Using 2.5 mm vs. 1.5 mm diameter copper only increases the gain by
about 0.1 dB. The feedpoint R is about 1 ohm lower, and the
bandwidth increases by about 4 kHz. The resonance moves up by 13 kHz
to 1935 kHz with the 430 pF cap. With a 482 pF series cap, the
resonance returns to 1907 kHz (the same as with the 1.5 mm steel
wire) and the SWR there is 1.09.
These small changes are probably not worth the extra money.
It's interesting that the resonant frequency moves up with the larger
wire. Dipoles or straight verticals usually move down, but
apparently having the bend in the inverted L reverses the behavior
(much like quad loops).
73, Terry N6RY
At 01:37 PM 2008-09-24, OZ1AXG Flemming wrote:
>>Flam,
>>
>>I modeled this with EZNEC (without the tower) and assumed the 1.5 mm
>>diameter stainless steel wire has a resistivity of 7.2 e-7 ohm-m and
>>relative permeability of 1.02. I estimated about 20 ohms of ground
>>loss resistance. Here's what happens at 1860 kHz:
>>Cu G=-0.43 dBi Z=52.6 -j 46.6 SWRmin=1.13 at 1922 kHz BW=97
>>kHz (50 ohm SWR 2:1)
>>SS G=-1.48 dBi Z=68 -j 36.2 SWRmin=1.44 at 1907 kHz BW=90 kHz
>>
>>The adjacent tower will modify these values depending on where it is
>>resonant, cross section, etc.
>>
>>With copper, you should get about 1 dB more signal, a better match to
>>50 ohms, and resonance moved up about 15 kHz. Adding another 27 pF
>>in parallel (457 pF total) should restore the resonant point.
>>
>>73, Terry N6RY
>
>Hi Terry
>
>Thanks a lot for the calculation, and your right, it has been a
>problem getting it close to 50 ohm with the current wire and ground
>loss. Very interresting, that the your calulated values is so
>consistent with my experimental findings.
>
>An extra question: If I where to replace the antenna wire with a
>2.5mm Cu wire - what implications would that have ?
>(The 2.5 mm is double up in price :=) or would that just make thing worse ?
>
>The serial capacity is a bit to large anyway, and last weekend i
>found out that the capacity varies with temperature.
>In daytime about 7% reflected power (0n 1824khz) and later on in the
>evening that was 12% - a bit to much for my taste.
>I will make some test with the capacitors ... but it looks like
>capacity increases with lower temperatures :=)
>
>--
>OZ1AXG Flam
_______________________________________________
Antennaware mailing list
Antennaware@contesting.com
http://lists.contesting.com/mailman/listinfo/antennaware
|