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#1
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How about a J-Pole? Low angle radiation. The bottom end can be connected to
earth ground. In fact, I constructed one using a mast with the 1/4 wave parallel section added 1/2 wave below the top. Real easy. No radials. thats also an option..i wonder if'n I'll need radials with a vertical dipole?? Steve kb8viv |
#2
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#3
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"Desmoface" wrote in message ... How about a J-Pole? Low angle radiation. The bottom end can be connected to earth ground. In fact, I constructed one using a mast with the 1/4 wave parallel section added 1/2 wave below the top. Real easy. No radials. thats also an option..i wonder if'n I'll need radials with a vertical dipole?? Steve kb8viv As others have replied, no you don't. I once made a 1/4 wave vertical and was listening to a local conversation on my IC-2AT when there occured a lightning stroke from a thunderstorm a few miles away. Although the mast and radials were grounded, the conversation disappeared for about 5 seconds and then faded back in. Electrical charge on the ungrounded vertical element. But I have never had this problem with a J-Pole since all parts of it can be at earth ground. I've also used folded vertical monopoles with the same results. In my old age I have probably become paranoid about ungrounded driven antenna elements. I know that some (maybe all) commercial antennas are designed with networks or other devices which does the grounding. If I were to build a simple ground plane today, I would make it a bit shorter than resonant and add a shunt inductor across the feed point. That way there can be no static accumulation on the element. By the way, we sometimes get sand/dust storms here. I have seen and heard the repeated discharges from the center pin to the shell of a PL-259 on a RG-8 line coming down from the roof during a sand storm. Anyway, this is not to convince anyone that a J-Pole is a cure-all or even the best choice. I just wanted to tell about what drives my choices and tell about a couple of my experiences. Anything you choose will be just fine, as long as you are aware of its properties. Have fun. John - KD5YI |
#4
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Lots of good ideas here, but I'll add the following since there has been an
occasional : A "dipole" is called a dipole because it has two "halves" or "poles" driven in the center. If we want to get a little more descriptive what we are talking about is a "center fed half wave antenna". This is the basic antenna. (the gran-daddy of all antennas, but I won't entertain arguments here about others including 1/4 wave ground planes) Just saying "dipole" could also (much less common) mean something other than the half wave length. When hams say "dipole" you can bet they're talkin' the half wave version. Turn it any way you want, it's the same thing, just keep the ends as far from things as possible. Half wave antennas like this need no ground plane, also called radials, when so constructed. If you "run the coax feed-line out through the center of one of the halves" you get the sleeve dipole. Since the coax runs right past the one end, there's a lot of capacitance and you have to adjust the length of that half a bit (shorter) for best SWR. In this case it may not be a true half wave long in total, but that doesn't matter. Anything close to a half wave, when it is getting power into it, will radiate like the full half wave. This can take the form of the coax braid pulled back or a tube or pipe. Going just a bit further, if you imagine spreading the very ends of the braid out to form a big a cone, so it gets wider at the end away from the center of the half wave - and keep going until it is all 90 degrees from the coax (and the other half (1/4 wave)) then you have a 1/4 wave ground plane antenna. Then there's the "J antenna" now-a-days always called the "J-Pole" (except by me) P.S. The original "J-Pole" Was, and I believe still is, a commercial antenna by, I believe Cushman, consisting of either 2 or, I believe 4, J's spaced vertically on a supporting "POLE" for increased omni directional gain. Thus the name "J-POLE". Getting descriptive again, this is an "end fed half wave antenna". This is the same radiating element, a half wave, just fed differently. To get the high impedance feed required at the end of a half-wave, what looks a lot like a shorted quarter wave stub matching section is used. This beast can also be placed vertically or horizontally. And by the way, there is no reason that the "1/4 wave" matching section has to be in-line with the radiating half wave. It can be at 90 degrees if necessary. Just keep in mind that the feed-point end is still at a high impedance and should be kept away from other things to minimize the capacitance and resulting de-tuning -- just like any end of any half wave antenna. (I won't entertain arguments of whether the matching section is really a true 1/4 wave or not) Now the thing which triggered my comments, that of Richard , KB7QHC You can follow my former instructions and simply make the upper portion 5/8ths wavelength (if you have the room). If this makes it too long (too close to ground) you will need radials. I believe he was originally describing a sleeve dipole. The upper part could indeed be 5/8 wave, HOWEVER that radiator has significant reactance, capacitive. The 5/8 antenna is usually done as a ground plane and you will see a "loading coil" at the bottom. It turns out that the 5/8 radiator has a close to 50 ohm real radiation resistance, but with capacitance. In the series representation it is 5 - Jx. I don't know the value (I have wondered what it is for years-anybody model it & let me know??), but it can be tuned out with a series inductor and nothing more. A good design, however, will use a coil to ground with the coax feed tapped up the coil. This is to get the radiating element grounded, but the net match is the same. If I'm not mistaken, the original "Extended Zepp" or was it just called the "Zep" or Zepplen",was a "di-pole" made up of two 5/8 elements on either side of center. Should have had a rather thin radiating pattern broadside and, yes some gain, like the 5/8 vertical does to obtain its "gain". Should have perhaps had two smaller lobes something like 50 degrees of the centerline (also like the 5/8) Zat help any? Whew! Steve K;9;D:C:I |
#5
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On Fri, 9 Jan 2004 16:52:38 -0600, "Steve Nosko"
wrote:\ I believe he was originally describing a sleeve dipole. The upper part could indeed be 5/8 wave, HOWEVER that radiator has significant reactance, capacitive. Hi Steve, I have built such antennas for 10M and the reactance was quite insignificant when swamped by the proximity of ground (being a foot below the lower element which was actually a tower section insulated above ground). The upper element was a 21 foot section of 1inch tubing. 73's Richard Clark, KB7QHC |
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