Long wire antenna random wire

 

Related Pages:

Groundplane
Verticals (they
are generally
end-fed 1/4 wave
radiators)

Verticals
and baluns  

End-fed
half wave

J-pole

Counterpoises

 

Technically a true “longwire” needs to be
at least one
wavelength long, but
Hams commonly call
any end-fed wire a longwire or random wire antenna.

Long wire or
random wire antennas
are very simple antennas. They can come close to half wave antennas in
efficiency, although efficiency decreases as they are made very long or
installed closer to earth. Like every antenna that exists, random or long wires have
advantages and disadvantages.

Because the radiating area is often brought into or near the operating
position, longwires often create RF
interference to consumer goods or RF
in the operating
room. The easy
installation part
comes from generally
needing only two
supports, and not
having a heavy
feed line hanging
from mid-span like a
dipole. The
long expensive feed cable
normally associated
with a doublet or
dipole is not
needed, the antenna
wire itself serving
as a “feed line”.

End Fed Halfwave

The end fed 1/2 wave antenna, much like the 43 foot vertical,  has
become a cult phenomena.

 

This is the feed impedance and SWR of an end fed 80 meter half wave with an
ideal 49:1 transformer and good stable counterpoise or ground. The black dots
represent exact harmonics of the 3.57 MHz fundamental resonance. The actual
resonance, 49:1 transformed SWR at resonance, and the antenna impedance at
resonance are shown:

1. Resonances in open ended antenna will not fall on exact harmonics.
   Resonances tend to fall higher in frequency. This is because of end effect.
2. Impedance is highest on the lowest band. Note it is 6850 ohms on 80
   meters, decreasing to 1360 ohms on ten meters. This is because more and more
   half waves are being fed from one feedpoint with increasing frequency.

  In a low loss system with good counterpoise, with a very good 49:1
feed transformer, in-band EFHW SWR is:

Freq 3.57MHz	7.33 MHz	11.12 MHz	14.86 MHz	18.66 MHz	22.41 MHz	26.12 MHz	29.9 MHz
SWR 2.82:1	1.59:1	1.10:1	1.33:1	2.05:1	2.08:1	1.91:1	1.81:1
Imp 6850 j0	3920 j0	2665 j0	1860 j0	1210 j0	1180 j0	1285 j0	1360 j0
In-band SWR 2.82:1	3:1	10:1	5.1:1	3:1	9.8:1	10.2:1	10:1

 

  

 

 

The antenna itself works just as well as any other wire of similar height and
length. Any or all problems are in the counterpoise and feed system. The difficult
problems associated
with random wire or
long wire antennas
are caused by ground currents
and radiation from the single wire feeder.

End-fed antennas, or antennas with the single wire feeder brought into the
shack, come with a little misconception. One
commonly repeated
myth or “theory” is
that half-wave
antennas, being
resonant, do not
require a
counterpoise, or that
some magical length of antenna will prevent RF in the shack. This does not mean
the antenna will be worthless and not make contacts, it simply means something
else replaces the missing counterpoise area and we also bring RF fields right
into the shack. The feed line, as well as everything connected to and surrounding
the single-wire feed line and counterpoise, becomes part of the radiating system.
This creates three potential problems:

1.) The feed line, mast, and things around the feed line connect through the
antenna into the receiver. This brings noise into the receiver.

2.) The feed line, mast, and things around the feed line become part of the
radiator. This brings voltage (electric fields) and current (magnetic fields)
directly into the shack.

3.) The feed line and grounding affects SWR and tuning.

Since we often do not have a baseline for noise, unnecessary additional noise
will often go unnoticed. The remaining two issues are more likely to be noticed,
but only if we run enough power to cause RF burns, power supply shutdown, or
other forms of RFI.

Transmitter power levels, feed line length and routing, and the
susceptibility of equipment to RF problems greatly influence things we most
likely notice. This is why some people (usually with QRP power levels)
swear by end-fed half-waves, while others (usually with higher power) avoid
end-fed antennas. The reason
for that is simple,
end-fed half waves
have common mode feed line current problems affecting their
performance, and
these common mode currents cause inconsistency
in
user satisfaction.

In nearly all cases, if we notice it or not, an inadequate counterpoise hurts
antenna pattern and efficiency. This is why high power stations often have more
efficient, more ideal, antenna systems. Higher power very often excludes use of
power wasting systems, because the wasted power often creates significant local
problems. If 5% of 10 watts is exciting the desk with RF, it isn’t any big deal.
If 5% of 1500 watts excites the desk with RF, the result can be hazardous.

I wouldn’t have a problem with a 1500 watt transmitter into a longwire
antenna with a tuner remote from the shack and house. I would likely have a
fire, or damage equipment, if I brought a single-wire feeder into the house!
With 5 or 10 watts, I wouldn’t care.

How the Longwire or Random Wire Antenna Works

The antenna element works the same as any other antenna.
Electromagnetic radiation comes from
current flowing over a spatial distance along the wire.

The single wire feeder not only radiates electromagnetic energy, it has very
strong


electric and
magnetic induction
or energy storage
fields
surrounding the wire for some distance out from the wire.

In order to force
current up into the
feed wire and
antenna, the
matching or feed system has
to “push against”
something else. For
every milliampere of
current flowing
into the feed wire
of the longwire
antenna, an exactly
equal current has to
flow into a ground
system of some type!
In any non-terminated antenna, currents and voltages are transformed along the
antenna. This transformation is caused by standing waves. This means ground lead
currents can increase or decrease along the ground wire and everything connected
to the ground wire or ground system. Voltage changes also along the ground or
counterpoise system, just as it does in antennas. The voltage caused by antenna
return currents, and the return current, will become stronger (more intense) or
weaker (less intense) because of standing waves on wiring and equipment cases.

These ground
currents,
displacement
currents, or common
mode currents cause
everything connected
to the matching
system to become
“hot” with RF. The result is
generally all sorts
of RF interference
to active devices or
even physical harm
to the operator,
such as actual burns
or on lower bands
like 160-meters…..
electrical shocks! These
unwanted but very necessary currents ideally should flow through the lowest impedance
path and widest area path we can manage.

Noise from Longwire Antennas

Radiation and fields surrounding the single wire feed system not only leak
out, unwanted noise and signals can also leak in.  Radiation from
the feeder and
everything connected
to the matching
system, as well as
common mode
currents, also
allows


common mode noise
ingress.
This deteriorates
receiving system
noise performance.

Common mode
currents and
induction field
coupling also
decreases
transmitting
efficiency. This effect adds
unnecessary loss to
the system.

If things are
just right (like
having a fairly good
RF ground in the
shack) and power is
fairly low, we can
often “get away”
with having a random
wire or longwire
brought directly into the
shack, but that is
more a matter of
good fortune than
good planning.

Curing RFI and Reducing Noise, and Optimizing Performance, with
Single Wire Feeds

Repeating and expanding on what was said above, radiation
and fields
surrounding the single wire feed system not only leak out, unwanted noise and
signals can also leak in. 
This is an
unsolvable problem
with a single wire
feed.
The very best we can do
is relocate these
problems to an area where they cause no noticeable problems. We can do this by
relocating the
feedpoint. Relocating the feedpoint can move
strong magnetic
and electric fields
away from the
operating position, house wring, consumer devices,
and our sensitive
equipment. This reduces noise into the antenna feed system, and RFI caused by
the antenna feed system.

Method 1, grounding

Ground rods have limited
effectiveness, except perhaps on very low frequencies.
This is not saying we should not have ground rods, but rather depending on them
for RF grounds is not a good idea. The RF
ground should if
possible contain
radials or
counterpoise wires
and not just ground rods.
Ground rods do very
little good for RF!
As a matter of fact
adding ground rods can decrease RF efficiency when
an insulated
counterpoise is
used.

With fairly low power and good luck, one way to mitigate noticeable problems is
a ground plane at room level. This ground system
can be as simple as strips of
foil laid under a
carpet. These conductive wide strips of foil would then be connected
back to a wide
station equipment
ground buss. Stained-glass hobby suppliers sell adhesive backed copper foil that
works very well, and is easily soldered. Another choice is aluminum foil, but
this requires pressure connections that tend to be less reliable over time. As an alternative to copper foil strips, a
metal screen or
grid of wires at floor level can be used. This system can be right under the
carpet, or can even be directly below the floor. it is important whatever is
used, that is have a low impedance RF connection to the cabinets or chassis all
desk equipment.

This type of counterpoise system makes
the entire room, including the operator, “rise”
in voltage and match
the equipment
chassis voltages. It also
disperses or spreads
the current and
voltage around, reducing intensity of localized electric and magnetic fields.
This is very effective for a second floor (or higher story) RF ground. Like
standing on a metal plate carrying high currents, there is little potential
difference between different areas of the high-conductivity counterpoise sheet.

 

 

The antenna lead
from the tuner
“longwire” terminal
to the antenna should parallel and
be reasonably close
to the outside
ground lead, if
possible. This forms a two-wire transmission line, which helps to reduce
external fields. the single wire feeder should
be kept away from
the operator and
RF sensitive equipment. The
lead to the longwire
antenna should be
as short and direct as reasonably possible, because the feed wire is a leaky
transmission line.

The grid of foil, metal screen, or wire grid does not need to be too dense. Below 30 MHz, spacing
conductors one
to two feet apart is nearly as effective as a solid sheet.

If we can’t do that, then sometimes a 1/4 wave counterpoise along the
baseboard will work. But it is fart better to disperse the current in a widely
spread-out path, making all areas of the operating area have similar RF
potential without concentrated currents.

 

 

 

also see desk grounding

Method 2, Keep Problems Outside

The best method
of taming a long
wire is to install a
good low-loss
current or choke
balun just outside
the operating room.
This effectively puts distance between the leaky feeder and radiating ground
leads outside and away from sensitive equipment.

Basic Simple System   

A system like this
is ideal for a minimal investment unobtrusive counterpoise.

This system, even with a minimal RF ground, keeps
common mode currents
out of the operating
area. This system reduces
noise and RFI. It
generally eliminates
the need for a shack
“floor groundplane”. The balun must be a reliable current balun with
high common mode impedance. A voltage balun, or a single core 4:1 balun, will
make things worse.

The counterpoise
can be a ground
system like radials
instead of a single
counterpoise. It
just cannot connect
back to the station
entrance ground, or
the balun’s ground.
If it is a single
wire or a few wires,
they should be
insulated from earth
and kept a little
distance above
earth. Ideally the
single wire
counterpoise should
be directly under
the longwire
antenna, and a few
feet above earth.
Remember the
counterpoise will
have considerable
current and voltage,
and might be an RF
burn or shock
hazard.

 

 

Because a counterpoise is less than perfect, and can even have a fairly high
impedance on some bands, the counterpoise system will try to “ground” back through the
station gear. Unun’s and voltage baluns have a low impedance path for
common mode currents, and will not isolate counterpoise currents from the shack
equipment. A current balun isolates the ground path from the counterpoise to the
shack. The balun must
be a high quality
current balun, not an unun or voltage balun.  It should be
a 1:1 ratio with
high common mode
impedance and high
voltage breakdown.
The coaxial feed line, since it operates at high SWR,
should be high
quality and as short as possible.

Ideally the counterpoise wire should be elevated above earth. This minimizes
earth losses, and the counterpoise should not be connected to a ground rod or
especially to the station ground. The required lightning and safety grounds must
all be on the coaxial side of the balun. Unless the ground system
is nearly perfect with near-zero RF impedance, it is best to keep the antenna’s ground or counterpoise
isolated from the feed line shield and station equipment.

Improving the System Above

At the expense of simplicity, a better ground will improve efficiency. A
better ground would be multiple radials, or multiple counterpoise wires. The ideal system, in which efficiency would nearly equal that of a balanced
center-fed system, would be a ground system similar to radials for a vertical.
The ground system can include existing wire fences or metal plumbing, or might
be a totally new system installed just for the antenna. A good enough ground
system, or large area counterpoise system, reduces RF voltage on the ground
terminal. If the voltage on the counterpoise or radial ground system is large
enough to present a low impedance, station equipment and the antenna ground systems
can be tied together. Isolation, such as a current balun, might not be required.
The counterpoise, in effect, becomes “less hot” with RF voltage.

Such a system would minimize RFI and electrical noise problems. With a large
system of multiple wires,
current density in lossy earth surrounding the radials is reduced.

One common misconception is a near-perfect ground needs 120 quarter-wave or
half-wave radials. This is not true. Even 15-20 radials can form a very low loss
ground. All we can do is install the best ground we can manage. The more wires
used in a counterpoise or ground, and the more spread out the conductors are, the less
critical and frequency sensitive the system becomes. The point of diminishing
returns is generally around 15-25 radials. If 40-60 reasonably long radials are
used, any further increase becomes meaningless.  A semicircle of 10-20
radials radiating outward from the feedpoint, generally following the antenna
direction, is usually good enough to make further work a waste of time. The best
systems would center equally on each side of the antenna, if possible. Small
systems should be suspended above ground, if possible, to minimize losses.

For more about end-feed systems follow the end-of-page links:

End-fed
Half wave

Counterpoises