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