On 6/27/10 13:07 , m II wrote:
RHF wrote:

- c: Same antenna with a 9:1 ratio transformer

First most of the very broadband 9:1 Matching
Transformers that are shown for Shortwave Radio
Listening (SWL) are also designed to cover the
AM/MW/BCB down to 500 kHz and SW Bands
up to 30 MHz. These usually recommend about
a 10-Turn Primary for the Feed-Line to the Radio
and about a 30-Turn Secondary for the Antenna
side.

Note - That you set a 5 MHz lower 'limit' so you
might want to cut the number of turns in half and
have a 5 -Turn Primary with a 15 Turn Secondary.



Both the examples you give have a three to one ratio. If this is a
nine to one example, I must be missing something in your explanation.

Please enlighten me.


The impedance ratio is what matters, here. More or less uniform
impedance across the desired spectra produces more or less uniform
performance at the radio input. The impedance ratio of a transformer
is equal to the square of the turns ratio.

A 3:1 turns ratio produces a 9:1 impedance ratio.

Now, a transformer's optimum frequency range will depend on a
number of factors. The specific material used in the core is one.
That's why there are so many different types of core material. The
actual number of turns is another. The type of wind is a third. For
these reasons, and that the impedance ratio is too narrow for the SW
bands, a 75-300 TV transformer is not exactly a good choice for
shortwave listening. Although it may produce passable results at
some frequencies if there are no other options.






mike