Jon,
A few minutes with MatchCad. See answers below.
Mike, W4EF.................
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From: Jon Ogden[SMTP:jono@enteract.com]
Sent: Monday, October 11, 1999 9:04 AM
To: amps@contesting.com
Subject: [AMPS] THE SWR FINAL EXAM!
OK,
Now, for the FINAL exam on SWR:
Joe Ham has a brand new amplifier that he plans to use exclusively on
14.225 MHz. His antenna is a beam that does not have the best match at
14.225 MHz. Therefore, Joe being a careful guy, has decided to put a 50
Ohm isolator at the output of his amplifier. Joe's coax is 50 Ohms and
for practical purposes we will assume his coax is lossless. The length
of the coax is random and unknown. His SWR meter is also 50 Ohms and has
zero line length. The impedance in Ohms of Joe's antenna at 14.225 MHz
is 69.5+j36 Ohms. This is measured INSIDE his shack at the input to his
coax.
Questions:
1.) If Joe puts his VSWR meter after his isolator what is the VSWR that
he sees?
1.976:1
2.) What percentage of Joe's power is actually radiated by his antenna?
89.2%
3.) If Joe moves his VSWR meter 1/16 wavelengths closer to the antenna
what is the VSWR he measures? What is the impedance that is read at this
point?
1.976:1
40.371 + j 29.681
4.) If Joe moves his VSWR meter an additional 1/8 wavelength closer to
his load what is his VSWR? What is the impedance measured at this point?
1.976:1
25.299 + j 0.068
5.) If Joe moves yet anothr 1/4 wavelength closer to the antenna now what
is his VSWR? The impedance?
1.976:1
98.819 - j 0.264
6.) If Joe moves his meter an additional 1/16 wavelength, will the SWR
and impedance be the same as in question 1?
Yes
7.) If Joe continues to move close to his antenna, will his VSWR change?
No
8.) What is the capacitance or inductance of Joe's antenna at the
69.5+j36 Ohm point?
402 nanohenries (series equivalent)
9.) What is the capacitance or inductance of Joe's antenna if he adds an
additional 1/16 wavelength of coax to what he starts with?
42 nanofarads (series equivalent)
10.) Assume Joe has added the additional length of coax in question 9.
Now he want to match that point with a 1/4 wave transformer at 14.225
MHz. What does the impedance of the transformer line section need to be?
How long is this line in meters?
70.3 Ohms
5.255 meters (assumes a velocity factor of 1.0 as in an air line)
11.) With the setup given in question 10, what percentage of power is now
radiated by Joe's antenna?
100%
12.) What will Joe's VSWR meter now read after his isolator? Does he
need the isolator any more?
1.003:1
No!!
Well, that's it! Good luck! Are you up to the challenge?
73,
Jon
KE9NA
-------------------------------------
Jon Ogden
KE9NA
http://www.qsl.net/ke9na <--- CHECK IT OUT! It's been updated!!!!!
"A life lived in fear is a life half lived."
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