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Planning a dual band inverted L for 160/80 meters

Modeling a quarter wave inverted L antenna for the low bands

In the fall of 2007, I was able to install my first amateur radio tower. While this certainly was a huge improvement over my humble Butternut multiband vertical, my new tower had a major shortcoming... a lack of antennas for the low bands: 160 meters and 80 meters.

A few years ago, I acquired a multiband commercial antenna called a half-sloper.  This antenna, the Alpha Delta DX-B, is a trapped, quarter wave single wire sloper for 160, 80, 40 and 30 meters and has been installed at the 55' level and slopes to about 25' off the ground. I bought this at an estate sale for $15. I did not have high expectations - but wanted to try to get a lowband antenna installed prior to the winter 2007/2008 season.  The Alpha Delta DX-B performance was disappointing - especially on 80 meters. It is definitely time for a new approach to 80 and 160 meters.

Tower/Site Considerations and Operating Goals

With every antenna installation, many factors must be considered: safetyN0HR ham radio tower, surrounding objects, costs, etc. Like many hams, I am somewhat limited in terms of space for lowband antennas. 

Here are a few factors that I need to consider: 

  • A 70 ft. self supporting ANwireless tower topped with a 4 element SteppIR and a Cushcraft XM-240 2 element yagi.
  • A distinct lack of high/natural support structures (no trees!). Our small neighborhood was a corn field 10 years ago and we simply do not have many large/solid trees quite yet.
  • No radials at the moment - but will plan to install several (at least 16, ground mounted).
  • Only running 100 Watts (for now) - will likely add an amp.
  • A medium-sized lot (0.6 acre) in a somewhat rural setting, surrounded in the back by a chain link fence.
  • DXing and contesting would be my primary needs for this antenna. Favoring, but not limited to CW.
  • XYL would prefer very few wires running across the yard. The sloper was OK.
N0HR tower site plan

Lowband antenna choices

Based on the factors above, I narrowed my antenna choices to the following:

  • Shunt feed the tower

  • K2KQ Double L  (looks like an interesting approach with several merits)

  • Dipole / Inverted V (lack of space, trees/supports and a reputation as a cloud warmer steered me away)

  • Inverted L

The popular inverted L seemed to be a good choice for ease of installation, low cost, and design flexibility.

 

Evolution of the dual band 160/80 meter inverted L using MMANA

Having driven my ham radio budget into the red with the tower project, I have resorted to using a freeware antenna modeling program: MMANA

 I started with a simple base fed, quarter wave, inverted L for 160 meters.  Wanting to make this a dual band antenna, I decided to put a trap at the top of the vertical section. Given the lack of tall supports for the horizontal wire, I am forced to have a sloped L.  The resulting design is a vertical wire parallel to my tower (hopefully about 3 ft away), 18.5m up to the trap and 19m sloping down.

MMANA 160/80 Meter Inverted L Design N0HR

The feedpoint is a 300 pF series cap with a balun to match the low impedance. The trap in the LC is at 2.9 MHz and consists of a 25.1 ÁH inductor and 120 pF cap.

MMANA setup inv L

 

Modeled Performance and SWR Plots: 80 meters

80 meter performance Inverted L

Gain

 

Modeled Performance and SWR Plots: 160 meters

160 meter SWR inverted L plot

160 meter inverted L gain plot

Remaining questions

  • How can the matching network be built such that the antenna can be remotely tuned across both 160 meters and 80 meters?
  • Will mounting the antenna near (<3 ft.) the self supporting tower significantly impact the design?
  • Are there improvements in the design to be made?
  • Where can I find components for the matching network and trap?

 

 



   
 

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