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Author Topic: 160M Inverted L  (Read 1726 times)
K9WW
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Posts: 28




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« on: November 26, 2012, 09:07:46 PM »

Greetings from the "Frozen Tundra" of northern Wisconsin,

I have erected an inverted L antenna for topband above a redial field of 27 Wave/4 radials on ground and some buried slightly in high traffic areas. The vertical element is a wire about five feet away and alongside my main tower for about 77 feet and then hung horizontally for 100' to the top of tower#2 at a height of 62 feet. according to what I have read I can use the longer length with a series variable capacitor to tune-out the inductive reactance posed by the longer wire (177') and provide a match across the entire band. My question is would I be better served with this arrangement or trim the wire for a 1 wave/4 at my desired frequency of operation in the DX window and use an L network to provide full band coverage? I have several Cardwell bread slicers which I can pad with a doorknob cap to get the correct value and still handle the voltages present in either case. I've read plenty of info in the ARRL Antenna Book and ON4UN's Low Band DXing book plus lots of posts online and am still a bit confused as to what will be the better antenna. Any opinions will be greatly appreciated!

Kirk, K9WW
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GW3OQK
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Posts: 145




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« Reply #1 on: November 27, 2012, 01:03:41 AM »

The antenna puts the current max about half way up the vertical section for max low angle radiation. Being longer than 1/4 wave it increases the radiation resistance and reduces ground losses. Should be great, will you be on in the coming 160m events to give it a try to EU?

I have a similar arrangement and use an Icom auto tuner at its base. To match it I think you need 300 pf and 2 uH in series with the wire, and 1000 pf in parallel with the 50 ohm feeder. (I think its about 37 ohms +j280. Perhaps someone with check my estimate!)

73
Andrew
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WX7G
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Posts: 6035




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« Reply #2 on: November 27, 2012, 01:18:33 AM »

The unknown (at this point) is the tower. The inverted-L will induce current into the tower and the input impedance will depend on the the tower electrical length. The tower should be connected to the radials.

For now let's look at the inverted-L without the tower. The EZNEC simulation is over average ground with radials 0.05' above ground. This approximates buried radials. The input impedance is 34 +j345 ohms. Tuned with a 265 pF series capacitor the 50 ohm 2:1 VSWR is 52 kHz.

Now for the 1/4 wavelength inverted-L. The input impedance is 29 ohms and the 2:1 VSWR is 56 kHz fed directly and fed through a 1:2 transformer (MFJ and DX Engineering have these) the bandwidth is 62 kHz.

Placing a 130' tower alongside each antenna shows the 1/4 wavelength inverted-L to be affected much less. I would go with the 1/4 wavelength inverted-L, feed it direct with coax and touch up the match at the shack.

I've used the tuner-at-the-shack method before to broadband a 38' top loaded vertical. I presently use a motorized inductor at the base of a 30' vertical that is fed through a 1:4 DX Engineering transformer.
« Last Edit: November 27, 2012, 01:27:20 AM by WX7G » Logged
K3VAT
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Posts: 709




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« Reply #3 on: November 27, 2012, 10:48:22 AM »

The unknown (at this point) is the tower. The inverted-L will induce current into the tower and the input impedance will depend on the the tower electrical length. The tower should be connected to the radials.

Correct, and the other unknown (less of an impact, but still noteworthy, especially if large) is what antenna(s) are at the top of the tower.  For instance, how far above the horizontal portion of the inverted L is any antenna and what antenna is it?  I've found that this does matter and it should be factored into your models.

Whatever length of wire you decide to use for your new Inverted L, it might be quite helpful and instructive if you could have a vector analyzer present to assist in the design of your matching network.  Every situation is different and even though yours and my inverted L may be very similar in physical characteristics (height and length of horizontal section) what we have in the way of towers, antennas on the tower, and objects adjacent to the near field play an important part in its design and final performance. Mutual coupling is not unlike mutual attraction; one needs to be careful  Grin

GL, 73, Rich, K3VAT
« Last Edit: November 27, 2012, 10:54:34 AM by K3VAT » Logged
KC4MOP
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Posts: 733




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« Reply #4 on: November 28, 2012, 03:29:19 AM »

Would the antennas at the top of his tower act like a capacity hat? Or is it a different line of thought being that he has an inverted "L"?
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