Re: LF: Re: [rsgb_lf_group] Impedance transformation on 136 kHz - advice needed

[Roger Lapthorn <rogerlapthorn@gmail.com>]

Robert,

3C90 42mm diameter toroids (and a range of others) are available on-line from Farnell if this helps

73s

Roger G3XBM

On 29 January 2013 11:28, mal hamilton <g3kevmal@talktalk.net> wrote:

 

----- Original Message -----

From: mal hamilton

To: mal hamilton

Sent: Tuesday, January 29, 2013 11:22 AM

Subject: Re: [rsgb_lf_group] Impedance transformation on 136 kHz - advice needed

PS

 

Robert

An audio amp is not the best method and inefficient even at 137 Khz.

A better approach is a purpose built amplifier with a couple of FETS class D or E. There is plenty of info available about building such amplifiers

 

 

----- Original Message -----

From: mal hamilton

To: rsgb_lf_group@yahoogroups.co.uk

Sent: Tuesday, January 29, 2013 11:19 AM

Subject: Re: [rsgb_lf_group] Impedance transformation on 136 kHz - advice needed

Robert

Use a capacitor of around 10 uF in series with the amp output then a 4 to 50 ohm transformer.

Old tv  LOPT are suitable or material similar to 3C9 material.

Prim turns around 8 and Sec 28 turns. This is a rough guide to get started

 

de mal/g3kev

 

----- Original Message -----

From: robert_la4ana

To: rsgb_lf_group@yahoogroups.co.uk

Sent: Tuesday, January 29, 2013 9:51 AM

Subject: [rsgb_lf_group] Impedance transformation on 136 kHz - advice needed

 

Hi Group,

As I am planning to use an audio amplifier as a PA on 136 kHz, I need to
transform the amplifier output (100 Watt at 4-8 Ohm) to 50 Ohm to match
it to the coax cable to the antenna, possibly with a 50 Ohm low-pass
filter in between. Not much information is available on the Internet
about such a specific configuration. I am fond of capacitor impedance
transformation, which I use with success for matching and tuning my loop
antenna on 475 kHz. Could I employ the same concept to step up impedance
from 4 Ohm to 50 Ohm at the output of my audio amplifier?

I have also looked into using ferrite toroids as transformers but the
sheer amount of information on this subject, often incomplete and
contradictory, is confusing me. It looks like many are using "3C90" as a
core material, but this type of core seems difficult to acquire - at
least for me here in Norway. I understand that "type 77" material could
also be used and this one is easier to get by. If I would opt for an
FT140-77 core, would this be suitable as a transformer toroid for 100
Watt on 136 kHz or would I need a bigger one? And how many primary and
secondary turns would such a tranformer need to step up from 4-8 Ohm to
50 Ohm?

I would be very grateful for some input on this subject before I start
ordering (the wrong) components.

73 de Robert,

LA4ANA

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