D. Peter Maus wrote:
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.
[...]


Good information all around.

But it's also important to remember that a random wire will have an
impedance which will vary hugely with frequency.

(If you get many wavelengths long, the impedance variations with
frequency do start to smooth out.)

The free demo antenna modeling program called EZNEC will figure the
impedance for you, in addition to actually showing you the antenna
pattern geometry for almost any wire configuration.

A matching transformer can help or hinder signal transfer (which
matters surprisingly little on the noisier bands below about 10
MHz), and definitely DC shorts and DC isolates the antenna and radio
input.

If you really want maximum signal transfer on a wide band of HF
frequencies with a random wire, the best way to go would be to add
an adjustable antenna coupler to peak things up (in other words, to
match impedances), varying the settings for each band of interest.


With all good wishes,



Kevin, WB4AIO.
--
http://kevinalfredstrom.com/