with them). snip You will have common mode currents of some magnitude with ANY GP type antenna. Clear, but by using a half wave radiator (end-fed) CM current is about 13 dB lower to start with, and that saves me a lot of aluminum that doesn't contribute to the radiation. Again, I know design/construction is more elaborate. Did you include the coax shield in your simulation? All antennas connected with coax will have a long conductor consisting of the coax shield running from the radials to about ground. I am familiar with Common Mode issues and how to measure, model and simulate them. The number quoted is from both measurement and simulation. You will be hard pressed to notice 1 dB difference in a typical amateur system. that was the reason I mentioned: "The effect of sloping angle on zero elevation gain is small, and you get hardly measurable more gain when they are almost vertical. Sloping radials have some other advantage: less birds." The biggest advantage to sloping radials is they move the impedance from 20 Ohms to much closer to 50 Ohms. I still can't reproduce, or find a reliable reference for your 3.67 dbi for a quarter wave with quarter wave 85 degr sloping radials. This will make the third time I will say that number is probably the result of small angles and closely spaced wires, i.e. garbage. Sorry, I didn't notice that. We drifted away somewhat from Irv's posting.... -- Wim PA3DJS Please remove abc first in case of PM |
Radials
In article ,
Wimpie wrote: I know the "classical" definition, but we live in 2014, and when the scope of some definitions changed over the last 20..30 years, it is good to know how they changed to avoid misunderstandings. Visit some antenna manufacturer's websites and see what changed over the years (that doesn't mean that I agree with them). 1. There are industry standards, but some antenna manufacturers use numbers and definitions to make their products more attractive. That has not changed over the last many years. 2. The feature that defines a ground plane antenna is the ground plane, not the vertical element. 3. Ground is relatively flat. Drooping radials to approximate a sleeve dipole is stretching the definition of a ground plane! Fred K4DII |
Radials
Fred McKenzie wrote:
In article , Wimpie wrote: I know the "classical" definition, but we live in 2014, and when the scope of some definitions changed over the last 20..30 years, it is good to know how they changed to avoid misunderstandings. Visit some antenna manufacturer's websites and see what changed over the years (that doesn't mean that I agree with them). 1. There are industry standards, but some antenna manufacturers use numbers and definitions to make their products more attractive. That has not changed over the last many years. 2. The feature that defines a ground plane antenna is the ground plane, not the vertical element. 3. Ground is relatively flat. Drooping radials to approximate a sleeve dipole is stretching the definition of a ground plane! The Radials of a ground plane antenna work entirely differently than the sleeve in a sleeve dipole, drooping or otherwise. -- Jim Pennino |
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