HF Antenna Types

Antennas started out as long wires, sometimes held up by kites.
Next step was the classic Marconi setup, an antenna and a grounded
counterpoise, mounted vertically or horizontally.  You couldn't
get too much metal into the air.  You've seen the old pictures of
several horizontal wires, the more and longer, the better.  The
antenna was regarded as the aerial half of a "condenser," the
ground making up the other plate.

They were sort of right.  The modern vertical antenna, the Beverage,
and many of the other designs we see, are still pretty much all
grounded and/or terminated Marconi antennas.  Most of these can be
mounted very low to the ground, or even on it.  In fact, if very
conductive soil or salt water is available, this is the best place
for achieving acceptable radiation angles.  Hence the tendency for
AM broadcast stations to favor flat, swampy areas.

Other designs, however, have their own counterpoise.  Typical is the
wire or metal dipole, which works best when suspended completely
free of the ground.  Most rotatable "beam" antennas are arrays of
dipoles with different phasing arrangements, so these, too, work
better high off the ground.  Finally there's the ground-plane
vertical and all its variants, where a metal screen or set of
radials stands in for the missing ground.  These antennas are common
on CB.

But all this is about transmitting.  What about listening?  Well,
the "reciprocity" principle means that the effects described in
antenna textbooks for transmitting will similarly affect receiving.
For example, an antenna designed to increase signal in a given
direction will also hear better this way, but will have to be
turned or phased for maximum signal from all other directions.

All but short, active antennas are supposed to use the resonant
tuning so well investigated by people like Tesla, where the
antenna length bears at least some relation to the wavelength
of the desired signals.  This means that HF antennas are rather
large.  It could be worse.  Some military VLF antennas have to
be several miles/km long, and towed by aircraft.  On ELF, antenna
farms can be the size of small countries.

Resonance, of course, brings up the first buzzword people use on
beginners so they'll keep thinking antenna design is a black art.
This is the dreaded "Impedance."  Well, impedance is a special sort
of resistance that takes into account the behavior of alternating
currents, but that's not on the quiz.  For us, it's just a measure
of how efficiently radio waves are transferred from the antenna to
the radio.  If the numbers match, this is good.  If they don't,
this is bad, but not as bad as it would be for transmitting, when
things smoke, burn up, and smell bad.

Of course, the typical hobby listening antenna is usually too low,
too short, or just too random to make much use of resonance.  This
is not a problem, but it can be an annoyance. As you go across HF,
about a 10 to 1 frequency ratio, both the receiver and the antenna
make huge peaks and dips in their impedance (impedance for us being
basically a measure of how efficiently the signals are transferred
into the radio.).  Thus you'll have some frequency bands where the
stations leap out of your speaker, and other places (usually your
favorite band) where they vanish into black holes. 

Enter such matching devices as baluns, coaxial sections, or antenna
tuners.  The idea here is to tune out the impedance mismatch with
various resonant circuits until the radio and antenna understand each
other and can proceed to have a good dialog.  Kind of like people.

So here's where the laws of physics kick in.  There is no free lunch,
ever, with antennas.  There's no black magic.  The tuner can't
increase the performance of the antenna.  All it can do is make the
lousy frequencies sound more like the good ones.

Now the balun, which stands for BALanced-UNbalanced, is not only a
matching transformer but a converter from unbalanced (Marconi) to
balanced (counterpoised) antenna designs.  For example, the dipole
is a balanced antenna, or at least it's supposed to be, but most
decent radios use unbalanced coaxial cable with a grounded shield
side to cut noise.  While no law (including those of physics) says
you can't just connect the coax shield to one side of the antenna,
this means that it's not a dipole any more, but a kind of a funny
bent Marconi, with the coaxial shield very much part of the antenna,
and so you get back some of the noise.

An aside: how can the shield be grounded to the radio, and then to
a water pipe or whatever, and still be part of the antenna?  Skin
effect.  The common-mode noise on the outside of the shield goes to
ground, or at least it's supposed to, and the RF on the inside
doesn't.  Neat, huh?

Baluns, though, have also become a bit overrated.  Some wire antennas,
like the T2FD or the Beverage, need special ones with a 9:1 matching
ratio.  Since these are pretty darn good antennas, they hear like
champs, and their baluns get too much of the credit. Like the tuner,
it's not a magic snake oil.  Also, there are good baluns and lousy
baluns.  Lousy ones, usually those with poorly-designed ferrite cores,
don't do anything except enrich the company selling them.

The only other antenna mumbo-jumbo is the decibel (dB).  This is a
ratio.  That's all it is.  One Bel, named for Alexander Graham Bell,
a very bright man who started the National Geographic Society, endowed
several schools for the deaf, and by the way invented the telephone,
is a 10:1 ratio of anything.  Your taco could be 10 dB, one Bel,
hotter than the mild one, in Scovill heat units or however they
measure these things.  You would have a ten dB taco ratio.  Your
taco could be advertised on the banner out front as a 10 dB taco.
(This could be a tacoBel, but I am not going there, nor did I make
this one up, so don't flame me!)

You can see why spec writers are in love with dB.  It creates the
impression that they know something.  Often, that's about where it
ends.  For hobby antennas, the published gain spec means that the
advertising department has asked engineering how much Jalapeno its
taco has, and they've sent back a number based on calculation from
a theoretical isotropic source.

An isotropic source, now, is an antenna with absolutely no direction.
It's so wretched that it only exists in theory.  You can't build one,
because even the structure holding it would radiate/hear better.  The
closest electrical analogue is a light bulb.  Light bulbs don't hear
much, though actually it is possible to transmit a radio signal on one.
This is the only "antenna" that will ever show a LOSS relative to the
theoretical isotrope, since quite a bit of the RF goes to light the bulb
up!  That's right.  You'll always see your lousy antenna in the dark.
Airplanes won't hit it.

If the gain spec shows the number in dBi (Taco Bells to an isotrope),
this is what's up.  Sometimes you see dBd (tacos to a real dipole in
"free space," whatever that is), but that's usually for transmitting.
At least one radio magazine, QST, makes the advertisers cover up the
gain specs (sometimes with a magic marker).  I would regard any dB
figures for antennas as a logarithmic quantity indicating how badly a
company wants you to buy their antenna, unless it's verifiable from a
real-world test range.

My own personal bias is toward wire antennas for listening.  If you
have the space, they get the most bang for the buck.  They get a lot
of RF voltage going, and they can be extremely simple and inexpensive.
The only drawbacks are size, and sometimes noise.

Here are descriptions of several of the hundreds of antenna types.
Antennas are the most fun part of the radio hobby.  Good luck, and
watch the power lines!

==================================================================

Active antenna-	Any of several types of short spikes, coils or
loops that use sensitive RF preamps to make up for the tiny amount
of signal, weak voltage fluctuation really, that gets picked up.
These can vary from horrible to pretty good, and can achieve a huge
saving of space. The laws of physics continue to assure that nothing
beats a nice wire, but the gap is becoming ever smaller.  These are
receive-only, of course, with transmitting having expensive
consequences.

Adcock array-	A four-antenna setup used for direction finding by
comparing signals at the different points.  Unlike a loop, it need
not be rotated.  Antennas can be anything from verticals to huge
rhombics.

Beverage-	You'll think I'm drinking same when I describe this
antenna, but it's a real design, named for its inventor.  It's a
terminated, Marconi longwire for the lower frequencies.  The good
news is that it can be made of just about anything that conducts,
and works best near or on the ground.  The bad news is that the best
ones are at least a mile (1.6 kM) long, and preferably way more than
that.  People in rural areas, where these antennas can be run atop
long fences, and so on, have heard standard, commercial, AM
broadcasting stations from halfway around the world.  A recent QST
described a smaller Beverage for HF.  It was an interesting
article, and the antenna could actually fit a very large back yard.

DDRR-	Stands for directional diversity ring radiator, a special case
of the horizontal loop that makes a very efficient, low-profile antenna
perfect for car roofs on VHF.  A bit large on HF.

Delta loop-	A three-sided wire antenna, usually mounted vertically,
often in pairs to achieve gain.  They are popular on the lower end
of HF, as they outperform dipoles without needing huge spaces.

Dipole-	Simple wire antenna composed of two tuned segments, usually
connected to the radio in the middle, often through a balun, an
balanced/unbalanced transformer, since a dipole is a balanced antenna
and coaxial feed systems are not.  It's possible to think of the dipole
as two tuned monopoles in doublet configuration without ground.  It's
possible, but confusing.  It does, though, explain why it's also
called a "doublet."

Dipoles get a bad press because they are hardly ever strung high enough
to get optimum performance either listening or transmitting on HF.  A
dipole in what engineers call "free space" is actually a pretty good
antenna, with a low pickup angle and a figure-8 pattern slightly
favoring signals arriving perpendicular to the wire.  A sharp null
appears at either end, which is sometimes useful for getting rid of
nearby interference.  Some very good DX is heard on simple dipoles.

Unfortunately, free space on HF requires very tall mounting supports.
The whole antenna has to be a certain fraction of the lowest wavelength
away from anything conducting electricity - other antennas, buildings,
even the ground.  This works out to something like 300 feet/100m at the
low end of HF - ouch!  And so the dipole's performance degrades, with
more of the pattern going straight up (good for space men) and less
efficient a match.  At some point, loops or vertical antennas become
better uses of the same labor and money.

Discone-	A wideband antenna consisting of a metal disk atop a
cone, which achieves a 10:1 frequency bandwidth, say from 3 to 30
MHz.  This is a real interesting antenna.  The cone resonates the
disk, using  more and more of its surface until finally the
whole cone is involved.  Above or below the cone's bandwidth, the
performance degrades.  While the discone has been described as an
antenna that performs equally mediocre on all bands, this is in
many cases better than one that works real well on one band and not
at all on the others.  Discones are real hot scanner antennas on
VHF/UHF, they are also popular with the military, and we even see
wire ones on HF, often folded to reduce the otherwise great height.

Dish-	HF dishes, in which a parabolic reflector concentrates great
signal levels onto a dipole or voltage probe mounted at the focus
center, are possible, but prohibitively large.  Typically, HF
radiotelescopes use phased arrays.

Fan dipole-	Several dipoles cut for different frequencies, fanned
out from a common center feed point, thus saving space and complexity.

Folded dipole-	This is basically a dipole that has been folded
back upon itself, using parallel wires with insulated spacers.  Not
only does the antenna become shorter, but its impedance changes,
often for the better depending on application.  Most people know
these antennas from the little twinlead dipoles provided with
consumer FM tuners.

On HF, the folded dipoles are very popular with the military, due
to their ruggedness, portability, and adaptability.  They can be
hung from flagpoles as an inverted V, stretched out straight, or
sloped, depending on the tactical need of the situation.  The classic
military dipole is by Barker & Williamson, and you can buy one too,
for a price.

Fractal antenna-	I don't know much about these except that they
exist.  A fractal is a self- similar curve.  By folding back on
itself in a manner that replicates the original curve in ever
smaller scale, it concentrates more surface area in less space than
was originally thought possible in this universe.  In plain English,
fractals grow to incredible lengths, without necessarily getting any
bigger.  This means you can get a hell of a lot of wire onto a small
circuit board, a "smart card,' or whatever your restricted-space
application is.  The wire interacts with itself, of course, and so
you don't really get a fractal dimension on RF.  This is not a Star
Trek subspace radio, but it works, up to a point.

Helical antenna- There's suddenly a lot of interest in this design
for HF.  The more common use we see every day is on VHF radios, where
a coil inside a flexible casing makes the popular "rubber ducky"
antenna.  However, several designs for HF antennas wound on broomsticks
or inside concentric, plastic pipes have been shown to receive extremely
well, with a better signal/noise than wires.  Both active and passive
designs exist.  You can spend a fortune, or roll your own from plans on
such Internet sites as dxing.com.

Inverted L-	Simple wire antenna popular for shortwave listening in
a limited space.  The several variations all share the basic, and well
named, shape of a wire running vertically up a wall (or whatever) and
then horizontally to a remote support.  Main advantage is the large
signal pickup area (aperture), and extremely low cost.

Inverted V-	Descriptive name for an upside-down vee, really a
drooping dipole where the ends are lower than the middle.  If done
right, the inverted V saves space while improving on the low dipole's
angle of radiation, wasting less signal.  The pattern begins to
resemble that of a vertical, but without needing to bury radials.
It also lowers the feedpoint impedance of the antenna to something
closer to the 50 ohms desired for most equipment.  However, it's
tricky to get all this right, and most people don't.

Inverted Vees are popular for multi-antenna installations where a
rotatable yagi or log periodic is mounted on a tall tower for the
high end of HF, and a long Vee is hung with this tower in the center.
I used to have something like this for 80 and 40 meter amateur, and
it worked out very nicely.

J-pole-	Descriptively named vertical dipole with a tuned stub at
the bottom, more popular at VHF than HF, where these get rather
large.

Log periodic-	Antenna typically using many phased elements of a
gradually decreasing size to achieve gain over a wide frequency
range.  It's hard to explain, but the clever phasing arrangement
makes only a few elements actually contribute at any one frequency,
but the many different lengths mean more frequencies with real gain.
The standard TV antenna is a VHF/UHF log periodic. HF log periodics
are sometimes rotated on large booms, or fashioned from wires between
poles.  The TCI company makes some real killers for HF broadcasting
stations.  No more going outside in the snow to throw antenna
switches.

Long wire-	Technically, a wire antenna so long as to lose all phase
cancellation effects on a desired frequency, becoming something of a
traveling-wave antenna and getting awesome signal/noise ratio via
this feature as well as through its enormous signal pickup aperture.

At HF, though, this is very long indeed, sometimes thousands of feet/
Hundreds of meters. Shorter wire antennas of random length, while
Usually called longwires, are technically random wires, because they
are still showing resonant peaking/cancellation over the HF spectrum.
Often, an antenna tuner is used to improve HF randomwire performance
by filling in the antiresonant dead spots.  It does nothing for the
peaks.

Loop-	Basically a loop of conductive material, resonant at or near
the desired frequency. They can be made of wire or, at higher
frequencies, metal hoops.  Vertical loops have a sharp null (lack
of signal) at their short sides, making them popular for radio
direction finding.  The same null makes the loop a good choice for
DXing crowded AM broadcast bands, as it can get rid of unwanted
stations from other directions.  (Nothing gets rid of stations in
the same direction except an anti-radiation missile.) Old AM loops
were air core, with many turns of wire on the large wooden form
seen in radio museums.  Today, they are wound on ferrite rods or
toroids (donuts), and remain very directional.  Loops don't have to
be round.

More practical for HF listening is the horizontal loop, which is a
full-sized loop antenna all the way around a backyard, or wherever.
this gets a lot of wire into the air, with very little directionality,
making a good listening antenna.

Noise antenna-	These were real popular on VHF mobiles in the tube
era, when car engines were much noisier than now.  The idea is that
a second antenna is optimized for noise pickup, as opposed to signal
pickup (voltage probes are perfect), and this noise signal is combined
out of phase in the receiver to create a filtering effect.  This has
been tried on HF, and it works like a charm, but it remains a design
road not taken.

Phased array-	Historically, these were multiple verticals, spaced
and phased by different-length feeders, to achieve either broadside or
end-fire gain.  AM broadcasting stations still use these, due to the
enormous antenna sizes required.  A newer type of phased array uses
many small antennas mounted in the same plane, and fed by precise,
often computer-controlled, networks of delay lines.  On HF, these
are popular mostly for research.  Finally, it's perfectly possible
to phase vees, loops, yagis and log periodics on HF.  Smaller
arrays can still be rotated, and they look cool.

Quad-	A four-sided loop antenna, with a balanced feed at the bottom.
Quads can be arranged in parasitic arrays on the same boom, like yagis,
in which case they perform better, though at a considerable cost in
size, and general maintenance hassle, especially in icy climates.  The
quad is better transmitting antenna than the yagi, especially on
the higher end of HF, where it's a practical size.  3- and 4-element
quads are popular with CBers and freebanders who want to get serious
about radio.  If you see a vertically polarized quad (diamond shape)
on HF it's probably one of theirs.  Horizontal (box shape) is used by
hams.

Quagi-	A quad with a square reflector and several directors,
combining characteristics of a quad and a yagi.  Popular on UHF, but
a bit large for HF.

Random wire-	The typical shortwave hobby antenna, just a wire
stretched out until there's no more room, thus a random length. Wire
antennas get lots of signal.  Sometimes a simple tuner/preamp helps
these, other times it just increases noise.

Rhombic-	Large wire antenna, shaped vaguely like a diamond, and
hundreds of feet/meters long on lower frequencies.  Typically
suspended from telephone poles, and capable of awesome signal gain on
HF, especially when terminated.  Old rhombics were matched through
ominous coils, preferably with a fire extinguisher nearby, as these
coils could really heat up at higher transmitter output powers. Full,
global, multi-frequency coverage requires many acres/hectares of 
rhombics, but if one has the real estate the antenna still delivers
surprising bang for the buck, easily outperforming yagis or just
about anything else short of Sterba curtains.  The rhombic is also
a good listening antenna, sometimes allowing a layer of stations that
you never knew were there.  Four rhombics in an Adcock array made up
a very nice, if very large, direction finding setup, though
Wullenwebers are used for this now.

Rubber ducky-	A coil, usually wound on a vertical pipe, whose name
comes from the flexible antennas popular on VHF walkie-talkies.  Yes,
you can make an HF rubber ducky.  I saw one wound on a 2-liter drink
bottle once.  With a lot of attention to matching, and stern words to
all bystanders about getting too close to the thing, you can even
transmit.  But, with some really cool loaded verticals available, why
would you want to?

Sense antenna-	A sense antenna is one, usually an omnidirectional
vertical, that is combined with the signal from a sharp loop to resolve
the otherwise ambiguous double null that is created in direction
finding.  In other words, it means that your bearing is the right one,
and doesn't have a 50/50 chance of being 180 degrees off.

Sloper-	A dipole arranged with one end near the ground, and the
other one higher than it would otherwise be.  This lowers the dipole's
otherwise high angle of radiation at any practical mounting height, and
and can also sometimes change the impedance match to something closer.
Four slopers around a pole or typical amateur tower give a nice,
directional setup when the desired one is connected. Such arrays are
common, though they need a lot of open space to work well.

Sterba curtain-	Enormous, complex, stacked, phased array, often made
unidirectional with a mesh reflector, and usually hung from two or
more very large towers.  Antenna gain on HF rivals that of rhombics,
in less horizontal space (vertical space is easier to come by).
Curtain antennas are very popular with super-power HF broadcasters.

T2FD-	A wire antenna vaguely resembling a sloped, folded dipole,
but fed through a special balun transformer.  This is a good
listening antenna, because it tends to reduce some of the noise
relative to simpler wires.  A Dutch company makes a very nice, and
very expensive, listening setup which is an active T2FD on steroids.

Trailing wire-	Aircraft have used these on HF in the past.  The
classic has to be the seven-mile version that a Navy TACAMO (Take
Charge And Move Out) command post airplane can unreel to allow
VLF transmissions to submarines - while the subs are under water!
(Water, especially salt water, shorts out radio waves really well,
except at ridiculously long wavelengths.)  They don't use this long
version over land, for obvious reasons, though in a nuclear war
they would. They figure that not too many people would care about
falling wires at that point.

Vee-	Basically a half-rhombic, with two wires arranged in a V
shape and either aimed in the desired direction or mounted
vertically for omnidirectional coverage.

Vertical-	Any of a class of several antennas that use a tuned
wire or pipe oriented vertically to the ground.  The simplest is a
grounded Marconi, a half- or 5/8-wave monopole with a counterpoise
of radial wires buried in the ground.  On the low end of HF, the
antenna becomes more efficient, and easier to tune and match, as the counterpoise grows, often into acres of buried, expensive copper.
At the high end of HF, "ground plane" verticals become small enough
to use metal radials and take their counterpoise up the tower with
them.

The vertical has the seemingly impossible ability to achieve gain in
all compass directions at once, explaining the ads for "gain verticals"
in ham radio magazines.  It's not black magic, just the result of the
antenna's vertical orientation, allowing the beam width to concentrate
nearer the ground (where the people are) and less in space (where they
don't use HF much anyway).  When everything's perfect, the vertical can
achieve a very low beam, giving nice DX reception all directions at
once.  Better still, the thing sure takes up less space than a field of
rhombics.

Unfortunately, unless you live on a gently sloping hill of salt water,
it'll only be absolutely perfect on a small boat in the ocean.  Worse,
it'll only be just right on a narrow band of frequencies. This is not a
problem for the local AM station, which never changes frequency, or at
least better not.  It's a problem for us, though.  Various traps, coils,
and top-hats are often added to verticals, but it's still not as broad
an antenna as some of the alternatives.

Worse, HF verticals see an effect from soil conductivity, for several
miles around the antenna in some cases.  The counterpoise won't change
this much.  In sandy soils, the vertical deserves its reputation in this
hobby as not a very good antenna. Many people will also tell you that the
vertical gets more powerline noise.  I haven't found that to be the case
here, but I can't speak for everybody.

Voltage probe-	Simply a metal spike stuck into the RF field, the
way you'd stick a wet finger into the wind.  The feeble voltage
fluctuations thus picked up are typically sent to hot FET preamps/
matching circuits, or used to this advantage by the hot front
ends of modern radios.  The whip antenna on the typical consumer
"World Band" receiver is most definitely a voltage probe, as are
small, active antennas marketed under such names as "World Probe."
Decent voltage probes can be an alternative to hanging 15 feet/
5 meters of wire from hotel windows. :-)

Windom-	Off-center fed, broadband, wire antenna named for
inventor Laurent G. Windom, who described it in a 1929 QST.  The
original Windom used a single-wire feed and a ground counterpoise,
developing an impedance that remained near 600 ohms on both odd and
even harmonics.  In other words, it was cut for the lowest ham
band you wanted to use, and all the other harmonically-related bands
fell into place.

Modern "Windoms" are usually 2-wire coaxial feed through a balun,
and without a ground counterpoise.  This makes them more like Zepps.
While the "Windom" name is wrong, it often sticks.

Wullenweber-	A circularly disposed antenna array, used for
direction finding on HF, that was invented in World War II.  From
a distance, one sees this huge, round collection of poles and wires,
the low-frequency end of the array.  Closer up, the rows of smaller
wires and spikes come into view.  From the air, one sees something
that looks very much like Stonehenge, and in fact Wullenwebers
are among the easiest man-made objects to spot from space.  The
antenna is sometimes called an "elephant cage," though the scale
more resembles a Godzilla cage.  These are very popular with the
government and military for High Priority Intercept, real spook
stuff, and the classic one is at Imperial Beach, CA, where the US
Navy trains crypto types.  It replaced a rhombic farm, four of them
in Adcock config, but actually it's not all that much smaller. It's
visible 10 miles away.  Directional bearings are obtained by
comparing signals as they enter the different segments of the circle.

Yagi-	Directional antenna, really a Yagi-Uda, as it is named for its
two inventors.  It uses the clever principle of parasitic excitation,
where one electrically hot dipole is phased with a passive reflector
and any number of passive directors.  All these are dipoles of just
the right electrical size and spacing to achieve gain by narrowing
the pickup and radiation beam and concentrating on one direction in
all 3 dimensions. Yagis are thus often referred to as "beam antennas."
Like log periodics, they can be mounted on a rotating boom or strung
with wires.  Yagis are popular because of their size, with a chief 
disadvantage being the very narrow bandwidth.

Like dipoles, yagis have to be away from conductive objects to work
as well as they did when measured on the antenna range.  The 50-60
foot (20 meter) metal tower with its multiband yagi on top has become
a symbol of ham radio.

For shortwave listeners, the yagi makes sense mostly as a second antenna
for one band (say 12 MHz marine boating) if and when you need to hear
absolutely everything out there.  Otherwise, it's probably too narrow.

Zepp-	Off-center, wire antenna with good frequency coverage, named
from its early use on Zeppelins.  Various types and phasings exist,
all the way up to the awesome "extended double Zepp," a very large
antenna array pair with bandwidth approaching that of the discone
and gain approaching that of the rhombic.

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