Vertical Antenna Notes
Matt Roberts - matt-at-kk5jy-dot-net
Published: 2024-07-11
Updated: 2024-07-26
There is much useful information online about building effective
HF vertical antennas. Unlike some of the other antenna types I
have written about, the vertical is a thouroughly studied device, and
this article is a brief collection of notes and best-practices
that I have found make the best-performing antennas at my house.
These notes apply mostly to ground-mounted verticals, that use an
on-ground radial system. Some of these items may apply to
other vertical antenna types, such as those with elevated radials.
Use as many radials as you can, even if they are short.
A ground-mounted vertical antenna is half of a vertical dipole.
When transmitting, the vertical part is one half, and the ground is the
other half. When we transmit, current oscillates between the vertical
element and the ground. Since most soil is very resistive, that's not
good for efficiency.
To improve efficiency for such antennas, we lay radial wires on (or just
beneath) the surface of the ground, to lower the impedance of the ground
around the antenna, so that the current passing through the ground around
the antenna has better paths back to the feedline.
Most of this RF current is concentrated in and on the ground within
roughly a λ / 10 radius from the base of the antenna.
If we look at a model of a typical λ / 4 vertical antenna with no
loads, the current distribution is highest at the radial hub, and rolls off
sinusoidally along the ground away from the antenna, regardless of whether
there are any radials along that path, and regardless of the length of any
radials that are there. Because of this, we want to concentrate
as much metal as we can on the ground close to the radial hub.
For a given length of wire one has to devote to radials, many short radials
will outperform a few long(er) radials.
Feel free to combine long and short verticals.
This can be particularly useful for installations where there isn't room
for radials of desired length all the way around the vertical itself.
The pattern eccentricity caused by a combination of long and short radials
on different sides of the antenna is going to be very small, less than 1dB
in nearly all cases.
Long and short radials can also be helpful on a multiband antenna, as
it allows us to concentrate most of our conductors close to the radial
hub, but at the same time have some radials further into the yard for
the longer wavelengths. While resonant radials are not needed for
a vertical antenna, remember that the area around the radial hub where
the RF current is highest is determined by the wavelength.
Put down the wire you can, where you can, and don't worry about the
wire lengths too much. There is no need to make any radial longer
than λ / 4 on the longest wavelength you use with the antenna.
If you have that much wire, make more radials instead of long ones.
Use insulated radials.
Electrically, there is very little difference between using radials
that are insulated or bare. However, using insulated radials
protects the copper wire from corrosion that results from
exposure to the soil. In some places, the soil pH is
particularly harsh on metals. If you doubt this, next time
you put a landscaping staple in the yard to hold down a cable or
radial wire, pull it back up after a year or two in the ground,
and look at how much of it is missing.
I use insulated wire, sealed at both ends from water intrusion.
At the far end, I form the wire into a small loop, maybe a few inches
in circumference, which allows me to mechanically anchor the end of
each radial when installing them. This makes it easy to quickly
lay out the radial fan in the correct shape before stapling the radials
to the ground. I use a single staple to secure the looped end of
each radial first, then go back and add staples along the length of
all the radials.
Use a radial hub.
Custom Radial Hub
With Choke
I have used two different ways to get a good radial hub.
For the low bands, I use an HF2V, modified to add
30m and 160m. At the base of this antenna, I added a
DXE stainless steel radial plate.
This device works very well on the bands this antenna supports.
At right is a picture of a custom hub that I hand-built for a second
vertical antenna at my house, used for higher bands. It is nothing
more than a piece of 1/8" aluminum plate, cut to a 6" square,
then a number of holes drilled about 1" from each edge, for the
bolts. This works just as well as the DXE plate, but the cost is
much less, and I can pick the size that I want. It would be just
as easy to make a 12" or 18" hub, as needed.
The main trade-off between build and buy is cost. You can get
a nice radial plate for very little cost if you build it, but that
requires tools and time. I was able to get the local metal shop
to cut the plate for me, which is most of the effort. They get
the plate in large pieces, and they cut it for each customer, selling
it by the square foot. Marking and drilling the holes myself
was fairly easy. A
drill press
will make the drilling process a snap.
Regardless of the hub selected or built, I like to insulate the
plate from the ground, and raise it slightly above the dirt.
This is mostly about protecting the plate from corrosion.
My preference is to cover the ground under the plate with some
kind of rock or gravel, leveling the rock bed, then placing the
hub on top of the rock.
Some people like to ground their vertical antenna radials at
the antenna. I prefer to completely insulate the hub and
radials electrically from the ground. I want the current
returning through the radials and the hub, not through the dirt,
so I don't put down rods into the dirt at the antenna. I
do ground the coax shield at the house entrance, to
protect the house from near-miss lightning strikes and static
build-up.
I prefer aluminum to stainless steel for the plate itself, especially
for higher frequencies. Copper plate would make an even better
radial plate, electrically, but it would be more challenging to
protect it from corrosion. I use stainless bolts to attach the
radials. I also use anti-sieze on the stainless bolts, in case
I ever want to replace the radials. Otherwise, the dirt, mud,
and water will quickly wick into the threads and they will become
impossible to remove.
Make your own yard staples.
Some folks like to bury their radials, and that's fine. I prefer
to attach the radials to the surface of the soil with landscaping
staples. After just a few weeks of lawn growth, the grass roots
will pull the radials into the soil, accomplishing the same thing
as direct-burial, but with a fraction of the labor effort.
You can buy nice staples to hold your radials to the ground, and they
are available from just about any landscaping retailer. However,
staples are are much less expensive to get in large quantities
if you make them yourself. Here's what I do:
Go to Atwoods or your local farm and ranch store, and buy a roll of
galvanized electric fence wire. This is the same kind of wire
used to make professional "landscaping" staples, but it
is in a roll. You can buy different gauges; I prefer 17 AWG
wire, because it is easy to work with, but if you will be punching
staples into hard or dry earth, get thicker wire, like 14 AWG.
Cut the wire into the desired length. I prefer staples that
are about 6" long, which requires wires cut to about 14"
lengths. Your needs may vary. Use a piece of PVC or
a round broomstick, or some other hardened form, to bend the
wire into a "U" shape.
Use safety glasses when cutting the wire.
Use gloves when bending the wire.
Be generous with yard staples.
I recommend at least one staple every 12" — that's about
three staples per meter, for those of you in metric-land. Don't skimp
on the staples for a new installation. It is very, very easy to
space out staples to the point where your mower can lift the wire between
staples and cut it.
This goes double for feedlines run across the ground. I know hams
who skimped on the staples, only to have their riding mower lift the
coax cable and cut it in two. For quality feedline, that can be
a very expensive mistake. The way to prevent this is to use
plenty of staples when doing your initial installation (or just bury
the feedline).
I like to go a step further, and after the staples are all installed on
the radials and the feedline, I go back down each radial and feedline,
along their entire length, and gently lift the wire or cable midpoint
between the staples. If I can lift the wire, even just a little
bit, I'll put another staple down at that point.
An extra staple here and there can save you a lot of money and labor
later on.
Once the lawn has time to grow a bit over your radials, the staples
become less important, because the runners from the grass plants will
grow over the cables, and pull them into the dirt. But for a new
installation, the only protection is the staples you put down.
When installing staples, I like to turn them diagonally across the
wire or cable. Turn them enough so that both of the legs of the
staple are flush with the sides of the wire or cable. If you
put the staples normal (straight across) the cable, there is still some
wiggle room for the cable under the staple head. Turning the
staple diagonally across the wire will remove the slack and prevent
movement. It also helps reduce pinch points when used on coaxial
cable.
Use anti-sieze on all metal-to-metal connections.
Anywhere you have two metal pieces touching each other at a mechanical
joint (i.e., other than a welded joint), exposure to the elements will
quickly contaminate the joint. If this happens to a fastener, like
a bolt-and-nut interface, the two will eventually sieze up and it will
be impossible to remove the nut without a saw.
For other joints, such as between tubing sections on a typical aluminum
telescoping vertical antenna, contamination will start to cause
intermittent electrical contact, especially at RF. This will often
show up as SWR "drift" when transmitting.
The best way to fix this — or better yet, to avoid it
altogether — is to use proper
hydrophobic
coatings on all metal joints. For fasteners, there are
lubricants specifically made for this purpose, such as the
Permatex anti-seize compound.
For other joints, such as between vertical aluminum tubing sections,
I prefer dielectric grease, such as the
Permatex dielectric grease.
The latter is used on vehicles to protect spark plug and light bulb
connections from the same kind of weather contamination that we face
with outdoor antennas.
Again, don't be cheap — apply enough grease of the appropriate
kind to make sure that all of the contact area between the metal parts
is coated. When the parts are then pressed together, as when
tightening fasteners, the mechanical force will push the extra lube
out from between the metal faces that need to make electrical contact.
This will leave grease on surfaces that are not making electrical
contact, and protect the entire joint from contamination.
Use a choke at or near the base of the antenna.
Seriously, just do it. Chokes are easy to build and install,
and the performance cost of a well-designed choke is a tiny fraction
of a dB on the HF bands.
A coaxial choke, sometimes and somewhat incorrectly called a
"common mode choke," is a device that prevents RF
current from flowing down the outside surface of the coax shield
conductor, while allowing the current to flow inside the cable
unhindered.
In an ideal world, with a perfect installation site clear of all
nearby structures, a full-sized mast made from low-loss materials,
a huge radial fan with lots of copper and silver, and a perfect
match to a deeply buried feedline, a vertical antenna usually
would not need a choke. However, most of us don't have such
sites. Even contesting clubs don't build verticals like a
high-quality commercial broadcast site. So if we want the
antenna to perform well, we need to take some precautions to make
sure the RF goes where we want it, and not where we don't.
The way to do this is to use a choke on the feedline, at or near
the feedpoint. This keeps RF on the antenna and off the
outside of the coax and out of the shack. It also keeps
RF current from return currents in the soil from using the
feedline as a long radial.
A choke can be as simple as a few turns (or in the case of longer
wavelengths, several turns) of coax cable wound into a loop,
placed near the feed point of the antenna. More compact
designs use a ferrite core to get more choking reactance with
a much shorter length of cable, such as the
BalunDesigns 1115 series, or the
DXE Maxi-Core Chokes.
For most bands, a choke can be hand-made very easily and inexpensively
by using a loop of coax with an appropriate number of turns. These
can be very effective if the number of turns is chosen to match the
band(s) of operation. The
G3TXQ Chokes Page
has recipes for many different choke designs,
if you want to make your own.
The effectiveness of a coaxial choke is frequency-dependent, so choose
or design a choke for the band or combination of bands you intend to
use with your vertical.
Use a hairpin as an easy way to improve the match to 50-ohm
cable.
This is mostly for multiband or shortened verticals that present
a low Z0
on one or more bands. Over most soils
present in the US, a full-sized monoband vertical will present
a Z0 of between 36 and 50 ohms, which doesn't require
any matching for a typical modern transmitter.
Shortened verticals may present Z0 of less than
36Ω — sometimes much less. For these
antennas, a small inductor placed between the base of the vertical
section and the radial hub will raise the impedance to something
closer to 50Ω for use with typical coax cable. The
value will depend on the wavelength, the antenna, the radials,
and the ground underneath, but values less than 1.0μH are
typical.
Often, a single hairpin value can provide a good match across two
or three adjacent bands, when using a multiband antenna. The
match may not be perfect on all bands, but if you match the middle
band for the best Z0 match, the other bands will show
significant improvement, too.
Use of a shunt inductor has a nice side-effect, which is that the
antenna feedpoint is DC-shorted at the base. This offers a
modest amount of static protection, since any static build-up is
quickly drained to ground.
Use capacitive loading when possible.
Capacitive end-loading of antennas (often called a "cap hat")
is a somewhat misunderstood concept, but it is not a difficult
concept. In a nutshell, capacitive loading electrically
lengthens an antenna, so that it resonates at a lower frequency.
This makes it similar to inductive loading, but the lengthening
effect is accomplished using a different mechanism.
Ideally, a cap hat is placed at the end of one or more elements
of an antenna — in the case of a vertical, that would be
at the top of the mast. For a given physical dimension,
a cap hat provides more frequency shift if the ends of the hat
wires are connected to each other. This is nothing more
than a simple lengthening of the wire between the hub of the
hat and the point of voltage peak.
However, cap hats don't need to be ideal to be useful. The
hat can be placed anywhere along the length of the mast. The
greatest effect of a hat of given size is obtained when the hat
is placed at the end of the element, but it is usable at other
locations if end-loading isn't practical.
Textbook cap hats use wires that project from a hub at right angles
to the main antenna element, but they don't have to. They can
sag, droop loop, or overlap, if needed. It is not unusual for
large verticals to (re)use guy lines as cap hats, for example.
For a given length of mast, a capacitive load almost anywhere
along the length of the mast will provide a more efficient
antenna overall than a base-loaded inductor. It will
also provide a higher resonant Z0, all else being
equal. This is accomplished by increasing the RF current
integrated along the length of the mast itself. In other
words, the sum of ampere-meters is higher.
Even if the cap hat wires hang down parallel to the main mast,
they can still be very effective. The reason is that
the ascending current in the mast is higher than the descending
current in the hat wires. While the cancellation of
the two currents is not zero, it is relatively small.
The gains in integrated current along the main mast is
still improved by such a hat.
Experiment, experiment, experiment.
This applies to all amateur antenna systems, and not just
verticals.
Be willing to read up on different design techniques and
try different things. But regardless of what anyone
else tells you, the best vertical antenna design is the one
that works best at your house. The only way to
know for sure what works best is to build and test.
—
Copyright (C) 2024 by Matt Roberts, KK5JY.
All Rights Reserved.
