SUBJ: Getting Your First Antenna Up


The following was originally posted as a series of messages to a new
ham who lived in an urban setting.  The same questions seem to keep
comming up, however, so I though I would load it in the data library
for anyone who might find it handy.


There are several possibilities for getting you on the air from an urban
rooftop.

The easiest would be to get a Cushcraft R-4.  This works on 10, 12, 15,
and 20 meters, and since it is a half-wave vertical which requires no
ground plane you can just put it up on a short pole and expect it to work
pretty well.  I haven't used one but it has a very fine reputation.  The
trouble is it costs $250.

Next easiest is a commercial quarter-wave multiband vertical, of which
there are a number on the market for around $100.  They are available in
3 band models for 10, 15, and 20 meters, and 5 band models which add 40
and 80 meters.  The 5 band models are considerably more expensive and
performance on the lower bands is a bit dubious.  They do work however.
These antennas require a ground plane extending about a quarter
wavelength from the base of the antenna.  If you have a metal roof you
might have it made, but most people have to run radials.  These are
horizontal wires running out from the base of the antenna.  There need
to be at least three wires for each band and more is better.  The length
in feet is 240 divided by the frequency in MHz.  That's about 63 feet on
80 meters.  You can cheat in various ways, but anything less than 3
wires running straight and horizontal at equal angles will degrade
performance.

Ground plane verticals have a reputation for picking up local electrical
noise which might be a serious problem in your area.  I don't know how a
half-wave like the R-4 would compare in this respect.

The next possibility would be a commercial multi-band trap dipole.
These are wire antennas that run from about 50 to 100 feet in length,
work on all bands, and cost from $50 to $100.  The trouble with these is
that they have to be kept away from obstructions, especially anything
that acts like a ground plane.  If you do NOT have a metal roof, you
might be able to string one from a couple of short poles.  A metal roof
would be a problem.

Dipoles are somewhat directional.  They have a four-leaf-clover pattern
which is strongest about 30 degrees off the ends of the wire.  They have a
strong null along the line of the wire and a less pronounced null
broadside to the wire.  This means you may be able to reduce your noise
level by orienting the antenna in a particular direction.

Finally, you can build an antenna.  A single band vertical is not
difficult, but much more trouble than a dipole or one of its varients
(which are also the cheapest of all antennas).  Antennas for the high
bands are likely to be more practical in your location.  A 10 meter
dipole is only about 16 feet long and needs to be 8 to 16 feet above the
ground or ground plane.  Such an antenna can be built in rigid form out
of something like wiring conduit if desired.  Then it can be stuck up on
a single pole and it is easy to experiment with rotating it for optimum
results.  Or it can be of extremely simple wire construction.

If you can put up a single fairly tall pole or have something like a
chimney to work from you can put up a dipole varient such as the "Inverted
Vee".  This is just a dipole with the wires drooped down from the center
at about 45 degrees.  The ends can come right down to the ground or roof.
It is less directional than a horizontal dipole, but many people think it
works better, especially for DX.

A dipole is just a more or less straight conductor, a half wavelength long
at the desired frequency, broken at the center to attach a feedline.  The
length in feet of the conductor is usually given as 468 divided by the
desired frequency in megahertz.  When the antenna is made of wire, the
ends will probably be looped or twisted around an insulator for support,
and it is important to understand that the formula gives the maximum
extent of the conductor in space, not the amount of wire used.  The parts
which are doubled back to form attachments don't count.

The formula is also an idealized approximation.  The correct length of the
antenna will be somewhat different depending on the type of wire used, the
method of attaching the ends, the height above ground, and the presence of
obstructions in the area.  Therefore the thing should be cut to come out
at least a few inches longer than the formula specifies so that it can
be trimmed experimentally.  The correct length is quite critical.
Non-insulated wire should be used where practical.

The simplest insulator for the ends of the wire is just a small rectangle
of flat plastic with a hole drilled near each end.  The wire is passed
through one hole and twisted back on itself.  A cord or rope is tied
through the other and led off to the supporting structure.  The wire is
also cut at the center and a similar insulator used to tie the two halves
together.  Some details are shown on pages 8-22 to 8-24 of the ARRL
Antenna Book (14th edition).

The feed should be 50 to 55 ohm coaxial (shielded) line.  The type
designated RG-58 is the nicest to use since it is small, flexible, and
relatively cheap.  If you have a run of more than 100 feet or so, it may
be worth considering the larger RG-8 or RG-213, but they are much more
expensive and much more trouble to work with.  They have lower losses for
long runs and will handle high power levels.

Don't confuse RF coax with audio cable; it aint the same thing.  Also, the
coax used for cable TV is 75 ohm and will not work as well although it is
not out of the question to use it.

The best way to connect a dipole like this is to use a commercial 1:1
balun transformer at the center of the dipole.  "Balun" is short for
"Balanced to Unbalanced" and this device serves to match the symmetrical
antenna to the coax which is considered "unbalanced" because one side (the
shield) is grounded.  Without the balun, RF current may flow down the
outside shield of the coax which can cause all kinds of problems (including
angry neighbors when you wipe out the Saturday cartoons).  The commercial
units are waterproof and sturdy enough to support the tension on the two
halves of the dipole.  They cost $15 to $20.

The center of the dipole can also be supported by an insulator and the
coax connected directly to it.  The shield goes to one side of the dipole
and the center conductor to the other.  In this case, roll up about 8
turns of the coax into a coil about 6" in diameter, as close to the
dipole center as practical.  Tape or tie the coil and let it dangle.
This forms a "choke" which discourages the RF from running down the
outside of the coax but does not affect the flow on the center
conductor.  This trick is shown on page 5-9 (figure 14) of the Antenna
Book.

The outer ends of a dipole can be bent off at an angle if there is not
enough room for the full half-wave length in a straight line.  The more
of the antenna which is bent out of plane, the more degraded the
performance will be.

In a pinch, dipoles can be mounted indoors, either strung in an attic or
stapled to a wall or ceiling.  This only works on wood buildings without
metallized insulation in the walls.  Wiring, pipes and gutters can also
cause trouble.  Indoor antennas never work as well as outdoor ones.

Page 3-22 (figure 37) of the Antenna Book shows how to connect the
standard PL-259 plug to RG-58 line with a little adaptor.  This stuff is
all available from Radio Shack, although their hardware is not of the
highest quality.

To do the final tuning of an antenna requires an SWR meter, also known as
a reflected power meter.  Radio Shack sells one that will work for $20.
It is nice to have a good one like the cross-needle meters sold by MFJ and
Daiwa for about $70.  If you are getting an antenna tuner anyway, get
one with the meter built in.

The trick to tuning an antenna with an SWR meter is to take readings at
opposite ends of the band, plus a couple of points in between to make sure
you know what is happening.  If the lowest reading is at the bottom
(low frequency end) of the band, the antenna is tuned too low and
therefore is too long.  Snip about an inch off each end and repeat the
measurements.  At some point the reading will drop abruptly at the
bottom of the band.  Then take measurements every 50 KHz or so to
determine where the minimum is.  Snip half an inch at a time off each
end and recheck until the minimum SWR occurs in the band segment you
want to use most of the time.

To minimize interference, use the lowest power level which will register
on the SWR meter, don't transmit any longer than you absolutely have to,
and never, never turn the frequency knob while you are actually
transmitting.  If you have the type of meter with a "sensitivity"
control, set the control in its maximum position.  Put the meter in
"forward" mode and increase transmitter power until the meter reads full
scale.  Flip the meter to "reflected" mode and read the SWR.

It helps a lot to have a helper and some means of communication between
the shack and the roof.  It could be simplest to set up the transmitter on
the roof.

An SWR of less than 2 will be fine as far as the antenna goes, but many
of the newer solid-state rigs won't put out full power at more than about
1.25.  If the antenna doesn't come out that good, an antenna tuner may
be needed at the transmitter.


=> Mike/W7KCB [75766,1455]