[TowerTalk] 160m Vertical

[K4SAV]

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
>>
>>
>>
>>
>>
>>
>>___________________
>>