[NCARC] Twin-Lead J-pole Questions

[Rick Huebner]

Floyd Moore wrote:

> OK all you antenna elmers...
> 
> After the Fest, and the next day of operations up at Pinewood Springs, I
> determined that I really need some better portable antennas.  I was really
> impressed by the twin-lead j-pole and decided to go off and investigate.
> 
> I found the following diagram (similar diagrams are found all over the
> net, but the design is the same):
> 
> ( http://k9xih.net.cit.cwru.edu/w8edu/projects/j-pole;lines=1-62 )
> 
> 
> 
>>  TV Twin-lead J-pole
>>  By Ed Humphries - N5RCK
>>  edh@hpuerca.atl.hp.com
>>
>>  The following is a description of a J-Pole antenna made from 300 ohm
>>  TV twin-lead. They have quite a few advantages which include improved
>>  performance for 2-meter HTs, portability, and low cost.
>>
>>                  |   | do not short this end.
>>                  |   | (when trimming for vswr, cut both sides)
>>                  |   |
>>                  |   |
>>                  |   |
>>              3/4 |   |        Technically-speaking, this is a 1/2
>>             wave |   |        wave end-fed antenna with a 1/4 wave
>>                  |   |        matching section.
>>                  |   |
>>                  |     1/4" gap
>>                  |   |         (trim for vswr _below_ gap)
>>                  |   | 1/4
>>                  |   | wave
>>                  |   |
>>  coax conductor=>*   *<= coax shield
>>           1 1/4"-|   |
>>                   -*-    solder the twin leads togethe
>>
>>  For a center frequency of 146 MHz:
>>  1. Start with @54" of TV twin lead (flat, NOT foam core)
>>  2. Strip 1/2" of insulation at bottom and solder wires together.
>>  3. Measure 1 1/4" from soldered wires and strip insulation on both
>>     sides. This is the solder point for a coax feedline.
>>  4. Measure 16 3/4" from coax shield solder point and cut out 1/4" notch.
>>  5. Measure 50 1/3" from coax center conductor solder point and trim
>>
> 
> Now, the question arises regarding the length of the legs and the total
> length of the antenna.
> 
> Note that the diagram says 3/4 wave and the instructions read "for 146MHz...
> Measure 50 1/3".
> 
> This does not make sense.  I calculate that the 2M wavelength at 146MHz is
> 80.9" and the 3/4 wave is 60.7".  What's up?
> 
> Can someone explain this to me?


You are calculating the distance the signal travels in one period *in a 
vacuum* not in a twin lead.  For a great study of how these work and how 
to make them work, check out my page: 
http://snow.prohosting.com/w0rcy/Jpole/jpole.html

Also, the basic program in that can be replaced by a c program written 
by http://www.k4lrg.org/Balloon/ATV_1.html at the bottom of his page.

I *love* that question!

Rick, W0RCY