Perhaps I might be of assistance by providing some orders of magnitude.

A 1/2-wave stub line, 1 metre (39.37 inches) long constructed of a pair of
polished copper tubes, each 1/2-inch in diameter, centers spaced 3 inches
apart, at 150 MHz (not very far from 2 meters) has the following
characteristics -

Zo = 299.8 ohms.
Attenuation = 0.0024 dB.
As a tuned circuit, Resonant Q = 5800.

When short-circuited -
Parallel input components of Zin -
Rsc = 0.08 ohms,
Xsc = -j*582 ohms.

When open circuited -
Series input components of Zin -
Roc = 1.11 megohms,
Xoc = j*38.6 ohms.

These and many other interesting values can be very accurately and rapidly
calculated, from 20 Hz to 1 GHz, by using program RJELINE2 available, free
issue, from website below.

Contributors to this newsgroup, who by no means are lacking in intelligence,
may then be able to get their feet on the ground.
-----
.................................................. ..........
Regards from Reg, G4FGQ
For Free Radio Design Software go to
http://www.btinternet.com/~g4fgq.regp
.................................................. ..........

On Sat, 12 Jun 2004 08:35:42 +0100, "Ian White, G3SEK"
wrote:

Roy Lewallen wrote:
I'd also like to add that the induced current isn't unpredictable, as
you stated. It has to follow rules like all other physical phenomena,
so it's entirely predictable.

Remainder deleted


Three questions emerge from my foggy brain wrt to a current project.

1. What sort of choke loading on a feed line would be most
effective at 70 cm, ferrite or coil wound from coax.

2. Could ferrite loading at a feed point take the place of say a
1/4 wave sleeve balun matching the unbalanced coax feed to a
centre-fed dipole element.

3. What grade of ferrite bead would best be useful at 70cm.

Thanks in advance.

MikeN ZL1BNB

MikeN wrote:

Three questions emerge from my foggy brain wrt to a current project.

1. What sort of choke loading on a feed line would be most
effective at 70 cm, ferrite or coil wound from coax.

Probably equally effective, although it might be tricky to get a coax
coil to resonate at 70 cm, while a ferrite core choke, which is
inherently broadband, would be easy.

2. Could ferrite loading at a feed point take the place of say a
1/4 wave sleeve balun matching the unbalanced coax feed to a
centre-fed dipole element.

Yes. A properly constructed sleeve balun can be made to have higher
impedance, but in the application you describe, a ferrite core choke
would be perfectly adequate.

3. What grade of ferrite bead would best be useful at 70cm.

I'd probably use type 43. That's a Fair-Rite designation, but ferrites
from other vendors with initial permeability of 700-800 or so would work
equally well. Cores from that material are readily available in a wide
variety of shapes and sizes. Some of the 60 series ferrites would also
probably be adequate at that frequency.

I suggest you visit the Fair-Rite website and take a look at the
impedances of various cores at the frequency of interest, and choose
ones that get you the impedance you need. (Unless you're running a lot
of power, you don't need to worry about whether the impedance is
resistive or reactive -- just look at its magnitude.) You can place
cores on the outside of the coax and get an imedance that's the product
of the number of cores and the impedance of one core. Or you can wind
the coax in multiple turns on a single core and get N^2 times the single
turn impedance, where N is the number of turns. 500 - 1000 ohms or so of
impedance is adequate for most applications. If you have an antenna
analyzer that operates at that frequency, you can measure it. Otherwise,
just go by the manufacturer's stated value of impedance at the frequency
of interest.

Roy Lewallen, W7EL