What are your objectives wrt an FM antenna???
You probably want a roughly omni-directional antenna for everyday
use, since FM station locations are scattered all over the place.
[Download *.kmz file fm www.tvfool.com
for display in GoogleEarth.]
But by it's very nature, these are low gain antennas and ALSO
are incapable of nulling out stations that interfere with the station
you DO want to receive. I'll discuss these in the section below.Directional antennas are found here:http://www.ham-radio.com/k6sti/
You might (also?) want a directional antenna, either in a fixed
direction to pickup a "favorite" hard-to-get station, or on a rotator
to seek out different directions (mostly nulling out interference).
I'm not sure what YOU mean by "Copper Cactus J-Pole"
, for example:http://www.iw5edi.com/ham-radio/?the...tus-antenna,94
This truly looks like a Cactus, with multiple "J" arms, each tuned to
a different Amateur Radio Band. [Click on gif's for larger size images.]This is a simple Half-Wave J-Pole:http://www.hamtechnet.com/jpole/jpole.html
The pdf contains additional info, incl. need for a choke coil at feedpoint.
Fol. analysis compares a simple J-Pole to a VERTICAL Dipole (AWG14 wires):http://techdoc.kvindesland.no/radio/...0163006789.pdf
Note windings (over PVC, not metal!!!) at the bottom of the J-Pole.
These windings form a choke coil needed when coupling directly to a coax.
It is equivalent to a 1:1 Balun, isolating the coax cable so that it doesn't
perturb the antenna pattern, while minimizing man-made noise pickup:http://www.moonraker.com.au/techni/feeders.htmhttp://www.w8ji.com/common-mode_noise.htmhttp://www.eznec.com/Amateur/Articles/Baluns.pdf
A choke balun can also be formed using either a series of Ferrite Beads
or by looping the coax several times through a larger Ferrite Core:http://www.audiosystemsgroup.com/RFI-Ham.pdf
[See Chapter 6, pg24+]
[Unfortunately, NEC is incapable of simulating this common mode problem.]
Although most Amateur Radio designs are for connection to a 50-ohm
transmitter, if a J-Pole is used with a 75-ohm FM/TV system, the VSWR
minimum will be a reasonable 1.5. Presumably they can be designed
to match a 75-ohm (or 300-ohm?) system, but I haven't seen any
designs for FM/TV Bands....cuz, J-Poles really aren't much of an antenna....
There is a simple formula for the antenna's characteristic impedance
as a function of the tube diameter and spacing and you adjust the
location of the feedpoint for desired impedance. If you want to change
from 50-ohm to a 75-ohm antenna, slide the feedpoint slightly higher:http://www.cvarc.org/tech/buildjpole.html
[Sounds like a 4nec2 modeling exercise for the student......]
FWIW: It appears 4nec2 comes with 144 MHz J-ant.nec file under Models/VHFsimple,
but the separation between elements is a lot smaller than above examples.
Be wary of any J-Pole design using thin wires rather than thick tubing
(e.g. 1/2-in copper)....they are designed for narrow band Amateur Radio
transmission and won't provide good SWR across entire FM or TV band.A Vertical Dipole (or Vertical Folded Dipole)
is like the driven element
in an FM Yagi, except ROTATED from horizontal to vertical orientation
to receive the Vertically Polarized signal (more popular for FM than TV)
and will have a nearly omni-directional antenna pattern as described
in the techdoc article. Free Space Gain is about 2 dBi (0 dBd), which can
be improved by as much as 3 dB due to ground bounce to a total of 5 dBi:http://electronics-diy.com/electroni...tic.php?id=543A Vertical Folded Dipole
can avoid many of the J-Pole's problems,
such as it's poor SWR match (esp. across the entire band), which can
be even more of a problem trying to match to a coax cable:http://www.digitalhome.ca/forum/show....php?p=1068465http://imageevent.com/holl_ands/loops/folded
I analyzed a VFD for the FM Band and provided links to other articles."Some J-Poles I Have Known" compared to Vertical Dipoles:http://www.cebik.com/content/radio.html
[FREE registration required]
Note that J-Poles and Vertical Dipoles are nearly always analyzed
when mounted at a certain height above the ground, rather than
"Free Space" like all other VHF/UHF antenna analysis work. The
ground is supremely important when modeling vertically polarized
antennas and plays a big role in why J-Poles are claiming gains in
excess of 5 dBi. However, we can also run similar simulations with
a horizontally polarized dipole, where we will find that there is a
"ground bounce" signal in addition to the direct path signal, which
adds as much as 3 dB more signal, resulting in a "Gain" of over 5 dBi.What's better than a J-Pole? Why, a Super-J-Pole of course...NOT!!!http://users.marktwain.net/aschmitz/...jpolecalc.htmlhttp://www.n7qvc.com/amateur_radio/copper.htmlNote the LACK of any windings at the bottom of this "Copper Cactus"!!!
In a Super-J-Pole, the 1/2-wavelength section grows to 5/8-wavelength
(or more) with a "hoop" added for phase matching.
But, as per Cebik's J-Pole article above, taller J-Poles obtain slightly
higher gain at the expense of very screwy azimuthal antenna patterns,
requiring you to ROTATE the antenna towards the desired station.....
Super-J-Poles (aka Co-linear J-Pole) presumably minimize this problem,
but Cebik didn't provide any azimuthal plots, only elevation....Better Omni:
If you want quasi-omni-directional coverage, try one of
the higher Gain designs from Area 51, such as SBGH WW-X0418-Mod0:http://imageevent.com/holl_ands/grayhoverman/widebw
The Free Space Gain is nearly 6 dBi, meaning the Gain over a ground
plane could be as much as 9 dBi. This UHF design would need to be
rescaled for use in the FM Band by a factor of about 5.8 LARGER.HOWEVER:
I'm not convinced a higher gain Omni will work any better
in your (or most anyone's) location....the stations are very strong to
begin with....the biggest problem is interference from other stations,
which can only be helped by using a directional antenna