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Shunt fed vertical?
Since things are a little slow here...
As I continue to experiment with my 28 foot vertical I attempted to try a shunt feed today. It totally escapes me how I might model it with EZNEC 4 but I do have an MFJ-269 to work with. I set up the radials and tied them to the base of the vertical. I moved up the vertical from the bottom 55 inches. No real reason, that is the length of wire I had handy I fed the wire with the MFJ 269 with the coax shield on the ground side. What I expected to see was some thing resonant between 6-8 mhz with a lot of reactance and who knows resistance. It did not happen.... What I did see was a resonance at 23 mhz with Rs= 48 and Xs=5 for a 1.1 SWR! Dabbling with EZNEC seems to suggest there may be something happening like a 5/8 wave radiator but many factors had to be forced into place. So that leads to my question: How do you shunt feed a 28 foot vertical? John Ferrell W8CCW |
#2
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Shunt fed vertical?
On Tue, 28 Nov 2006 16:24:07 -0500, John Ferrell
wrote: What I expected to see was some thing resonant between 6-8 mhz with a lot of reactance and who knows resistance. It did not happen.... I don't really understand that, what is resonance, and why would there then be lots of reactance? Owen -- |
#3
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Shunt fed vertical?
John Ferrell wrote:
Dabbling with EZNEC seems to suggest there may be something happening like a 5/8 wave radiator but many factors had to be forced into place. So that leads to my question: How do you shunt feed a 28 foot vertical? If you feed it at zero feet, you get a feedpoint impedance of zero. At the frequency at which the radiator is resonant, i.e. about 234/28 = ~8.4 MHz, the feedpoint resistance will increase as you raise the feedpoint. Somewhere on the way up, it will be 50 ohms with some inductive reactance so you need a series capacitor. It's a Gamma feed. It is easy to model with EZNEC. -- 73, Cecil http://www.w5dxp.com |
#4
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Shunt fed vertical?
On Tue, 28 Nov 2006 16:24:07 -0500, John Ferrell
wrote: What I expected to see was some thing resonant between 6-8 mhz with a lot of reactance and who knows resistance. It did not happen.... Hi John, It is hard to imagine your antenna showed a strictly resistive, non-reactive load over that interval where the radiator is only a quarter wave at best. Fed at the base it should have some capacitive reactance until resonance somewhere near the 40M band. Fed with a wire at the height you indicate should show an inductive reactance over the entire span. What I did see was a resonance at 23 mhz with Rs= 48 and Xs=5 for a 1.1 SWR! Sounds like your antenna base was not connected to the radials (you were driving the antenna through the length of the gamma wire, and the structure above it). So that leads to my question: How do you shunt feed a 28 foot vertical? For 80M? About a 1000pF in series with the added wire. This still says nothing of the resistance being sub 1 Ohm IFF your gamma wire is on par with the diameter of the vertical element. On the other hand, if it is very much thinner (and using half the capacitance), it stands to elevate the resistive portion (to 30ish Ohms) into a match. A more complete specification would tighten up the variability in this. 73's Richard Clark, KB7QHC |
#5
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Shunt fed vertical?
On Tue, 28 Nov 2006 22:25:45 GMT, Owen Duffy wrote:
On Tue, 28 Nov 2006 16:24:07 -0500, John Ferrell wrote: What I expected to see was some thing resonant between 6-8 mhz with a lot of reactance and who knows resistance. It did not happen.... I don't really understand that, what is resonance, and why would there then be lots of reactance? Owen I expected a dip in the swr at the 1/4 wave length but not a 50 ohm match. John Ferrell W8CCW |
#6
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Shunt fed vertical?
On Tue, 28 Nov 2006 22:33:43 GMT, Cecil Moore
wrote: John Ferrell wrote: Dabbling with EZNEC seems to suggest there may be something happening like a 5/8 wave radiator but many factors had to be forced into place. So that leads to my question: How do you shunt feed a 28 foot vertical? If you feed it at zero feet, you get a feedpoint impedance of zero. At the frequency at which the radiator is resonant, i.e. about 234/28 = ~8.4 MHz, the feedpoint resistance will increase as you raise the feedpoint. Somewhere on the way up, it will be 50 ohms with some inductive reactance so you need a series capacitor. It's a Gamma feed. It is easy to model with EZNEC. I will go back to EZNEC 4 and see if I can determine what I am missing on the Gamma match. John Ferrell W8CCW |
#7
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Shunt fed vertical?
On Tue, 28 Nov 2006 23:06:21 -0500, John Ferrell
wrote: On Tue, 28 Nov 2006 22:25:45 GMT, Owen Duffy wrote: On Tue, 28 Nov 2006 16:24:07 -0500, John Ferrell wrote: What I expected to see was some thing resonant between 6-8 mhz with a lot of reactance and who knows resistance. It did not happen.... I don't really understand that, what is resonance, and why would there then be lots of reactance? Owen I expected a dip in the swr at the 1/4 wave length but not a 50 ohm match. John, Your terminology has me confused. When you say you expect to see resonance between 6-8MHz, I understand resonance to mean zero reactance at that frequency. Then you tell us you expect to see lots of reactance. They are inconsistent. Then you tell me that you expect to see a dip in SWR at the 1/4 wave length. I am guessing, but I think you probably mean the frequency where the vertical is a quarter wavelength long electrically. If you drive the vertical at the base in series, you would expect to observe resonance (ie no reactance) somewhere around the quarter wave frequency. If you shunt feed it further up, the shunt feeding arrangement acts as an impedance transformer, and it is unlikely that you will see zero reactance (ie resonance) at the same frequency as if you series fed the vertical at the base. A feature of shunt feeding typically is that it is reactive, and you have to tune out the reactance, adjusting the tap point up or down to get the desired feed impedance. Owen -- |
#8
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Shunt fed vertical?
Hey John... Glad to see you are still experimenting with the
vertical... One answer is that you are shunt feeding a vertical monopole near it's third harmonic tapped at about 1/8 wave above ground... Depending upon shunt spacing and the diameter ratios it looks like you found the magic spot for that frequency to transform to nearly a 1:1 match... As one fella said to me long ago, I would rather be lucky than good, any day... denny - k8do |
#9
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Shunt fed vertical?
On Tue, 28 Nov 2006 14:47:29 -0800, Richard Clark
wrote: So that leads to my question: How do you shunt feed a 28 foot vertical? For 80M? About a 1000pF in series with the added wire. This still says nothing of the resistance being sub 1 Ohm IFF your gamma wire is on par with the diameter of the vertical element. On the other hand, if it is very much thinner (and using half the capacitance), it stands to elevate the resistive portion (to 30ish Ohms) into a match. A more complete specification would tighten up the variability in this. 73's Richard Clark, KB7QHC I will pursue your suggestions. John Ferrell W8CCW |
#10
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Shunt fed vertical?
On Wed, 29 Nov 2006 04:43:14 GMT, Owen Duffy wrote:
John, Your terminology has me confused. When you say you expect to see resonance between 6-8MHz, I understand resonance to mean zero reactance at that frequency. Then you tell us you expect to see lots of reactance. They are inconsistent. Then you tell me that you expect to see a dip in SWR at the 1/4 wave length. I am guessing, but I think you probably mean the frequency where the vertical is a quarter wavelength long electrically. If you drive the vertical at the base in series, you would expect to observe resonance (ie no reactance) somewhere around the quarter wave frequency. If you shunt feed it further up, the shunt feeding arrangement acts as an impedance transformer, and it is unlikely that you will see zero reactance (ie resonance) at the same frequency as if you series fed the vertical at the base. A feature of shunt feeding typically is that it is reactive, and you have to tune out the reactance, adjusting the tap point up or down to get the desired feed impedance. Owen Sorry for the confusing terminology but I am confused! The testing I have done so far with this 28 foot antenna has been series fed. The results obtained have been consistant with the Eznec models. I believe my current direction requires I puzzle out the Gamma match in Eznec. John Ferrell W8CCW |
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