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Referring to the schema by Chris Trask



I ask for help on the 1:3 transformer. What kind of (Amidon) ferrite
ring core were suitable for the purpose? How many turns? Wound
interleaving or on opposite sides of the core?

If a dipole is attached to the circuit, what kind of interface is best?
Maybe a balun. Could you please give some ideas for that.

Thanks in advance.
---
Timo

Referring to the schema by Chris Trask



I ask for help on the 1:3 transformer. What kind of (Amidon) ferrite
ring core were suitable for the purpose? How many turns? Wound
interleaving or on opposite sides of the core?

If a dipole is attached to the circuit, what kind of interface is best?
Maybe a balun. Could you please give some ideas for that.

My mistake on that drawing. It should have been a 1:2 transformer,
which allows you to twist three pieces of wire together for the best
possible coupling coefficient. For HF applications, Fair-Rite type 43
material is well-suited, or you can use type 61 if you're not concerned with
MF frequencies.

Binocular cores give better performance than toroids as there's less
leakage inductance. For a 1/4" square core, use 3 or 4 turns of the
trifilar twist (#32 or #34 wire) wound between the holes.

If using a dipole, I would like to offer an alternative circuit, shown
here:




The transformer is again made with a binocular core, with 3-4 turns of
twisted bifilar pair wound on the outside of each hole, then the wires
interconnected to give a pair of centre-tapped windings. Be mindful of the
phasing when making the interconnections.

Chris

,----------------------. High Performance Mixers and
/ What's all this \ Amplifiers for RF Communications
/ extinct stuff, anyhow? /
\ _______,--------------' Chris Trask / N7ZWY
_ |/ Principal Engineer
oo\ Sonoran Radio Research
(__)\ _ P.O. Box 25240
\ \ .' `. Tempe, Arizona 85285-5240
\ \ / \
\ '" \ IEEE Senior Member #40274515
. ( ) \
'-| )__| :. \ Email: christrask@...
| | | | \ '.
c__; c__; '-..'>.__

Graphics by Loek Frederiks

----- Original Message -----
From: "minninfaija" <timo.nieminen@...>
To: <loopantennas@...>
Sent: Monday, June 18, 2007 12:51 PM
Subject: [loopantennas] Active antenna transformer


Referring to the schema by Chris Trask



I ask for help on the 1:3 transformer. What kind of (Amidon) ferrite
ring core were suitable for the purpose? How many turns? Wound
interleaving or on opposite sides of the core?

If a dipole is attached to the circuit, what kind of interface is best?
Maybe a balun. Could you please give some ideas for that.

Thanks in advance.
---
Timo





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At 12:52 pm ((PDT)) Mon Jun 18, 2007, timo.nieminen wrote:

Referring to the schema by Chris Trask


I ask for help on the 1:3 transformer.

[Now a 1:2 transformer]

What kind of (Amidon) ferrite
ring core were suitable for the purpose? How many turns? Wound
interleaving or on opposite sides of the core?

This is like asking us how much string you will need
when you won't tell us the size of your parcel ;-)

However, assuming you want your transformer as broadband
as possible, this may help you to 'roll your own'

"A Second Look at Fabricating Impedance Transformers
for Receiving Antennas"
John Bryant with Bill Bowers and Nick Hall-Patch, VE7DXR
DXing.info, July 2003

[Supersedes:
"Fabricating Impedance Transformers for Receiving Antennas"
z_transformers.pdf or impedance_transformer_bryant.doc]

While this gives much more experiment and testing...

"Impedance Matching Transformers for Receiving Antennas
at Medium and Lower Shortwave Frequencies"
Bill Bowers, John Bryant and Nick Hall-Patch, VE7DXR
DXing.info June 29 2003, revised July 24 2003


Regards, LenW
--
Content of a follow-up post should exceed quoted content. (rfc1855)

At 2:41 pm ((PDT)) Mon Jun 18, 2007, christrask wrote:

If using a dipole, I would like to offer an alternative circuit, shown here:

Don't you need _very_ well matched JFETs
to ensure drain current balance in that circuit?

Could this be avoided by giving each JFET source
an independent primary and 330 ohm resistor,
then linking the "centre-tap" with the capacitor?

(Also giving extra degeneration to common-mode.)


Regards, LenW
--
From Yahoo! Groups Help: ... trim all the irrelevant quoted text
out of your message (as a courtesy to the other members of
the group to make the digest easier to read).

Don't you need _very_ well matched JFETs
to ensure drain current balance in that circuit?

Could this be avoided by giving each JFET source
an independent primary and 330 ohm resistor,
then linking the "centre-tap" with the capacitor?

If you were making a differential amplifier for instrumentation
purposes, yes. But in this application you are making a pair of source
followers, and the amount of error in using an unmatched pair in such an
application would hardly be noticeable. However, if you want to go to the
extreme of precisely matching (meaning "very well") devices down to 0.01%
you may feel free to do so.

Your second suggestion would be sufficient to overcome the adversity of
using unmatched FETs if you feel that it's necessary to be that exact. I
had thought of doing so, but the application is not that demanding since the
load impedance presented by the 2N5109 as seen through the transformer is
quite high.

Chris

,----------------------. High Performance Mixers and
/ What's all this &#92; Amplifiers for RF Communications
/ extinct stuff, anyhow? /
&#92; _______,--------------' Chris Trask / N7ZWY
_ |/ Principal Engineer
oo&#92; Sonoran Radio Research
(__)&#92; _ P.O. Box 25240
&#92; &#92; .' `. Tempe, Arizona 85285-5240
&#92; &#92; / &#92;
&#92; '" &#92; IEEE Senior Member #40274515
. ( ) &#92;
'-| )__| :. &#92; Email: christrask@...
| | | | &#92; '.
c__; c__; '-..'>.__

Graphics by Loek Frederiks