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