# User manual | ANTENNA TYPES

```ANTENNA TYPES
dipole
What’s a better way
of doing this?
What’s missing here?
(looking down)
min.
signal
max.
signal
max.
signal
min.
signal
Dipole voltage & current
- receiving
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
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
•
•
•
•
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 
• must be resonant ¼  length
• must be insulated from ground
• drooping radials will raise impedance closer to 50 
Vertical Antennas
• Variants
•
•
•
•
•
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