Antennas for 160mwashcoares.org/wp-content/uploads/2015/08/160m-antennas.pdfTitle Antennas for 160m Author WHF Office User Created Date 8/12/2015 8:03:30 PM

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.


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.


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


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


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


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.


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.


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.


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.


Full Sized Antennas

• Dipole = 240’

• Loop = 510’ circumference (75 ohm)

• Twist loop into figure of 8 to add 80m


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


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.


End Loaded Folded Dipole

• End wires set resonance

• Folded dipole center raises feedpoint

impedance for better SWR.

feedpoint


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


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


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


Multiband Antennas

• Need NVIS radiation on each band.

• Like to have low SWR on each band.


Add-on to 80m dipole

• ~30’ extension to end of 75m dipole or inverted vee.

• Retains 75m performance.


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’


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


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


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


Documents

Antennas for 160mwashcoares.org/wp-content/uploads/2015/08/160m-antennas.pdfTitle Antennas for 160m Author WHF Office User Created Date 8/12/2015 8:03:30 PM