KB6NU's Ham Radio Blog

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Champion again!

Posted: 11 Jan 2016 01:59 PM PST
http://feedproxy.google.com/~r/kb6nu...m_medium=email


This just showed up in my mailbox:



How cool is this? WithÂ*21 CW QSOs, 13 phone QSOs, and 29 multipliers,
yielding a total score of 1,972, I placed first from the state of Michigan,
and #27 overall in the 2015 Minnesota QSO PartyÂ*(MNQP). You can see all of
the results, plus a couple of interesting profiles of MNQP participants, by
downloading this PDF.

The 18th annual Minnesota QSO Party, presented by the Minnesota Wireless
AssociationÂ*takes place onÂ*Saturday, February 6th, 2016 fromÂ*8:00 AM CST
(1400 UTC) Through 6 PM CST (2400 UTC). If you live in Minnesota, please
listen for me. Ill be in there, defending my title.



The post Champion again! appeared first on KB6NUs Ham Radio Blog.


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2016 Extra Class Study Guide: E5A - Resonance and Q

Posted: 11 Jan 2016 11:28 AM PST
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Its hard to believe that its been four years since I wrote the first
edition of my No Nonsense Extra Class study guide. It has been, though, and
now its time to update the study guide to cover the 2016 2020 question
pool.Â*Heres the first updated section.

There have been several question changes in this section. They removed
questions that asked you to calculate the resonant frequency or half-power
bandwidth and added questions talking about Q and the consequences of
having lower or higher Q.

Heres the updated section:

E5A Resonance and Q: characteristics of resonant circuits; series and
parallel resonance; Q; half-power bandwidth; phase relationships in
reactive circuits

Resonance is one of the coolest things in electronics. Resonant circuits
are what makes radio, as we know it, possible.

What is resonance? Well, a circuit is said to be resonant when the
inductive reactance and capacitive reactance are equal to one another. That
is to say, when

2Ï€fL = 1/2Ï€fC

where L is the inductance in henries and C is the capacitance in farads.

For a given L and a given C, this happens at only one frequency:

f = 1/2π√(LC)

This frequency is called the resonant frequency. Resonance in an electrical
circuit is the frequency at which the capacitive reactance equals the
inductive reactance.(E5A02)

Lets calculate a few resonant frequencies, using questions from the Extra
question pool as examples:

The resonant frequency of a series RLC circuit if R is 22 ohms, L is 50
microhenrys and C is 40 picofarads is 3.56 MHz. (E5A14)

f = 1/2π√(LC) = 1/(6.28 x √(5010-6 x 4010-12)) = 1/(2.8 x 10-7) = 3.56 MHz

Notice that it really doesnt matter what the value of the resistance is.
The resonant frequency would be the same is R = 220 ohms or 2.2 Mohms.

The resonant frequency of a parallel RLC circuit if R is 33 ohms, L is 50
microhenrys and C is 10 picofarads is 7.12 MHz. (E5A16)

f = 1/2π√(LC) = 1/(6.28x√(5010-6 x 1010-12)) = 1/(1.410-7) = 7.12 MHz

When an inductor and a capacitor are connected in series, the impedance of
the series circuit at the resonant frequency is zero because the reactances
are equal and opposite at that frequency. If there is a resistor in the
circuit, that resistor alone contributes to the impedance. Therefore, the
magnitude of the impedance of a series RLC circuit at resonance is
approximately equal to circuit resistance. (E5A03)

The magnitude of the current at the input of a series RLC circuit is at
maximum as the frequency goes through resonance. (E5A05) The reason for
this is that neither the capacitor or inductor adds to the overall circuit
impedance at the resonant frequency.

When the inductor and capacitor are connected in parallel, the impedances
are again equal and opposite to one another at the resonant frequency, but
because they are in parallel, the circuit is effectively an open circuit.
Consequently, the magnitude of the impedance of a circuit with a resistor,
an inductor and a capacitor all in parallel, at resonance, is approximately
equal to circuit resistance. (E5A04)

Because a parallel LC circuit is effectively an open circuit at resonance,
the magnitude of the current at the input of a parallel RLC circuit at
resonance is at minimum. (E5A07) The magnitude of the circulating current
within the components of a parallel LC circuit at resonance is at a
maximum. (E5A06) Resonance can cause the voltage across reactances in
series to be larger than the voltage applied to them. (E5A01)

Another consequence of the inductive and capacitive reactances canceling
each other is that there is no phase shift at the resonant frequency. The
phase relationship between the current through and the voltage across a
series resonant circuit at resonance is that the voltage and current are in
phase. (E5A08)

Ideally, a series LC circuit would have zero impedance at the resonant
frequency, while a parallel LC circuit would have an infinite impedance at
the resonant frequency. In the real world, of course,Â*resonant circuits
don’t act this way. To describe how closely a circuit behaves like an ideal
resonant circuit, we use the quality factor, or Q. Because the inductive
reactance equals the capacitive reactance at the resonant frequency, the Q
of an RLC parallel circuit is the resistance divided by the reactance of
either the inductance or capacitance (E5A09):

Q = R/XL or R/XC

The Q of an RLC series resonant circuit is the reactance of either the
inductance or capacitance divided by the resistance (E5A10):

Q = XL/R or XC/R

Basically, the higher the Q, the more a resonant circuit behaves like an
ideal resonant circuit,and the higher the Q, the lower the resistive losses
in a circuit. Lower losses can increase Q for inductors and capacitors.
(E5A15) An effect of increasing Q in a resonant circuit is that internal
voltages and circulating currents increase. (E5A13)

Q is an important parameter when designing impedance-matching circuits. The
result of increasing the Q of an impedance-matching circuit is that
matching bandwidth is decreased. (E5A17) A circuit with a lower Q will
yield a wider bandwidth, but at the cost of increased losses.

A parameter of a resonant circuit that is related to Q is the half-power
bandwidth. The half-power bandwidth is the bandwidth over which a series
resonant circuit will pass half the power of the input signal and over
which a parallel resonant circuit will reject half the power of an input
signal.

We can use the Q of a circuit to calculate the half-power bandwidth:

BW = f/Q

Let’s look at some examples:

The half-power bandwidth of a parallel resonant circuit that has a resonant
frequency of 7.1 MHz and a Q of 150 is 47.3 kHz. (E5A11)

BW = f/Q = 7.1 x 106/150 = 47.3 x 103 = 47.3 kHz

What is the half-power bandwidth of a parallel resonant circuit that has a
resonant frequency of 3.7 MHz and a Q of 118 is 31.4 kHz. (E5A12)

BW = f/Q = 3.5 x 106/118 = 31.4 x 103 = 31.4 kHz

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