November 14, 2018
Steve Dick, K1RF

1

About Half-wave Antennas

A half-wave antenna is a resonant radiating element with an electrical length of
one half-wave. Total length in feet. ~468 / freq in Mhz. Its’ feed-point affects its
impedance. High current, low voltage at center; low current high voltage at
ends.



The most common half-wave antenna is the center-fed dipole, whose
impedance is approximately 72 ohms. A dipole is basically a mono-band
antenna. It is sometimes used on its 3rd harmonic with coax, or used
multiband as a doublet with balun, ladder line and wideband tuner.)

If fed off center, say at 30%-70% point, impedance is approximately 200 ohms
and can be used on multiple bands with an antenna tuner. Works with a 4:1
UNUN. See this link for more info. 4:1 impedance ratio is 2:1 turns ratio.



If fed at the end (a.k.a EFHW), antenna impedance is in the 2000-4000 ohms
range. It requires a high impedance matching device: Either a tapped resonant
circuit, a Zepp type coupling circuit, or a high impedance ratio UNUN (49:1 or
64:1) It can be used on multiple bands. With a 49:1 or 64:1 UNUN, no tuning
is required and no antenna tuner is required (or perhaps a “touch up” tuner
with up to 3:1 VSWR capability that many modern rigs have built-in.)2

What are the advantages of an EFHW fed withan UNUN compared to other wire antennas?
Ease of installation. Only a single high point required Many configurations possible to suit your installation:
Horizontal, Inverted V, Inverted L, Sloper, etc
 No hanging feedline. Feed point is near the ground.
 Minimal ground system or counterpoise needed – thecoax feed itself can act as a counterpoise Resonant on 80/40/30/20/17/15/12/10m with low
VSWR. No tuner needed or just a 3:1 “Touch-up” tuner
 One simple length adjustment – no interactions
between bands.
 Grounded at D.C. No static buildup.
 Shortened versions possible for limited area3Multiband operation of a HalfWave Antenna
 If the half-wave antenna

can be impedance
matched on all of its
harmonics, you now havea multiband antenna.
 On the fundamental
frequency, antenna
pattern is identical nomatter where you feed it.

3.57 x 1 = 3.57 MHz
3.57 x 2 = 7.14 MHz
3.57 x 3 = 10.71 MHz
3.57 x 4 = 14.28 MHz
3.57 x 5 = 17.85 MHz
3.57 x 6 = 21.42 MHz
3.57 x 7 = 24.99 MHz
3.57 x 8 = 28.56 Mhz

80M
40M
30M
20M
17M
15M
12M
10M

But a key characteristic that does change is the radiation pattern
on each harmonic. This IS affected by where the half-wave is fed.
4Conventional 1:4 and 1:9 UNUNs








These UNUNs belongs to a class ofSecPri
devices known as “transmission linetransformers”, which are formed by
Pri Strt
Sec Strt
winding bifilar turns, multi-filar turns,
coaxial cable, or stripline cable (twostrips of the flat conductor with a
Pri Enddielectric material between the strips)
Sec Endon a core having high permeability.
Standard Transformer
1:4 UNUNs (Zout=200 ohms) are
typically used for off-center-fed (OCF)
dipoles, Zepps, and 43 foot multiband
Pri Strt
Sec Endverticals and random length (nonresonant) wires
Sec Strt
Pri End
1:9 UNUNs (Zout=450 ohms) are
typically used with random lengthnon-resonant wire antennas
Balun Designs
At low frequencies, like all
Transmission line (twisted or parallel pair)transformers, they require adequate
Bifilar Wound Transmission Line Transformer
primary inductance at lowest freq.

Transmission line transformers exhibit very wide bandwidth (1-54 MHz),high power capability (2-5KW), and high efficiency

Some Applications of the 4:1 UNUN
 Variants of the modern W3EDP Zepp

Courtesy Tom, K1TA

 Off-Center-fed Antenna• 160 through 6 meters!
• 4:1 balun is wideband andallows this to work.
• Requires wideband tuner:
High SWR on even harmonics
and more feedline radiation

Courtesy Balun Designs

These usually require wide range antenna tuner
6

Application of 9:1 UNUN
Emergency Amateur Radio Club of Hawaii (EARCHI) marketed amatchbox-kit for a 6-40 mtr multi-band end-fed antenna. Uses nonresonant antenna (Recommended antenna wire length: 24-30 ft (60ft max) and powdered iron toroid, efficient transmission linetransformer
T130-2

Short Non-resonant antenna requires wide range antenna tuner
7

High Impedance UNUN for EHFW antenna The UNUN has a high impedance ratio (49:1

to 64:1) – not feasible to use transmission linetransformers whose practical limit is about
16:1. It is a conventional transformer.
 Uses a ferrite toroidal core or cores (usually
Type 43 or type 52 material). Most 1:4 and 1:9transmission line UNUNs use powdered metalcores (such as -6) having much lower
permeability, not suited for an EFHW UNUN.
 Bandwidths support about a 10:1 frequency
bandwidth (3-30 MHz) Two important UNUN parameters are coreprimary inductance and efficiency.
Less bandwidth (80-10M) and more lossy than the 4:1 or 9:1 UNUNs --But it still works fine and does the job.
8

High Impedance UNUN for EHFW antennacont’dPrimary inductance If primary inductance is too small at the low frequency end,
Mismatch loss is significant and high VSWR results.
 If primary inductance is too high, the high frequency
performance suffers and VSWR climbs at the high end.
 My rule of thumb: Inductive reactance (XL) should be in the
88 ohm- 200 ohm range at the lowest frequency of use (XL =6.28 * F * L where (XL is inductive reactance in ohms, F isfrequency in MHz, and L is inductance in microhenrys. ). This
corresponds to 4-9 microhenrys at 3.5MHz or 2-4.5
microhenrys at 7 MHz .
 Toroid primary inductance increases with size of toroid. Two
FT140 sized toroids have about the same inductance as asingle FT-240 sized toroid for the same number of turns.
 Toroid primary inductance is proportional to the number oftoroids.
9

High Impedance UNUN for EHFW antennacont’dEfficiency
 Good designs should support efficiencies in the rangeof 80-90%+
 For signal strength, It makes little difference.
 For dissipated power in the toroid(s), it makes a big
difference.
 At 50 watts continuous and 75% efficient toroid(s),power dissipation in the toroid is 12.5 watts.
 At 50 watts continuous and 85% efficient toroid(s),power dissipation is 7.5 watts: 60% of the 75% efficienttoroid!
 CW duty cycle is ~ 44% of continuous power
 SSB duty cycle is ! 25% of continuous power
10

EFHW UNUN Construction
I recommend
at least 2 toroids
for better efficiency,
higher primary
inductance, andsplitting power
between toroids

Number of turns= number inside
the toroid stack.
Crossover counts
as one turn“Crossover”

I recommend 2 type 43
material for less than~250 watts for lower
cost, Fair Rite P/N
5943003801
3 type 52 material for~500-1000W
higher efficiency andhigher temperaturecapable. Fair Rite
P/N 5952003801

11

EHFW Antenna Construction
The coax going to the UNUN
can serve as a counterpoise.
A separate counterpoise wire is
optional.
A choke balun is optional but must be installednear the rig, never near the UNUN
(Optional)

Coax
Rig

Choke
balun

(Optional)

Compensation
coil

Inverted L shown(Recommended).
Never run antenna
wire next to a tower.
Mount UNUN near top
of tower instead.

Coax
UNUN

Counterpoise(optional)

Mount UNUN
1-10 ft from ground.
Lower is better.

Ground rod
12

EFHW CounterpoiseCurrent flowing into the antenna's end must be equaled, at that end point, by thesame amount of current flowing into a ground or counterpoise of some type.



In spite of what some manufacturers will tell you, an end-fed 1/2 wave ALWAYS
has a counterpoise. If you don't specifically provide one then the coax shield will
act as the counterpoise.
For a suspended counterpoise wire, the "ideal" length would be 1/4 wavelength.
That would provide the lowest impedance on the common side of the feed point
and send the greatest amount of power into the antenna and minimize any RF on
the outside of the coax shield. Because the feed impedance on an end-fed 1/2
wave is high, you can get away with a lot shorter counterpoise. The higher yourcounterpoise impedance, the more current will flow in the outside of the coax
shield, so its a trade off. Many get away with just using the coax shield as thecounterpoise (which some manufacturers advertise as "no counterpoise").



If you are near a wire fence, connect the ground to it. that makes a great
counterpoise.
If you have a good radial system, that will be a great counterpoise with minimal
currents flowing on the coax.

More info on counterpoise here13

Is a Choke Balun needed?
 Generally speaking, no. Depending on

the quality of your counterpoise, current
splits between the counterpoise andcoax shield.
 You can use a choke balun, but it must
be used at the rig end of the coax feed,
NOT next to the UNUN or you will haveno counterpoise and the EFHW will not
work.

14

Ferrite Materials
Material

InitialPermeability

Relative costCore loss

Curie
Temperature

Comments

Fair Rite
43800

Moderate

Moderate

>130
degC

Power<200
W CW

Fair Rite
52

250

High

Low

>250
degC

Power>400W
CW

Fair Rite
61

125

High

Low

>300
degC

DO NOT USE
Permeability
too low!

Inductive part

Permeability is actually complex- it has an inductive component (u’) anda resistive (lossy) component (u’’)

Inductance typically gets smaller withfrequency
Resistive part

Never use powdered iron cores for a
EFHW UNUN – permeability is way low.
15

100pF compensation capacitor








A 100pF capacitor is part of the UNUN antenna matching circuit. It is placed
across the primary of the UNUN to improve high frequency UNUN performance.
Helps to compensative mainly for UNUN primary leakage inductance.
The value of 100pF works well for most UNUNs, small or large, low power orhigh power.
Typically uses a ceramic disk “blue cap” 3KV or more. These caps typically havea high current rating as they must handle several amps of RF current.
Voltage rating can be increased by putting two 200 pF caps in series. Current
rating can be increased by putting two 47pF caps in parallel.
For QRP rigs, a silvered mica 100pF 500V or 1KV rated cap is fine. Search
EBAY for 100pF silver mica, mouser.com, digikey.com
Sources of 100pF high voltage caps:
 EBAY (search for 100pF capacitor. Select a “blue cap”, 3KV or more
 Mouser P/N 81-DHR4E4C221K2BB (220pF, put two in series. Ceramic disk
15KV rated, $1.23 each

16

UNUN performance without andwith compensation cap (100pF)
Increasing SWR due to UNUN leakage inductances

SWR improved in upper frequencies

SWR 3.8

SWR 2.1

Good SWR here
SWR climbs at low frequency end ifprimary inductance is too small

QRP Unun 3/21 wind Rload=2450

QRP Unun 3/21 wind Rload=2450

VSWR without 100pF capacitor

VSWR with 100pF capacitor

The compensation capacitor compensates mainly for UNUN primaryleakage inductance and improves VSWR at upper end
17SWR plot on real antenna vs.
resistive load
SWR plot looks very
different from a resistive
termination and typically
improves when driving areal antenna!
 That’s because if theantenna operates
slightly above
resonance, it looks
capacitive. That
capacitance seriesresonates with unun
secondary leakage
inductance, effectively
cancelling it out!


80 40 30 20 17 15 12 10
3.5 7 10.5 14 17.5 21 24.5 28

Look Ma! Typical SWR plot with
No antenna tuner! Pretty nice!

18

What does that small “compenation coil” do?
The small coil, used on some commercial antennas, is used
to compensate the resonant point of the high bands.
~1.5microHenrys 6T on 1.25” OD inch PVC form positioned
at 78 inches from the feed point whether 80M EFHW or 40M
EFHW. See Steve Ellington’s youtube video on this topic. It
lowers resonant point more and more at increasing
frequencies.



80M – lowers 22KHz
40M – lowers 57 KHz
20M – lowers 170 KHz
15M – lowers 400 KHz
10M – lowers 1 MHz







Recommendation: For homebrew antennas, use coil if you’re primarily
a CW operator, don’t use coil if you’re primarily a SSB operator. Or
leave it out and use “touch-up” antenna tuner.
19

If 80 meter resonant point is too low(for SSB) on 80 meters
 You can add a small capacitor roughly inthe middle of the antenna wire. Value
250pF-500pF. This is mounted at a highcurrent point. The “blue” ceramic highvoltage caps work.
 This cap has little effect on the higher
frequency bands.

20

Shortened EFHW Antennas

UNUN side

34uH coil: 90T, 110uH coil: 260 turns, 1mm (AWG #18) wire on 19m PVC tube.
Adjust the long wire first for the high end bands. Then adjust the short wire
for the lowest band.
Courtesy Jos Van den Helm, PA1ZP, RSGB February 2016.
“A Three of Five Band End Fed Antenna”

21Adding 160 Meter Capability
to an 80M-10M EFHW Antenna simply

Disconnect UNUN from coax feed. Connect jumper between
coax feed and antenna, leaving UNUN “float”.
The EFHW becomes a quarter wave antenna on 160 meters.
Needs a good ground system to work properly.
More into at:https://www.youtube.com/watch?v=u_SFS5FlM-w
22

Air venting the UNUN housing Why Air Vent?
 Equalizes temperature and pressure

 Minimizes internal condensation
 Transformer runs cooler

 For ~100W rigs, two small 1/16” holes drilled onbottom of housing is sufficient For high power (>500w) multiple screened air
vents are recommended Commercial vent:Amphenol VENT-PS1YBK-N8001 $2.90
Some commercial UNUNs now use thistype of venting for pressure equalization
23

Use of 1,2,or 3 toroids
 Primary inductance is proportional to the number oftoroids. But…you can’t simply add toroids for more
power handling. You also have to pay attention toefficiency and primary inductance – both are ferrite
material dependent.
 More than 1 toroid generally improves efficiency: Single FT240-43, 2T/14T wind is 66.5% efficient on 80M

and has inadequate primary inductance/high VSWR. For a
100W rig on CW, dissipates 14.7 watts – much too high.
 Two FT240-43 2T/14T wind is 83.25% efficient, for a
100W rig, dissipates 7.4W on CW (3.7W per toroid) withgood primary inductance/VSWR. Runs cool as theproverbial cucumber.
Spend that extra few bucks, if necessary, for better performance24

Recommended UNUN configurations
Approx. Power rating, W Toroid size/matl Toroid qty
DigitalCW

MFGR

Secondary capacitor
AWG Wire Size Primary turns TurnspF

MFGR P/N

Efficiency
80M-10M

For:

Comment

SSB

QRP

low cost, tiny,
hard to wind,efficient5

12.5

20 non-standard

1 Fair-Rite

2643625002

22

3

21

100 90.1-93

QRP

recommended

15

37.5

60 FT140-43

1 Fair-Rite

2643802702

16

3

21

100 81.55-86.95

QRP Plus

2X power thansingle FT140-43
and compact

30

75

120 FT140-43

2 Fair-Rite

2643802702

16

2

14

100 79.25-85.3

100w+

recommended

85

212

340 Ft240-43

2 Fair-Rite

5943003801

14

2

14

100 84.1-88.1

good for 40-10.
high efficiency.
Primary
inductance too
low on 80 meters
High power (3uH)

170

425

680 F240-52

2 Fair-Rite

5952003801

14

2

14

100 98.2-88

High power recommended

300

750

1200 FT240-52

3 Fair-Rite

5952003801

14

2

13

100 98.6-92

Notes:
1. All UNUNs listed have >80% efficiency and adequate primary inductance unless otherwise specified. Relative cost
taken into account for recommendations.
2. An alternate to the FT240-43 is a similar size toroid made by Laird Technologies with their type 28 material. 28B2400-000. Digikey P/N 240-2120ND
This material has higher permeability than Type 52 but lower permeability than type 43. It may be substituted for the FT240-43 and has similar
efficiencies.
3. The 100pF cap should be at least 3KV for 100+ watt rigs, at least 6KV for high power. For QRP, a silvered mica cap, 500V-1KV rated can also be
used.

All UNUNs listed have >80% efficiency and adequate primary
inductance. Green are recommended for relative cost and ease of build
25

Common Errors in building
homebrew EFHW UNUNs











Using wrong toroid materials. Stay away from type 61 ferrite cores andany powdered iron cores like -2 or -6. Primary inductance too low
and it won’t work.
Putting insulating tape on toroids. (doesn’t need it and degrades
performance by adding an air gap from wire to toroid. Ferrite toroids
used in these UNUNs have very high resistivity.
Counting wrong number of turns (turns are turns that pass through theinside, including the crossover turn)Running parts of the antenna next to a metal tower (Put the UNUN at
top of the tower, not at base with antenna next to tower)
Putting a choke balun right next to the UNUN. If used, it should be near
the rig or the antenna has no counterpoise and won’t work.
Confusion about what antennas are appropriate for 4:1 and 9:1
UNUNs. These are used with non-resonant antennas, not EFHWs
Putting too much power into an undersized UNUN. Many
manufacturers have exaggerated claims for power handling capability.
Toroids will overheat, SWR climbs as Curie temperature reached. But
the toroid(s) will recover after cooling.
26

Commercial EFHW antennas and
UNUNs for EFHW antennas
 Recommended: https://myantennas.com/wp/
 https://www.hyendcompany.nl/
 https://www.gwhip.co.uk/ https://www.lnrprecision.com EndfedZ antennas
 https://qrpguys.com/ (for low cost QRP) Not Recommended: http://www.mfjenterprises.com/ MFJ198XXP Most stuff on EBAY

27

QRPGuys.com QRP antenna

Low cost ($15.00)
Clever design for portable use.
Rated <2:1 SWR, 10 watts
But… Uses an FT82-43 toroid. According to my analysis, only
about 70-76% efficient and primary inductance on 80 and 40 too low.
28

Some EFHW UNUNs I have built

QRP UNUN>90% efficient
80-10M
5W continuous,
10W CW,
20W SSB

Workhorse UNUN
2 FT240-43
84-88% efficient
80-10M
85 W continuous,
212W CW.
340W SSB

Test setup with 2 FT140-43
Small “QRP Plus” UNUN
80-10M
80-85% efficient
30W continuous
75W CW
120W SSB

29

Sources of materials










Toroids – arrow.com, mouser.com,
digikey.com, kitsandparts.com
1 post stainless wire rope clip:
Amazon.com
Enamel wire: Amazon.com. Search on
14 gauge copper magnet wire. TEMco,
Essex good brands.
Stainless steel ¼” hardware: Home
Depot, Lowes, mcmaster.com.
4”X4”X2” PVC junction box (Carlon
E989NNJ-CAR) Home Depot or Lowes
Cable Tie Base Saddle Type Mount
Wire Holder 100 pcs – Enay or Amazon
100pF high voltage capacitors – Ebay
Wire rope thimble (optional) Amazon or
marine supply house. Relieves strainaround eye bolt

30

Resources













Facebook group End Fed Half Wave Antennas >7000 members
Review of MyAntennas End Fed Half Wave Antenna 80-10 meters byJoel Halas, W1ZR, QST magazine, Mar. 2016 (Needs ARRL member
login)
Endfed Halfwave antenna patterns (look under EFHW)
Toroids.info Dimensions and inductance computation
Calculate ferrite cored inductor parameters by Owen Duffy
Ferrite complex permeability tables for commonly used materials
All Owen Duffy on-line calculators
Good resource on End-fed Antennas
W3EDP Multiband Zepp type antennaAll-band inverted L by Cebik before EFHW but shows antenna patterns
Good insights into inverted L antenna advantages over doublet
Info on EFHW CounterpoiseVideo on protective vent31