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#1
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which J-Pole?
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 Hosting the Jade Warrior home page: http://www.radagast.org/jade-warrior I do _not_ wish to receive unsolicited commercial email, and I will boycott any company which has the gall to send me such ads! |
#2
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Dustin wrote: 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. Good show! ...want to build a J-Pole ... For starters, check the info and links on the theory and practice of J-Pole, Zepp and end-fed antennas at Steve Yates, AA5TB's web site http://www.qsl.net/aa5tb Cheers, 73, Ron McConnell WGS-84: N 40º 46' 57.9" W 74º 41' 21.9" Magnetic Variation/Declination = 13º in early 2004 FN20ps77GV75 per w2iol or FN20ps77GU46 per K2RIW http://home.earthlink.net/~rcmcc |
#3
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Hi Dustin,
Here's another site you might want to check out, http://www.arrowantennas.com/inst/inst.html Look under "open stub j-poles" and then look at the OSJ 146/440 info. It's easy to build and has excellent results on both bands. 73 Richard WB8KRN |
#4
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On Mon, 01 Mar 2004 13:19:58 GMT, "Richard Heindel"
wrotF: Hi Dustin, Here's another site you might want to check out, http://www.arrowantennas.com/inst/inst.html Look under "open stub j-poles" and then look at the OSJ 146/440 info. It's easy to build and has excellent results on both bands. 73 Richard WB8KRN I built a copy of the Arrow dual bander and it does work well. I also have a copper collinear fed with a coax 4:1 balun that works very well, but I was never able to get a really good match to it. I use the collinear for packet and the Arrow clone for local repeaters. Mike kc9doa |
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