Hi

I am new to this forum and have a (most likely) dumb question

I recently aquired a S350DL receiver. I will be using this rig until I can buy a rela communications receiver. I will be using it to receive Shortwave only. I have constructed a T2FD (Tilted, terminated, folded dipole) with a design feed point impedance of 300 ohms. The S350 DL only has an unbalance high impedance external antenna point and a ground on it. How can I attach the T2Fd to the S350Dl and acheive a good impedance match.

1) Is the whip antenna on the S350DL a low impedance feed point (i.e. 50 or 75 ohms)? If so, could I use a 4:1 balun at the feedpoint of the T2FD to convert the feed line to 75 ohm coax (which would be unbalance line now) and just clip the center conductor to the whip and the sheild to a grounding rod?

2) I could use 300 ohm ladder line to connect to the T2FD but the problem is that the S350DL does not have a balanced esternal input point on it.

I guess what I am asking is what would be the best way to couple A 300 ohm T2FD antenna to a Crappy Radioshack S350DL receiver and achieve reasonable impedance matching.

I do not what to use an end fed long wire.

Any help would be appreciated.

On Tue, 26 Aug 2008 22:19:42 +0100, Bubblesdee wrote:

Hi

I am new to this forum and have a (most likely) dumb question

I recently aquired a S350DL receiver. I will be using this rig until I
can buy a rela communications receiver. I will be using it to receive
Shortwave only. I have constructed a T2FD (Tilted, terminated, folded
dipole) with a design feed point impedance of 300 ohms. The S350 DL
only has an unbalance high impedance external antenna point and a ground
on it. How can I attach the T2Fd to the S350Dl and acheive a good
impedance match.

1) Is the whip antenna on the S350DL a low impedance feed point (i.e. 50
or 75 ohms)? If so, could I use a 4:1 balun at the feedpoint of the
T2FD to convert the feed line to 75 ohm coax (which would be unbalance
line now) and just clip the center conductor to the whip and the sheild
to a grounding rod?

2) I could use 300 ohm ladder line to connect to the T2FD but the
problem is that the S350DL does not have a balanced esternal input point
on it.

I guess what I am asking is what would be the best way to couple A 300
ohm T2FD antenna to a Crappy Radioshack S350DL receiver and achieve
reasonable impedance matching.


On HF, matching imp for receiving should not make much difference give-or-
take a few 100%

I suspect whip antenna is high imp. ~2k and unbalanced. You would need
to make a one2one or four2one matching xformer placed near the radio with
ladder line coming from antenna. This will stop common mode noise from
entering your radio via feedline. Two 300-75 ohm TV xformers back to
back should work.

twinlead 300 xformer 75 short coax 75 - 300 twinlead

gator clip the xformer output to your collapsed whip
antenna and ear jack ground.

If you get allot of intermod from front end overload, place some
resistors between between whip and ear jack ground.

Thanks for your reply No Spam.

I was wondering if you might be able to clarify some points that you made.

1) Since I will be feeding the antenna via 300 ohm feedline, transforming it down to 75 ohms (coax) to eliminate noise and then transforming it back to 300 ohms, could I not just take one leg of the output of the transformer and connect it to the External antenna connector on the back of the radio instead of the whip? and then connect the other leg from the transformer to the ear jack ground. and then finally ground the radio via the GROUND connecter on the back of the set.

2) when you refer to to the ear jack ground, are you referring to the ground of the headphone jack. Is this the same point as the GROUND connector on the back of the radio or are the two points different?

Thanks again

Bubblesdee wrote:

Thanks for your reply No Spam.

I was wondering if you might be able to clarify some points that you
made.

1) Since I will be feeding the antenna via 300 ohm feedline,
transforming it down to 75 ohms (coax) to eliminate noise and then
transforming it back to 300 ohms, could I not just take one leg of the
output of the transformer and connect it to the External antenna
connector on the back of the radio instead of the whip? and then
connect the other leg from the transformer to the ear jack ground. and
then finally ground the radio via the GROUND connecter on the back of
the set.

2) when you refer to to the ear jack ground, are you referring to the
ground of the headphone jack. Is this the same point as the GROUND
connector on the back of the radio or are the two points different?

Thanks again

My S350DL has unbalanced, 500 ohm antenna terminals on the back; I
would imagine yours to be the same.

A 1:1 balun (balanced-to-unbalanced) at the dipole and coax (75 ohm) and
a 6.666:1 UNUN (unbalaced-to-unbalanced) between the coax and the S350DL
would be the ideal setup.

Regards,
JS


Half-a-Brain-McCain'n Insane; So Lawdy Mama, It Looks Like Obama!

John Smith wrote:


My S350DL has unbalanced, 500 ohm antenna terminals on the back; I
would imagine yours to be the same.

A 1:1 balun (balanced-to-unbalanced) at the dipole and coax (75 ohm) and
a 6.666:1 UNUN (unbalaced-to-unbalanced) between the coax and the S350DL
would be the ideal setup.

Regards,
JS


Half-a-Brain-McCain'n Insane; So Lawdy Mama, It Looks Like Obama!

Oh chit, it is folded dipole, duh!

A 1:1 current choke and forget the slight mismatch!

Regards,
JS
Half-a-Brain-McCain'n Insane; So Lawdy Mama, It Looks Like Obama!

John Smith wrote:
John Smith wrote:


My S350DL has unbalanced, 500 ohm antenna terminals on the back; I
would imagine yours to be the same.

A 1:1 balun (balanced-to-unbalanced) at the dipole and coax (75 ohm)
and a 6.666:1 UNUN (unbalaced-to-unbalanced) between the coax and the
S350DL would be the ideal setup.

Regards,
JS


Half-a-Brain-McCain'n Insane; So Lawdy Mama, It Looks Like Obama!

Oh chit, it is folded dipole, duh!

A 1:1 current choke and forget the slight mismatch!

Regards,
JS
Half-a-Brain-McCain'n Insane; So Lawdy Mama, It Looks Like Obama!

Actually, that is in error. Since the folded dipole is balanced and the
500 ohm terminals on the receiver show they are unbalanced, the 1:1
balun should suffice. It seems you will be going from 300 twinlead
(balanced) to the 500 ohm terminals.

450 ohm feedline with a 1:1 balun is yet another option ...

Regards,
JS

Half-a-Brain-McCain'n Insane; So Lawdy Mama, It Looks Like Obama!

On Wed, 27 Aug 2008 22:05:35 +0100, Bubblesdee
wrote:


No Spam;643798 Wrote:
On Tue, 26 Aug 2008 22:19:42 +0100, Bubblesdee wrote:
-
Hi

I am new to this forum and have a (most likely) dumb question

I recently aquired a S350DL receiver. I will be using this rig until
I
can buy a rela communications receiver. I will be using it to
receive
Shortwave only. I have constructed a T2FD (Tilted, terminated,
folded
dipole) with a design feed point impedance of 300 ohms.

We can only presume this 300 Ohms comes from a resistor, otherwise an
antenna does NOT present one impedance across all frequencies; and not
even across the bandwidth around 10% of 1 frequency.

The S350 DL
only has an unbalance high impedance external antenna point and a
ground
on it. How can I attach the T2Fd to the S350Dl and acheive a good
impedance match.

This begs the question: "Why do you want a match, when you have
already inserted a resistor that absorbs the power?" Another
question: "A match to what?"

1) Is the whip antenna on the S350DL a low impedance feed point (i.e.
50
or 75 ohms)? If so, could I use a 4:1 balun at the feedpoint of the
T2FD to convert the feed line to 75 ohm coax (which would be
unbalance
line now) and just clip the center conductor to the whip and the
sheild
to a grounding rod?

This would presume a match to the resistor on the T2FD. However, is
the receiver a 75(50)Ohm input? 50 Ohms source Z is a convention of
transmitters, and specified as so; but it is not always a given for
receivers' input Z. Some literally supply the 50 Ohm resistor
internally and at the gate or base of the input transistor. The gate
or base is functionally a much higher Z, but the necessity of bias and
other considerations sets this characteristic to something lower (but
may still be quite high).

Hence, without knowing this characteristic, you maybe discarding your
effort.


2) I could use 300 ohm ladder line to connect to the T2FD but the
problem is that the S350DL does not have a balanced esternal input
point
on it.

Being a "tilted" antenna, it will NEVER be balanced either!

I guess what I am asking is what would be the best way to couple A
300
ohm T2FD antenna to a Crappy Radioshack S350DL receiver and achieve
reasonable impedance matching.

I think what you are asking is can you improve performance - matching
be damned. A receiver demands much less attention to this as there is
usually excess capacity in gain within it.

On HF, matching imp for receiving should not make much difference
give-or-
take a few 100%

I suspect whip antenna is high imp. ~2k and unbalanced. You would need

to make a one2one or four2one matching xformer placed near the radio
with
ladder line coming from antenna. This will stop common mode noise from

entering your radio via feedline.

A matching transformer is not always a choke. A choke is not always a
matching transformer. A choke reduces common mode currents, but it
has to present an impedance that is in excess of the impedance of your
input (still little understood). Chokes and transformers do not
reduce noise without also reducing signal - UNLESS that noise arrives
from your home, and travels into your receiver through the inherent
imbalance of the system. A choke WILL aid you here, but it won't cut
down on the crackle of tropical storms.

73's
Richard Clark, KB7QHC

Hi

Thanks to everyone that responded.

I am a little confused but this is most likely my fault.

So i will simplify the question.

How could I transform from 300 ohm balanced antenna to 300 ohm unbalanced line without using coax. I will do the transformation right before the balanced (one antenna and one ground connector) on the S350DL.

I think that all I need to do is come off the T2FD with 300 ohm Twin lead and then into a 1:1 balun that does not use coax inside it but a ferrite choke. Does this sound right? If so, then my next question is DOES anyone know how to build a 1:1 balun not using coax to transform 300 ohm balanced to 300 ohm unbalanced?

Thanks again

On Thu, 28 Aug 2008 22:35:32 +0100, Bubblesdee
wrote:

I think that all I need to do is come off the T2FD with 300 ohm Twin
lead

Fine enough.

and then into a 1:1 balun that does not use coax inside it but a
ferrite choke. Does this sound right?

Not as stated. Ferrite is a material, the line running through it
becomes choked because of that material.

my next question is
DOES anyone know how to build a 1:1 balun not using coax to transform
300 ohm balanced to 300 ohm unbalanced?

Stuff the (insulated paired) wires through the ferrite bead/torus/tube
just like you would a coaxial line. Total length of ferrite (a
combination of about 40-60 beads or equivalent) on the order of a
foot. A large torus can support several wraps through it to the same
effect.

73's
Richard Clark, KB7QHC

Richard Clark wrote:
. . .
Stuff the (insulated paired) wires through the ferrite bead/torus/tube
just like you would a coaxial line. Total length of ferrite (a
combination of about 40-60 beads or equivalent) on the order of a
foot. A large torus can support several wraps through it to the same
effect.

Bear in mind that the impedance of multiple turns is proportional to the
square of the number of turns. So 5 turns, for example, through a single
core gives you the same impedance as 25 cores strung along the line.

Roy Lewallen, W7EL