Matching transformer comparison?
 

On Jun 27, 10:56*pm, m II wrote:
RHF wrote:

Thanks to both of you for the 'square of the turns ratio' bit. I have
found some old computer mother boards and these things are crawling
with ferrite cores suitable for rewinding. Some of them have a
trifiliar (weird word) wrap on them which will be coming off.

I've pulled them out and will be winding them soon.

tertiary mike III

Mike,

These are 'unknown' material Ferrite Cores that
you are getting-off an old PC Mother Board; and
they may or may not work with the specific Number
of Turns that you try on them to cover the Shortwave
Bands 5MHz~30MHz.

You may get 'lucky' with 10T&30T or you may come
up with something weird. So experiment with what
'appear' to be the same Size, Shape and Color Ferrite
Cores. 10T&30T; 6T&18T; 3T&9T; and if you get all
the Turns on one Core 15T&45T.

TIPS:
http://users.catchnet.com.au/~rjandu...ts/tut_5a.html
* Sort these 'unknown' material Ferrite Cores
for Size, Shape and Color {ideally you will have
a few of the same to do similar and dissimilar
things to} in building a Matching Transformer
that covers and does what you want done.
* Use different 'colored' Wires -if- you are using
http://www.w8ji.com/Beverages/Beverage_transformer.jpg
Insulated Wires for the Windings. Or put a
piece of Black Electrical Tape on the Primary
[Coax/Radio] 10T Winding to 'mark' it and easily
identify it from the Secondary [Antenna/Ground]
30T Winding. If you are using Magnet Wire
-aka- 'Enameled Copper Wire'
http://en.wikipedia.org/wiki/Magnet_wire
http://www.radioshack.com/family/ind...goryId=2032240
to make your Windings then again use a piece
of Black Electrical Tape on the Primary [Coax/
Radio] 10T Winding.
*Pre-Cutting your Wires for the Windings
Measure the Cross-sectional Perimeter of
your Ferrite Core Rap by wrapping a String
around it two to three times and getting the
average Distance of One Turn. Multiply this
One Turn Distance by the Number of Turns
and add 3"~6" for each of the two Tag-End
Leads {that's an extra 6"~12" total}
-example- 10T x 1"/T = 10" for the Windings
+ 6"~12" for the Leads : Total 16"~22" Wire
-example- 30T x 1"/T = 30" for the Windings
+ 6"~12" for the Leads : Total 36"~42" Wire
-so- you can see that a Two Winding Matching
Transformer will take about 5-Feet of Wire.
* Space your Wraps all around the Core -or-
http://www.regionalmfg.com/component...nsformer-1.jpg
Make All your Wrap on one-side for the Primary
and across from it for the Secondary
http://www.coilws.com/images/toroidal/CM%20Choke.gif
-but- Keep your Wraps "Tight" :
Tight Wraps = good electrical magnetic 'coupling'
Loose Wraps = poor 'coupling'.

hope this helps - iane ~ RHF

Matching transformer comparison?
 

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/