To confirm Terry's model, my 160M 4-square uses 122.5 foot military
towers similar in cross section to Rohn 45. The towers are mounted on
12 inch base insulators intended for AM broadcast stations. 60 radials
approximately 120 feet long terminate at the bottom of the base
insulators, approximately four inches above the ground. An 8 inch wire
runs from the bottom of the tower to the transmission line at the
bottom of the base insulator, making the overall vertical height just
over 123 feet. The towers are guyed at three levels using non
conductive guys; however, the tower is slight loading by 9 three foot
long conductive guy grips.
The towers resonate (R=29, X=0) at 1790 kHz (just below the band, as
required in a 4-square) as measured by an LP-100. The LP-100 also
measures SWR (50 ohms), but I made no record of SWR since its not
useful in determining resonance.
My opinion -- FWIW -- is that all of Steve's perceived problems derive
from using inappropriate test equipment for measuring the impedance of
a full size 160 meter vertical. There's no apparent problem in any
aspect of the design of Steve's antenna, although there are a few small
compromises that we all make, but they should have little effect on the
results.
73!
Frank
W3LPL
---- Original message ----
>Date: Wed, 03 Oct 2007 01:28:14 -0700
>From: Terry Conboy <n6ry@arrl.net>
>Subject: Re: [TowerTalk] 160m Vertical
>To: towertalk@contesting.com
>
>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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