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Author Topic: 14 foot vertical with top hat  (Read 1483 times)
KU7I
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« on: June 18, 2017, 06:21:52 AM »

I have a 13 foot vertical up right now. It uses 39 feet of wire on a bamboo pole. It is a serpine winding so it goes up 13 feet, back down 13, and then up again 13 feet. I am feeding this via a 4:1 balun at the feedpoint to 300 ohm ladder line. It plays very well on 40 to 6 meters. Extensive ground system installed including multiple 8 foot ground rods throughout with over 60 radials 16 feet long.

I am wondering if I would increase efficiency much on 40 and 30 by converting this to a 14 or 15 footer with a small 4 foot diameter 4 spoke top hat? I would possibly use the same serpine winding of 39 feet but with a 14, 15, or even 16 foot overall length so the last 1,2, or 3 feet would not be part of the winding, it would go straight up to the top hat. Top hat may only be a 3 or 4 foot t bar on top, say 2 feet either side.....or I could do the small 4 foot diameter 4 spoke top hat.

Just looking for ideas. I dont know if it is worth the work to make changes. I enjoy the simplicity of this ant. No switching needed. All 39 feet is used on all bands but I am aware some parts of this are cancelled out due to phasing depending on the band.

Again, looking to optimize 40 and 30 meters if possible by adding some overall vertical length with possibly a small T bar on top or simple 4 spoke top hat but not a big one.

Comments please.

Lane
Ku7i
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KC4ZGP
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« Reply #1 on: June 18, 2017, 07:12:20 AM »


Lane,

Try it and let us know.

_ _ ... ... _ _

Kraus
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N3QE
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« Reply #2 on: June 18, 2017, 07:23:38 AM »

If you had 39 feet of wire spiraled around a 14 foot long tube I might feel that this would help load it up on the lowest band. The serpentine, I do not think, helps it be electrically longer than 14 feet.

Lane, it is not obvious that this antenna is effectively 39 feet on any band. It feels more likely it works just like a 14' vertical. That would be almost a quarter wave vertical on 20M and a little more than a quarter wave on 15M and would be easy to load with a tuner on 20 and 15m.

A top hat would make a 14' vertical much easier to load up on 40 meters but would also affect its ability to be loaded up on 20M and 15M, possibly deterimentally.

If you could put up a taller pole and guy it at the top with wires transitioning to rope, that could serve as a top hat that would make the antenna substantially better on 40M. Again, not sure how this would affect behavior on 20M and 15M.

Tim N3QE
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KD6RF
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« Reply #3 on: June 18, 2017, 08:30:49 AM »

Cap hat is the most efficient way to go for shortened antennas.

TomW8JI recently generated a good article on short antennas that delves into the principles involved - well worth a read "Classic W8JI Limited Space Antennas Article" ===> http://vtenn.com/Blog/?p=2074

Another classic is from Sevick "Classic Sevick Shortened Vertical Article" ===> http://vtenn.com/Blog/?p=46

An optimum system should end looking almost like this ===>



Since the time of the Sevick article, it has been found that moving the loading coil down a bit away from the capacity hat helps bump up the efficiency a bit.
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VTenn Antennas
Bay Area Technical Equip Rental and Test Range
http://vtenn.com/Blog/
KH6AQ
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Posts: 7677




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« Reply #4 on: June 18, 2017, 08:44:03 AM »

EZNEC won't correctly simulate adjacent wires that close together so I placed them 2" apart. It shows two resonances at 8 and 17 MHz. The input impedance at 7 MHz is 3.4 - j112 ohms.

At 7 MHz a single 14' wire presents an input impedance of 4.4 -j700 ohms.

Adding four 2" top hat spokes makes the input impedance 8 - j380 ohms.

Estimating your ground to be 10 ohms, the radiation efficiency (neglecting feedline, tuner, and antenna wire loss) is 25% for the original vertical and 44% for the top hat vertical. The gain difference is 2.4 dB.

The current along the top hat vertical is not uniform, but with a top loading inductor of 19uH the resistive part of the input impedance is 14 ohms. With this the radiation efficiency is 58% for a gain of 3.6 dB over the original vertical.

A top hat loaded vertical can be made to work on several bands. The MFJ-1795 is a good example of such and antenna.





« Last Edit: June 18, 2017, 08:47:47 AM by KH6AQ » Logged
KU7I
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« Reply #5 on: June 18, 2017, 01:00:15 PM »

Thank you Dave.

Lane
Ku7i
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W5DXP
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« Reply #6 on: June 18, 2017, 04:20:00 PM »

The feedpoint of an electrical quarter-wavelength vertical is the highest current point. Maximum radiation is achieved when putting that highest current through the longest possible straight vertical conductor before encountering any loading.

I would take your 39 feet of wire and run it 16 ft straight up. Then take the other 23 feet and make it into a pancake inductor top hat. A pancake inductor is a two-dimensional spiral that needs an insulating support structure, e.g. PVC pipe or fiberglass rods. (Pancake inductors were first patented by Tesla.)

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AH7I
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« Reply #7 on: June 18, 2017, 04:32:04 PM »

Top hat with a perimeter wire offers more capacitance than spokes alone. If you make the hat big(~8' diameter) enough, you won't need much of a loading coil. Use all that serpentine wire to make a three or four wire cage for the vertical element for a bit more bandwidth. Radials sound good.

73, -Bob ah7i/w4
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KU7I
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« Reply #8 on: June 18, 2017, 06:37:56 PM »

Dave KH6AQ,
I can easily add a four spoke capacity hat to the top, each spoke being 2 feet long. I can add a perimeter wire around the spokes. I am hoping for 60% efficiency with the current 39 foot serpentine wind but dont think that just adding this top hat will get me there.

Lane
Ku7i
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G8HQP
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« Reply #9 on: June 19, 2017, 03:22:57 AM »

Quote from: N3QE
If you had 39 feet of wire spiraled around a 14 foot long tube I might feel that this would help load it up on the lowest band. The serpentine, I do not think, helps it be electrically longer than 14 feet.
To a first approximation, the useful electrical length (for radiation purposes) of almost any antenna is the physical length - no matter how many times it folds back etc. This assumes that the antenna is essentially 1-dimensional. Extra wire running alongside the other wire can modify the feedpoint impedance: basically you have incorporated a distributed matching network in the antenna, which will add reactance. It may also widen the bandwidth, as you have a thicker antenna.

Once you start adding capacity hats or using wider spacing or more wire going off at right angles then the electrical length can increase, as you are now using 2-dimensions.

I seem to recall seeing an academic paper on a fractal monopole antenna, which showed that the electrical length was not much different from that of a simple straight monopole which extended the same distance away from the ground plane.

Quote from: KH6AQ
EZNEC won't correctly simulate adjacent wires that close together so I placed them 2" apart. It shows two resonances at 8 and 17 MHz. The input impedance at 7 MHz is 3.4 - j112 ohms.

At 7 MHz a single 14' wire presents an input impedance of 4.4 -j700 ohms.

Adding four 2" top hat spokes makes the input impedance 8 - j380 ohms.
Yes, so radiation resistance stays essentially unchanged when folding back occurs but folding adds some inductance which reduces the capacitive reactance of a short monopole. Then adding a capacity hat nearly doubles the radiation resistance by nearly doubling the average current.

You could combine the two features. Maybe 14' out, plus the same back to form an open folded monopole. This is 14' long for radiation purposes, but has added inductance due to the transmission line mode of the pair of wires shorted together at the far end. Then add a capacity hat to increase the radiation resistance and further reduce the inductance.
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KH6AQ
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« Reply #10 on: June 19, 2017, 09:21:57 AM »

At 7 MHz the input impedance of the 14' vertical with four 2' top hat spokes is 8 -j380 ohms.

The input impedance of the 14' vertical with four 2' top hat spokes and a perimeter wire is 10 -j270 ohms.

Add a 6.2uH base loading coil and the latter is 10 -j0 ohms.
Add a 10.9uH top loading coil and the latter is 14 -j0 ohms.

For a coil Q of 300 the coil loss of the top loaded version is 1.6 ohms. Using the 10 ohm ground loss estimate the radiation efficiency is 55%. The antenna current is uniform along the 14' vertical and a larger top hat will not increase the radiation resistance. It will reduce the loading coil reactance which leads to higher efficiency and greater SWR bandwidth. I've built top loaded verticals with and without perimeter wires, with spiral structures, and with a short top hat cylinder. They all work and for simplicity I would use six top hat spokes as long as you can make them. Aluminum rod from the hardware store is available in 3' lengths.

I recall there were several comments on another thread about the ladder line feed. The comments were subjective with no numbers assigned such as an estimate of feedline radiation loss or the required feedline choke common-mode impedance. To simulate this we would need to know how far above ground the feedline is routed and its length. Or, feed the antenna with coax and the problem is taken care of.
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