QRZ! Ham Radio 1/QRZ Ham Radio Callsign Database - December 1993.iso/arrl/ants_ind.oor

Text File | 1993-11-21 | 16KB | 345 lines

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Prepared as a membership service by the American Radio Relay
League, Inc., Technical Information Service.

file: \public\info\tis\antind.txt updated: 12-10-92

Reprinted from: December 1992 QST "Lab Notes."
Copyright 1992 American Radio Relay League, Inc.
All rights reserved.

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--------------------------- cut here ----------------------------
This ARRL Technical Information Service handout has been derived
from the December 1992 QST "Lab Notes" column. Information has
been added to keep it current. You should also periodically check
the ham-classified section of ham and electronic publications.

Lab Notes           Conducted by Steve Ford, WB8IMY, Assistant
                    Technical Editor and the ARRL Laboratory Staff

----------------------------------------------------------------

Limited Space Antennas

Don't you wish you owned a monstrous tower with monoband Yagis for
every amateur band from 40 meters through 70 cm?  How about some
sprawling 80- and 160-meter rhombics to round out the collection?
Antenna farms like these exist only in the dreams of most hams.
When it comes to antennas, the greatest obstacle isn't always
cost, it's s-p-a-c-e!  ARRL Laboratory Engineer Zack Lau, KH6CP/1,
has spent years grappling with the challenge of operating in
limited-space environments.  In this month's column Zack applies
his expertise to solve some difficult problems.--WB8IMY

Q: What is the smallest antenna I can buy that has lots of gain?

A: An optical telescope.  A little 4-inch telescope has about 94
dBd of gain.  An 8-inch telescope has 100 dBd of gain.

Q: I'm serious!  I was thinking about the HF bands--something to
put in my apartment.  How much gain can I expect?

A: When squeezed into small apartments, most antennas will have no
more than the same maximum gain as a dipole--0 dBd--less any
losses.  Losses can be relatively low in wood-frame buildings, and
very high in concrete and steel structures.  I've also heard bad
things about stucco and wire mesh!

Q: How can I determine losses?

A: Try an antenna and see if it works!  No kidding.  It's sort of
like measuring the loss of a windowpane by looking through it.
There are usually too many factors involved to evaluate indoor
antennas with theoretical models.  It's quicker and less difficult
to optimize indoor HF antennas empirically through trial and
error.

Q: If I run 1500 watts to an indoor dipole, won't the fields
around the antenna be awfully strong?

A: That's an understatement!  (How's your fire and medical
insurance?) I recommend using low power with indoor antennas.  In
fact, I suggest 5 watts output, though some folks run as much as
100 watts.  While studies haven't conclusively linked low-level RF
exposure to health problems, it's prudent to limit exposure if you
can.  If you're concerned about RF in your home, check Chapter 36
of the 1993 ARRL Handbook.

Q: But if I run low power to an indoor antenna, how can I compete
with stations running 1500 watts to huge outdoor antennas?

A: You can't.  However, you can have lots of fun with a modest
setup.  One of the challenges of radio is seeing what you can do
with what you have.  When you erect gigantic antennas and produce
huge amounts of RF, you expect to work any station you want--and
become gravely disappointed when you don't!  This is hardly the
best formula for enjoying Amateur Radio.

Q: Okay.  I decided to try it your way.  I set up a 20-meter
dipole in the attic of my apartment building and carefully
measured the wire lengths.  Even so, the best SWR I can get is 4
to 1.  What am I doing wrong?

A: Your antenna is probably being detuned by nearby objects.  You
could either vary the length of the antenna, or use an antenna
tuner to reduce the SWR at the transmitter.  A tuner makes a lot
of sense if you can afford it.  In many cases you'll be able to
operate on several bands with the same antenna--even with a high
SWR on the feed line.

On the downside, operating with a high SWR can result in
considerable feed-line loss.  When there is a mismatch at the
antenna, a portion of the transmitted RF energy is reflected
rather than radiated.  This reflected RF travels back and fourth
many times between the antenna and the antenna tuner.  The higher
the SWR, the more trips are required to get rid of it--either in
the form of heat in the feed line or radiation at the antenna.  In
good-quality feed line, many trips are possible before any loss is
noticeable.  By contrast, a poor feed line may dissipate most of
the energy in a single trip!

Q: If the RF energy is making all those trips up and down the
coax, won't my signal become distorted at the receiving end due to
the delays?

A: You're forgetting how fast radio waves travel.  For example, a
radio wave zips through 16 feet of RG-213 coax in 49 nano-seconds.
Even if it made 100 trips before finally being radiated, that's a
delay of only 4.9 microseconds.  The delay distortion that occurs
is too small for anyone on the receiving end to detect.  ATV (fast
scan television) enthusiasts have some cause for concern though.
Delayed ATV signals appear as ghosts on TV screens.

Q: You were right about adjusting the length of my dipole.  All it
took was a little trimming and the SWR came right down.  I'm
running about 100 feet of coax to the antenna.  I don't really
need that much feed line, but I'm too lazy to cut it.  Although
the antenna tuner tunes easily, I get lousy signal reports.  Is
the tuner affecting my signal?

A: Not likely.  The problem isn't the antenna tuner--it's your 100
feet of coax!  An antenna tuner isn't a miracle worker.  Putting
it very simply, a tuner is a device that couples RF to the antenna
system and reflects reflected power back to the antenna.  It
functions as an extension of the output tuning network in your
transceiver.

In your case, you're wasting a great deal of power heating 100
feet of coax with precious little radiated at the antenna!  You'll
never know this by the behavior of your antenna tuner, however.
It happily matches your transceiver to this RF heating system and
you're left wondering why you get such terrible signal reports.
In this instance, a low SWR indication at your tuner doesn't add
up to a terrific signal.

Q: An antenna tuner is a lot of money for my low-budget station.
Are there cheaper alternatives?

A: Parts from old transmitters can often be fashioned into tuners.
Some of the vintage rigs featured hefty output networks.
Cannibalize the parts and they'll make great antenna tuners.

The cheapest antenna tuner for the higher HF bands can be made
from two quarter-wavelength pieces of coax and three inductors
made of #14 wire (see Fig 1).  While I've played with this system
in the lab, I haven't actually used it on the air since it's
physically large on any band below 12 meters.  (The system is
similar to that used for microwave work.  By placing three screws
a quarter wavelength apart in a waveguide, you can match just
about anything at a single frequency.)  The losses might be a
little higher than those you'll find in a conventional antenna
tuner.  The typical quality factor (Q) for coils is a tenth that
of decent capacitors, but should still be acceptable if good coax
such as RG-213 is used at 100 watts or less.

Q: When I shortened my coax, my carefully adjusted antenna went
from a 1:1 to a 2:1 SWR.  What happened?

A: The outside shield of the coax was functioning as part of the
antenna.  By shortening it, you effectively changed the antenna!
You need too decouple the feed line from the antenna.  If
possible, bring the coax away from your dipole at a 90\o/ angle.
If the coax is running parallel to your antenna, RF coupling is
likely to occur.  In addition, try placing a balun in the feed
line at the antenna.  You'll see a number of baluns advertised in
QST.  Baluns decouple the feed line from the antenna but, like
antenna tuners, they aren't cure-alls.  You may need to experiment
a bit to achieve maximum feed-line decoupling.  (Try some ferrite
beads on the coax, for example.)\1/

Q: Why don't they sell VHF/UHF antenna tuners?

A: They're available, but the market is awfully small since most
VHF/UHF antennas are designed to present good loads to 50-$OMEGA
feed lines.  Also, tuners usually give poor results at VHF/UHF.
Operating at VHF/UHF with a high SWR almost always results in
horrendous feed-line loss.  Using an antenna tuner won't solve the
problem, and manufacturers aren't inclined to sell products that
don't provide much benefit.  However, if you must have a 6-meter
antenna tuner, it's possible to modify an existing HF unit by
substituting smaller coiled and capacitors.

Q: Does it matter which wire size I use for my antennas?

A: If you want to comply with the National Electrical code, yes.
It species #14 hard-drawn copper wire for lengths under 150 feet.
To make antennas nearly invisible, however, amateurs have
successfully used wire as fine as #32 with monofilament fishing
line as supports and insulators.

Q: Since my whole antenna system is indoors, do I really have to
spend the extra money for high-quality connectors?

A: Probably not.  Apartment-dwelling hams have been known to use
clip-lead connectors with excellent results on HF--as long as the
clip leads are reliable.  I've seen a lot of shoddy ones with
poorly crimped connections.

Q: How about a small transmitting loop?  Is it true that such an
antenna will work as well as a full-size dipole?

A: We'll it's only comparable to a full-size antenna in a similar
location.  If I shielded both of them within a house or apartment,
I wouldn't expect either to work all that well.

Loops can be quite efficient if their losses are kept to a minimum
through the use of thick tubing for the radiators and low-loss
capacitors for the matching networks.  Low-loss variable
capacitors are often expensive, though.  Even if money isn't a
problem, small loop antennas aren't practical at 80 and 160 meters
where the bandwidth of an efficient loop can become too narrow to
pass an SSB signal!

Q: Is there an antenna that offers high angles of radiation on 80
and 40 meters and low angles on the higher bands for DX work?

A: An antenna that meets this requirement is the full-wave
horizontal loop.  Unfortunately, an 80-meter loop is about 70 feet
on a side, by no means a small antenna.  If you only operate on 40
through 10 meters, a 40-meter loop may be feasible depending on
the size of your dwelling.  Or, you could use a vertical loop for
the higher bands and a short horizontal dipole and an antenna
tuner for the lower bands.  An antenna popular among novice hams
in New Zealand is a loop of wire wrapped around the house!  I
haven't tried it, but it would make a pretty invisible antenna,
especially if you put it up just prior to painting your home.

Q: I'm on the 10th floor of an apartment building.  How do I get a
ground?

A: Are you sure you need one?  (I can't think of any satellite
stations that are grounded to the earth!)  If you absolutely must
have a ground because of stray-RF problems, 15 to 20 square feet
of sheet metal on a concrete floor works about as well as anything
else.  It acts as a lossy ground, getting rid of stray RF.

You might also try a 1/4-wave counterpoise wire attached to the
ground lug on your rig.  You'll need a wire for every band on
which you experience grounding-related problems.  However, this
approach can backfire.  The counterpoise wires might also act as
antennas, radiating even more RF into your apartment.  Even wide
copper straps are bound to be good radiators unless they're placed
near lossy materials such as concrete.

Q: Can I use the sheet-metal ground for my long wire antenna?

A: You could, but much of your power may end up as heat.  You're
better off with balanced antennas, though people have effectively
used long wires in limited-space environments (high-rise buildings
in particular).  When working against poor grounds, much of your
signal is lost as heat.  Even so, the remainder that is radiated
may be adequate if the wire is 100 feet up!

Q: I just heard a DX station using a trap vertical at a beach-
front location.  His signal was terrific!  I used a vertical too,
but my signal wasn't nearly as good on his end.  Why?

A: The quality of the ground determines how well a vertical
performs at low angles of radiation--the lower the angle the
better for DX!  Sand saturated by salt water is a terrific ground
plane for verticals.

The ideal ground plane for HF verticals should extend for
hundreds, if not thousands, of feet around the antenna.  The DX
station you heard is operating in a near-ideal environment (in
more ways than one!).  Some hams try placing copper screens under
their verticals, but it doesn't offer much improvement.  Unless
you can move to a tropical island or seaside resort, you can only
work with what you have available.

Q: The trap vertical I bought seems to work, but I can't adjust
the SWR to 1:1 on all bands.  Should I be concerned?

A: No, Many designers find it a challenge to build an efficient
antenna that exhibits an SWR under 2:1 on all HF bands.  The
easiest way to reduce the SWR is by increasing the losses, either
by putting a resistor or using lossy matching techniques.  The
classic double-bazooka antenna uses the latter--the coaxial stubs
increase the bandwidth by making the antenna convert RF into heat!
The acceptability of these techniques is subject to much debate
and we don't have space to cover it here.  If I were you, I'd use
an antenna tuner and stop worrying about the SWR.

Q: I'd prefer to use a multiband trap antenna so I won't need an
antenna tuner.  What are the disadvantages of doing so?

A: Traps generally reduce the bandwidth of the antenna and limit
power-handling capability.  In most antenna designs, they're the
weak link in the system.  For multiband HF operating, I prefer an
antenna with a minimum of weak links--no traps or baluns.  This
allows experimentation with an antenna tuner to determine which
bands the antenna will work on.

Try a coax-fed dipole and "force feed" it via an antenna tuner.
Make the antenna as long as practical.  Use good-quality coax and
keep it as short as you can.  You may even want to try feeding
your dipole with 450-ohm ladder line since, compared to coax,
open-wire feed line has extremely low loss at HF.  The SWR may be
quite high on some bands, but don't let it bother you.  Use your
tuner to couple the RF to the antenna system and most of it will
be radiated.

We welcome your suggestions for topics to be discussed in Lab
Notes, but we are unable to provide individual replies.  Please
send your comments or suggestions to: Lab Notes, ARRL, 225 Main
Street, Newington, CT 06111.

*** FOOTNOTE #1 <below> *** \1/R. Healy, "Feeding Dipole
Antennas," QST, Jul 1991, pp 22-24.

Fig 1--Here's a cheap antenna tuner design for single-band
operation.  The inductors (L) are wound with #14 wire.  Their
diameter and number of turns depends on the impedance of the
antenna system you have in mind.  In other words you have to
experiment!  The 1/4-wave-length coax sections should be made from
RG-213.

The members and HQ staff would like to thank the following people
for their contributions to this information file:

KH6CP

Send any additional information or changes to [e-mail].

73 from ARRL HQ.