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Author Topic: building a trap 75/40/20 meter vertical  (Read 1329 times)
DAVIDVD59
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« on: August 29, 2007, 08:51:57 PM »

Does anyone know of a way, or an article on how to build a trap vertical for 75-40- and 20 meters?
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N3OX
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« Reply #1 on: August 29, 2007, 08:59:54 PM »

How tall can you go?

Do you have any trees?

Do you want to do it to save money over buying a commercial vertical?

Do you want to do it to increase performance over a commercial vertical?

Do you not care about any of this stuff and just want to build something?

I'll get back to you in a second.

Dan
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73,
Dan
http://www.n3ox.net

Monkey/silicon cyborg, beeping at rocks since 1995.
KE3WD
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Posts: 5689




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« Reply #2 on: August 29, 2007, 09:06:44 PM »

It would be a heckuva lot shorter if you go for a resonant vertical for those lowbands instead of a trapped situation.  

Unless you're planning on building a 63 foot tall vertical...


!
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N3OX
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« Reply #3 on: August 29, 2007, 09:17:42 PM »

"Unless you're planning on building a 63 foot tall vertical... "

With wire and a suitable tree, eaaasy.

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73,
Dan
http://www.n3ox.net

Monkey/silicon cyborg, beeping at rocks since 1995.
N3OX
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« Reply #4 on: August 29, 2007, 09:42:57 PM »

OK, 40 foot wire, #14AWG

20m trap coil 2" diameter, 21 turns #16 wire spaced over 2" - about 16 microhenries

20m trap capacitor 8pF at 3kV

20m trap installed 16 feet 10 inches from bottom of wire vertical.

- - - - - -

40m trap coil 2" diameter, 27 turns #16 wire spaced over 2.75"  - about 20 microhenries

40m trap capacitor 24pF at 3kV

40m trap installed 24 feet 5 inches from bottom of wire vertical.

- - - - - -

If you're actually crazy enough to build an antenna I designed in 20 minutes at 12:30 a.m. after I had a cup of coffee too late in the evening, then be aware that I designed it in 20 minutes at 12:30 a.m. after I had a cup of coffee too late in the evening.

On the other hand, it's not too tall... a lot of people have 40 foot trees, and you can get 40 foot fiberglass pole supports otherwise.

I don't know if you can get 8 and 24 pF capacitors... 25pF would probably work for the 24... can you get 8?  Maybe not... maybe have to change it a bit for 10pF which would require reducing the coil inductance a little on the 20m trap.

Also, the bandwidth is pretty narrow on 80; tuning will be critical.

SWR isn't going to be great on coax if you put a good ground system in... I didn't design anything to match it up to 50 ohms... resonant impedance is about 23 ohms on 40 and 17 ohms on 80.

Maybe someone can link to a more practical construction article ;-)

73,
Dan

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73,
Dan
http://www.n3ox.net

Monkey/silicon cyborg, beeping at rocks since 1995.
AB2KT
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Posts: 62




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« Reply #5 on: August 30, 2007, 12:36:18 AM »

How about half of this?

http://degood.org/coaxtrap/

Each leg is 28' 8" long. It covers the pre-WARC bands + 30m.

73
Frank
AB2KT



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WA3SKN
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Posts: 5526




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« Reply #6 on: August 30, 2007, 04:10:10 AM »

David...
There are lots of people here that can design vertical, horizontal,and all kinds of antennas.
If you describe your situation, and what you are trying to do, they can help you!
Since you are not in the ULS yet, are you planning on transmitting soon, or just listening?
Are you in a house, apartment, etc?
Do you have any trees, or other supporting structures nearby, or does the antenna need to be self-supporting?
Does it have to be a vertical?
Info, please!

-Mike.
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DAVIDVD59
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Posts: 122




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« Reply #7 on: August 30, 2007, 06:45:26 AM »

I am living in a duplex, and have one support that is a 30 foot pole by the corner of the building. No I am not a ham yet, but have been studying and when an exam session opens in my area I am going, so this antenna will have to be a transmit antenna also, I have been a long time swl.

I understand verticals aren't the best, but I wasn't too impressed with my inverted vee.
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AA4PB
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« Reply #8 on: August 30, 2007, 07:10:46 AM »

Verticals can be excellent low angle radiators (for DX) IF they have a sufficient radial system, are mounted in the clear (not surrounded by buildings, etc), and they are not surrounded by noise sources. While a good vertical is great for DX, it is usually not too good for the close-in work (out to several hundred miles) often done on the lower bands.

A 1/4 wavelength on 80M is about 60-feet so you'll need a vertical that tall unless you provide loading coils for the lower bands. This can all get rather complex mechanically. Given that, you might want to consider a commercial vertical or some other type of wire antenna - or a combination.

You could simplify the electrical and mechanical design by using a shorter vertical that uses a tuner at the base (either one of the SGC automatic tuners or a home-built manual tuner). It still requires a good radial system however.
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KT8K
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Posts: 1490




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« Reply #9 on: August 30, 2007, 09:50:08 AM »

Personally, I prefer verticals to inverted vees.  A vertical with a sufficient number of radials will work fine.  The more "in the clear" it is, the better, of course, but having a truly omnidirectional capability can be very nice.  I have a dipole and two verticals right now, and find the ability to switch between vertical and horizontal is great, as the patterns, angles of maximum radiation, and sometimes even the polarity can make a very noticeable difference.

Once you get the vertical up, perhaps you can put a loop of wire around the eaves of the house for a horizontal antenna - it can be practically invisible - and then you'll have the kind of choices I enjoy.

My 20/40/80 fan dipole is sagging badly, and I hope to re-string it in the next month, but right now it is a "W" shape on 80m and an inverted vee on 20 and 40.  I have found the difference between nearly-horizontal and inverted vee to be noticeable, and the vee configuration is NOT as good as having the dipole strung out horizontally, even though I was able to pull the feedpoint from 35' to about 60' off the ground.  I hope it will work better when I can reposition the ends to make it nearly horizontal again.

Happy antenna building!  
Best rx & 73 de kt8k - Tim
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DAVIDVD59
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Posts: 122




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« Reply #10 on: August 30, 2007, 10:24:18 AM »

I will throw this one in the mix, can a person put up say a 25 foot vertical with a capacitance hat, and put an air wound coil in the bottom, then run 3 separate coax feeders to the coil at 3 different positions and use the appropriate cable for each band without the other two trailing coax cables being a problem?Huh??
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WA3SKN
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« Reply #11 on: August 30, 2007, 12:40:05 PM »

Theoretically possible, but complex and with many inter-reactions and adjustments!  I certainly would NOT recommend it!
Consider that same vertical, mount an auto-tuner on a weatherproof box at the base, and a single feed line.
But don't forget a radial system of some sort with the vertical!
And, the cheap method would just put a switch at the base with a tapped coil... you would just have to walk outside to change bands... but simple!

-Mike.
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N3OX
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« Reply #12 on: August 30, 2007, 03:33:01 PM »

"just put a switch at the base with a tapped coil... you would just have to walk outside to change bands"

Not necessarily:

http://www.n3ox.net/projects/sixtyvert/moonlander_lg.jpg

http://www.n3ox.net/projects/sixtyvert/moonlander_drawings.pdf

Dan
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73,
Dan
http://www.n3ox.net

Monkey/silicon cyborg, beeping at rocks since 1995.
DAVIDVD59
Member

Posts: 122




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« Reply #13 on: September 02, 2007, 06:03:21 AM »

Thanks for the responses.
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