SM2YER Goran's Homepage

- 160/80/40 meter band - trapped dipole -
2007.03.05 Is trying to build a trapped dipole for 160/80/40 meter bands. I read a nice article by A. C. Buxton, W8NX in QST July 1992 Issue.

I have previously made other trapped dipoles using the standard coax-trap design. So this new developped 'Super coax-trap' is new to me, it's always nice to find that almost everything is possible to improve, i also read some nice articles about doubble coax-traps but with only one layer in the ARRL book 'More wire antennas'.

On this page i describe my efforts to put up a trapped dipole usin the 'Super coax-trap' described by A. C. Buxton, W8NX in QST July 1992 Issue

One side of the antenna

- Technical data -

SPECIFICATIONS
========================================
Lenght A ................... 9.82m (32.2')
Lenght B ................... 3.35m (11')
Lenght C ................... 5.58m (18.3')

Res freq for 40m trapp ..... 7.17MHz
Res freq for 80m trapp ..... 3.85MHz

Ant res freq on 40m band ... 
Ant res freq on 80m band ...
Ant res freq on 160m band ..

Trap design
The QST article describe the trap design. I'm using this design mainly for the simplicity and the superior power handling capability compared with 'normal' coaxial traps.

First working trap resonant @ 7.174MHz

2007.03.13 Made the first trap ready today, it was some test and errors before i got the trap to resonate as i wanted, seems like my wire gives little more capacitance than the QST article description. The traps is made out of the dielectric and center lead from normal RG-58U coaxial cable. The outer jacket and the shield is stripped away. The remaining center lead/dielectric is used to make two interconnected coils on the PVC plastic core. One inner and one outer coil. Se fig. 1. Note! The inner conductor from the RG-58U coaxial cable must be stranded, solid cu-lead wont work!

Principle for the dual layer trap coil

2007.03.14 Now also got one ready made trap to resonate @ 3.856 - Now i only need to duplicate two more in the same manner.

Second working trap resonant @ 3.856MHz

Trap data

Trap making is on the way...

2007.03.17 made some 'Lexan' plastic isolators to put inside the traps. The isolators also keeps the antenna wire in center. Also applied heat shrinking tube seal on the traps, the tube seal is UV and weather resistant, it also protects the traps from mechanical damages. I use 'REYCHEM MWTM' type tube. This tube is also treated with heat glue inside and make a 100% water thigh seal. I use the following dimensions, 40m Trap 63/16 - 80m Trap 140/42 (Millimeters before/after heat treatment)

40m trap before applying the heat shrinking tube seal, viewed from the end.

Using 'RAYCHEM MWTM 63/19' on 7.17MHz trapps

Using 'RAYCHEM MWTM 115/38' on 3.856MHz trapps

2007.03.19 Got the 40m wire section and the 7.17MHz traps upp to day, i'm using the 'miniVNA' when i adjust the antenna section wires. Needed to adjust the wires by shortening about 45cm (1'6") on each dipole half compared to the given lenght in the article. (My messurements is given from center to trap center)

Did not understand from the article how the given lenghts is messured.

I'm about to add the 80m section wires and the 3.85 traps. I discover now also that my 'Radio Works B1-2K PLUS'balun that i intended to use with this antenna only cover 3.5-50MHZ ! It won't work! - I'm going to build this 1:1 Air-Core balun instead.

- Documentation -

QST Article

Go Back!