[Antennas] J-Pole antennas

[Ron Youvan]

> Well, I said there could be some strong opinions
> about this, but one has to see that the J-Poles that
> are shorted at the bottom are fed a small distance
> up from the bottom, and require a balanced feed
> because the J-Pole is balanced at that point.
 > Being as coax is an unbalanced thing, arrangements must
 > be made to connect the unbalanced coax into a balanced
 > J-Pole quarter wave section.  It could require a Balanced-
 > To-Unbalanced transformer, but how does one build a
 > balun that will physically fit in or on the J-Pole?

   Not so.
   The "J" pole antenna is a dipole (1/2 wl) END fed with a 1/4 wl stub, which can be fed many ways 
including a magnetic loop, or even by the shorter element being a 1/4 wl whip off of the top of a 
car.  The stub can be in line with the dipole, the most common configuration, but bending it in any 
direction changes little.
   My normal way to feed them is to run the "foam RG8/U" up inside of the shorter element to a hole 
made at the correct location (determined empirically earlier) where both the inner and outer coaxial 
cable exit, the other to be soft soldered there and the inner goes across to the longer element 
where it is soft soldered.  I make both sides of the "short" at the bottom of the stub with 
plumbing "T"s and under the short element I add enough copper tubing to be 18 inches long below the 
"short" this is a "sleeve balun" and my antennas do not suffer from the usual sensitivity to the 
feedline dressing.
   When I solder the coax I clamp the wires to the copper pipe with very small aviation hose clamps 
(screw clamps) and I do a final test moving the inner up and down slightly for the best match were I 
want it, (usually at 146.00 MHz) then I soft solder the wire to the (per-cleaned) copper pipe under 
the clamps, which does not stick to the stainless steel, the clamps come off easily.
   I use 5% silver 94% copper hard solder to put the "J" pole together.  (1% phosphorus)
   The impedance of the feed line is not critical, any impedance can be matched the way I build them.
I sweep my antennas with a "return loss bridge" to tune and match them, before during and after 
construction.
-- 
    Ron  KA4INM - The only real problem is death is so final.