From netcomsv!decwrl!olivea!spool.mu.edu!howland.reston.ans.net!gatech!bloom-beacon.mit.edu!mcrcim.mcgill.edu!sifon!CC.UMontreal.CA!cumin.telecom.uqam.ca!hobbit.ireq.hydro.qc.ca!barde!vaillan Tue Sep 28 18:26:21 PDT 1993 Article: 955 of rec.radio.amateur.antenna Newsgroups: rec.radio.amateur.antenna Path: netcom.com!netcomsv!decwrl!olivea!spool.mu.edu!howland.reston.ans.net!gatech!bloom-beacon.mit.edu!mcrcim.mcgill.edu!sifon!CC.UMontreal.CA!cumin.telecom.uqam.ca!hobbit.ireq.hydro.qc.ca!barde!vaillan From: vaillan@ireq.hydro.qc.ca (Clement Vaillancourt) Subject: Re: Wire J-pole? Message-ID: Sender: news@ireq.hydro.qc.ca (Netnews Admin) Reply-To: vaillan@ireq.hydro.qc.ca Organization: Hydro-Quebec (IREQ) Date: Tue, 28 Sep 1993 17:37:17 GMT Lines: 154 Read this one from Fred: Clem. -- Clement Vaillancourt, | Institut de Recherche d'Hydro-Quebec Analyste, | Varennes, P. Quebec, Canada, J3X 1S1 Informatique scientifique | Tel:+1 514 652 8238 Fax:+1 514 652 8309 vaillan@ireq.hydro.qc.ca | Radio-amateur: VE2HQJ@VE2CRL.PQ.CAN.NA > In article <16382@hacgate.UUCP> tony@hacgate.UUCP (Tony Reeves) writes: > >Can someone provide me with the formula on constructing a j-pole. > >I'm interested in the formula, not the ready made answers like > >"make it x long for this". > > > > Well, my formula is far from scientific, but if you're > going to build one, I'll guarantee that it will work for > VHF and above. Here's my simplified, seat-of-the-pants > J-Pole design strategy that I've used with excellent results: > > First, a J-Pole is defined as a 1/2 length radiating > element, fed by a 1/4 wavelenght parallel transmission > line. This yields the following picture (viewed from the > side): > > ------------------------------------------------ > base -> | > ---------------- > > It is related to the end-fed Zepp trailing wire antenna, > once used on Blimps. The main idea calls for the longer > element to be (1/2 + 1/4) 3/4 wavelength long and the > shorter one 1/4 wavelength at the frequency of interest. > > The Lower part of the J forms a parallel transmission line > which is used as a matching section for the 1/2 wavelength > free end of the long element. In my design, the elements > are 1/4 inch in diameter and are spaced about 2.5 inches > apart yielding a transmission line impedance of around 400 > ohms give or take.. > > A coax line is attached to the two sides of the parallel > transmission line at a point above the base where a good > match is obtained. The method and positioning of this > attachment is one of the greatest area's of discussion and > dissent amongst J-Pole builders. > > Some builders connect a coax like this: > > one side of coax here v > ----+---------------------------------------- > | > ----+----------- > one side of coax here ^ > > With this method, the builder will usually make a hard > electrical connection at each point the coax attaches to > the pole. Many designs using copper pipe for the elements > find it handy just to solder directly to the elements. > Most other designs will attach to a couple of screws. The > design in the Antenna Book (ARRL), uses a sliding bar which > can be moved up and down the J and secured with set screws > at the matching point. > > There is often argument as to which part of the coax (the > center conductor or the shield) should be attached to which > arm of the J. In the ARRL Antenna Book, their schematic > shows the center conductor on the short arm while the photo > of the finished product shows the center conductor attached > to the long arm. I always attach the center conductor to > the long arm. I'm fairly sure, however, that it does not > matter because what the RF sees is really something like > this: > > ============--------------------------------- > > Which is parallel line transitioning to a single radiator. > Whether this is true or not is irrelavent, however, since > it's safe to say with certianty that if you conect your > center conductor to the long arm it WILL work. > > > My J-Poles use a different feed arrangement than the one > just shown. I construct my J-Poles with a SO-239 mounted > in the base of the J, which becomes the attachment point > for the coax shield. I then take a 6 or 8 inch piece of > number 12 solid insulated wire and wrap in about 4 times > around the long element, about 4 inches up from the base. > There is no DC connection between the feed wire and the > element. The end of this wire is soldered into the center > pin of the SO-239 which is sticking up from the bottom of > the base plate. > > ------0000------------------------------- > | | > SO-239 -> [[[----- > | > ----------------- > > This same feed arrangement is quite popular around the San > Jose, CA area where it has been sold in large numbers by > the American Legion at the Foothills Amateur Flea Market > for a number of years. It is a capacitive coupling > arrangement, despite the look that the wire is wound into a > coil around the long radiator. It's a bit touchy to > adjust, and the SWR is affected by bending the feed wire > left or right between the radiators as necessary, or by > adjusting the tightness of the coiled wire (the capacitor) > around the radiator. > > At least one other design that I've seen uses a variable > capacitor with this same arrangement, mounted in the middle > of the feed wire which is electrically attached to both the > SO-239 and the radiating element. > > Matching the antenna to the line is not terribly > difficult. In my design, I made the base out of a flat > piece of aluminum bar stock through which set screws were > used to hold the elements in place. This also allowed for > some experimental lengthening and shorting of the short > element when tuning. > > In a nutshell, if you make the long element 3/4 wavelengths > long, and make the short element 1/4 wavelength long with > some provision for adjusting it's length, then in just a > few minutes you'll find it very easy to locate the matching > point for the feed wire using an SWR meter. During my > short production run I was able to achieve a good match > which was easily repeatable, by carefully measuring the > feed wire and standardizing my forming method. This > process required no further adjustment after construction, > and did not require the use of an SWR meter. > > My final design resulted in a somewhat shorter than > expected short element (17 inches instead of 19) but a > perfect 1:1 match nonetheless. I've built about 25 of > these antennas in both 2M and 440 versions and have heard > nothing but satisfaction from my customers. > > There are two additional things which come to my attention > on the subject. First, it ought to be possible to achieve > a 5/8 J-Pole which would have (3dB) gain. Secondly, there > seems to be no commercial manufacturers of J-Poles although > the Ringo Ranger seems to be is closely related to it. > Ham's agree that J's work well, and will almost never pay > more than $20 for one (which seems to be a magic price). > > So there you have it, a non-technical description of J-Pole > design, derived from reading, observation and practice. I > hope you find it helpful. > > 73, > | Fred Lloyd AA7BQ Fred.Lloyd@West.sun.com |