Antenna Myths and Magic
Antenna Myths and Magic
SLVARC April 11, 2014
Glen Worstell, KG0T
glen@worstell.com
Agenda
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Computer Modeling Tools
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A True Story
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Antenna Myths and Facts
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Transmission Line Magic
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Bonus CD
Too many slides!
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I will skip some.
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Ask questions. The answers will be short.
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Ask for a slide to be discussed if desired.
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Read the presentation and other stuff on the
bonus CD.
OK to throw rotten tomatoes, but no hard green
ones, please. They hurt.
Computer Programs I Use
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They all work on Linux (use wine for 1 st 2).
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MMANA-GAL (antenna analysis).
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Transmission Line Details (TLD).
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SimSmith (Smith Chart + SWR plots)
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T-Network Tuner Simulator (google it)
True Story-This would have been
nice...
This is what I had
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Free from someone's junk box
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I restored it and made it work
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AM, dual 813 final
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Dual 811 plate modulation
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Young kids like Rich
never heard of AM or 813
Ask Tom, who knows
–
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about AM and tubes
A little knowledge …
Dipole pattern:
I built an armstrong rotatable diople
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Strongest signal broadside to antenna wire
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Weak signal off end
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Tested, QSO with Net Control station in L.A.
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Using 813 xmtr, 50 miles.
Very disappointing result
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NO DIFFERENCE!
Why not?
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MYTH!!!
Answers to Myths
You may fall asleep after this slide :-)
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It is much more complicated than that.
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Must consider entire
system, not just the antenna.
– Height above ground, ground conductivity, slope.
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Nearby buildings and other objects.
– Losses in antenna, ground, xmsn line, balun, tuner, swr, etc.
– Azimuth
and angle of propagation to desired
station(s).
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Phase of the moon and other stuff (maybe :)
My favorite myths:
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My antenna has <blah, blah> gain.
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My antenna has <blah,blah> SWR.
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My antenna is good because it has low SWR.
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My antenna is good because I worked <blah, blah)
dx.
I have an antenna tuner.
A coax air-wound choke balun is a good choke and
a good balun, at least for one or two bands.
MYTH: My antenna has
<blah, blah> db gain.
Facts:
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Antennas have only loss (but they have directivity).
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db is a Ratio – dbi and dbd are common.
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dbd is often bogus (what dipole)?
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dbi “gain” is bogus because it could just as easily be loss!
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Dipole has large loss
in some directions, maybe.
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“gain” means “loss”
at other azimuth.
Why my armstrong dipole had no
directivity:
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Best azimuth angle is high for local contacts.
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No directivity at high angle (85 degrees).
My SWR is <blah, blah>
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Fact: no, it almost certainly is not!
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SWR is the ratio of the maximum voltage on an xmsn line to the
minimum voltage on the line (at a different place on the line). VSWR,
ISWR, and SWR are all the same value.
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Swr is a function of anrenna R+jX and line Zo – nothing else!
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SWR is not affected by:
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Length of line.
Characteristic impedance of the line (except as above).
The frequency, except if the load depends on the frequency.
Antenna “tuner”.
Phase of the moon, or almost anything else.
Your “SWR” meter DOES NOT measure SWR.
What does an “SWR” meter
measure?
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The degree of mismatch between a source Z
and a load Z.
The source Z is usually assumed to be 50
ohms resistive.
This value is good for one thing only:
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It tells you if your solid state amplifier will be
happy. :-)
How to measure SWR?
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Use a vector impedance meter at the antenna to
get it's (complex) Z at the frequency of interest.
–
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With some math, can get value from other end of line.
Measure the (complex) Z of your xmsn line, or use
the Mfgr's spec. May usually assume that the
characteristic Z is real.
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Calculate it from ZL and ZO.
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Or, measure V all along the line and calculate ratio.
For the rest of this presentation:
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I'll use “SWR” to mean the common (wrong)
definition of the measure of the match between two
impedances.
I'll use “antenna gain” to mean (something) relative
to the radiation of an isotropic antenna in free space.
I'll use “antenna tuner” to mean an impedance
matching device.
I'll use “My antenna is good because I worked <blah,
blah> dx to mean that you worked some dx.
Myth: Low SWR means good
antenna.
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SWR (Standing Wave RATIO) is relative to some (complex) impedance.
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Antenna, transmission line, input to xmsn line; usually relative to 50 ohms resistive.
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Often SWR meters are relative to 50 ohms resistive.
50 ohm dummy load has good broadband SWR
relative to modern xmtr output Z.
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Long lossy xmsn line may
give low SWR at xmtr.
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Must treat as SYSTEM.
Folded dipole, 21.3 MHz.
50 0hm xmsn line.
SWR at antenna 6:1
SWR at xmtr depends on line length.
TLD ->
Myth: <blah, blah> DX contacts
means good antenna.
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This is a very, very common myth.
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Poor antenna is like QRP with good antenna.
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I love my KX3.
QRPers often work lots of stations.
May not be able to work some stations because
of azimuth.
Relative to what other antenna?
Myth: My SWR is <blah,blah>
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Relative to what? SWR is a RATIO.
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Where?
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At input to antenna?
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Along feedline? (The same everywhere)
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At xmtr end of feedline?
Myth: Throw up a random length of
wire at least ½ WL on lowest band,
feed with random length of open
wire line, use a tuner: good all-band
antenna.
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ARRL publishes a lot of junk – and lots of good stuff also. Read with discrimination.
No such thing as a “random length” - it is what it is.
If your “random” antenna is 1 WL on some band, and your “random” feedline is 1/2 WL, very
poor results.
Try using TLD, and use MMANA-GAL for pattern.
The “SWR” and/or voltage at the tuner may be too high.
The loss in the tuner may be significant.
Pick the length of the line for min. loss in the tuner for the bands of interest.
No good solution for 80/40.
Antennas have reciprocity
(path loss A to B is same B to A)
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Usually, but not always true.
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Complicated, I an not an expert.
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Read the book on the CD.
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Best receive antenna #= best xmit antenna.
Transmission Line Magic
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Open wire xmsn line (2 and 4 wires)
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Extended Double Zepp (not really magic)
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1 / 4 wave xmsn line matching
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1 / 2 wave xmsn line matching
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1 / 12 wave xmsn line matching
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400 ohm xmsn line matching
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75 meter broadband
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“450” ohm ladder line
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Cheap, available.
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Velocity factor changes when wet.
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Actually about 400 ohms.
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Lots of plastic between wires.
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Slightly higher loss than open wire, but often
insignificant.
4-wire open wire line
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4-wire has advantages:
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Lower loss. Z0 = 83.7*log((2D/d)*1.414))
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Lower noise and radiation.
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Lower Zo (sometimes useful).
Open Wire Line
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2-wire Zo :
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407 ohms, #12 stranded wire, 2” spacing.
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682 ohms, #18 solid wire, 6” spacing.
4-wire Zo :
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154 ohms, #12 stranded wire, 2” spacing.
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219 ohms, #18 solid wire, 6” spacing.
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123 ohms RG-62 (coax) useful for ¼ wave matching to folded dipole (300 ohms).
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450 ohms very useful for matching to almost anything.
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100 ohms good for long line to loop, quad, 40 meter dipole at 45 feet
Zo = 100 ohms, use 1 / 4 wave 75 ohm coax to get 50 ohms at xmtr..
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Part of the 75 ohm coax can be used for a choke balun.
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100 ohms = 2 50 ohms in parallel (open wore)
Common Z0 for matching
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100 0hms (2 50 ohms in balanced parallel)
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25 Ohms (2 50 Ohms in unbalanced parallel)
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150 or 37.5 (2 75 Ohms as above)
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93 RG-62 or 186 balanced parallel
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Mixed unbalanced parallel eg 75 || 50 = 30
Extended Double Zepp
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For best results, add capacitor (see article on CD).
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Simulation: About 380 ohms, 40 meters, 40 feet high, 170 feet long.
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Very low loss over entire 40 meter band using 400 ohm line and low-loss
9:1 balun.
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450 ohm ladder line is usually about 400 ohms.
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Not too bad without capacitor and proper length of 400 ohm open-wire line.
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Some gain over dipole, see Myths re gain.
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At 45 degrees elevation, 40 feet high, broadside, 7.1 dbi.
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Dipole, same height and az-el ,5.4 dbi; EDZ 1.7 db better.
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High angles, any azimuth, dipole 4.8 dbi, EDZ -.4 to -2.1 dbi; dipole is
better.
Dual section xmsn line matching
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Length L1 of Z1 line connected to antenna (ZL).
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Length L2 of Z2 line connected to Z1 line.
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Any length of Z0 (50 ohms) line connected to xcvr
or tuner.
Z0, Z1, Z2 assumed real (R+jX, x = 0).
Can match a very wide range with proper Z1 and
Z0.
Not broadband but often useful anyway.
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1 / 4 wave xmsn line matching
Example of dual line matching with L2 = 0,
L1 = 1 / 4 wavelength
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Don't forget velocity factor.
75 ohm line makes good match for 100 ohm
antenna
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Dipole at some heights
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Full WL loop
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quad
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1 / 12 wave xmsn line matching
Example of dual line matching.
With some length of 50 and some length of 75
ohm Zo, can match about 25 to 150 0hms.
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L1 and L2 are close to 1/12 wavelength.
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Good to match 75 ohm dipole to 50 ohms:
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Z1 is 50 0hms.
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Z2 is 75 ohms.
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L1 and L2 are 1/12 WL.
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400 ohm xmsn line matching
Another case of dual line matching.
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L2 = 0
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See “My Xmsn Line Tunes my Antenna”.
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“notuner.htm” on CD; recommended.
1 / 2 WL matching
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Another case of dual line matching.
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L2 = 0
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L1 = 1 / 2 WL
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Zin = Zout for any Z1
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Example: feed dipole with 1 / 2 wl of window or
open wire line: low SWR, low loss, cheap.
SWR on line is about 5:1 or 6:1; don't care.
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75 meter broadband
Several methods:
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Fat (multi-wire cage)
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Fan
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Parasitic element
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LC network
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Just use good automatic tuner.
One method is a special case of dual line
matching.
Conclusion: how to put up a good
antenna.
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What is azimuth and angle of path to stations you want?
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Select an antenna to fit space and $.
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Model it to see if it has the desired azimuth and angle.
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Put it up and measure Zo.
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Use appropriate xmsn line, balun, and matching network
for the measured Zo.
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A dipole is often a good, easy, cheap antenna.
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BEST: compare it to another (real, not modeled) antenna.
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