Antennas and Feedlines
An Overview with Useful Notes
9 April 2015
Dale Hunt WB6BYU
Terminology
First, let’s define some terms.
• SWR
• Gain
• Directivity
• Bandwidth
• Narrow band vs. Wideband vs. Broadband
• Single band vs. multi-band
• Beamwidth
• Polarization
SWR
• SWR is a measure of how well an antenna (or other load) matches the
feedline.
• It does NOT by itself indicate anything about how well the antenna
works (though it often is the first sign of a broken antenna.)
• SWR is a ratio – it is always given as the ratio of a larger number to
one, like 3 : 1 or 1.27 : 1. A perfect match is 1 : 1.
• There are probably more myths, misunderstandings, and simply
incorrect statements about SWR and related topics than any other
part of ham radio.
SWR (continued)
• SWR tells you how far you are off, but not the direction you have to
go to get a perfect match.
• Many modern rigs reduce output power when SWR is high to protect
the transmitter. Typically this is around 2 : 1.
• SWR by itself does not damage transmitters, but it may indicate a
condition where the transmitter can not be operated at full power for
a long period of time without damage.
Gain
• GAIN is a comparison of the signal radiated by one antenna compared
to some other antenna as a reference.
• The reference must be stated (or clearly implied), or the number is
meaningless.
• Gain is generally expressed in dB relative to the reference. Common
abbreviations are:
• dBi - dB with respect to an isotropic antenna (a convenient theoretical
construct that radiates equally in all directions)
• dBD – dB with respect to a dipole, assuming the same orientation and
location.
• You cannot have GAIN without DIRECTIVITY.
Directivity
• DIRECTIVITY is a measure of how much more power an antenna
radiates in one direction than in another direction.
• Imagine a partially-inflated round balloon – it sticks out roughly the
same in all directions. They only way to make it stick out more in one
direction is to squeeze it so it doesn’t stick out in other directions.
• That is how we get antenna gain – we reduce the power in some
directions to focus it in the desired direction.
• The higher the gain of an antenna, the less power it radiates in
undesired directions because of the required directivity.
Gain vs. Directivity
This shows how the gain of an
antenna is increased in one direction
(red trace) by reducing the power
radiated in other directions.
The blue plot is a single delta loop
antenna with a bidirectional pattern.
The red shows the effect of adding a
reflector to make a 2-element beam
using the same geometry. More
power can be radiated towards the
right because less power is radiated
to the left.
Bandwidth
• BANDWIDTH refers to the range of frequencies over which the
antenna will perform within some specific operating limits.
• The most common use is the SWR Bandwidth: the frequency range
over which the SWR is within certain bounds. This is particularly
popular because most hams aren’t equipped to measure other
parameters of interest.
• Other bandwidths of interest may be where the antenna meets some
criteria for gain, Front-to-Back ratio, efficiency, etc.
SWR bandwidth examples
This narrow-band antenna
only covers 144.7 to 146.3
MHz at under 2 : 1 SWR.
This is typical of a yagi
designed for maximum
gain at one frequency.
This wider bandwidth antenna
covers most of the whole 2m
band at under 2 : 1 SWR.
This wideband antenna covers 136 to
157 MHz at under 2 : 1 SWR, a
bandwidth of 21 MHz.
However the change in the radiation
pattern with frequency limits its use in
practice to about 145 – 147 MHz.
Single Band vs. Multi-band
• A single band antenna is designed to operate on just one ham band.
• A 40m dipole is an example, though it may also have a low SWR on 15m.
• A multi-band antenna is designed to operate on multiple ham bands,
usually (but not always) without any manual adjustments or changes
to the antenna.
• Common examples are antennas using traps or multiple elements in parallel.
• On HF, a single antenna can often be used on many different ham
bands by feeding it with ladder line to a wide-range antenna tuner in
the shack. The low loss ladder line reduces the feedline losses due to
high SWR. The radiation pattern likely will vary from band to band.
Single band, multi-band, wideband
20m dipole only has low SWR
relative to 50 ohm coax cable
on one HF band.
Multi-band dipole has low
SWR on 20m, 15m and 10m.
Wideband antenna covers all
frequencies between 14 and
30 MHz, both inside and
outside the ham bands, with
SWR less than 2.5 : 1
BEAMWIDTH
• BEAMWIDTH is the angular range over which an antenna provides
maximum gain.
• It is commonly measured between the directions in which the
radiated power is half that of the peak (down 3dB).
• Higher gain antennas have narrower beamwidths.
• The beamwidth for horizontal polarization is generally narrower than
that for vertical polarization from the same yagi, but that isn’t
necessarily true of other types of beam antennas or combinations.
Example yagi patterns: directivity and beamwidth
These plots show the radiation pattern of
3 yagi antennas with different numbers
of elements, and how gain is achieved at
the expense of radiation in other
directions. The purple trace (9 element)
has the highest gain (sticks out furthest
to the right) and the narrowest
beamwidth, so must be aimed more
accurately. The blue plot for the 3element yagi has a wider beamwidth, but
isn’t as strong in the peak direction.
Generally the gain of a yagi depends
mostly on the length of the boom – the
longer the boom (in wavelengths), the
higher the gain and the sharper the
pattern.
Polarization
• POLARIZATION refers to the orientation of the antenna elements relative to
ground.
• Vertical polarization (antenna elements primarily vertical) is used for
VHF/UHF FM operation as it is most convenient for omnidirectional
coverage from a mobile vehicle that may be turning along a street.
• A dipole is omnidirectional when vertically polarized.
• Horizontal polarization (elements primarily horizontal) is used for VHF/UHF
weak signal work (SSB and CW) because it provides better signal strength
over difficult paths.
• A dipole has nulls off the ends when horizontally polarized.
• When signals are cross-polarized they will be very weak.
• Circular polarization is often used for space communications where there is
no common reference for “vertical” or “horizontal”.
Omnidirectional Gain Antennas
• Omnidirectional Gain Antennas are common for VHF/UHF FM use.
• On the surface, it may appear that these would violate the directivity
requirement since they provide gain in all directions.
• The gain is achieved by reducing the radiation at other vertical angles
and concentrating it at the horizon.
• This is achieved by using a tall stack of radiating elements – a simple
tall radiator won’t work because the radiation from parts of it will be
out of phase with that from other sections.
• The gain is limited by the height of the element stack in wavelengths.
Omnidirectional (collinear) gain antenna patterns
This shows the vertical (side view) radiation pattern of
stacks of 1, 2 and 4 collinear dipole elements. The
single dipole (purple trace) has more radiation at higher
angles and less gain at the horizon, and is a bit over 3’
long on 2m. It is stronger than the others at vertical
angles greater than 15 degrees.
The blue trace (2 elements) has about 3dB gain over the
single dipole and is about 8’ long on 2m.
The 4-element stack achieves about another 3dB gain,
and stands about 20’ tall on 2m to do so.
The progressive flattening of the pattern is clear from
these plots – like sitting on the balloon to squish it out
sideways.
Generally speaking one must more than double the
length of a vertical antenna to get 3dB more gain.
Yagi antennas
• A Yagi (or Yagi-Uda: Mr. Uda did the work, but Dr. Yagi published
papers describing it in English) is a common type of beam antenna.
• They are simply constructed of parallel elements roughly ½
wavelength long, often held in position by a central boom (but other
construction methods are possible.)
• They can be built for any band, though they can get mechanically
complex on 80m or 160m, and construction tolerances may be an
issue at 10 GHz and higher.
• Yagis can be built in many sizes. 2m dimensions can vary from 2
elements spaced 4” apart to 40 elements strung along a 100’ rope.
Yagi antennas (continued)
• There is no single set of dimensions for yagi antennas.
• A 3-element yagi has 5 important dimensions: the lengths of the 3
elements and the spacings between them. (The diameter of the
elements also make a difference.) These dimensions can be adjusted
for various trade-offs among gain, beamwidth, SWR, bandwidth,
Front-to-Back ratio, or other parameters of interest.
• Yagis with higher gain in a given length tend to have a lower feedpoint
impedance, so some sort of matching network is commonly used to
give a lower SWR when fed with 50 ohm coax.
Pattern comparison of 3 element yagis
This shows the variation in azimuth patterns
among four different 3-element yagi designs.
Boom lengths vary from about 18” to 48” on 2m.
The green trace has a very deep Front-to-Back
ratio, but somewhat less gain.
The purple trace was optimized for high gain with
a low SWR on 50 ohm coax. The Front-to-Back
ratio is poor, and it uses a relatively long boom.
The blue and red traces were designed for other
combinations of parameters.
The first step in choosing a yagi design often
should be to determine which parameters are
most important to your specific application.
Affect of antenna height - VHF
• Antenna height above ground makes more difference than antenna
gain for most terrestrial VHF/UHF operation.
• Signal strength increases with height even beyond the point where
there is a line-of-sight path between the two antennas.
• Compared to an antenna at 5’, raising it to 10’ gains 7dB (equivalent
to 5 times the power) on a signal from 10 miles away. Raising it
further to 20’ gains another 4dB.
• These numbers assume flat ground with no obstructions – in practice
it can be more as raising the antenna allows signals to clear buildings,
small hills, etc.
• I recommend 12’ to 20’ as a practical height for portable VHF/UHF
antenna supports. Greater height is useful for longer distances.
Height vs. relative gain on 2m
Relative gain of an antenna on 2m
over flat, unobstructed ground at
a distance of 10 miles, as a
function of antenna height.
The biggest increase in gain is in
the first 20’, though gain
continues increasing beyond that.
For portable VHF/UHF antennas,
supports of 12’ to 20’ seem to
provide the best trade-off
between performance, ease of
setup, and portability.
Height of Stacked (Collinear) Gain Antennas
• Collinear (stacked) omnidirectional gain antennas don’t always provide the
expected gain if they aren’t mounted high enough.
• The average height of a collinear antenna is in the center, and that is the height
that should be used in relative gain calculations.
• For example, if the maximum height for an antenna + mast is 22’ off the ground,
the average height of a 20’ collinear array is only 12’, with the base 2’ off the
ground. Using the previous chart, relative gain would be 6dBd gain + 0dB relative
gain = 6db.
• An 8’ stack with nominal gain of 3dBd would have an average height of 18’, for a
total of 3dBd + 2.5dB relative gain = 5.5dB.
• A dipole with an average height of 20’ would have 0dBd + 3.5dB relative gain =
3.5dB.
• In this case, an 8’ antenna on a 14’ mast works very nearly as well as a 20’
antenna with the same top height, in spite of the nominally higher gain of the
longer collinear antenna.
Affect of Antenna Height - HF
• Because ionospheric HF propagation works for higher vertical angles
than VHF/UHF propagation, the effects of height above ground are
rather different.
• For vertically polarized antennas there is not a lot of advantage to
installing them more than ¼ wavelength above ground. The primary
advantage of raising the base above the ground is to reduce ground
losses that restrict antenna efficiency.
• For horizontally polarized antennas, the height above ground affects
the angle of maximum radiation, which correlates to specific
propagation distances depending on the height of the ionosphere.
HF Antenna Height (continued)
This plot shows the vertical radiation pattern of a
horizontal antenna above ground as a function of
the antenna height in wavelengths.
At ¼ wavelength (green trace) maximum radiation is
straight up, suitable for short range NVIS paths on
the lower HF bands.
As the antenna height is increased the radiation at
lower angles increases. At ½ wavelength (black
trace) maximum radiation is at a vertical angle of 30
degrees, suitable for single hop distances of around
1000 miles or so.
At 1 wavelength (red trace) maximum radiation is at
15 degrees, and the radiation at lower angles is
10dB stronger than at ¼ wavelength. This is why
higher horizontal antenna are better for working DX.
SWR (further discussion)
• A low SWR is not necessarily a good thing
• Improving the efficiency of a ground-mounted HF vertical antenna will often
make the SWR higher rather than lower.
• Lossy coax will lower the SWR on an antenna, making it look better.
• When feeding an antenna with 50 ohm coax and measuring with a 50
ohm meter, if the SWR changes with the length of the coax then
something is probably wrong with your antenna. The most common
cause is lack of an effective balun, so that the outside of the coax is
actually part of the antenna. So changing the length really is changing
the antenna.