N6RY wrote:
...."I used a single 8 pf insulator at the junction with the top guy
sections and grounded the far end. ".....
I'm not sure this is correct. Steve said he said the top wire had 3
insulators. The next lower one had 2 insulators, and the bottom two
wires had one insulator. The numbers you got were very similar to what
I got when I used only one insulator, but I don't think this is what he has.
...."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. "....
This was confusing because he made us guess at where the guy points
were. With the lengths he gave I figured he must have two different
distances for the guys, about 75 and 97 feet from the base. Is this
correct Steve? I implemented the un-insulated lengths Steve listed, but
the end lengths are different from yours because the guy point locations
are different.
With the new model based on his last data (which is different from
yours) I get a resonant frequency of 1.94 MHz and an SWR of 1.23
(R=40.5). So, no revelation here. I'm still looking at this, but I
don't see the problem yet. I don't think the 45 ft tower is a problem.
I don't see how you could load this enough to be resonant on 160. I
even tried other things like a chain link fence or electric fence that
might happen to be resonant on 160, but there is so little coupling to a
horizontal wire that I don't think this could be a cause either.
Here are a few other minor points relative to the model:
For an equivalent simulation diameter for Rohn 25, I used the formula
given here:
http://lists.contesting.com/_antennaware/2002-05/msg00004.html
Equivalent Cylindrical Diameter = 2 * CUBEROOT [(T * F^2)/2]
where for R25, T = tube diameter = 1.25 and F = Face width = 11.25.
So equivalent diameter for R25 is 8.58 inches.
It's only a minor contributer, but I thought I would list it just for
informational purposes.
For an equivalent ground resistance using 40, 120 ft long radials, I
used a graph from the ARRL Antenna book. That gives an equivalent of
7.5 ohms. Adding another 20 radials drops it to 6.8 ohms. This is also
just a minor contributor.
I also assumed the guy anchor points were not tied into the radial
system, so I added a best guess at the impedance of the anchor points
with one ground rod. It didn't make any significant difference even
with large variations.
I'm not sure what the capacitance of a 15-3 insulator is and I don't
have one to measure, but trying various reasonable values made very
little difference.
Agreed that the feed point being 4.25 ft above ground is not a problem.
See info in my first post.
Jerry, K4SAV
Terry Conboy wrote:
>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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