In article ,
H. Dziardziel wrote:

On Thu, 10 Jun 2004 20:53:30 -0400, "lsmyer"
wrote:

I enjoy AM dxing, and I've noticed that the radios with larger
ferrite rods seem to be better for dxing than those with small ones.

I did some research on the subject, and I kept running into terms
such q-factor and flux. My 45-year-old mind only has room for so
much, and I'm afraid that anything I try to learn about electronic
theory will just push out some of the more important stuff already
stored in there such as anniversary dates and current wife's name.

So maybe somebody can answer a couple of questions for me in simple
English, and maybe help me save what few brain cells I still have
left.

1) Is there an optimal length for an internal ferrite antenna?

2) If so, then why don't all radios have one that size?

3) Is ferrite expensive or something?

4) How can my little Degen 1102 (tiny ferrite antenna) do so well on
MW while my ATS505 does so poorly?

Thank you in advance for all who reply.


It's been years so the erudite here will please set me straight.

The coil-antenna combines signal gathering with tuning and
directivity. The ferrite core just increases the antenna-coil
inductance. This results in a smaller coil for tuning frequency
coverage but less antenna (the physical coil proper) for actual
signal reception. It adds to core losses but there is less wire loss.
It makes a much smaller transformer. So, there are several tradeoffs.

The most efficient AM band ferrite coils for external antenna use
were only about an inch and a half or so as I recall. They are
variable inductance too for tweaking. Some old radios may have them.

This sounds right to me. I don't see how a larger ferrite rod would
necessarily work better other than change the number of turns for a
given frequency range. A larger rod would have more inductance per
turn.

I would add the type of ferrite is important, which will determine the
inductance per turn and core loss.

1. No optimal length.

2. The antenna rod works with other components chosen in the design of
the radio and so must be different.

3. Ferrite is not expensive but it is brittle and the longer the rod the
easier it is to break.

4. The design of the front end of the radio of which the rod antenna is
just one part of determines the difference in performance.

--
Telamon
Ventura, California