Think of the symbol "Pi" (\(\pi\)). It's the same shape, with two lines down to the ground.
Also see Wikipedia article section and accompanying images: https://en.wikipedia.org/wiki/Antenna_tuner#Low-pass_π_network
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A capacitor has a response that increases as frequency increases and an inductor has the opposite response, it decreases as frequency increases. In the circuit described the inductor is between the signal path and ground and the capacitor in the signal path.
So, the capacitor impedes the passage of low frequencies in the signal path and the inductor allows the passage of low frequencies to ground leaving the higher frequencies as the only ones that pass through the T-network described.
Hint: a Touchdown pass is thrown HIGH
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One of the most common issues with transmitting into a multi-band antenna system is the creation of harmonic distortion that can cause cross interference with the outgoing signal. Using some sort of filter network just prior to the last stage of the amplification process can suppress harmonics within that particular frequency transmission. Of these many different Filter Networks the Pi-L (π) network is one of the most effective methods to suppress the harmonics in the final stage.
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Hint: 4 "C's" Circuit + Complex = Cancels + Changes
The term “impedance matching” is rather straightforward. It’s simply defined as the process of making one impedance look like another. Frequently, it becomes necessary to match a load impedance to the source or internal impedance of a driving source. It’s crucial that the reactive components cancel each other. An example is the delivery of maximum power to an antenna. Impedances in radio-frequency transmitters must be matched to pass maximum power from stage to stage. Most impedance include inductances and capacitance that must also be factored into the matching process. Antenna impedance must equal the transmitter output impedance to receive maximum power.
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Chebyshev filters are analog or digital filters with a steep roll off at the edge of their passband and a ripple within the passband or stopband.
To rule another option out: "Butterworth filters are as smooth as Butter". See the graphics when you scroll down to : "Comparison with other linear filters".
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An elliptic filter (also known as a Cauer filter, named after Wilhelm Cauer, or as a Zolotarev filter, after Yegor Zolotarev) is a signal processing filter with equalized ripple (equiripple) behavior in both the passband and the stopband. The amount of ripple in each band is independently adjustable, and no other filter of equal order can have a faster transition in gain between the passband and the stopband, for the given values of ripple (whether the ripple is equalized or not). Alternatively, one may give up the ability to adjust independently the passband and stopband ripple, and instead design a filter which is maximally insensitive to component variations.
As the ripple in the stopband approaches zero, the filter becomes a type I Chebyshev filter. As the ripple in the passband approaches zero, the filter becomes a type II Chebyshev filter and finally, as both ripple values approach zero, the filter becomes a Butterworth filter. - K4AGO
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A cavity filter is the best choice for use in a 2 meter repeater duplexer because it has a very high Q, can handle high power and is mostly stable to temperature changes. It provides a "steep" notch to only pass the band of interest with little loss.
The other answers given are worse choices because:
LC filters suffer from less than ideal L and C behaviors of their components.
Crystal filters typically cannot handle higher power.
DSP filters are not ideal because they would require analog-to-digital conversion which limits the power and requires regeneration of the signal (at a high power) after processing. Remember, DSP means digital signal processing, and a high power signal must be converted to low power digital then regenerated to be passed through DSP. A DSP filter would waste a lot of power and require more circuitry. They are mostly used when you start with a weak signal, then process it before sending it to a power amplifier.
https://www.amateur-radio-wiki.net/what-is-a-cavity-filter/ has an explanation of cavity filters and their use in amateur radio applications.
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Since a Pi network contains two capacitors in shunt at the input and output, and a series inductor, two L networks connected back-to-back would create that if the two inductor value are added together to create one inductor.
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Pi-L, L for inductance. So, its a Pi with an inductor. (jmsian)
Hint: It's the only answer that has "Pi" in it, which comes from the loose suggestion it looks like the Greek letter pi (π).
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Just two elements! That seems simple enough. Do we always use these L-networks when constructing lossless matching networks?
Nope. L-networks have two major drawbacks:
The disadvantage of the L circuit - it can match loads equal or less than 50 Ohm. If the L circuit is reversed it can match loads equal or higher than 50 Ohm. It can not match on both sides. For example If the load is changing from 35 to 100 Ohms a reversed L network will match only from 50 up to 100 Ohms and will not match from 35 to 50 Ohms.
So the answer: The Q of Pi-networks can be varied depending on the component values chosen
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Many digital modulation methods depend on the phase shift of the baseband signal to convey information.
If you have a filter in a receiver that has a different phase shift depending on frequency, then the decoder/demodulator may have a difficult time extracting the original and meaningful phase shift.
Modulations that have quadrature components (QAM etc) are susceptible. FSK is more robust.
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A crystal lattice filter is A filter with narrow bandwidth and steep skirts made using quartz crystals.
Note: There is a slight difference in layout between crystal lattices and ladders. There are pairs of crystals within lattice networks. Resonance modes are paired with each crystal in the lattice that facilitate an intended bandpass envelope (shape) to pass.
As far as the "skirt" jargon - When viewed graphically, some filter transitions are said to resemble one or both sides of a woman's skirt, so sharp transitions are known as steep skirts.
Silly test tip: I just remembered this as, “Crystal likes to wear steep skirts.” -N5MAJ
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