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comet vert?no radals
An idea,
so if one took a random length say 30 foot length of twin feed, stuck a big 50 ohm resistor on one end and a ballun to bring it to 50 ohm resonance at the other, in the tuner via coax. would it be 50 ohms wide banded because it sees the 50 ohm 'dummy load' at the far end as a match, and radiates a signal 'of sorts' from the twin feed. If one used the tuner to miss match the 'antenna' The worse the match the more signal would squirt from the feeder. |
comet vert?no radals
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comet vert?no radals
John Ferrell wrote:
Is a resistor used in matching a bad thing? My intuition is that it is, but I have no math to back up the position. One of my current projects is to understand the matching problem for an electrically short vertical. Using EZNEC+, when I run the SWR function with the radiator (simulated real ground) I get an indication of Z = R + j. If I add an inductive load at the bottom of the radiator equal to the j value it brings the match into a reasonable range. At this point I have not grasped the relationship of the "R" in the equation. If I use the figure for R equal to 10% of the XL, the match gets even better, but not 1:1. 10% comes from the suggested estimate in the help file. Question 1: What is this "R" thing? I started out thinking it was series DC resistance and lower is better. My interpretation of the EZNEC results contradict this position. More "R" seems better. Let's suppose that your antenna was a black box instead of an antenna. You connect a source to it and look at the impedance looking into the black box. If the resistive part of the impedance (the R) was zero, the box would contain the equivalent of only an inductor (if the X was positive) or a capacitor (if the X was negative). Although you would measure voltage and current at the box's terminals, you'd find that they were 90 degrees out of phase with each other, and no power would be delivered from the source or consumed by the box. Not the situation you want with an antenna -- you need to deliver power to it so that power can be radiated. If, on the other hand, you see some R when you look into the box, you can calculate that the power being delivered by the source and consumed by the box is I^2 * R, where I is the RMS current at the box terminals. So R is always present at an antenna feedpoint at any time the antenna is radiating power, since part of the power "consumed" by the antenna goes toward radiation. This R is called the "radiation resistance". Some part of the power supplied by the source is also actually consumed by heating the wire and ground. This increases the R seen at the feedpoint. That part of the total R is called the "loss resistance". What you want to do is to maximize the fraction of the power that's radiated, and minimize the fraction that's wasted as heat. So you want to make the ratio of radiation to loss resistance as large as possible. If you have a short antenna, you'll find you can raise the radiation resistance by lengthening it or adding a top hat. Center loading with a coil also raises the radiation resistance, but you can end up increasing the loss just as much due to coil resistance. With EZNEC, you can set all losses to zero by setting wire loss and the loss of all loads to zero and, if using a grounded vertical, using MININEC type ground. Then the R you see is only the radiation resistance, and the field strength is that of a 100% efficient antenna. Reintroduce the losses and you'll see the loss resistance added to the radiation resistance at the feedpoint. The field strength will drop according to the reduction in efficiency. Question 2: Is this an effective method of matching a 50 ohm line to the vertical radiator? Any resistance you intentionally add will increase the loss resistance without changing the radiation resistance. More of your rig's power will be spent heating the resistor and less will be radiated. In some instances, such as military frequency-hopping applications, the trade of greater bandwidth for less efficiency might be worthwhile. You'll have to decide for yourself whether it is for you. There are certainly a host of other matching methods which are less lossy, but few which can achieve the extreme broadbanded match of a resistor. But don't be fooled into thinking that because the match is broadbanded that you're radiating the same amount of power over the band. The wider the bandwidth, the larger the fraction of your power you'll dissipate in the resistor. I do understand this is a math model and it ignores radiation from the inductor. "Loads" don't radiate. Inductors modeled as wires using the helix creation feature do. Question 3: Looking ahead, I think the next matching technique should be an inductor in series with the ground end of the radiator and the ground. The 50 ohm line would feed a tap on the coil 50 ohms up from ground. That's not likely to work. Do I compute XL from the bottom to equal 50 or is this strictly a cut & try thing? You don't want an XL of 50, you want the feedline to see an R of 50 with no X, that is, 50 + j0. I have yet to physically model anything with these tests but I am getting close to it. I know there are better matching methods than what I am attempting, this is an academic exercise for me! Look into making an L network. It would be a good opportunity to learn how to use the Smith chart, and you'd end up with a low loss matching solution. Roy Lewallen, W7EL |
comet vert?no radals
Thank you!
I believe you have pointed the way to get me back on track. I have only studied Smith Charts enough to pass tests. I think I best start there and with the L-Network. I also better revisit the antenna material in the ARRL antenna course. I missed some things very basic there. On Sun, 22 Jan 2006 14:02:03 -0800, Roy Lewallen wrote: John Ferrell wrote: Is a resistor used in matching a bad thing? My intuition is that it is, but I have no math to back up the position. One of my current projects is to understand the matching problem for an electrically short vertical. Using EZNEC+, when I run the SWR function with the radiator (simulated real ground) I get an indication of Z = R + j. If I add an inductive load at the bottom of the radiator equal to the j value it brings the match into a reasonable range. At this point I have not grasped the relationship of the "R" in the equation. If I use the figure for R equal to 10% of the XL, the match gets even better, but not 1:1. 10% comes from the suggested estimate in the help file. Question 1: What is this "R" thing? I started out thinking it was series DC resistance and lower is better. My interpretation of the EZNEC results contradict this position. More "R" seems better. Let's suppose that your antenna was a black box instead of an antenna. You connect a source to it and look at the impedance looking into the black box. If the resistive part of the impedance (the R) was zero, the box would contain the equivalent of only an inductor (if the X was positive) or a capacitor (if the X was negative). Although you would measure voltage and current at the box's terminals, you'd find that they were 90 degrees out of phase with each other, and no power would be delivered from the source or consumed by the box. Not the situation you want with an antenna -- you need to deliver power to it so that power can be radiated. If, on the other hand, you see some R when you look into the box, you can calculate that the power being delivered by the source and consumed by the box is I^2 * R, where I is the RMS current at the box terminals. So R is always present at an antenna feedpoint at any time the antenna is radiating power, since part of the power "consumed" by the antenna goes toward radiation. This R is called the "radiation resistance". Some part of the power supplied by the source is also actually consumed by heating the wire and ground. This increases the R seen at the feedpoint. That part of the total R is called the "loss resistance". What you want to do is to maximize the fraction of the power that's radiated, and minimize the fraction that's wasted as heat. So you want to make the ratio of radiation to loss resistance as large as possible. If you have a short antenna, you'll find you can raise the radiation resistance by lengthening it or adding a top hat. Center loading with a coil also raises the radiation resistance, but you can end up increasing the loss just as much due to coil resistance. With EZNEC, you can set all losses to zero by setting wire loss and the loss of all loads to zero and, if using a grounded vertical, using MININEC type ground. Then the R you see is only the radiation resistance, and the field strength is that of a 100% efficient antenna. Reintroduce the losses and you'll see the loss resistance added to the radiation resistance at the feedpoint. The field strength will drop according to the reduction in efficiency. Question 2: Is this an effective method of matching a 50 ohm line to the vertical radiator? Any resistance you intentionally add will increase the loss resistance without changing the radiation resistance. More of your rig's power will be spent heating the resistor and less will be radiated. In some instances, such as military frequency-hopping applications, the trade of greater bandwidth for less efficiency might be worthwhile. You'll have to decide for yourself whether it is for you. There are certainly a host of other matching methods which are less lossy, but few which can achieve the extreme broadbanded match of a resistor. But don't be fooled into thinking that because the match is broadbanded that you're radiating the same amount of power over the band. The wider the bandwidth, the larger the fraction of your power you'll dissipate in the resistor. I do understand this is a math model and it ignores radiation from the inductor. "Loads" don't radiate. Inductors modeled as wires using the helix creation feature do. Question 3: Looking ahead, I think the next matching technique should be an inductor in series with the ground end of the radiator and the ground. The 50 ohm line would feed a tap on the coil 50 ohms up from ground. That's not likely to work. Do I compute XL from the bottom to equal 50 or is this strictly a cut & try thing? You don't want an XL of 50, you want the feedline to see an R of 50 with no X, that is, 50 + j0. I have yet to physically model anything with these tests but I am getting close to it. I know there are better matching methods than what I am attempting, this is an academic exercise for me! Look into making an L network. It would be a good opportunity to learn how to use the Smith chart, and you'd end up with a low loss matching solution. Roy Lewallen, W7EL John Ferrell W8CCW |
comet vert?no radals
I have been using this antenna for about 4 months now with excellent
results. It works very well on all the ham bands up to 6 meters. Unlike the test in the QST article where they mounted the antenna at ground level. The trick is to get it up in the air 30 to 35 feet. In my case, I have the base at about 30 feet in the air. If you can't mount the antenna at the recommended height then look at one of the GAP verticals. Jack w4grj |
comet vert?no radals
Quoth:
On 80m and 40m I have used a 33' vertical in the bed of my truck fed with an "L" network and had good results. *Leads me to believe that the 33' would work well with a modest ground system. For 80 Meters:I have had decent results in end feeding (connect the conductor and shield together to make a single conductor) a 40 meter dipole xmission line into a grounded Pi network (your average MFJ tuner); what you have is an end fed long wire with a really big capacitance hat. Works best if the xmission line is 1/4 wave (i.e. 66 ft), but it's not critical. Also, the better the RF ground the better the efficiency. And hey, it even works to some extent on 160, as long as you keep the power output in check. Rockinghorse KI6AZS |
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