J-pole theory
Allen
they work? I understand dipoles and yagi's pretty well, but this antenna is
outside of my understanding. Is the small pole a director for the larger
driven pole or since it is connected electrically to the larger one does it
not act in this way? Any bits of explanations would be helpful and
appreciated.
Thanks,
Allen
Ralph Mowery
feed line is attached to is a part of the transmission line and matching
section. It should not have any effect on the actual radiation of the
antenna. The function of the part at eh end is to match the very high
(several thousand ohms) to the low (50 ohm) transmission line.
AC6V
http://www.frii.com/~rick/Jpole/jpole.html
Ralph Mowery wrote:
--
73 From Rod AC6V In San Diego
700 Ham Topics & 6,000 Links
http://ac6v.com/
I Doubt, Therefore I Might Be!!!
Roger Sparks
"Allen" <wook...@hotmail.com> wrote in message
news:Ll2D6.14634$ep.43...@news1.rdc1.tn.home.com...
I see you have some good responses so far and hope you can use some more
thoughts about the J antenna.
If you add the total length of material in the J, you will see that it is
nearly one wave length long. Of course you can feed a one wavelength
long antenna at either high current point and the feed resistance will be in
the 75 to 100 ohm range, depending upon height above ground. It makes a
great horizontal antenna and works very well when folded as in the J
antenna. The feed resistance is lowered when folded so the J has a
resistance near to 75 ohms.
One problem with the offset fed antennas (this is one of those awkward
things) is that the antenna is unbalanced and the short side has a reflected
wave about one wave length (time wise) ahead of the long side. This means
you need a good balun if you want to direct feed the beast.
If you want to feed it directly with 52 ohm coax, you should plan on
stepping up the impedance with something like a series section impedance
matching section. The formulas are in the ARRL Antenna Book.
Your next question must be "Have you done this?" No, I have not but I think
it must work that way. It might be a fun experiment for you!!
The folded short element does radiate. It is easily observed with Eznec.
The pattern change is very slight unless the spacing becomes fairly large.
Roy Lewallen's Eznec is a great antenna modeling program. You would have a
lot of fun modeling this and other antennas.
73, Roger, W7WKB
AB2HR
wrote:
One element acts as the radiating element, the other as the
counterpoise. It doesn't matter which is which, either way will work,
but I usually connect the longer element to the center conductor of
the coax, the shorter element to the shield... I just built one for a
friend of mine for use on 2-meters, with a 3/4 wave radiator, received
signals went from S1 to S10 (granted, the J-Pole had a 10-boot
elevation advantage over the 1/4 wave ground plane - and in a hilly
environment, so the signal increase was not solely due to the antenna
design), and transmitted signals to the test station from S5 to full
scale (30 dB over S9). Believe me, these things work - with gain, and
are not very directional...
73 de
Ray / AB2HR
SysOp: The Lost Chord BBS
607-733-5745
telnet: 24.95.131.236 port 6000
http://www.geocities.com/SiliconValley/Park/9257/lostchrd.html
Keith McQueen
matching section. Normally a half wave antenna when fed at the end has a
very high impedance. The 1/4 shorted matching section converts the high
impedance to 50 ohms. It is not as stated earlier a "counterpoise". The
1/2 wave section is resonant like a dipole. The only difference is that
whereas the dipole is center-fed and has a low impedance, the J-Pole is
end-fed and requires impedance matching. The gain of this antenna is the
same as a vertical 1/2 wave dipole.
"AB2HR" <ab...@arrl.net> wrote in message
news:ft5rdt0hmhcn7mli4...@4ax.com...
Ian Jackson
<kb...@arrl.net> writes
in Merrycan 'two cents worth' ....
Viewed from the HF world, the J-pole is a 'Zepp' antenna*. This is an
endfed halfwave, fed by a quarterwave of balanced high-impedance (ie
low-loss) transmission line. This is short-circuited at the bottom end
(a point of very low impedance), and the coax feeder is tapped up from
the short circuit end at the point where the impedance equals the
impedance of the coax. You really should not use coax (you should twin
feeder) but, in practice, you 'get away with it'.
Maybe it is best to think of it as starting as a fullwave antenna, fed
one quarterwave from the end (Fig 1).
Fig 1.
1/4w 1/4w 1/2w
0--------0 0---------0------------------------0
FEED
POINT
(low impedance)
Now pull the 'quarterwave' parts of the antenna together, to form a high
impedance (eg 600 ohm), balanced transmission line (Fig 2). This is the
'traditional' Zepp antenna.
Fig 2. (high impedance) 1/2w
0| |0-------------------------0
| |
| |1/4w (say) 600 ohm twin feeder
| |
0 0
FEED
POINT
(low impedance)
A variant of this arrangement is tapping the feedpoint up the
shortcircuited quarterwave 600 ohm feeder
Fig 3. 1/2w
0| |0-------------------------0
| |
| |1/2w (say) 600 ohm twin feeder
\ |\
|_\| \
Short/ \ \Inner <---Coax feeder (connect at point where
Circuit \Braid the antenna system is 50 ohms)
If you want a vertical for VHF, then 'Make it so'!!!
Fig 4.
0
|
|
|
|
|
|1/2w THIS IS A J-POLE
|
|
|
|
|
0| |0
| |
| |1/4w
\ |\
|_\| \
Short/ \ \Inner <Coax feeder
Circuit \Braid
The UK version of the J-Pole is the 'Slim Jim'. This is constructed like
this (Fig 5).
Fig 5. |--|
| |
| |
| |
| |1/2w THIS IS A 'SLIM JIM' ANTENNA
| |
| |
| |
0 |
|
0 |
| |
| |1/4w
\ |\
|_\| \
Short/ \ \Inner <Coax feeder
Circuit \Braid
* An antenna which was used on the German Zeppelin airships, where there
was need for an 'artifical ground'. This arrangement allowed for a low
impedance feed from a transmitter to a highly efficient halfwave endfed
antenna.
Do they work?
Yes...
Very, very well.
73, Ian.
Ian Jackson
<ianja...@g3ohx.demon.co.uk> writes
Fig 3. 1/2w
0| |0-------------------------0
| |
| |1/4w (say) 600 ohm twin feeder
\ |\
|_\| \
Short/ \ \Inner <---Coax feeder (connect at point where
Circuit \Braid the antenna system is 50 ohms)
Ian.
