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Center Fed " Zepp " with Coaxial Balun ?



 
 
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  #1  
Old January 4th 05, 11:35 AM
Gary
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Default Center Fed " Zepp " with Coaxial Balun ?

I'm mulling over different antenna designs for this spring / summer
and a friend of mine has suggested for 80m on up a full size 80 meter
"dipole" 66' X inches per leg, center fed with 300 Ohm twinlead. He
has this arrangement and the twinlead comes into his house and is
connected to two 10' sections of RG-8X both rolled into a coil about
6" in diameter. He then takes and connects the braid and shield of one
of the coax coils to one leg of the twindlead and I'm not exactly
certain how the other leg is connected. He claims it's a 4:1 coaxial
balun. I wonder if some of the experts here are familiar with this
type of arrangement. He claims it has lower losses than connecting it
to the 4:1 balun in his tuner. Any thoughts on the subject ?

Thanks in advance

73 Gary K8IQ

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  #2  
Old January 5th 05, 12:28 AM
JGBOYLES
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a friend of mine has suggested for 80m on up a full size 80 meter
"dipole" 66' X inches per leg, center fed with 300 Ohm twinlead


Hi Gary, A classical Zepp or Zepplin antenna is not center fed. I thought they
were end fed and hanging down from the airships of WW2.
If you have the room for a full sized 80m dipole, use 300 ohm or 450 ohm
ladder line with a choke balun (1:1 W2DU) and an antenna tuner. Will work
great on all HF bands. Don't mess with a 4:1 balun. Doesn't provide enough
benefit for this type of antenna.
The best "all band" antenna I have ever used was a 100' dipole, high as
possible, fed with ladder line and a tuner. Don't know if the 1:1 balun I used
did much.
73 Gary N4AST
  #4  
Old January 5th 05, 03:47 PM
Bob Miller
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On Tue, 04 Jan 2005 06:35:12 -0500, Gary wrote:

I'm mulling over different antenna designs for this spring / summer
and a friend of mine has suggested for 80m on up a full size 80 meter
"dipole" 66' X inches per leg, center fed with 300 Ohm twinlead. He
has this arrangement and the twinlead comes into his house and is
connected to two 10' sections of RG-8X both rolled into a coil about
6" in diameter. He then takes and connects the braid and shield of one
of the coax coils to one leg of the twindlead and I'm not exactly
certain how the other leg is connected. He claims it's a 4:1 coaxial
balun. I wonder if some of the experts here are familiar with this
type of arrangement. He claims it has lower losses than connecting it
to the 4:1 balun in his tuner. Any thoughts on the subject ?

Thanks in advance

73 Gary K8IQ

Remove extra @'s to reply directly


Gary, I have a kit #835 from the Wireman that is a 4-1 balun -- it
uses two pieces of paralleled coax with ferrite beads. That is the
closest I've heard to the twin coax deal you describe above. Don't
know that you really need such a balun -- a standard single coil of
coax, or a W2DU type with ferrite beads on a short length of coax, or
the 4-1 balun that comes with your tuner -- any of these should work
with your center-fed dipole. Right now, my 130 foot dipole with
ladderline works best with my tuner's built-in 4-1 balun.

I'd just put the antenna up, and then experiment with different ways
of feeding it to your tuner -- I imagine every installation will work
a little differently depending on how high your antenna is and how
long your ladderline is.

bob
k5qwg


  #5  
Old January 6th 05, 12:57 AM
Cecil Moore
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Gary wrote:
Thanks Gary. I hadn't heard of a "center fed Zepp" before either and
your description of them being end fed and hanging down from airships
of WW2 correlates with what I'd heard.


The Zepp is an end-fed 1/2WL antenna. The Double Zepp is a center-fed
one wavelength antenna. An Extended Zepp is a 5/8WL end fed antenna. An
Extended Double Zepp is a 10/8WL center-fed antenna.
--
73, Cecil http://www.qsl.net/w5dxp

  #6  
Old January 6th 05, 09:43 AM
Gary
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On 5 Jan 2005 16:57:56 -0800, "Cecil Moore" wrote:

Gary wrote:
Thanks Gary. I hadn't heard of a "center fed Zepp" before either and
your description of them being end fed and hanging down from airships
of WW2 correlates with what I'd heard.


The Zepp is an end-fed 1/2WL antenna. The Double Zepp is a center-fed
one wavelength antenna. An Extended Zepp is a 5/8WL end fed antenna. An
Extended Double Zepp is a 10/8WL center-fed antenna.


Thanks Cecil and to all who responded, I think I'll just put up a
center fed dipole and feed it with twinlead or ladder line and then
use the 4:1 balun in my antenna tuner.

73 Gary
  #7  
Old January 6th 05, 01:55 PM
Cecil Moore
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Gary wrote:
Thanks Cecil and to all who responded, I think I'll just put up a
center fed dipole and feed it with twinlead or ladder line and then
use the 4:1 balun in my antenna tuner.


That is a very popular configuration and works for a lot of hams. You
may need to change the length of the feedline for optimum operation.
Some ideas and information are available on my web page at
http://www.qsl.net/w5dxp/notuner.htm

One case where a 4:1 balun doesn't function well is on dipoles that are
less than 1/2WL long. For instance, the impedance looking into the
feedline for a 102 ft. dipole used on 80m may range down to 10 ohms. A
4:1 balun tries to take that 10 ohms down to 2.5 ohms. Antenna tuners
are inefficient when trying to match such low impedance values.
--
73, Cecil http://www.qsl.net/w5dxp

  #8  
Old January 6th 05, 03:27 PM
Jim Leder
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After reading numerous posts, I built an EDZ for 20 using the '450 ohm'
ladder line and coax balun approach. Maybe I got lucky, I don't know but
when I hooked up the antenna analyzer I was surprised to see an SWR of 1.3:1
at 14.030 and the impedance was around 51 ohms. So far it does outperform my
old G5RV by a fair amount to it's favored direction (SW or NE). The antenna
is at 35 feet (wish my trees were higher, but they aren't). The only
downside I can see is this antenna has a very narrow beamwidth and the nulls
are quite pronounced, but that's what the G5RV is now for.


"Cecil Moore" wrote in message
oups.com...
Gary wrote:
Thanks Cecil and to all who responded, I think I'll just put up a
center fed dipole and feed it with twinlead or ladder line and then
use the 4:1 balun in my antenna tuner.


That is a very popular configuration and works for a lot of hams. You
may need to change the length of the feedline for optimum operation.
Some ideas and information are available on my web page at
http://www.qsl.net/w5dxp/notuner.htm

One case where a 4:1 balun doesn't function well is on dipoles that are
less than 1/2WL long. For instance, the impedance looking into the
feedline for a 102 ft. dipole used on 80m may range down to 10 ohms. A
4:1 balun tries to take that 10 ohms down to 2.5 ohms. Antenna tuners
are inefficient when trying to match such low impedance values.
--
73, Cecil http://www.qsl.net/w5dxp




  #9  
Old January 6th 05, 03:55 PM
[email protected]
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This is an excellent antenna that will serve you well over all HF
bands, including 6 meters. I use this antenna with good 300 ohm line
purchased from Davis RF, and a Radio Works Remote Current Balun. From
there I use about 4 feet or so of LMR400UF low loss coax to the rig.
The autotuner in my FT-920 will tune this antenna flat on most bands.
I do need to use an external tuner on 30 and 10 meters, but this is
easily acomplished by a 2 position coax switch. This system has
produced the best results of any setup I have had in 40+ years as a
Ham.
Unless you are real certain about the behavior of that coax balun, I
would invest in either a Radio Works, or DX Engineering balun of known
repute. Then get on the air and be amazed.
Jim/K2TL

  #10  
Old January 6th 05, 05:00 PM
Cecil Moore
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Default

Jim Leder wrote:
After reading numerous posts, I built an EDZ for 20 using the '450 ohm'
ladder line and coax balun approach. Maybe I got lucky, I don't know but
when I hooked up the antenna analyzer I was surprised to see an SWR of 1.3:1
at 14.030 and the impedance was around 51 ohms.


Odds are that your feedline is a multiple of half-wavelengths
plus 0.2 wavelength. If you know the VF and length, it would
be interesting to calculate how close you came to N.2 wavelengths.
--
73, Cecil http://www.qsl.net/w5dxp


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