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

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

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.

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

"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: ---

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.

"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

"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

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)

"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