Post on 07-Mar-2021
Antennas for 160m
Dale Hunt, WB6BYU
25 October 2008
Copyright 2015 by Dale Hunt WB6BYU. Non-commercial use permitted if this notice is retained.
Why use 160m?
NVIS operation requires a frequency
below the Critical Frequency, or signals
will not be reflected back to Earth.
Here in Oregon, the Critical Frequency
often drops below 80m during the
winter, so 160m is the only option.
Copyright 2015 by Dale Hunt WB6BYU. Non-commercial use permitted if this notice is retained.
The Problem
• Half wave dipole is 240’ long, too large
for many hams to put up.
• Proportionally closer to the ground than
for other bands.
• Feedpoint impedance and bandwidth
drop as antennas are made smaller.
Copyright 2015 by Dale Hunt WB6BYU. Non-commercial use permitted if this notice is retained.
Ways to shorten an antenna
• Bend the wire to make it fit
• Capacitive end loading
• Loading coils
• Linear loading stubs
• All of these reduce the radiation
resistance and the feedpoint impedance
Copyright 2015 by Dale Hunt WB6BYU. Non-commercial use permitted if this notice is retained.
How to match low impedances
• Folded dipole / “T” match
• Shunt reactance / “Beta” match
• Series capacitor
• Tapped / link coupled coil
• Off center feed
• Remote antenna tuner
• Feedline matching sections
• Increase the losses
Copyright 2015 by Dale Hunt WB6BYU. Non-commercial use permitted if this notice is retained.
Can’t I use my antenna tuner?
• 75m dipole fed with 100’ of RG-8 coax
– Feedpoint impedance = 8 - j1100 ohms
– Feedline loss = 22.8 dB
– Tuner loss = 1.2 dB
– Total loss = 24dB
– 100 watts becomes 400mW radiated power
Twinlead has only 15.2 dB loss = 3 watts out
Copyright 2015 by Dale Hunt WB6BYU. Non-commercial use permitted if this notice is retained.
How to minimize loss
• Keep the antenna as high as possible -
especially the ends
• Use high-Q loading coils
• Use large diameter wire
• Use designs with a high radiation
resistance.
Copyright 2015 by Dale Hunt WB6BYU. Non-commercial use permitted if this notice is retained.
Antenna tuner guidelines
• Improve match at antenna where possible.
• Aim for SWR less than 5 : 1 (10:1 max).
• Use setting with maximum capacitance, minimum inductance.
• With common T tuner, one capacitor should be at maximum where possible.
• A 1: 1 balun may give higher efficiency.
• Tuner losses are highest with low resistance loads.
Copyright 2015 by Dale Hunt WB6BYU. Non-commercial use permitted if this notice is retained.
Design approach
• Pick physical dimensions that fit the space
and supports available.
• Choose a method to tune it to resonance,
• Choose a method to match the impedance.
• Computer modeling saves a lot of time!
• Same principles can be used to build
shortened antennas for 80m or other bands.
Copyright 2015 by Dale Hunt WB6BYU. Non-commercial use permitted if this notice is retained.
Design assumptions
• Resonant on 1.950 MHz. (Also 3.95,
5.35, 7.25 MHz for multi-band use.)
• Optimize SWR on 160m (where my
autotuner doesn’t work.)
• Antennas modeled at 20’ over poor
ground.
Copyright 2015 by Dale Hunt WB6BYU. Non-commercial use permitted if this notice is retained.
Full Sized Antennas
• Dipole = 240’
• Loop = 510’ circumference (75 ohm)
• Twist loop into figure of 8 to add 80m
Copyright 2015 by Dale Hunt WB6BYU. Non-commercial use permitted if this notice is retained.
Universal short dipole
• Can be made almost any length
• Adjust center coil to resonance
• Coax attaches to link coil
• Adjust link turns for SWR at resonance.
loading coil in antenna wire
link coil connected to coax
as long as possible
Copyright 2015 by Dale Hunt WB6BYU. Non-commercial use permitted if this notice is retained.
Folded End Wires
• End load in same space as 80m antenna.
• Uses more wire than a straight dipole.
• Lower feedpoint impedance, but usable.
• Adjust end wires for resonance.
• Can fold ends of wires many ways.
Copyright 2015 by Dale Hunt WB6BYU. Non-commercial use permitted if this notice is retained.
End Loaded Folded Dipole
• End wires set resonance
• Folded dipole center raises feedpoint
impedance for better SWR.
feedpoint
Copyright 2015 by Dale Hunt WB6BYU. Non-commercial use permitted if this notice is retained.
More Short Folded Dipoles
• Need to be longer to tune bottom of 160m band.
• Best fed with 75 ohm coax cable.
• 30 to 50 kHz SWR bandwidth.
160’
140’
folded dipole with one loading wire
folded dipole with two loading wires
Copyright 2015 by Dale Hunt WB6BYU. Non-commercial use permitted if this notice is retained.
Smaller Tuned Loop
• Requires high voltage tuning capacitors
• Can have reasonable efficiency
• Can be used on other bands by
changing the matching at the feedpoint.
total width 82’
height
30’
feedpoint
tuning capacitors
each 75pf
Copyright 2015 by Dale Hunt WB6BYU. Non-commercial use permitted if this notice is retained.
Really Small Loaded Folded
• 7 kHz SWR bandwidth
• -10dB gain
• Shifts 14 kHz for one turn on each coil
• Shifts 40 kHz for 1’ on each wire.
• May not be practical to build and keep tuned.
• Impedance depends on ground, coil losses.
total width 40’
height
20’
feedpoint127uH
loading coil
127uH
loading coil
Copyright 2015 by Dale Hunt WB6BYU. Non-commercial use permitted if this notice is retained.
Multiband Antennas
• Need NVIS radiation on each band.
• Like to have low SWR on each band.
Copyright 2015 by Dale Hunt WB6BYU. Non-commercial use permitted if this notice is retained.
Add-on to 80m dipole
• ~30’ extension to end of 75m dipole or inverted vee.
• Retains 75m performance.
Copyright 2015 by Dale Hunt WB6BYU. Non-commercial use permitted if this notice is retained.
Add-on to 40m dipole
• ~30’ addition to 40m dipole/inverted vee
• Gives dual band operation in the same
length as a 80m dipole.
40m dipole
64’
100uH loading coils
added
portionadded
portion
2.5’ extension wire
27’ end wire
total length 124’
Copyright 2015 by Dale Hunt WB6BYU. Non-commercial use permitted if this notice is retained.
End fed single wire antennas
• Bending wires gives NVIS pattern on 160, 80, 60 and 40m.
• Use with remote auto-tuner.
70’
70’
150’
100’
BIRD’S EYE VIEW OF SINGLE-WIRE FED ANTENNAS
BENT WIRE
BENT WINDOM
feed here with
single vertical wire
Copyright 2015 by Dale Hunt WB6BYU. Non-commercial use permitted if this notice is retained.
More single wire antennas
• These also provide multiband NVIS
operation with a remote auto-tuner.
DINKUM SQUARE
each side is 60’ long each side is 65’ long
5’ gap (rope)
20’ wire
40’ wire
WINDOM LOOP
Copyright 2015 by Dale Hunt WB6BYU. Non-commercial use permitted if this notice is retained.
Field Dipole
• Simple tunable field antenna.
• Operates 40, 60, 80, 160m.
• Manual band switching.
• Tunable ends allow for adjustment.
• Can cover other HF frequencies as well.
feedpoint
66’16’
A B
Copyright 2015 by Dale Hunt WB6BYU. Non-commercial use permitted if this notice is retained.