Amateur Radio

Home Page

  Contact us

Astronomy
Articles
Astronomy Links
Amateur Radio

AMTOR
Portable Operating
Batteries for portable use
Portable in Portugal 2002
K1 notes
K2 notes
K3 review
Issues with the FT857D
Removing bugs from a laptop screen
Simple vertical antennas
6m quad antennas
6m J-Pole antenna
Simple spectrum analyser
Universal Receiver
Companion Transmitter

Weather
Weather page

A 6m portable J-Pole antenna by G4AON

There are many of the popular J-Pole antennas described on the Internet, most are either ribbon cable antennas for the 2m band, or quite elaborate copper/aluminium tube versions for 6 metres. I was unable to find a 6m ribbon cable one, so I set about making one. This antenna is based on the original 2 metre design by F C Judd, G2BCX, which appeared many years ago in several publications.

These antennas were quite popular at one time, with the introduction of low cost multi-band commercial vertical antennas for 6m/2m/70cms they seem to be rarely used these days. For a low cost, "build it in minutes" antenna the ribbon cable J-Pole or "Slim Jim" as it was known, is hard to beat. The antenna shown here includes a coaxial cable choke feed to remove RF currents from flowing on the outer of the cable. There are alternative methods of achieving the same decoupling, but wrapping a length of cable around a plastic tube is both low cost and easy to assemble.

Materials required to build the antenna are :

1 off 450 Ohm slotted ribbon cable, 4500 mm long. JSC part number 1318, or equivalent, http://www.jscwire.com/
2 to 3 metre length of RG58 coax.
120 mm x 43 mm diameter off cut of plastic waste water pipe, not critical.
Coax plug/socket to fit RG58 coax.
3 off small cable ties (Ty-Raps).

Assembly

Begin by drilling a couple of small holes at each end of the plastic waste water pipe, these are to take cable ties to secure the coax cable. A hole size of 2 to 3 millimetres diameter should suffice. Secure one end of the coax with a cable tie and wrap the rest round the pipe securing with the 2nd cable tie. The amount of coax and number of turns aren't particularly critical. Form one end of the coax into short tails, fit a plug or socket of your choice on the other end.

Carefully cut the ribbon cable to 4305 mm. Solder a piece of spare wire across each end. Measure 142 mm from one end and carefully remove the insulation. Solder the coax tails to the ribbon cable at this point, see diagrams below.

On the side where the coaxial braid connects to the ribbon cable, measure 1380 mm from the bottom end and cut the wire, measure a further 80 mm along the ribbon and cut the wire again. Remove the wire between the cuts. Secure the coax to the ribbon with a cable tie as per the image below left.

Tuning and performance

When made from the components and dimensions given here, this antenna should give a low SWR around 50.150 MHz. If you require an antenna for higher in the band the 4305 mm length would need reducing. At the dimensions and frequency quoted, my antenna gives an SWR of 1.4:1 at the feed point and looks even better on the end of a 10 metre length of RG213 coax. If the match isn't satisfactory, the connection point of the coax feed will need to be moved up or down for best SWR. Unfortunately sometimes having to move the feed point is one of the drawbacks of using ribbon cable for these antennas. I was lucky and found a good match "2nd go". If you use the same JSC ribbon as I used, the dimensions should be sufficient to obtain a good match. Before deploying the antenna outdoors, the open end of the coaxial cable should be sealed against moisture using silicon rubber sealant or hot melt glue.

This antenna is quite long, the image further up the page shows it secured to a fibre glass fishing pole with garden ties. At this height the antenna is too low to perform well. Mounted with the lower end some 12 feet above ground, plenty of good contacts all around Europe and further out to 3000 miles have been made using 100W of mostly CW. Clearly under good conditions almost any antenna will work, but results have been excellent considering the height. Mounting options could include installing inside sections of a fibre glass fishing pole as these are quite plentiful and cheap.

Protected by Copyscape Web Copyright Checker

 

Copyright © 2008 Dave's Astronomy Magazine. All rights reserved.