At 07:28 PM 2007-10-02, Steve AB5MM wrote:
>I can't believe you guys are still hanging in there with us on this
>tower/antenna project. We think we may see some light at the end of the
>tunnel. (yeah I know, it's probably a train)
Steve,
Here a few comments based on some general observations and the EZNEC+
5.0 model of your tower that I built...
That 4'3" base of your tower is definitely part of the antenna, so
it, and the 4" of insulators add to the 120.5' above them and make
your antenna very close to 125.1 feet tall. You are then feeding it
a little above ground, which shouldn't be too big a deal. Even so,
you would think that this should be resonant around 1.87
MHz. However, there is some capacitive loading from the insulators
on the guys which seems to lower the resonant frequency and the feed impedance.
Maybe someone has some real numbers, but I assumed the insulators
were about 8 pF each (so two in tandem are 4 pF, etc.)
I modeled the tower as a 5" diameter cylinder and added 3 ohms for
ground loss. I added guy wires to the 4 lengths you mentioned to
reach a point on the ground 120 feet from the tower in three
places. I used a single 8 pf insulator at the junction with the top
guy sections and grounded the far end. I used 0.375" diameter guys.
All this results in an antenna model that is resonant at 1.785 MHz
with a feedpoint impedance of 19.4 ohms or SWR = 2.58, close to what
you measured (1.788 and 2.6 SWR, I think). The bandwidth of this
referenced to 19.4 ohms is about 115 kHz at SWR = 2:1 (which is what
you would get in a 50 ohm system matched with a perfect 19.4-to-50
ohm transformer).
Since the impedance is lower than 50 ohms, you need an L-network that
has the series reactive arm connected to the radiator and a shunt
reactance across the feedline. I chose a series C of 1600 pF & shunt
L of 3.81 uH to match to 50 ohms at 1.9 MHz. The SWR bandwidth is
still very close to 115 kHz between 2:1 points. The L-net shouldn't
narrow the bandwidth since its Q is 1.1 (BW ~= 1.7 MHz). Using a
highpass L-net will give some attenuation of AMBC stations in the
lower part of their band.
A simpler approach is to use a Lowpass L-network and use the
inductive reactance of the antenna itself when operated above
resonance. My model shows that a 1870 pF capacitor across the
feedpoint will result in a 50 ohm match at 1.89 MHz and a SWR
bandwidth of about 125 kHz. (You would have to a small coil between
the radiator and the shunt C if you wanted the minimum SWR to be
lower in the band.)
I hope this helps! Good luck!
73, Terry N6RY
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