Monthly Archives: November 2013

Trapped Inverted L Antenna for 160/80/40

Here’s a simple antenna which will get you on the air on top band and 80m (40m as well at a pinch) in a small garden.

Following on from an article in Practical Wireless Feb 2004 by GM0ONX which showed how to build an 80/40 trapped inverted L antenna I modified the design to cover 160/80/40.  The traps are constructed from plastic tubing and regular 50 ohm coaxial cable. For the tubing use washing machine outlet pipes obtainable from any DIY store for a few pounds and cut to size using small hacksaw. Support the vertical section using a non-metallic extendable fishing rod. Here’s how to add top band in easy steps:

  1. First construct antenna as in original article but reduce length of 6.55m section to 5.55m and do not attach end insulator.
  2. Build a 21 turn trap following instructions in article. You can play around with trap design using the free program CoaxTrap from http://www.qsl.net/ve6yp/index.html
  3. Attach 21 turn trap to end of 5.55m section and add another 9.3m of wire terminated with the insulator to other side of trap. Don’t solder wire connections on trap yet.
  4. Hoist antenna into air and check swr on 40, 80 and 160. 40 should not have changed from original design and should have minimum swr around 7.050.  Adjust, firstly, 5.55m wire to move minimum swr point on 80m where you want it. 80 is a large band so if you are into CW/RTTY aim for 3500-3600 while ssb is somewhere between 3.600 and 3.800 depending on whether you want to inter-G ragchew about arthritis (and every other ache and pain known to man) or go for DX. Secondly adjust length of 9.3m wire for top band. The dimensions I have given above in (3) give resonant points at 7.050, 3.750 and 1.940. Just adjust for your band of interest. Finally solder wires, waterproof traps and stick horizontal portion of antenna as high as possible.

The antenna works very well but swr bandwidth of <1.7:1 is around 40 kHz on top band but using an atu will give you all of band. II have worked almost 40 DXCC countries on top band (with contest in progress). You probably won’t work any continental DX on top band but the whole of Europe is very possible. You will be a VERY loud signal on the Sunday morning AM net! It also works very well on 80m but performance on 40m is pedestrian due to most of antenna being vertical and it will be 10-20dB down on a half-size G5RV mounted high and clear.

To get improved TX output you will need to add some radials to earth rod to improve signal strength on top band. This will make no difference to RX performance. The great thing with this design is that you get a working top band antenna into a horizontal space less than 20m long.

Here’s the original article from Feb 2004:

Download (PDF, 918KB)

GM0OMX published a follow-up article in January 2005 showing how to add top band to his design. However I came up with my design in the summer of 2004. Here’s the later article:

Download (PDF, 488KB)

 

 

 

Low VHF in Carcassonne

Some beautiful weather in Carcassonne last week with temperatures up to 28C! The MUF (maximum usable frequency) obliged as well and several NA and SA utility comms were heard between 29.7 and 40.0 MHz.

Here’s 33.6 MHz at 1515 UTC on 30 October 2013:

 

High MUF 39.040 MHz 15:20 UTC 30 October 2013 – sounds Brazilian

 

33.78 MHz at 15:29 UTC 30 October 2013:

 

and 33.74 MHz at 1517 UTC 1 November 2013:

 

SFI was getting up to 160 and above last week and it’s great to see the F2 propagation bring in some interesting DX.

Equipment used was the Icom 9100 in NFM with 12 kHz filter and Cushcraft R8 vertical. Audio fed out of icom into Acer laptop mic input and recorded with the versatile freeware Audacity.