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: safety, 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.

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.

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 µH inductor and 120 pF cap.

Modeled Performance and SWR Plots: 80 meters

Modeled Performance and SWR Plots: 160 meters

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?