jhb@zeus.UUCP (John H. Burgess) (01/29/85)
First of all, thanks to those of you who took the time to respond to my query.
Lots of good information came out of it, which I will forward to the net:
1) The Sept. 1983 QST has an article on page 56 that describes the (then new)
Novice test procedure. It lists the FCC's novice test bank of 200 ques-
tions from PR Bulletin 1035-A. It also invites people to send an s.a.s.e.
to ARRL and ask for their "ARRL Novice Test". It is a "multiple-choice
version of the question pool along with complete instructions to examiners
and applicants". I sent for that yesterday.
2) Apparently, some time ago, some hard-working person typed in the entire
question pool, with correct answers only (not mult choice) and sent it to
the net. This was forwarded to me. Its handy being machine-readable.
3) A single multiple-choice test, with an answer key was provided. This solved
my immediate need of giving the test to an applicant.
One of the questions prompted a fair amount of controversy and friendly rivalry
among the people I showed it to. Let's see how the net votes:
2D-7.7 If an SWR bridge reading is higher at 3700 kHz than it is
at 3750 kHz, what does this indicate about the antenna?
A. The antenna is too long
B. The antenna is too short
C. The antenna is too high
D. The antenna is too low
Again, my thanks to those who provided responses.
John Burgess, N7DCQjans@mako.UUCP (Jan Steinman) (01/30/85)
In article <453@zeus.UUCP> jhb@zeus.UUCP (John H. Burgess) writes: >One of the questions prompted a fair amount of controversy and friendly >rivalry among the people I showed it to. Let's see how the net votes: > > 2D-7.7 If an SWR bridge reading is higher at 3700 kHz than it is > at 3750 kHz, what does this indicate about the antenna? > > A. The antenna is too long > B. The antenna is too short > C. The antenna is too high > D. The antenna is too low > Poor question, not enough information! Which frequency is the operating frequency? Without making assumptions, either A or B could be right. (By making MAJOR assumptions, D could be right. Could C ever be correct?) -- :::::: Jan Steinman Box 1000, MS 61-161 (w)503/685-2843 :::::: :::::: tektronix!tekecs!jans Wilsonville, OR 97070 (h)503/657-7703 ::::::
jhs%Mitre-Bedford@d3unix.UUCP (01/31/85)
Re: Novice test question 2D-7.7:
"One of the questions prompted a fair amount of controversy and friendly
rivalry among the people I showed it to. Let's see how the net votes:
2D-7.7 If an SWR bridge reading is higher at 3700 kHz than it is
at 3750 kHz, what does this indicate about the antenna?
A. The antenna is too long
B. The antenna is too short
C. The antenna is too high
D. The antenna is too low"
Technical questions normally should not be settled by voting. There should
be a correct answer. However, in this case, there seems to be no way to
decide, nor is there any assurance that ANY of the above answers is correct.
There is not enough information given. What is the desired operating
frequency? Is it 3750 KHz, for example? And what is the VSWR at the desired
operating frequency? Is it 1.000000 to 1 at 3750? If the answers to these
two questions are "Yes!" then I would say that none of the above answers
is correct.
However, since the question is addressed to presumably-aspiring Novices,
who may be presumed to want to operate over the entire Novice band of 3700 to
3750 KHz, maybe we can adduce some "common sense" facts and circumscribe the
problem enough to get a practical answer. Let's assume that coverage of the
Novice band is desired, and that the antenna is a half-wave center-fed type
with a feed line whose characteristic impedance closely matches the feedpoint
impedance of the antenna at resonance. Then the SWR should be close to
1 to 1 at resonance. As one tunes away from resonance, it should increase
as a function of the ratio of actual measurement frequency to resonant
frequency.
It would be desirable to put resonance somewhere near 3725 KHz and
have nearly equal VSWR at 3700 and 3750. Now the question begins to make
more sense. If the VSWR is higher at 3700 than at 3750, then the resonant
point must be higher than the midpoint, 3725. It might be close to 3750, in
which case the VSWR would be very good at the high end of the Novice band but
poorer at the low end. Or, resonance might even be above the Novice band, in
which case the VSWR might be very poor over the whole Novice band, and not get
down to a reasonable value for operating without a tuner until we get up to
3900 or 4000 KHz.
In either case, we can see that the resonance point is at too high a
frequency. Using the standard formula, length = 468 / f(res), for a half-wave
resonant antenna, we can see that if f(res) is too high, then length is
too short. We can even calculate the correct length. It is about 125' 7".
So the antenna must have been cut to less than this length by mistake.
With regard to answers C. and D. above, note that this formula assumes that
the antenna is far away from objects which might detune it by stray
capacitance to ground. In particular, if one were to cut an antenna to
125' 7" and string it up only a few feet above ground, it might exhibit a
resonance well below 3725 Hz because the stray capacitance to ground would
be too high. But this would make the VSWR higher at 3750, not 3700.
This is the usual case that one runs into with practical antenna
installations, so probably the antenna will end up this way unless cut for a
theoretical resonance higher than 3725. So answer D. can be ruled out.
Stringing the antenna up too high would only tend to bring it closer to the
theoretical resonant frequency. So answer C. can also be ruled out.
In summary, answer B. seems to be the correct answer, taking these practical
considerations into account.
73,
John Sangster, W3IKGBob Clements <clements@bbncd1.arpa> (02/01/85)
Re the posted question:
2D-7.7 If an SWR bridge reading is higher at 3700 kHz than it is
at 3750 kHz, what does this indicate about the antenna?
A. The antenna is too long
B. The antenna is too short
C. The antenna is too high
D. The antenna is too low
None of these answers are right. The correct answer is: The antenna is
exactly right.
The reason is that this Novice intends to use his Novice license long
enough to get to 13 WPM and get his General. He then intends to retire
to 75 Meter phone and ragchew forever. Therefore, he has cut the antenna
for 75 Meters and is just using it in the Novice band until he upgrades.
73,
/Rcc
(K1BC)rees@apollo.uucp (Jim Rees) (02/04/85)
2D-7.7 If an SWR bridge reading is higher at 3700 kHz than it is
at 3750 kHz, what does this indicate about the antenna?
A. The antenna is too long
B. The antenna is too short
C. The antenna is too high
D. The antenna is too low
The correct answer is certainly D. There is not enough information
here to decide whether the antenna is too long or too short, and
unless the antenna blew down in the last storm, it is never high
enough off the ground.