At 08:55 AM 2007-09-28, Brad Smith wrote:
>The antennas are spaced 1/4 wavelength are are fed with equal length
>lines of 50 ohm
> coax and a 1/4 wave 50 ohm line is inserted in one line. Should
> I have used 75 ohm?
Brad,
I had a little more time to look at this, and if your individual
elements are indeed close to 50 ohms with the other element open
circuit, the W7EL Simpfeed system could work quite well. I plugged
the impedances of the array with equal currents, 90 degrees out of
phase into Roy's Simpfeed.exe program and it spit out 75 ohm line
lengths (VF=0.66) of 65.21 and 87.61 feet. (The program couldn't
find a solution with two 50 ohm lines.)
Note that if your elements aren't exactly 50 ohms at resonance (when
excited one at a time), the line lengths will need to be adjusted somewhat.
I entered the first two line lengths in the model and the gain is 2.1
dB and F/B=24 dB at a 25 degree takeoff angle. (The phasing lines
are assumed to be lossless.) The currents are essentially equal and
the phase difference is 90.04 degrees. The SWR at 3.8 MHz is 1.1 and
it stays below 1.7 across the 80m band. Of course, the pattern
deteriorates a lot at the band edges. The F/B is 10 dB or better
from about 3.72 to 3.9 MHz. It approaches an omni pattern at 3.5 MHz.
Adding the nominal loss of RG-11 line to the model (a nice feature of
EZNEC 5.0) adds about 0.4 dB loss and lowers the SWR to 1.55 at 3.5
MHz and 1.4 at 4 MHz.
Changing directions would involve switching a 87.61-65.21=22.4 foot
length of 75 ohm line along with two fixed 65.21 foot 75 ohm
lines. (BTW, in the arrays I've built, I haven't seen problems where
only the center conductor is switched to change directions. This is
a very common method and is used by all the commercial designs I've
seen. I'm not sure why it would be necessary to also switch the
shield. It might imply some common mode currents on the outside of
the shield.)
It is permissible to use equal 1/2 wavelength lines (or multiples of
1/2 wl) of any impedance from each of the two elements to connect to
the 75 ohm phasing lines. Oddly enough, with halfwave 50 ohm lines,
this reduces the SWR at the band edges (but the pattern still
deteriorates at roughly the same rate vs. frequency as without them).
The Simpfeed program also found an alternate solution with 75 ohm
lines (VF=0.66) of 30.17 and 73.37 feet. I tried these in the model
and the currents and pattern are fine, but the common point impedance
is 31.84 + J 12.86 ohms (SWR=1.735) at 3.8 MHz.
73, Terry N6RY
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