N6BT's "Vertical dipole"
Anybody with experience with this antenna? It appears to be a quasi-vertical
dipole with two "radials" attached to the bottom end and fed at the bottom. At least this appears to be the case; I could be wrong. What issues arise when feeding a dipole at one end (the bottom in this case)? This can't be a truly balanced design, can it? Al W6LX |
N6BT's "Vertical dipole"
On 8/5/2010 11:33 AM, Al Lorona wrote:
Anybody with experience with this antenna? It appears to be a quasi-vertical dipole with two "radials" attached to the bottom end and fed at the bottom. At least this appears to be the case; I could be wrong. What issues arise when feeding a dipole at one end (the bottom in this case)? This can't be a truly balanced design, can it? Al W6LX Most likely, it is something similar to this: http://www.earchi.org/pdf/ENDFED.pdf Pick up a reasonable matchbox, or build your own, use with the above 80-10m. I'd be surprised if what he has warrants the expense. Regards, JS |
N6BT's "Vertical dipole"
On Aug 5, 1:33*pm, "Al Lorona" wrote:
Anybody with experience with this antenna? It appears to be a quasi-vertical dipole with two "radials" attached to the bottom end and fed at the bottom. At least this appears to be the case; I could be wrong. What issues arise when feeding a dipole at one end (the bottom in this case)? This can't be a truly balanced design, can it? Al *W6LX I've run plenty of base fed half waves, but never saw a need for two lower elements.. You can't really call them radials in that case. They have nothing to do with lowering ground loss. But it's hard to tell, being I've never seen the antenna. But if I were base feeding a half wave, I would not bother with two lower elements spread apart. No real point in it. The main difference in feeding at the base vs feeding in the center is impedance. A much higher Z when base fed, so you need a matching device. And for that I usually prefer a single turn coil and a coax capacitor. Some have called this a "gamma loop" matching device, which is fairly close I suppose being as you tap the single turn coil at the point of best match, much like the usual gamma match. Truly balanced? Nope.. But that really doesn't mean much. Even a center fed half wave can have common mode problems. So it's not like that method of feeding cures all feed line radiation problems. They will both require decoupling sections for optimum performance. But saying that, you can often run a half wave with no real decoupling and have decent results. It will vary to each installation. |
N6BT's "Vertical dipole"
On Aug 6, 7:38*pm, wrote:
On Aug 5, 1:33*pm, "Al Lorona" wrote: Anybody with experience with this antenna? It appears to be a quasi-vertical dipole with two "radials" attached to the bottom end and fed at the bottom. At least this appears to be the case; I could be wrong. What issues arise when feeding a dipole at one end (the bottom in this case)? This can't be a truly balanced design, can it? Al *W6LX I've run plenty of base fed half waves, but never saw a need for two lower elements.. You can't really call them radials in that case. They have nothing to do with lowering ground loss. But it's hard to tell, being I've never seen the antenna. But if I were base feeding a half wave, I would not bother with two lower elements spread apart. *No real point in it. The main difference in feeding at the base vs feeding in the center is impedance. A much higher Z when base fed, so you need a matching device. And for that I usually prefer a single turn coil and a coax capacitor. Some have called this a "gamma loop" matching device, which is fairly close I suppose being as you tap the single turn coil at the point of best match, much like the usual gamma match. *Truly balanced? *Nope.. But that really doesn't mean much. *Even a center fed half wave can have common mode problems. So it's not like that method of feeding cures all feed line radiation problems. They will both require decoupling sections for optimum performance. But saying that, you can often run a half wave with no real decoupling and have decent results. It will vary to each installation. I think there has been some efforts to improve feedline decoupling when feeding a .5wl antenna using radials. This may be such an attempt. Jimmie |
N6BT's "Vertical dipole"
"Al Lorona" a écrit dans le message de ... Anybody with experience with this antenna? It appears to be a quasi-vertical dipole with two "radials" attached to the bottom end and fed at the bottom. At least this appears to be the case; I could be wrong. What issues arise when feeding a dipole at one end (the bottom in this case)? This can't be a truly balanced design, can it? Al W6LX Hi Al Go there and you'll found the "real" of explanation about this antenna model. www.kg4jjh.com/sigma.html http://www.n6bt.com/n6bt-AAOL-3rd-Ed...1c-201-205.pdf 73 DMB --- news://freenews.netfront.net/ - complaints: --- |
N6BT's "Vertical dipole"
On Aug 8, 8:04*am, "DMD" wrote:
Go there and you'll found the "real" of explanation about this antenna model. www.kg4jjh.com/sigma.html http://www.n6bt.com/n6bt-AAOL-3rd-Ed...1c-201-205.pdf That's totally different than what I had in mind. That is just a loaded vertical dipole with capacity hats on each end. And looks to be center fed, not base fed. |
N6BT's "Vertical dipole"
"DMD" wrote Go there and you'll found the "real" of explanation about this antenna model. www.kg4jjh.com/sigma.html DMB Thank you for the references. Not good. I didn't realize that the feedpoint contains a 50 ohm resistor ( 40m EZNEC model on KG4JJH's web site cited above). Thanks again, Al W6LX |
N6BT's "Vertical dipole"
"Al Lorona" wrote in
: .... Thank you for the references. Not good. I didn't realize that the feedpoint contains a 50 ohm resistor ( 40m EZNEC model on KG4JJH's web site cited above). He does not mention a "50 ohm resistor" in his review document, not show it in his feedpoint picture. Are you quite sure of what you say? He does include load of 50+j500 in shunt with the feedpoint, presumably modelling the shunt coil... has that got you confused? The next question is whether such a load is a good characterisation of the actual coil. The value of R seems rather high. Owen |
N6BT's "Vertical dipole"
Owen Duffy wrote:
He does include load of 50+j500 in shunt with the feedpoint, presumably modelling the shunt coil... has that got you confused? The next question is whether such a load is a good characterisation of the actual coil. The value of R seems rather high. Wouldn't that correspond to a Q of 10, which is pretty bad for a air core coil. I'd say something like 30 or 100 would be more realistic (unless it's wound with really fine wire) (I don't have eznec, so I can't open the .ez file) |
N6BT's "Vertical dipole"
"Owen Duffy" wrote in message ... Are you quite sure of what you say? Beats me... I've never seen one of these antennas and am going only by the EZNEC model on that web site, as I said in my last post. Says that there's 50 + j500 at the feedpoint. That's a 50 ohm resistor in series with an inductor, isn't it? Or, as you say, a coil with a real low Q. Either way, it's an extremely lossy device that adversely affects the results. Try adjusting the R to a more reasonable value for an average coil and re-simulating in EZNEC and take a look at the result. KG4JJH has measured data in that review that definitely coincides with a 50 +j500 device at the feedpoint. Al W6LX |
N6BT's "Vertical dipole"
On Aug 9, 6:53*pm, Jim Lux wrote:
Owen Duffy wrote: He does include load of 50+j500 in shunt with the feedpoint, presumably modelling the shunt coil... has that got you confused? The next question is whether such a load is a good characterisation of the actual coil. The value of R seems rather high. Wouldn't that correspond to a Q of 10, which is pretty bad for a air core coil. I'd say something like 30 or 100 would be more realistic (unless it's wound with really fine wire) (I don't have eznec, so I can't open the .ez file) I opened the document in 4NEC2. Whilst my version of EZNEC cannot read or write .NEC decks, 4NEC2 can read .EZ format. If you look at the pic of the coil, a Qi of 10 seems very low. Nevertheless, it doesn't consume much power. Owen |
N6BT's "Vertical dipole"
On Aug 10, 10:35*am, "Al Lorona" wrote:
"Owen Duffy" wrote in message ... Are you quite sure of what you say? Beats me... I've never seen one of these antennas and am going only by the EZNEC model on that web site, as I said in my last post. Says that there's 50 + j500 at the feedpoint. That's a 50 ohm resistor in series with an inductor, isn't it? Or, as you say, a coil with a real low Q. Either way, it's an extremely lossy device that adversely affects the results. Try adjusting the R to a more reasonable value for an average coil and re-simulating in EZNEC and take a look at the result. KG4JJH has measured data in that review that definitely coincides with a 50 +j500 device at the feedpoint. I though about this while on the road this morning. Sometimes enlightenment comes when there is not a keyboard to confuse things. I assumed that he intended to model the shunt coil, and correctly modelled it, but in fact his load is in series with the feedpoint... so it hasn't captured the shunt coil. Removing the load, and running the model gives a feedpoint Z of 16- j500. A shunt coil with Qi around 100, and L around 29uH will transform that to 50+jX. That coild has an equivalent shunt R of 120k, so negligible power is lost in it. I haven't looked closely at the model in all respects, but it seems to do some crazy things that certainly give unrealistically low efficiency. Owen |
N6BT's "Vertical dipole"
Thanks, Owen. That makes sense. I agree it looks like a modeling error.
Regards, Al "Owen Duffy" wrote in message ... On Aug 10, 10:35 am, "Al Lorona" wrote: "Owen Duffy" wrote in message ... Are you quite sure of what you say? Beats me... I've never seen one of these antennas and am going only by the EZNEC model on that web site, as I said in my last post. Says that there's 50 + j500 at the feedpoint. That's a 50 ohm resistor in series with an inductor, isn't it? Or, as you say, a coil with a real low Q. Either way, it's an extremely lossy device that adversely affects the results. Try adjusting the R to a more reasonable value for an average coil and re-simulating in EZNEC and take a look at the result. KG4JJH has measured data in that review that definitely coincides with a 50 +j500 device at the feedpoint. I though about this while on the road this morning. Sometimes enlightenment comes when there is not a keyboard to confuse things. I assumed that he intended to model the shunt coil, and correctly modelled it, but in fact his load is in series with the feedpoint... so it hasn't captured the shunt coil. Removing the load, and running the model gives a feedpoint Z of 16- j500. A shunt coil with Qi around 100, and L around 29uH will transform that to 50+jX. That coild has an equivalent shunt R of 120k, so negligible power is lost in it. I haven't looked closely at the model in all respects, but it seems to do some crazy things that certainly give unrealistically low efficiency. Owen |
N6BT's "Vertical dipole"
On 8/5/2010 11:33 AM, Al Lorona wrote:
Anybody with experience with this antenna? It appears to be a quasi-vertical dipole with two "radials" attached to the bottom end and fed at the bottom. At least this appears to be the case; I could be wrong. What issues arise when feeding a dipole at one end (the bottom in this case)? This can't be a truly balanced design, can it? Al W6LX Looking for something else, I found this: http://www.qsl.net/wb1gfh/zepp.html He also had a page offering this: http://www.qsl.net/wb1gfh/antenna.html Mainly, his dialog was humorous. Anyway, thought you might enjoy it. Regards, JS |
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