Active Ferrite Rod Antenna
 

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

Re: Active Ferrite Rod Antenna
 

Very neat Ian I may look into making one of those it beats the four foot frame aerial, and very portable ! how many ferrite rods did you need ,Mick.

Re: Active Ferrite Rod Antenna
 

How much of a performance advantage does it offer over a much more compact standard ferrite rod aerial with buffer amp?

Re: Active Ferrite Rod Antenna
 

Here's my version of the 'ferrite sleeve loop'. I wrote this up in Radio User mag a couple of years ago. There are 39 rods around the hollow centre and about 25 turns tuned with a standard variable cap. It covers 500 KHz to about 2 MHz and works like Billy-O. It's very high Q and can be tricky to get 'on the nose' at the HF end, but it's very effective. There are no active electronics involved, the aerial is completely passive.
The FSL is the invention of the late Graham Maynard, who had a very fertile brain and seemed to ping up ideas on all matters.
His original writings on the FSL can be found here http://www.zen22142.zen.co.uk/Media/fsi.htm but be prepared to hack your way through lots of tangential meanderings!

Re: Active Ferrite Rod Antenna
 

If it's not a silly question, surely ferrite rods are polarised. How do you know which way round to fit them?

Re: Active Ferrite Rod Antenna
 

As far as I know ferrite rods do not have any polarity, Dave.

Re: Active Ferrite Rod Antenna
 

Ferrites can be classified into two groups:

"Hard" Ferrites are materials developed to be permanently magnetised, they are found as ferrite magnets at the back of some loudspeakers. Also in core memories found in old computers (one in tfe Apollo spacecraft), and some jukeboxes.

"Soft" Ferrites are materials developed for linear operation with very low hysteresis. These are the ferrites used in inductor/transformer/antenna cores.

Both types are mechanically hard to the point of being brittle.

Hard ferrites are made to be magnetised, which makes them polarised.

Soft ferrites follow whatever field is applied to them and carry no polarisation of their own. Some interesting (and brain-twisting) tricks can be done with soft ferrites polarised by a separate permanent magnet in their vicinity. Look up the microwave 'circulator' for some weirdness...

The standard book is "Soft Ferrites Properties and applications" by E C Snelling usually referred to a Snelling, Soft Ferrites. It's amusing how often people's eyes convert that n to an m and leave the reader surprised.. Smelling soft ferrites... some new perversion? What's it doing in Dewey decimal 621.xxx shelves? People-watching can be fun.

David