Hi Roger this can be done and was written up by Derek G3GRO I  think in one
of the early (Blue cover) LF Source books that Peter Dodd edited. The
message seemed to be to adjust the coil so that for tuning there is minimum
penetration of the ferrite into the coil.

If you do it this way you do not necessarily need ferrite. I did some
experiments with a 36mm coil and I used a couple of iron-dust toroids,
cemented onto a plastic rod. The diameter was just such as to fit into the
coil former. This was for adjusting the L in a Class E stage so the
inductance would probably not be enough for aerial loading (about 100uH) on
137 but might work for 500kHz.

Alan G3NYK

----- Original Message ----- 
From: "Roger Lapthorn" <[email protected]>
To: <[email protected]>
Sent: Tuesday, August 16, 2011 12:57 PM
Subject: Re: LF: Re: Ferrite Loops


> Hi Jim (et al)
>
> *Ferrite rods as 5-20W TX loading coils?*
>
> As long as the ferrite doesn't saturate am I right in thinking that the
use
> of ferrite rods as coil formers for 137 and 500kHz is basically "a good
> idea"?
>
> From personal experience with 5W this worked well at 500kHz so I assume
that
> the idea could be translated to 136kHz if using separate rods for each
500uH
> of inductance so the cores of each do not saturate. Am I right in thinking
> that if you bundle x cores together (in parallel) the core will saturate
at
> x times the power? Making a ferrite rod based variometer would be
> straightforward - PVC tube with cores sliding together lengthwise for
> example.
>
> Engineering large air-spaced loading coils is quite a feat whereas making
> up, for example, 8-10 separate ferrite coils with a range of taps on each
is
> quite easy (and small). Less wire would be needed so the losses in the
coils
> would be lower compared with the air-spaced equivalent.
>
> Is there mileage in this, say up to 15-20W RF?
>
> 73s
> Roger G3XBM
>
> On 16 August 2011 11:53, James Moritz <[email protected]>
wrote:
>
> > Dear Tom, LF Group,
> >
> >  what do you think about an array of many parallel mounted ferrite rods,
> >> each of them carrying only a few windigs, all windings connected in
series
> >> (and then perhaps tuned) and the rods arraged in such a way that the
> >> individual apertures dont touch? Or will this lead to the dimensions of
a
> >> comparable air loop ;-) ?
> >>
> >
> > I am sure this would work, but I think you have also identified the
> > limitation ;-) Fundamentally, if the signal has a particular power
density
> > at the receive site, the antenna must intercept the signal from a
certain
> > aperture area in order to deliver a certain power to the receiver. So
there
> > is a limit to how small it can practically be, although the actual shape
can
> > vary to obtain the same aperture - one could make a rough comparison
between
> > the short, wide loop vs. the long, thin ferrite rod, and a long yagi vs.
a
> > broadside array of dipoles.
> >
> > I think an array of ferrite rods might be attractive in some
circumstances
> > - for instance, you could have numerous small rods stacked vertically,
to
> > produce a "ferrite rod vertical" with a small turning circle but a
> > relatively large effective area.
> >
> > Cheers, Jim Moritz
> > 73 de M0BMU
> >
> >
> >
> >
>
>
> -- 
> http://g3xbm-qrp.blogspot.com/
> http://www.g3xbm.co.uk
> http://www.youtube.com/user/g3xbm
> https://sites.google.com/site/sub9khz/
>