Hello, I'm new to Ham, and I just took and passed my Technician test
yesterday and am going back in April to take my General.
Congratulations on passing the Tech test, and best wishes on your
studies for the General!
I made a Yagi Beam
antenna yesterday, but want to build a J-Pole Copper Cactus antenna for 2
Meter. I have come across 2 different designs and wanted your opinions on
which one to use. One has a tuner in the upper section of it such as a Ringo
Ranger except it does a 3/4 circle, and the other one doesnt use a tuner.
Which one do you recommend? and also, from what I understand I set the SWR
on this by the placement of the Coax ends on the 1/4 and 3/4 wavelength
rods? is this correct? Thanks for any help, I have included pics of the 2
proposed antenna's.
Also,
Anyone got any specs for these like 0 dBd gain, beamwidth etc.....
Well, let's see... a bunch of questions here.
The term "Copper Cactus" is usually used to refer to a specialized
sub-class of copper J-pole antennas. The Cactus is a J-pole with two
or more quarter-wave matching stube - smaller ones near the top for
higher frequencies, larger ones near the bottom for lower frequencies.
A copper J-pole with only one matching stub isn't really a Copper
Cactus, I think - it's just a J-pole. These are quite popular for
2-meter operation. A 2-meter J-pole _will_ load up and transmit on
440 as well, but it's not very efficient - a lot of the power takes
off at a high radiation angle, and isn't aimed effectively at the
horizon.
I've seen two feedline arrangements for a true two-arm Cactus. One
uses individual feedlines from the transceiver (or from a diplexor),
often fed up through the copper pipe itself - this is the variety I
use. Another uses a single feedline to the antenna, and some sort of
phasing harness consisting of quarter- and half-wave sections of coax
inside the pipe to make connections to the matching stubs - I've never
been able to make sense of the design rules for this sort, and neither
has anyone I've spoke with.
I've never seen a copper J-pole with a Ringo-type "tuner". The
Ringo's circular loop at the bottom is an impedance-matching device,
which serves a purpose very similar to the quarter-wave matching
section at the bottom of a standard copper J-pole.
I _have_ seen collinear "Super J" antennas, which have a quarter-wave
matching stub at the bottom, a half-wave radiator (as usual), a
half-wavelength phasing-section stub in the middle (sometimes it
sticks out at right angles, sometimes it's wrapped into a 3/4 circle),
and another half-wave radiating section at the top. The 3/4-circle
phasing stub isn't an impedance matcher (as in the Ringo) or a "tuner".
Rather, it allows the two half-wave radiators to radiate in phase
with one another rather than out of phase, and gives the antenna more
directional gain. The disadvantages are that the antenna is twice as
high, and significantly more complex to make.
A standard J-pole has a radiation pattern which closely approximates
that of a 1/2-wavelength dipole - there's some disruption in the
classic dipole pattern due to radiation from the matching stub. You
can probably consider it to be a 0 dBd antenna and not be wrong enough
to matter, in most applications.
A J-pole can be tuned for best SWR at a specific frequency by fiddling
with some combination of four variables - the attachment points of the
two coax conductors onto the two sides of the matching stub, the total
length of the matching stub, and the total length of the radiator. If
you build a copper-pipe J-pole from plans on the Net (or in the ARRL
handbook), and use the exact lengths and spacings and diameters of the
pipes that the authors suggest, then you can eliminate two of these
variables (the lengths of the stubs and radiators) and just fiddle
with the attachment points. It's quite possible to get the SWR down
to within a gnat's whisker of 1:1 at some point within the 2-meter
band, and if you're using 1/2" or 3/4" pipe you'll find that the
antenna is so nicely broadbanded that it'll have a very acceptable SWR
through the whole band. If you're just starting out with antenna-
building, I'd encourage you to use one of these "cookbook" designs.
If you decide to experiment with different pipe diameters or spacings,
then I'd suggest putting copper end-caps over the ends of the radiator
and matching arm, and _not_ soldering them into place initially. You
can slide the caps up and down a fraction of an inch at a time to vary
the lengths of the pipes, and then solder them when you've gotten the
best match. Another approach is to drill holes in the copper caps,
braze or solder some brass nuts on top, and run a brass machine screw
down through the nut - the screw becomes an extension of the pipe, and
you can tweak the electrical length of the pipe by adjusting the
screw. If you do this you'll want to cut the pipes an inch or so
shorter than the final desired length, and use the screw length to
make up the rest and leave room for adjustment.
A couple of hints:
- Make sure that the J-pole is well clear of any obstructions or
interference when you tune it up. J-poles are easily "de-tuned" by
nearby metal or other conductive objects.
- If you decide to solder the coax directly to the pipe (or to copper
brackets or flanges), make sure you don't overheat the conductors
enough to melt the center insulation (this would short out the
antenna), and make sure you coat all exposed portions of the
conductor, braid, and insulation with a waterproofing material
afterwards (outdoor-rated metal-compatible black silicone sealant
is a good choice) to keep rainwater from getting into the coax.
--
Dave Platt AE6EO
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