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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: Building your own tuner is an economical alternative.  Tuner
circuits can be found in almost any edition of the ARRL Handbook
and in many other Amateur Radio publications.  Parts can usually
be found at most hamfests and at dealers who specialize in surplus
equipment and components.  Construction of an antenna tuning unit
is an easy first project for the novice builder since layout and 
construction methods are simple.*

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.

* = Note: the text of this paragraph has been changed from the 
    original published text.

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 mtracy@arrl.org. 

73 from ARRL HQ.

