Re: PAR Electronics EF-SWL Antenna

On Apr 8, 1:55 pm, hallicraf...@xxxxxxxxxxx wrote:
On Thu, 8 Apr 2010 10:40:56 -0700 (PDT), Wimpie <wimabc...@xxxxxxxxx>
wrote:





On 8 abr, 06:42, bpnjensen <bpnjen...@xxxxxxxxx> wrote:
On Apr 7, 2:44 pm, hallicraf...@xxxxxxxxxxx wrote:

On Wed, 7 Apr 2010 14:19:12 -0700 (PDT), bpnjensen

<bpnjen...@xxxxxxxxx> wrote:
On Apr 7, 2:16 pm, hallicraf...@xxxxxxxxxxx wrote:
On Wed, 7 Apr 2010 13:28:18 -0700 (PDT), bpnjensen

<bpnjen...@xxxxxxxxx> wrote:
On Apr 7, 1:20 pm, dave <d...@xxxxxxxxx> wrote:
bpnjensen wrote:

Dale at PAR advertises that his matching transformer is wound on a
binocular core...but he does not go into precise detail. You'd have to
buy one and disassemble the unit casing to find out.

Bruce

Maybe we shouldn't have chased him away...

We've lost more smart people that way...

I think it was the part about buying one and disassembling it. Could
be using one of these. Probably type 43.http://www.surplussales.com/Inductors/FerMisc/FerMisc-5.html

Jim

Indeed!  I bet I know which one Dale uses ;-)

Maybe even cheaper with instructions and diagram:http://www.aytechnologies.com/TechData/9-to-1_XFMR.htm

Jim

Thanks Jim - I have two compound questions about this diagram that he
does not answer in the FAQ:

1 - In this wrapping technique, does the 50 ohm node at upper left
lead electrically to the 450 ohm node at upper right, or the feedline
ground?  I assume the latter, but...this technically creates a half-
wrap somewhere, which would give either 2.5 or 3.5 turns.  Same for
the 450 ohm node.  It is not obvious from the diagram or the text.

2 - Is it automatically assumed that the feedline ground also goes to
a ground rod (somewhere?), or just the coax outer conductor?  What
happens if there is no radio ground, just one at the antenna ground,
or vice-versa?  What if both grounds are at the same rod?

Just wonderin'... :-)

Bruce

Hello Bruce and others,
[large text]

You make one complete turn when you pass both ferrites.

When you start from the left side (that is the 50 Ohms side), you end
at the left side also. So when you use green wire for the left side,
you have two green wire ends at the left side.  For the secondary
side, you start from right en ends at the right side.

One full turn is like putting a hairpin through the cores (as
mentioned in the article).

The purpose of this type of transformer is:

#1. To get more output from a wire (and also more interference that
comes from outside). Mostly (in the receiving case), {wire length} <
0.25 lambda. Therefore the wire has relative high impedance (>> 50
Ohms). The transformer reduces the mismatch. Where the wire length is
about 0.25 lambda + n*0.5 lambda, your wire has relative low
impedance. In that case the transformer increases mismatch and this
will result in less output. This is mostly not a problem as
interference/noise coming from the antenna > receiver's noise level.

#2. To separate the antenna circuit from the common mode coaxial
circuit to reduce indoor interference from domestic equipment.

When you connect both grounds together and have a high impedance
ground, the coaxial common mode circuit partly shares the antenna
circuit (they have the ground impedance in common).   When indoor
interference couples to your coaxial cable, this interference is
coupled to your antenna via the common ground impedance.  By
connecting "coaxial ground" only to the coaxial braid (and not to the
ground provision outside), there is no common ground anymore. This
reduces the coupling between common mode indoor interference on your
cable and the antenna.

The challenge with unbalanced antennas is to find a (floating) ground
that is relatively clean. Large metal surfaces can act as ground, even
when there is no physical connection to mother earth. In my experiment
I used a large metal window frame. I will try to connect this to the
window frame on the lower floor.

What if you have no large metal surface at hand?  You can create one
artificially with wire (wires in star connection).  You can omit the
ground and connect both grounds of the transformer together (so you
get an autotransformer). In that case, the cable that runs from the
transformer to your receiver functions as ground provision. If this
cable is several meters long, this will result in reasonable receive
signal strength. But there is a big however.

You decided to use an outdoor antenna (maybe) to reduce noise/
interference level. By using the cable braid as ground, your antenna
is partly inside your house again, as the cable is now part of your
antenna.  When you like, you can use this transformer with a dipole
also. For the case that {dipole size} << 0.5 lambda, you get more
signal and noise output.

The problem with the design of these transformers is the trade-off
between low frequency response, capacitance between sec. and  prim.
and high frequency response. When you use a ferrite toroid and keep
sec. and prim. apart (no overlap), you create lowest coupling
capacitance, but worst high frequency performance because of leakage
inductance.   When you want good low frequency performance (for
example down to long wave), you need sufficient turns to get
sufficient inductance, however this comes with increased coupling
capacitance, hence reducing the transformer's ability to separate the
common mode cable circuit from the antenna circuit (at higher
frequencies).

There are several reviews about this type of transformers that they
reduce interference significantly. If it does, it is mostly not
because of the transformer ratio, but because of the isolation between
primary and secondary (when you don’t connect the grounds together).

When the interference that reaches your antenna comes from outside,
the transformer increases signal as well as noise, and you will only
gain S/N ratio when in the old situation the receiver's noise was
dominant.

Best regards,

Wim
PA3DJS
www.tetech.nl
in case of PM, please remove abc first.

Wim,

In a few paragraphs you have covered so much. Excellent information
about grounding and excellent points about ferrites and coupling at
different frequencies.

If we had a few more people like you Wim, we would have a lot more
people in this newsgroup. Hope you stay around. Thanks.

Jim

Please allow me to echo what Jim said. Wonderful information, thanks,
Wim!

Bruce
.

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