From: oops@msg.ti.com (Mitchell Carson)
Newsgroups: rec.radio.amateur.antenna
Subject: Re: Copper Cactus Antenna -- Tri-bander plans
Date: 22 May 1995 14:25:05 GMT
Organization: Texas Instruments
Reply-To: oops@msg.ti.com (Mitchell Carson)
NNTP-Posting-Host: mcarson.sc.ti.com
X-Newsreader: IBM NewsReader/2 v1.09
Here ya go -- a tri-banded J-pole design:
THE Copper pipe Tri-Band "J" ANTENNA !!!
_ _ _ _
|-| <cap | | |
| | | | |
| | 1" | | |
| | _ _ | | |
| | |-| | | | |
| | | | | 18.5" | |
440 | | | | | | | |
|*| |*| - 6" | | |
|-|\ /|_| | 1.25" | | | |
| |_|_| | - | | | |
| _|_|__| _ - | |
| | ^ | |
1.5"|-| ^ | |
_ | | elbow _ | |
|-| | | | 37.5" |
| | | | | | |
| | | | | | |
| | | | | | |
| | | | 12.5" | 57.5"
220 | | | | | | |
|*| |*| - | | |
|_|\ /|-| | 1.75" | | |
| |_|_| | - | | |
|__|_|_ | <copper T - - |
| | |
|-| |
| | _ _ |
| | |-| | |
| | | | | |
| | | | | |
| | 2.5"| | | |
| | | | | |
| | | | | |
| | | | 19" |
146 | | | | | |
| | | | | |
|*| |*| - | |
| |\ /| | | 4" | |
|_| \ / |_| | | |
| |__|__| | - | |
| _|_|_|__| - |
|_| | -
| | \
| | \ RG-58u
| | \
| | \ This Design is by:
1/2"-1">>>| | `----=- 146 Mhz.
copper |_| "Kuby", N6JSX
pipe \\
\`----------=- 220 Mhz. Dale Kubichek
`----------=- 440 Mhz. Rowland Hts., CA
Note: All matching elements were placed 120 degrees from one another.
(copyright) 1994, Kuby Kommunications, Rowland Hts., CA
The "J" antenna design goes back many-a-year, way before I became a HAM.
What interested me about the "J" antenna design is:
* Very low - nearly straight-out angle of radiation
* NO ground plane required
* Possibility of multiple bands in one antenna with separate
feedlines for each band
* Simplicity of construction
* Inexpensive to make
The "J" antenna low angle of radiation gives the user the most distance.
This will prove to be a great antenna for the Midwest or the flat Central
Plains of the USA. Conversely, the "J" is not the best for near mountain top
repeater usage.
A "J" requires NO ground plane or special counter poise. This makes the
"J" ideal for mobile use. Doing some fact finding I found claims that the "J"
has approximately 3db of gain over a 1/4 wave ground plane antenna and can be
made from almost any material; copper pipe, steel whips, & even TV twin lead.
What is a "J"?
Technically, the "J" antenna is an end-fed 1/2 wave antenna that uses a
1/4 wave matching stub. To some of the old old timers it might be called an
"end-fed ZEPP" bent 90 degrees. In actuality, the conductor is 3/4 wave length
long and the match is 1/4 wave length long. The matching stub creates the 1/2
wave loading point. This makes the antenna effectively an omni 1/2 wave
length. The "J" is in best terms a balanced 1/4 wave matching stub feeding an
unbalanced 1/2 wave load.
A "J" can be fed by ladderline or coax. I found using coax cable RG-58u to
be the best.
I have designed a Tri-band and Dual-band "J" antenna with 1/2" rigid copper
pipe, copper "T"'s, 90 degree elbows, and copper pipe caps. Using rigid copper
pipe makes it a snap to assemble. Simple plumbers soldering techniques are
used for solid construction. These materials will withstand a lot of abuse and
weather. Each band has its own feedline and all three radios can be run
simultaneously.
I have made a mobile 2 meter "J" from a Radio Shack 104" CB whip. The
whip has a mounting bolt on the bottom for easy attachment to mounting clamps.
I fabricated a plastic insulator and metal support bracket for the 1/4 wave
matching section. The coax was connected by using electrical split bolt
connectors. Also the split bolt connectors give mobility in adjusting the
VSWR.
Below is a table of measurements on the "J" construction - these
measurements are not absolute but a very close proximity to the antenna you
might build.
VHF & UHF "J" vertical antenna dimensions
-------------------------------------------------------------------
146 223 446 Mhz.
-------------------------------------------------------------------
Mathamatical
1/4 wave 19.3 12.6 6.3 electrical
1/2 wave 38.5 25.2 12.6 computations
--------------------
Antenna
radiating element 57.5 37.5 18.5
matching element 19 12.5 6
--------------------
coax tap off's
combo 2m,220 5 2.25
combo 2m,220,440 4 1.75 1.25
--------------------
spacing between
elements ID 1.5-3 1-2 .75-1.5
(these dimensions can be as small as 1/4" but the coax tap off's
are effected)
NOTE: all dimensions are in tenths of a inch, combo 2m & 220 dimensions are
also good for mono band antenna construction.
-----------------------------------------------------------------
Designer Construction Notes
* On the 3 J antenna combo I took the matching elements and placed them 120
degrees from one another, this appears to help in eliminating the cross band
RF desensitizing. It is advisable to tune the 440 section first and then the
220 section with the 440 coax disconnected from the radio and so on.
* Insulators between the match stubs maybe required to keep the elements
equidistant.
* Mounting brackets can be obtained cheaply from Radio Shack
* Silicon the antenna feed points and open coax for preservation.
* Try to keep the coax feeders at electrical 1/4 wave length multiples this
seems to make a large difference in the matching.
Std. disclaimer: I speak only for myself.
Windows95, Windows96, Windows97...Win-ever <grin>.