ANTENNA TYPES
ANTENNA TYPES
dipole
What’s a better way
of doing this?
What’s missing here?
½  dipole radiation pattern
(looking down)
min.
signal
max.
signal
max.
signal
min.
signal
Dipole voltage & current
- receiving
receiver
Dipole voltage & current
- transmitting
transmitter
How long should the dipole be?
• From prev. slide: ½ wavelength ( 2 x ¼ )
• If I want my dipole to resonate at 7.100 MHz, how
long should it be?
• Formula:
– Below 30 MHz: ½  dipole should be 468/f (MHz), in feet
– Why is this shorter than a half wave in space?
– effects of (a) wire thickness & (b) end capacitance
• My antenna should be 468/7.1 = 65.915’ = 65’ 11”
• What is a good practice when first measuring the
wire?
– Cut it a little long – easier to trim than lengthen
Dipole Impedance & SWR
• What is the approximate impedance of a ½  dipole
(at resonance)?
– Theoretically 73 
– In practice, depending on height, proximity to buildings, etc.
it could be closer to ~ 65 
• If the impedance of my dipole is 65 , and I feed it
with RG58 coax, what will my SWR be?
–
SWR = Zantenna / Zfeedline = 65/50 = 1.3
Dipole variants
• Inverted Vee
Dipole variants
• Inverted Vee
– Big bonus: only needs one support
– Radiation is more omni-directional than dipole
– Drooping the legs
• brings impedance closer to 50 
• makes end capacitance effect larger
– Antenna needs to be SHORTER than dipole (~ -5%)
– (note: App 2 in study guide is wrong, says +5%)
• can cause hazard to people on the ground so keep ends ~8’
above ground
– Leg angle should be > 90° to avoid interference between
field from each leg
Dipole variants
• Fan dipole
•
•
multiband with only one feedline
off band dipoles have high impedance – transmitter current
flows to desired dipole
• some interference between dipoles – makes adjusting lengths
difficult
Dipole variants
• Trap dipole
Dipole variants
• Two band trap dipole
– Traps are parallel L-C circuits
– They resonate near the lower edge of the higher band
– At resonance the trap has very high impedance & acts like an open
circuit so only the inner portion of the antenna is active. It is cut
for the higher band
– Below resonance, the traps act like loading coils, so by using
proper length of the outer portion, the antenna can be resonated in
a lower band
– Trap dipoles can cover several bands
Dipole variants
• Cage dipole
• “Cage” of wires simulates large diameter element (remember
skin effect?)
• Gives more usable bandwidth
• Larger effective dia. requires shorter ant. for same resonance
(At ARRL HQ, a cage dipole with 4’ spreaders covers the entire 80m band!)
Dipole variants
• Others
–
–
–
–
Sloping dipole (“sloper”)
Off Centre Fed dipole
Folded dipole
Staggered inverted vee
Vertical Antennas
• ¼  Vertical
Antenna
¼  vertical
radiation
pattern
view looking
down:
omnidirectional
side view over
perfect ground
over real ground,
elev. angle is higher
3-D view
Vertical Antennas
• ¼  Vertical Antenna
– Needs either ground radials or elevated radials
– Ground radials
•
•
•
•
can be random length
either bare or insulated wire is OK
can be shallow buried or laid on the surface
Impedance of vertical with ground radials is ~ 36 
– Elevated radials
• must be resonant ¼  length
• must be insulated from ground
• drooping radials will raise impedance closer to 50 
Vertical Antennas
• Variants
– Short Loaded Vertical
•
•
•
•
•
loading coil resonates antenna on desired freq.
coil can be at antenna base or midpoint
popular for HF mobile (e.g. screwdriver antenna)
mobile CB antennas are this type
loading can also be done via a capacitance hat at top – not too
popular these days
– Trap Vertical
• theory same as described for trap dipole
Directional Antennas
(Yagis & Quads)
• x
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