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Author Topic: approximate top hat sizing  (Read 1340 times)
KU7I
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Posts: 122




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« on: February 27, 2014, 09:37:46 PM »

Looking for advice here so please respond with your ideas.......is it safe to assume a top hat is roughly equal to the missing length of a vertical? For example, let's say a 1/4 wave vertical for 40 meters is 33 feet long but I can only go vertical 21 feet. This means I am 'missing' 12 feet. If I divide this missing number by the number of spokes in my top hat, say 4, that means I need 4 top hat spokes that are 3 ft long.....correct or am I way off? Just trying to get a rough idea.

Thanks

v/r


Ku7i, JH1JCM
LCDR Lane Zeitler, NC, USN
Post Anesthesia Care Unit
Director, EMT-Basic Program
USNH Yokosuka, Japan
DSN 315-243-5078
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WX7G
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« Reply #1 on: February 27, 2014, 11:43:38 PM »

That is not the method to determine top hat length. The simple way is a NEC simulation and it shows that four 5' top hat spokes on a 21' monopole resonates at 7.15 MHz.
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W8JI
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« Reply #2 on: February 28, 2014, 04:58:42 AM »

+1 on this answer...



That is not the method to determine top hat length. The simple way is a NEC simulation and it shows that four 5' top hat spokes on a 21' monopole resonates at 7.15 MHz.


That half the missing length thing makes no sense. That isn't how hats work.
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W5DXP
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« Reply #3 on: February 28, 2014, 08:01:38 AM »

Taking EZNEC's 40m resonant vertical antenna, VERT1.EZ, and adding two 10' top hat spokes shifts the resonant frequency down by 2.22 MHz. Adding another two 10' (4 total) top hat spokes shifts the frequency down another 750 kHz. Adding another two (6 total) 10' top hat spokes shifts the frequency down another 360 kHz (to about 3.8 MHz). Adding another two (8 total) 10' top hat spokes shifts the frequency down another 240 kHz (to about 3.58 MHz). The resonant frequency effect of the first two top hat spokes is  almost a magnitude higher than that of the last two of eight top hat spokes, i.e. it's a non-linear function.
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73, Cecil, www.w5dxp.com
The purpose of an antenna tuner is to increase the current through the radiation resistance at the antenna to the maximum available magnitude resulting in a radiated power of I2(RRAD) from the antenna.
G3TXQ
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Posts: 1507




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« Reply #4 on: March 01, 2014, 01:12:35 AM »

Just to add what others have already said ......

Consider a couple of examples at the limits of your hypothesis: a 21ft vertical with a single 12ft 'hat wire' will be a 33ft inverted-L and resonant quite close to the 40m band; a 21ft vertical with 48 3" 'hat wires' will be a 21ft vertical with a small "blob" at the end - it'll be resonant somewhere between 10MHz and 11MHz.

Clearly, 'hat wires' of length 12ft/N are not equivalent for all N !

Steve G3TXQ
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N4UFO
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Posts: 193




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« Reply #5 on: March 01, 2014, 02:02:51 AM »

Lacking an NEC program, I had to learn the hard way what the wise gents above have shared... Cheesy Fortunately, I experimented with cheap aluminum electric fence wire, so no big $$ loss. (No worries guys, only 100 watts here... no wire fires.)

I would suggest a 'T' if you have a place to run the wires to horizontally... Otherwise 'guy' the vertical at the highest sturdy point (the tip top of some verticals would be too thin to support the downward pull) and run 3-4 wires down to insulators then the rest of the length use rope to anchor points. Also, you may want to guy half way up the vertical with JUST rope unless what you are using for a vertical is super sturdy, like pipe. (Only guyed at the top, it could wobble in the middle like a broken popsicle stick.) I have done exactly this with a refurbished/reworked Butternut vertical that is 25 feet tall. It is inductively loaded near the base with coils and capacitively loaded at the top with "umbrella wires", as they are called. (The wires are attached at around 17-18'.) It functions as a fairly good 160m & 80m vertical. If I were to loose the base loading coils and shortened the wires on top, I would have a 40m vertical just like you are looking for.

In contrast, I also have a 30m vertical with a top hat that is another 'rework a commercial antenna' design. I added 3' to the length of the vertical (at the base) and then added wire LOOPS at the top. The vertical is 20 feet tall and a 1/4 wave vertical for 30m needs to be 23 feet. So I added the loops to basically make up for the missing three feet... but there is much more than three feet of wire there. (And the shape and spacing of the loops is a bit touchy, by the way.) You would have to make loops considerably larger than mine and unless the vertical was extremely sturdy, it would be considerably top heavy. Not only top heavy but 'top loaded' in the wind...  Shocked A top hat acts like a bit of a 'sail' in other words. I would recommend trying umbrella wires or a 'T' in your case.

Pics and description of my antennas on my QRZ page, if you care to have a look:  http://www.qrz.com/db/N4UFO

73 and thank you for your service,

Kevin N4UFO
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W5WSS
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Posts: 1687




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« Reply #6 on: March 01, 2014, 03:51:53 AM »

Hello Kevin K4UFO you have good ideas.

I like your antenna farm!

73
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K4SAV
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Posts: 1837




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« Reply #7 on: March 01, 2014, 11:23:23 AM »

It's a little more complicated than the information given so far.  The amount of top hat needed is also a function of the diameter of the vertical element.  The easiest way to determine the top hat is with a NEC simulation.  ON4UN gave a manual method of calculating the top hat in his Low Band DXing Book but some assumptions are needed to simplify that method.

Jerry, K4SAV
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W9GB
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Posts: 2614




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« Reply #8 on: March 01, 2014, 12:24:14 PM »

Mark Edwards, W0QL went through a similar exercise for his shortened 40 meter vertical.
http://www.w0ql.com/vertical.html

Today, he uses a Bird Feeder for his vertical element. Photos on his QRZ web page.
http://www.qrz.com/db/W0QL

Jerry Sevick,W2FMI (SK, 2009) wrote a series of articles on Short Vertical Antennas (SVA) for
ARRL/QST magazine in 1970s, after his retirement from Bell Labs.

Short Vertical Antenna and Ground Radial, W2FMI, 64 pages, published 2003, CQ Communications.
http://www.universal-radio.com/catalog/books/0554.html
« Last Edit: March 01, 2014, 12:32:12 PM by W9GB » Logged
N5GZH
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Posts: 87




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« Reply #9 on: March 02, 2014, 11:19:59 PM »

The only generalization I see without computer modeling is that the efficacy of the top hat seem directly proportional to its wind loading.  Huh

I'm wondering:  Given a side by side comparison of a straight vertical and a somewhat (say 2/3 length) top hat vertical,  both operating at resonance, is it possible the top hat might be a better performer by virtue of its current distribution being moved upwards?   Would this reduce losses from proximity to nearby obstacles ?

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