In the fall of 2007, I was able to
install my first
radio tower. While this certainly was a huge improvement over
Butternut multiband vertical, my new tower had a major shortcoming...
a lack of antennas for the low bands: 160 meters and 80 meters.
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
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
L (looks like an interesting approach with
Dipole / Inverted V (lack of space,
trees/supports and a reputation as a cloud warmer steered me
The popular inverted L seemed to
be a good choice for ease of installation, low cost, and design
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
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 ÁH inductor and 120 pF cap.
Modeled Performance and SWR Plots: 80 meters
Modeled Performance and SWR Plots: 160 meters
- 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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