[Hermit6345]

We decided to make a portable aerial for use with any domestic radio (LW MW). I found a design with artwork on the web designed by Raymond Haigh in Everyday Practical Electronics (September 2000 issue).

Beware. Somehow we managed to make a perfect PCB for this project although it was inverted! It made the construction just that little bit more difficult, but possible.

This project relies on using several ferrite rods in an array/bunch. The only place that seemed to offer ferrite rods of the size / length required was China. So, the ferrites arrived and a disappointing crumbled mess was found within the package.
Not daunted, more were ordered and we proceeded with some stubby rods that were found in spares. Sean set about gluing the 3D kit of ferrite dust back together.

Right from the start, the tilt and turn design was dismissed - a simple rotation option was implemented. 35mm domestic waste pipe was utilised to house the antenna ferrite rods. The vertical piece of the antenna is 22mm water pipe. The coil was formed onto the pipe directly and threaded through to the jack plug (normal stereo 6mm). Stereo plug was used as there is a feedback loop also within the design. The turning of the coils is the easy part. Getting the wires down to the jack plug and not damaging them when the ferrite rods are introduced proved difficult. We ended up wrapping the ferrites with shrink wrap bands to keep them compact and less likely to stray when being introduced into the pipe.
Joining the 22 to 35mm pipes would have been really easy if we could have found a ‘T’ piece to do this. However, we ended up drilling a hole into the 35mm pipe and using PVC weld to join the two in the form of a large ‘T’. We shaped the end of the 22mm crudely to allow as much space as possible for the ferrites.

This design relies on a dual gate MOSFET device (BF981) which proved to be rather delicate/sensitive to static – in spite of wearing an earthing strap when working on the PCB.
The housing was drilled for control pots and the top of the box was drilled to accept a jack socket (to allow rotation of the antenna). A die cast metal box houses the PCB to shield it. The wiring to the switching is possibly the most difficult part of the build as there is a lot of action at the on/off selector switch.
The pots and switch control must all have similar splined shafts for matching knobs to be added. This involved a bit of searching for parts on the web and a bit of sawing of over length splined shafts! VR6 (1K Log) is possible to find – the parts list suggests it is may not be.
The design was modified to allow a 9v external power supply to be used, the socket for which isolates the internal battery when the power jack plug is inserted.
It was tested on several radios (MW old at first) and worked best with the Eddystone EB35 MK III. This has a 75 ohm antenna socket which suits the output impedance of the antenna.
A short wire was attached to the antenna bypass socket and signals that were almost totally inaudible on this were demonstratively improved by tuning the active antenna coil (via a varicap diode) and using the feedback winding to adjust the feedback just before oscillation (Q Multiplier). Noise cancellation is excellent by turning the antenna on its jack plug. Although LED bulbs seem to permeate no matter what! It is an excellent alternative for a long wire and has the benefit of being compact, portable and noise cancelling.
We then went on to construct a LW version of the antenna. By now the glue had set on the broken ferrite rods! This required a bit of experimentation on the number of turns used and quantity of feedback loop. Main LW coil needed to be around 2.5mH, but this can be checked if you are interested in building one.
Artwork was made using Front Panel Design software.

We did not consider building a plug in dual LW/MW antenna. This was due to Sean’s fear that one coil would affect the other (like it did with a frame aerial design we made) and also that the jack plug does not have enough segments. It would involve even more switching to achieve this and two coils this close together could compromise the performance.
For those who need a compact antenna for a Woodie or Bakelite beast without a frame aerial, this is the neatest way to achieve a decent reception for LW/MW.


IAN