|
|
The GPO Mobile Antenna
by
KB8GPO Ken Libby
The GPO Mobile Antenna is a duel band antenna for ten and two, and is suitable for flying the old Radio Free Waldenburg Alien Pirate flag from. I have been running this antenna on my vehicles for over ten years. For those of you who know me from the 147.180 USECA Repeater you already know that my powerhouse station can put out an awesome signal from all over the Detroit Metro area. What not many people know is my Ten-Tec Kit two meter rig puts out one watt, that’s all I use. It’s all in the antenna.
About ten years back I ran many tests on a couple of different 5/8 wave, ¼ wave, "J" pole and stacked "J" pole antennas. My conclusion was, that you can not beat a simple "J" pole for two meter mobile performance.
The "J" pole is simply a ½ wave antenna with a built in matching circuit. Most people know that a half wave antenna is a gain antenna. What a lot of people don’t realize is that a half wave antenna radiates at right angles to itself, or if it is mounted vertical then it radiates "line of sight", the perfect antenna for two meters. The simple gain numbers do not begin to tell the real gain of this antenna.
The antenna is ¼ wave, 10 meter, end fed, stainless steel whip that has a two meter "J" pole arrangement built onto the top portion of it. To construct the two meter "J" pole section onto the whip was just a mater of mounting a "shorting bar" and two meter ¼ wave section, ¾ wave (of two meter) down from the top of the whip. Sorry about that, you try to explain it. I will provide numbers later.
The ten meter use of the antenna is just your basic ¼ wave whip, but it is more broad banded, covering the top half of 11 to the top of 10 meters in under 1.5:1 SWR. The two meter coax must be removed from the antenna to use this antenna on ten, and the ten meter coax must be at least removed from the radio to use the antenna on two meters….. Switching the coax from two to ten will work just fine, but watch the slack that you aren’t dragging your coax on the road.
I used all stainless steel parts, including nuts, machine screws, shorting bar, and connection tabs. Other metals can be used, but for long life in Detroit road salt, stainless is best. The stainless steel shorting bar is the only tricky part. To drill and tap holes in stainless you must use a drill press, running at lower speeds then you would use for steel and cool the bit and hole with mix of light oil and alcohol. To tap the screw holes you must use a cutting fluid that is recommended for stainless and a small tap "T" handle (so you can feel the tap). It can be done, I made two shorting blocks without dulling a bit or breaking a tap. The ARRL printed instructions on building the "Super-J Maritime Antenna" in many of it’s publications. This article is the bases of my shorting block design. If you are wanting to experiment, you might try the Super J design, I did.
The two meter connection point is a little tricky. Most "J" pole instructions will tell you to hook the conductors up opposite of the way that I use. In the mobile installation I found that there is almost no coupling between the coax and the lower part of the whip if you hook the center conductor of the coax to the whip and the shield of the coax to the ¼ wave section (short rod).
For the connection "clips" I folded a 1.5" long piece of ½" by 1/16" flat strip stainless around the antenna rods, drilled and secured them with machine screws and nuts to clamp on the rods. One of the clip screws (on the short rod) also secures a small "L" bracket that is screwed to a SO-239 connector. The other clip (on the long rod) has an "eye" connector that is soldered to the center conductor of the SO-239. Your hook up could be wired coax to clips, but you must seal the coax real good with silicone. In the picture below you will notice a piece of cord around one of the rods, it is used to help hold the Antenna upright under high wind loads. Not a pretty picture, many Michigan winters.
The two rods should be stabilized so as not to touch under in high wind loads. I found that they can be held nicely with a 1 ¾" piece of 3/16" plastic gas line. I punched two small 1/8 to 3/16" holes in the tube ¼" from each end and slipped it unto the two rods, very snug fit. The ball at the top of most whips will pull off and can be reused. To keep the tube from "walking" off the short rod you should grind a notch about ¾" from the top.
There are many articles on "J" poles that can explain tuning in finer detail, but for the most part, if you go by my measurements the only tuning that should be done is to slide the connection "clips" up or down a little at a time for a max movement + - ½", for best SWR at 146.000 MHz.
My measurements are not what the formulas will tell you, but with much experimenting this is what I found to work the best.
101 ¼" steel CB Whip, they are about .20" wide so a #7 drill bit works good for the Shorting Bar holes.
60 ¼" from the top of Antenna to the top of the Shorting Bar.
19 ¾" from the top of the short rod to the top of the Shorting Bar, you need a 20 ¾"x .20 Rod.
2 ¼" from the top of the connection clips to the top of the Shorting Bar.
1" Spacing between the rods.
1 ¾ x 1 x ¼" shorting bar
Links for more information about J Pole antennas
Design Your Own J-Pole Antenna, by Buck Rogers K4ABT
Ken Libby
Last up-dated Sunday September 29, 2002