World of Ham Radio 1997/WOHR97_AmSoft_(1997-02-01).iso/mods/misc_mod/jp0le.doc

Internet Message Format | 1997-02-01 | 8KB

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
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From: [e-mail] (Clement Vaillancourt)
Subject: Re: Wire J-pole?
Message-ID: <[e-mail]>
Sender: [e-mail] (Netnews Admin)
Reply-To: [e-mail]
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:[phone] Fax:[phone]
   [e-mail]          |  Radio-amateur: [e-mail]


> In article <[e-mail]> [e-mail] (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                        [e-mail] |