Help with J antenna design
 

Plan is to build a J antenna for 20 meters using 1 inch EMT conduit.
Antenna will be supported at the bottom by a 4x4 post in the ground and
at the 30+ foot level with a fiberglass bracket bolted to the eve of
the house. The upper most section will be a fiberglass 1/2 wl antenna
thta used to be used for CB. I am looking for a program that will allow
me to calculate deminsions for a project like this. Feedline will be 50
ohm coax.

Help with J antenna design
 

JIMMIE wrote:
Plan is to build a J antenna for 20 meters using 1 inch EMT conduit.
Antenna will be supported at the bottom by a 4x4 post in the ground and
at the 30+ foot level with a fiberglass bracket bolted to the eve of
the house. The upper most section will be a fiberglass 1/2 wl antenna
thta used to be used for CB. I am looking for a program that will allow
me to calculate deminsions for a project like this. Feedline will be 50
ohm coax.

You don't need a program. The long element is 3/4 wavelength long. The
shorter parallel element is 1/4 wavelength long. You can short the
two elements together at the bottom and attach your feedline at the
50 ohm point on the two parallel elements. Or you can feed the long
element with the center conductor and the short element with the
braid of the coax. 3/4 wavelength on 20m is about 50 ft.

The free demo version of EZNEC from www.eznec.com will probably model
that antenna.
--
73, Cecil http://www.qsl.net/w5dxp

Help with J antenna design
 

Thanks Cecil, I was concerned about spacing of the stub more than
anything else. I guesstimated this should be about a foot. I also know
I could be totally wrong. By looking at some plans the distance between
the short and long tubes seems to vary with the frequency and size of
the tube. Doubt if it is too critical. I downloaded EZNEC a few years
ago and could never figure out how to use it.

Help with J antenna design
 

"JIMMIE" wrote in message
oups.com...
Plan is to build a J antenna for 20 meters using 1 inch EMT conduit.
Antenna will be supported at the bottom by a 4x4 post in the ground and
at the 30+ foot level with a fiberglass bracket bolted to the eve of
the house. The upper most section will be a fiberglass 1/2 wl antenna
thta used to be used for CB. I am looking for a program that will allow
me to calculate deminsions for a project like this. Feedline will be 50
ohm coax.

Why not build an end fed 1/2 wave antenna and forget about the mechanics of
the 1/4 wave matching section and the problems of its height? IMHO, a simple
L or Pi network in a WX proof box with a 33' vertical would be a lot easier
to construct and adjust than a 50' tall antenna.

Dale

Help with J antenna design
 

JIMMIE wrote:
Thanks Cecil, I was concerned about spacing of the stub more than
anything else. I guesstimated this should be about a foot.

The bottom 1/4WL is a quarter-wave matching stub, either open
or shorted. If it is open, you have a Zepp antenna, and they
used to use open-wire transmission line for that. One foot
spacing may be a little much. Rule of thumb - 1% of a wavelength
would be about eight inches on 20m.
--
73, Cecil http://www.qsl.net/w5dxp

Help with J antenna design
 

On Mon, 23 Jan 2006 14:36:51 GMT, "Dale Parfitt"
wrote:

Plan is to build a J antenna for 20 meters using 1 inch EMT conduit.
Antenna will be supported at the bottom by a 4x4 post in the ground and
at the 30+ foot level with a fiberglass bracket bolted to the eve of
the house. The upper most section will be a fiberglass 1/2 wl antenna
thta used to be used for CB. I am looking for a program that will allow
me to calculate deminsions for a project like this. Feedline will be 50
ohm coax.


Yes, I may I suggest you take a look at this one. You'll get two bands
instead of one.

http://k6mhe.com/files/DualBandVert.pdf

Danny, K6MHE




email: k6mheatarrldotnet
http://www.k6mhe.com/

Help with J antenna design
 

While the lumped element match is a viable solution. Here's some info (I
don't see Cecil's response)

The spacing of the ~1/4 wave matching section is not critical - here's
why. First, it can be viewed as nothing more than a section of transmission
line, obviously shorted at the bottom (though this section does not have to
be vertical, it's just convenient in many cases).
The spacing and thickness of the conductors will determine the
characteristic impedance (Z0). Normally the Z0 of T-Line is critical, but
this is a bit of a different situation. You are trying to get a match
between the 50 ohms side and something around 2000-3000 ohms (I'll use 2500)
which is at the bottom end of the (vertical) 1/2 wave antenna. Now, on this
design, we have the "liberty" of connecting the 50 ohm line anywhere we want
on the matching section.
And, second, if you view the resulting impedance along the line as
varying from zero at the bottom (shorted end) to 2500 at the top (antenna
feed point), there certainly will be some location (nearer the short) which
is at or close to 50 ohms.
The higher the Z0 of the matching section, the higher up you'll have to go
to get the best match. I'm pretty sure in is my 1947 handbook (non ARRL)
that has one, but it is wires and a wood frame.

There will be, however, discussion regarding the balanced connection on the
matching section and the unbalanced coax, but I won't go into there. I'm
sure others will...as well as the fact that it doesn't matter which side the
coax center conductor should be connected to (doesn't matter).

You can also do a search on "Arrow Antenna" and see an interesting (and oft
argued) variation that does not use a short at the bottom.

73, Steve, K9DCI


"JIMMIE" wrote in message
oups.com...
Thanks Cecil, I was concerned about spacing of the stub more than
anything else. I guesstimated this should be about a foot. I also know
I could be totally wrong. By looking at some plans the distance between
the short and long tubes seems to vary with the frequency and size of
the tube. Doubt if it is too critical. I downloaded EZNEC a few years
ago and could never figure out how to use it.

Help with J antenna design
 

The associate ground system is not practical at this location

Help with J antenna design
 

Thanks, the impedance of the half wave section is what I really
needed.I was figuring 5 to 10K. The info you and Cecil gave me got me
back on track
Installing a ground system here is very impractical. I wanted to see
how well the J antenna worked. If performance is acceptable I plan to
further experiment with multiple frequenciey antennas, an HF version of
the Copper Catcus. For now I would be happy just to get this antenna
working on one frequency.

Help with J antenna design
 

"JIMMIE" wrote in message
oups.com...
The associate ground system is not practical at this location

What associated ground system are you referring to?

Help with J antenna design
 

Careful about assuming that a 1/4 wave stub will do the trick...

To match a 50 ohm line to a 2500 ohm load using a shorted stub at the
transmission line end and a section between the 50 ohm line and the
2500 ohm load requires (using sections shorter than 1/4 wave for each):

50 ohm stub & section: 8.2 degrees & 81.9 degrees
75 ohm: 8.3 & 82.1 degrees
100 ohm: 8.5 & 82.2 degrees
150 ohm: 9.0 & 82.6 degrees
200 ohm: 9.8 & 83.3 degrees
300 ohm: 15.0 & 85.7 degrees
350 ohm: 45 & 88.9 degrees
353.55 ohm: 90 & 90 degrees

Note that the last on is the case of not using a stub (90 degree stub
looks like an open circuit). The 1/4 wave line impedance should be
sqrt(Zin*Zout).

You can adjust the length to tune out reactance, too.

But if you're matching resistive load to resistive line, the total
length of the stub part and the series part will be longer than 1/4
wave, and more so as the impedance of the stub and series part rise.
You can't use this arrangement if the stub and series section
impedances are higher than sqrt(Zin*Zout). SO--beware of spacing the
two too far apart!

Cheers,
Tom

Help with J antenna design
 

K7ITM wrote:
Careful about assuming that a 1/4 wave stub will do the trick...

To match a 50 ohm line to a 2500 ohm load using a shorted stub at the
transmission line end and a section between the 50 ohm line and the
2500 ohm load requires (using sections shorter than 1/4 wave for each):

50 ohm stub & section: 8.2 degrees & 81.9 degrees
75 ohm: 8.3 & 82.1 degrees
100 ohm: 8.5 & 82.2 degrees
150 ohm: 9.0 & 82.6 degrees
200 ohm: 9.8 & 83.3 degrees
300 ohm: 15.0 & 85.7 degrees
350 ohm: 45 & 88.9 degrees
353.55 ohm: 90 & 90 degrees

Note that the last on is the case of not using a stub (90 degree stub
looks like an open circuit). The 1/4 wave line impedance should be
sqrt(Zin*Zout).

You can adjust the length to tune out reactance, too.

But if you're matching resistive load to resistive line, the total
length of the stub part and the series part will be longer than 1/4
wave, and more so as the impedance of the stub and series part rise.
You can't use this arrangement if the stub and series section
impedances are higher than sqrt(Zin*Zout). SO--beware of spacing the
two too far apart!

Cheers,
Tom

Thanks Tom this is the kind of info I need. Hacked a test antenna
together this morning usin chain fence top rail. Spacing i 8 inches for
the stub.Planed on tuning the length of the stub by adjusting a
shorting bar at the bottom so antenna was intentionaly made a little
long. Final length of the 3/4 wl part is adjusable too. Since it is
mounted to the house I figure with the help of friends we can lower
it, tweak it and have it back up again in 15 minutes or less. Probably
be the weekend before I can do more to it.

Help with J antenna design
 

JIMMIE wrote:
I figure with the help of friends we can lower
it, tweak it and have it back up again in 15 minutes or less. Probably
be the weekend before I can do more to it.

You could get the same performance out of a center-fed
vertical dipole and do all your tweaking at the transceiver. :-)
--
73, Cecil http://www.qsl.net/w5dxp

Help with J antenna design
 

Thanks everyone for all the help, I am sincerely appreciative of the
help and suggestions I have recived from the group. I am also aware
that there are antennas that may seem to be better suited for the job
but in my situation I think I have made the best choice.

Help with J antenna design
 

Nope. As I said in my post "...(though this section does not have to
be vertical, it's just convenient in many cases).
"

73, Steve, K9DCI
"Ron McConnell" wrote in message
...

...

By the way, the matching section for the proposed vertical antenna
doesn't have to vertical, does it? Can't it be run horizontally
and the overall height reduced from about 50 ft to about 33 ft?
As mentioned a L network with an inductor and capacitor would also
work instead.

Cheers, 73,

Ron McC.


Ronald C. McConnell, PhD