Oops, I lost one of my insulators on the numbers I gave. The resonant
point is now 1.97 MHz SWR=1.18 (R=42). It's interesting that the
coupling to the broken up guys made the resonant frequency go up instead
of down.
Jerry, K4SAV
K4SAV wrote:
>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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