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160 Meter Antenna Top Hat Redesign




 
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Author Topic: 160 Meter Antenna Top Hat Redesign  (Read 426 times)
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N1BCG
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« on: March 12, 2020, 12:15:02 PM »

I'm hoping the antenna braintrust can come together with some suggestions for swapping out one capacity hat design for another.

I have a 30' tall vertical with a center loading coil and a top hat consisting of six evenly spaced 72" horizontal elements.  While this works well, I need to add a guying system at the top due to excessive movement in high winds.

My thought is to replace the six elements with three downward sloping stainless steel cables broken up by insulators. The uppermost sections would be electrically connected to the top of the antenna and serve as the new capacity hat while the rest would be for guying only.

The question is, how long do the three sloping top sections need to be to have the same capacitive effect as the six horizontal elements?

I would LOVE to experiment with various lengths, but due to it being mounted on a tall building (thus the wind issue) it's hard to take down and re-install. I'd like to do this as few times as possible.

The counterpoise is a combination of multiple runs of conduit that run straight down 60' and terminate in a utility room and bonding to twelve iron plumbing vent pipes spread across the roof that tie into the entire building's drain system. This results in a near 50Z j0 match, incredibly, which I attribute to the mainly vertical orientation of the counterpoise.

Thoughts?
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W1ITT
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« Reply #1 on: March 12, 2020, 01:32:17 PM »

My 160m antenna is a Tee with the top as part of a catenary between two tall pine trees.  However, with modeling, I was able to determine that I could increase the base R by dropping relatively short downleads from the Tee ends, up to a certain length before cancellation of fields starting eating up the advantages.
If you take a similar tack and model with MMANA, an excellent, free, and fairly intuitive program, you can play with a number of iterations of the guyed top hat method.  EZNEC and the others are FB-OM as well. The length, number, and slant angle of the guy top hat will influence the efficiency and base R of the antenna as the downward slant generally tries to fight with you.  In my case, I did what I could to maximize the base R and took care of a bit of reactance and brought the R to 50 ohms with an L network.  Measuring with the HP network analyzer, the MMANA prediction was closely realized.
73 de Norm W1ITT
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R. Fry SWL
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« Reply #2 on: March 13, 2020, 04:30:28 AM »

... how long do the three sloping top sections need to be to have the same capacitive effect as the six horizontal elements? ...

An ~accurate response will need the depression angle for those sloping top sections, the horizontal dimensions of the roof, the X-Y location of the vertical on the rooftop, and the operating band(s).

If you will post those, I will use NEC4.2 to model the whole system including an estimate of the 60' paths to r-f ground (Earth potential), and post the results.
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N1BCG
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« Reply #3 on: March 13, 2020, 09:53:33 AM »

If you will post those, I will use NEC4.2 to model the whole system including an estimate of the 60' paths to r-f ground (Earth potential), and post the results.

That is a fantastic offer, thank you!  This data isn't going to be as clear-cut as "120 quarter wave radials spaced with 3 degree separation at right angles to and centered at the base of the vertical radiator". Until now, that's been all of my Part 73 vertical work. I was hoping to determine the capacitance of the current top hat and replicate that using three sloping elements where the counterpoise is of a fixed value.

I understand that this is a function of surface area and distance and the slope adds an additional variable, increasing the complexity.

Of the variables you mentioned:

The depression angle for those sloping top sections - I'd go with 45 degrees. A proper value would require an exact measurement of the right triangle angles.

The horizontal dimensions of the roof - The roof contains 12 horizontal runs of varying lengths of grounding cables that terminate at iron vent pipes. Their extent below the roof is unknown.

The X-Y location of the vertical on the rooftop - The roof is shaped like a [ with the antenna positioned at the center of the open side, equidistant from the inside corners.

The operating band(s) - 1800kc

My apologies for the limited usefulness of this information. I'm beginning to realize that it's simply going to take time and experimentation to determine the proper lengths of the new capacity hat. Since there are six 6' radials at a 90 degree angle, three 12' radials of a similar diameter should present the same capacitive value. However, their slope downward will simultaneously reduce their exposed area to the counterpoise while also reducing the distance.

Fascinating stuff...
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AJ1G
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« Reply #4 on: March 13, 2020, 11:58:19 AM »

Do you have enough space to replicate the ORIGINAL 1BCG antenna, dimensionally scaled appropriately for the present day 160 meter band?  

You might want to consider using elevated folded counterpoise elements, I have had them recommended to me for 160 where it will be difficult to fit full quarter wave plus counterpoise wires on the property.  The overall length of the 160 meter FCP radial elements are each only about 33 feet long.

If you do go with elevated counterpoise elements, it might be better to dispense with your “counterpoise of opportunity”, it may increase ground losses.  When getting smart about how to design my 40 meter quarter wave wire vertical/80 meter inverted L with elevated radials, I learned that such counterpoise should not be grounded at the feed point, i.e. the counterpoise is as much not connected to ground as the side of a dipole that isn’t connected to the coax shield.  And be sure to include a common mode choke or balun just below the feed point to minimize ground losses through the feed line flanking path especially if it’s laying on the ground or buried, and to minimize stray RF in the shack.  I had really bad shack RF issues when I first put the vertical on service without one.

My plans for 160 are to hang a nominal 30 foot helical wound vertical with top hat that I will have on loan from my buddy Steve, KL7JT from Sudbury MA, in place of the quarter wave 40 meter vertical radiator working against at least 4 elevated FCP counterpoise elements.  The helically loaded radiator was described in a QST article a couple of years back.
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Chris, AJ1G
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R. Fry SWL
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« Reply #5 on: March 14, 2020, 05:28:14 AM »

... Since there are six 6' radials at a 90 degree angle, three 12' radials of a similar diameter should present the same capacitive value. However, their slope downward will simultaneously reduce their exposed area to the counterpoise while also reducing the distance.

Below is a NEC calculation of the feedpoint Z of a system resembling your description when using 3 top hat wires each 12 feet in length with a depression angle of 45 degrees.

I don't have enough detail to model any other conductors at/near the rooftop, but the feedpoint Z of this configuration is rather close to what it is when using a top hat of 6 horizontal wires each 6 feet in length.

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N1BCG
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« Reply #6 on: March 14, 2020, 09:17:45 AM »

I don't have enough detail to model any other conductors at/near the rooftop, but the feedpoint Z of this configuration is rather close to what it is when using a top hat of 6 horizontal wires each 6 feet in length.

Wow, thanks!! VERY interesting.  For some detail, there's a loading coil mounted 6' up from the physical base of the antenna (about 10' up electrically when including the feedline). I calculated the inductance to be around 104uH based on turns, diameter, and length.

Despite having a T match, none of the components are needed as the antenna presents a 50Z j0 load which I attribute to coincidence and the predominantly vertical counterpoise.

The antenna is an HPR.0990 for those who are curious:

http://www.theradiosource.com/products/antenna-hpr0990.htm
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