|
HF
PORTABLE INVERTED L ANTENNA
Fast deployable HF portable end fed inverted L antenna (1.8
- 54MHz).
This
quick and easy-to-set-up multi-band antenna for portable operations
consists of a simple long wire antenna, a counterpoise arrangement,
a 1:9
Voltage Unun
or 1:9 Voltage Balun,
and an auto-tuner, providing access to all HF bands.
The portable supporting mast is a 7m squid pole, and the antenna is
a simple 18.7m length of lightweight wire. Other wire lengths, such
as 25m and 38m, can also be used in this configuration, as long as
they avoid 1/4 wavelengths and multiples of this length for all
intended operational bands. Special attention is given to avoiding
1/4 wavelengths and multiples for the lower bands being used.
The use of a 1:9 Voltage
Unun or 1:9 Voltage Balun
presents a range of impedances to the auto-tuner that are more
easily matched than when the tuner is used on its own. I am using a
Kenwood AT-50 auto-tuner, which, in combination with the 1:9 unun or
1:9 balun, can easily match all HF bands.
Figure 1 depicts a typical 1:9 unun connection with the inverted 'L'
antenna, which works fine. However, having a common ground as part
of the antenna makes it difficult to block common-mode currents,
even with an additional choking balun. When using this arrangement
with my camping trailer, where the ground is the frame of the
trailer, a lot of noise is picked up from the portable refrigerator,
and the transmitter interferes with the dimmable LED lights.
Figure 2 shows an alternate arrangement using a 1:9 balun with the
inverted 'L' antenna. This arrangement performs as well as the
figure 1 arrangement and has the additional advantage of better
isolation of the antenna and feedline, especially when a choking
balun is included in the feedline before the connection with the 1:9
balun. The only drawback is that a longer counterpoise may be
required, especially on the lower bands. It is recommended to use a
common-mode choke for optimal results with this antenna arrangement.
The main advantage of the figure 1 arrangement is that it can be set
up quickly, and the trailer frame serves as a sufficient ground when
running a separate counterpoise wire is not practical.
Figures 4 and 5 depict SWR and impedance measurements for various
antenna configurations.
Figure
1 Typical 9:1
voltage Unun with inverted L wire antenna configuration and
counterpoise wire.
Figure
2 Typical 9:1
voltage Balun with inverted L wire antenna configuration and
counterpoise wire
| Antenna |
Length
(m) |
Counter
poise (m) |
Note |
| 18.7M 5MCP |
18.7 |
5.0 |
|
| 25M 8MCP |
25.0 |
8.0 |
|
| 25M 8MCP FN |
25.0 |
8.0 |
Counter
poise also grounded to a steel fence |
| 38M 18.7MCP |
38.0 |
18.7 |
|
Figure
3 Table
showing various Inverted L antenna dimensions and general arrangements that
have been trialed.
The 18.7m wire length has been successfully used on all HF bands, although its efficiency is low for the 80m band. The 25m and 38m lengths provide better coverage of the lower HF bands, with the 38m length being well suited for operations on the 160m band.
While the 18.7m wire length antenna in the above arrangements can achieve a match for the 160m band, no attempts to make contact have been
attempted.
Figure
4
Table
showing SWR presented to the auto tuner. SWR in the vertical axis and frequency in MHz in the
horizontal axis for the various antenna configurations.
BAND
FREQUENCY MHz |
SWR for various Inverted L antenna
|
| 18.7M
5MCP |
25M 8MCP |
25M 8MCP FN |
38M 18.7MCP |
| 1.85 |
21.0 |
15.5 |
3.2 |
6.6 |
| 3.60 |
11.9 |
1.6 |
7.8 |
4.4 |
| 5.4 |
8.5 |
2.6 |
11.2 |
2.7 |
| 7.15 |
4.8 |
1.8 |
6.0 |
2.0 |
| 10.10 |
3.3 |
6.4 |
1.9 |
3.8 |
| 14.15 |
4.8 |
1.9 |
1.4 |
2.2 |
| 18.1 |
2.4 |
2.4 |
4.6 |
1.5 |
| 21.2 |
1.6 |
2.6 |
2.8 |
2.3 |
| 24.95 |
3.3 |
1.5 |
1.8 |
1.6 |
| 28.5 |
1.9 |
2.5 |
3.0 |
1.3 |
| 29.5 |
2.4 |
2.1 |
2.2 |
1.5 |
| 50.50 |
1.5 |
1.4 |
1.6 |
2.7 |
| 53.00 |
2.8 |
3.0 |
3.3 |
2.8 |
Figure
5
Table
showing SWR recorded per antenna configuration per band. The yellow
highlights in the table are indication of the better suited antenna length per
band based on efficiency and radiation patterns.
Figure
6
Table
showing Impedance presented to the auto tuner, Impedance, Resistance and Reactance in the vertical axis and
frequency in MHz in the horizontal axis for the 18.7m wire antenna
with a 1:9 Balun.
Figure
7 MMANA
antenna modelling for the 18.7m inverted 'L' antenna showing the
radiation pattern for 28.5MHz
Figure
8 MMANA
antenna modelling for the
18.7m inverted 'L' antenna showing the 3D
radiation pattern for 28.5MHz
Also see 1:9 Unun: 1:9
Voltage Unun
1:9 Balin: 1:9 Voltage Balun
TOP
OF PAGE
Page
last revised 20 November, 2020
Page
last revised 15 January, 2021
|
