Hi All,
After reading the page on Pennant and Flag antennas,
I decided I would attempt to build a transformer with less
capacitance between primary and secondary than the
working design on the Pennant and Flag page.
I used a 4229 Pot core of 3F3 material. I wound 13 turns
of # 30 wire on the bobbin. I then cut Styrofoam to fill the
space from the center of the bobbin to the outside edge.
The Styrofoam is shaped like a donut with flat sides, I then
cut the donut so I had two halves, like this ( ), these were
sandwiched around the primary. I then wound 3 turns over
the Styrofoam as the 50 ohm winding.
.
I measured the capacitance as 10 pf.
When I connected the pot core to earth
ground the capacitance measured 1.5pf.
I then removed the 3 turns and the Styrofoam and insulated
the 13 turns with tape, I wound a layer of .002" brass
( as a shield ) around the 13 turns, ( being careful to insulate
the end to prevent a short). I brought a brass tab out of the
pot core to ground the shield. I reinstalled the Styrofoam and
wound 3 turns over the Styrofoam. This was all put into
a small box made of copper clad PCB material, with a SO-239
connector and two brass screws for connecting the antenna.
A third brass screw was used for grounding.
When I connected the shield and the pot core to earth ground
the capacitance between windings measured as 1/10 of 1 pf.
Comments???
----------------------------------------------------------------------------
------------------
The RESISTANCE and the REACTANCE of the TRANSFORMER
The core was gapped to have XL=3200 ohms at 540Khz on the
antenna 13 turn side.
I terminated the 3 turns with a 49.9 ohm 1% carbon film resistor.
4229 pot core 3F3 material 13 T and 3 T
540 Khz R=850 ohms XL=290 ohms
1Mhz R=900 ohms XL=180 ohms
2Mhz R=900 ohms XL=173 ohms
3Mhz R=1025 ohms XL=183 ohms
4Mhz R=1050 ohms XL=540 ohms
5Mhz R= 1025 ohms XL=510 ohms
6Mhz R=620 ohms 31.5* phase shift
I could not measure any further, using the reactance
canceling method.( I didn't have any small polystyrene caps)
Previous measurements also showed something running out
around 6 Mhz.
FT 143-43 toroid core 35 T and 8 T
540Khz R=750 ohms XL=370 ohms
1Mhz R=850 ohms XL=500 ohms
Mhz R=950 ohms XL=825 ohms
3Mhz R=1175 ohms XL=1525 ohms
4Mhz R =1700 ohms XL= 2350 ohms
I could not measure any further, using the reactance
canceling method.
First, how do my measurements compare to others for the
FT140-43 toroid core?
Second, does having a less reactive transformer add anything
positive to the effort?
Pot core---2Mhz R=900 ohms XL=173 ohms
vs
Toroid------ 2Mhz R=950 ohms XL=825 ohms
Comments??
I am working outside the limit of my understanding of transformers,
any info and help is appreciated.
Mike
--
FAQ on WWW: http://www.contesting.com/topband.html
Submissions: topband@contesting.com
Administrative requests: topband-REQUEST@contesting.com
Problems: owner-topband@contesting.com
|