Making Waves – The Shorty Forty

I was taking part in a Twitter conversation today with someone building the helical antenna published in this month’s RadCom, the RSGB member’s magazine, and having issues with the matching of it.  He was trying it out due to lack of space and a poor earth (clay) at his QTH.

I set to thinking and remembered an antenna design that I used to hand out to Foundation and Intermediate Licence trainees when I was mentoring them through their studies and assessing their practical assignments.  This was the Shorty Forty antenna because we don’t all have the requisite 20.28m of free space to string a dipole across.  I shared the plans with him and later thought “Why not share them for everyone?” so here goes:

The Shorty 40 – Helical Whip for 7MHz

So you want to get onto 40m but don’t have room for a dipole (20.28m)? Then this could be just the answer if you have a little time on your hands and enjoy home construction.

The Shorty 40 is a helical whip for 40m wound on a 3m long, 32mm diameter piece of PVC tubing (the sort available in most DIY stores). You will need 21m of 1.2mm diameter enamelled copper wire, 80cm of 2mm diameter ECW and 10 or 15m of 1.5mm diameter insulated copper wire (the sort used for lighting circuits or earth wire). You will also need a SO239 connector and a piece of angled aluminium.
ShortyForty

The picture says it all really but, just in case, begin by winding the 21m of copper wire along the length of the pipe, using tape or adhesive to secure it along the way. Cut the 80cm of 2mm ECW in half and push through the holes drilled in the top of the pipe. Solder together in the centre and solder the end of the coil here also. Drill a 16mm hole in the aluminium bracket for the SO239 socket and then attach it to the pipe using machine screws. Then solder the other end of the coil to the centre pin of the socket. Connect pipe to mast and connect two or three 5m radials to the solder lug on the aluminium bracket. Attach coax, raise mast and away you go.

Disclaimer  I cannot claim to be the first person to develop an antenna such as this but I have researched ideas on the internet and in books on the subject – from ARRL, PW Publishing and RSGB publishing – and changed them to suit modern metric measurements and make them easier to understand and build. 

 

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