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18 votes
2 answers
3k views

Why do repeaters use cavity filters?

Repeaters apparently often use large, expensive cavity filters to connect the receiver and transmitter to the same antenna. Why aren't typical electrical circuits, such as inductors and capacitors ...
Adam Davis's user avatar
  • 12.1k
17 votes
4 answers
19k views

Where are the baluns for VHF and higher? How can one be made?

In another question, commenters recommended that I look into adding a 1:1 balun to a dual-band dipole to eliminate the feedline as a radiator. As I understand it, a 1:1 balun also functions as a choke,...
William's user avatar
  • 803
16 votes
2 answers
4k views

How does selecting the opposite sideband when receiving CW on HF reduce interference?

I am studying for my FCC General Class license, and question/answer G4A02 reads as follows: Q: What is one advantage of selecting the opposite or "reverse" ...
Bryson's user avatar
  • 263
13 votes
4 answers
1k views

Why do today's transceivers use LC filters rather than crystal filters?

Going through a number of schematics for today's receivers (Elecraft KX3, for example) I note that most use LC circuits for filtering. The wikipedia article for Crystal Filters suggests, however, ...
Adam Davis's user avatar
  • 12.1k
12 votes
1 answer
3k views

What is a "roofing filter"?

What are roofing filters? Are there different kinds of things that are called roofing filter? Why would a circuit or component be called a roofing filter instead of just a IF bandpass filter? Why ...
hotpaw2's user avatar
  • 13.4k
10 votes
2 answers
1k views

What is meant by "poles" when discussing filters?

I was looking over the product page for the K3 (dreaming I know) and noticed that they offer a variety of crystal filters. Some are 5-pole and some are 8-pole, other than price, what is the ...
WPrecht's user avatar
  • 6,492
9 votes
5 answers
834 views

What limits a PIN diode or varicap from being used in a "Software Defined [Radio] Filter"?

A PIN diode varies its capacitance based on a DC bias, and a Varicap exhibits this effect even more clearly. What limits the use of "solid state variable capacitors" like these for ...
natevw - AF7TB's user avatar
9 votes
2 answers
301 views

How to effectively test and interpret the "working range" of a ferrite core with a signal analyzer?

I have a couple of ferrite beads that are advertised as effective for suppressing conductive noise around 150kHz. I'm trying to monitor their actual performance with noise filtering. In my set up I ...
GHGC's user avatar
  • 93
9 votes
1 answer
456 views

How do you choose an inductor wire gauge for high-power RF?

At 100 watts in a 50-ohm system, the current flowing through an inductor would be about 1.41 A, thus a 26-AWG wire should be plenty since they are chassis-rated for 2A and for just a few windings that ...
KJ7LNW's user avatar
  • 723
8 votes
2 answers
709 views

How to calculate if low-pass filtering is sufficient?

An answer on this question Suggested method for boosting 0.05 watt transmitter up to 10+ watts? suggests putting a filterless 19dB amplifier between the HackRF (or similar general-purpose SDR) and a ...
natevw - AF7TB's user avatar
7 votes
1 answer
613 views

Super-low-tech VHF low-pass filter construction?

Related to this question: Is it legal to broadcast an FM radio signal with a Raspberry Pi Zero? Is it possible to build a super-low-tech low-pass filter for VHF harmonic suppression without a PCB or ...
hotpaw2's user avatar
  • 13.4k
6 votes
2 answers
2k views

Looking for a schematic of a DIY circulator

I would like to build a simple HF, VHF or UHF circulator to get a better understanding of how these devices work. Sadly at the moment I'm unaware of any articles or books that provide corresponding ...
Aleksander Alekseev - R2AUK's user avatar
6 votes
3 answers
2k views

"Coaxial capacitors in line” in E4E04?

I've been studying the U.S. license examination question pools. For the most part, it is reasonably obvious what the questions and answers mean even when I am not yet able to come up with the answers ...
Kevin Reid AG6YO's user avatar
  • 24.6k
6 votes
2 answers
3k views

Small band-pass filter for HT?

I'd like to add a band-pass filter to my Baofeng UV-82 to make it comply with the spurious emissions requirements. Ideally it would be a rigid device with SMA or BNC connectors that would fit between ...
mrog's user avatar
  • 1,023
6 votes
2 answers
891 views

Filtering for PSK31 demodulation

What's the ideal filter for PSK31 demodulation? That is, something that minimizes ISI, while having a narrow bandwidth to reject adjacent channels, while being reasonably matched? Most PSK modes use ...
Phil Frost - W8II's user avatar
6 votes
2 answers
748 views

Choosing capacitor for high-pass filter with cutoff of about 160 MHz

A strong (100 kW) local FM transmitter causes interference to or overloading of receivers in the VHF/UHF bands. It seems like a single, properly chosen, capacitor could make an effective passive high-...
Jamie Cox's user avatar
  • 163
6 votes
1 answer
196 views

How to achieve highest Q-factor in inductors for 6-160m amateur bands?

When building filters for ham bands (direct sampling SDR frontend) - it is common to build filters using 2,6,10,12 material toroids. Are there any recent developments that offer lower losses / higher-...
BarsMonster - R2AYN's user avatar
6 votes
1 answer
1k views

Resonant RF Transformer Substitution

I'm looking at a RF transformer filter design that appears to be using the primary-side transformer inductance as part of a second-order band-pass filter (first schematic). This isn't the only filter ...
W5VO's user avatar
  • 1,901
6 votes
0 answers
216 views

Choice of PCB coil design for filter inductor at ca 20 to 400 MHz, 5 W

Looking at the diplexer schematic from this nice question, I realized this can completely be done in PCB-formed elements (minus the connectors). (As a kind of counter-action to the original question'...
Marcus Müller's user avatar
5 votes
2 answers
563 views

SSB filtering in the frequency domain?

I'm not sure whether it's more appropriate to ask this question here, or in the dsp.stackexchange. But I'll try here first. There seems to be a lot of information available on the filtering and ...
hotpaw2's user avatar
  • 13.4k
5 votes
2 answers
766 views

Wide band TX capable band-pass filter for VHF and Airband

I want to share the same antenna for VHF (TX/RX 144-148 MHz) and for Airband RX-only (116-136 MHz). I live on a very congested area where filter is a must, so I'm looking for the best solution for my ...
Aldoir's user avatar
  • 51
5 votes
2 answers
3k views

How can the design of an RF filter also encompass impedance matching?

I'm installing an antenna which will require a matching network. (A fixed network: the load will not change.) I'm also building some low-pass filters to reduce transmitted harmonics. It would be a ...
Phil Frost - W8II's user avatar
4 votes
2 answers
1k views

How to measure quartz crystal motional parameters using a VNA?

I want to measure quartz crystal motional parameters with a VNA. Unfortunately all Google searches I've done have lead me to nothing in concrete. I know how to get an impedance curve and measure the ...
S.s.'s user avatar
  • 153
4 votes
1 answer
92 views

How to properly simulate a crystal filter being pulled?

After reading k1zmt's answer to the question of how to change the resonant frequency of a quartz, I wondered how to simulate a quartz filter to check that result. I went with the following model of a ...
Marcus Müller's user avatar
4 votes
1 answer
1k views

Looking for origins of a crystal filter schematic used in uBITX

Here is a crystal filter used in uBITX transceiver: I've never seen such a topology in any other transceiver and decided to invest some time in exploring it. Turned out it's quite interesting. Eight ...
Aleksander Alekseev - R2AUK's user avatar
4 votes
2 answers
476 views

SDRPlay RSP1 and broadcast band (AM) interference

I heard the SDRPlay RSP2 has an AM notch filter, but I don't want to spend that much money. How is the RSP1 for broadcast band interference? Will I need a preselector? I already have an MFJ-1020c ...
K8KV's user avatar
  • 377
4 votes
1 answer
965 views

DIY Triplexer for HF (low), 6m (med), and 2m/440 (high)

I'm looking at picking up the TYT TH-9800, which is a Quad-Band FM radio on 10m/6m/2m and 440 (I'm lucky enough to have some 6m and 10m repeaters in the area). I know there exists quad band antennas ...
Michael KC1MJP's user avatar
4 votes
1 answer
1k views

Cellular VHF Interference

I just got my license a week ago. Installed a new VHF/UHF radio in my car and everything seems to be working great. I've been heard at distant locations and picked up signals from repeaters that are ...
Matt KG7UDH's user avatar
3 votes
3 answers
1k views

Under what circumstances does intermodulation become an issue in a filter after LNA setup?

I'm reading about using software-defined radio for picking up ADS-B signals at 1090MHz. There is this blog which considers (among others) a setup as follows: ...
JJJ's user avatar
  • 133
3 votes
4 answers
711 views

Are directional couplers ever used in duplex repeater systems?

I understand that most single-band repeaters will use a tuned cavity duplexer to isolate the received signal from the transmitted signal, since the frequencies are very close and extremely sharp ...
natevw - AF7TB's user avatar
3 votes
2 answers
848 views

SSB Transceiver Architecture - USB with Crystal Filter

I'm planning to build a 20m SSB transceiver and I want to get the overall architecture straight in my head first. Here's what I'm thinking so far, a single conversion superhet design with a 9MHz IF ...
Buck8pe's user avatar
  • 989
3 votes
2 answers
5k views

Baud rate/frequency/bandwith/characters per second

I'm trying to filter a noisy serial line that comes in to a radio (mostly as a thought excercise in line filtering). The data comes in on a RS-232 line, 9600 baud, 8n1. I've found a lot of ...
Dieter Vansteenwegen's user avatar
3 votes
1 answer
141 views

Tradeoffs: varicap vs. air variable capacitor for Rx bandpass?

What might be the tradeoffs between choosing varicap diodes versus air variable capacitors in the design and construction of tunable bandpass filters for HF receivers?
hotpaw2's user avatar
  • 13.4k
3 votes
2 answers
45 views

Problem of two SAW fitler?

When I series two SAW filter, why the attenuation less than sum both of them in stop band region. Each of them (in stop band) have 60dB attenuation but when series them, the attenuation is equal to ...
Mahdiehtesham's user avatar
3 votes
1 answer
91 views

Help understanding transmission loss of trivial circuit

I was trying to DIY a VHF/UHF diplexer, and got some weird readings from the VNA. By debugging I reached this trivial circuit: Which has the following frequency response: I'm trying to figure out ...
Rimio's user avatar
  • 137
3 votes
1 answer
447 views

How can I determine if out of band interference is affecting my AIS reception?

AIS I am receiving ship AIS messages. These are low power, digital messages periodically sent on two marine channels close to 162mhz. This frequency is surrounded by FM broadcast, weather radio, and ...
hazzey's user avatar
  • 229
3 votes
1 answer
718 views

Tuning a SAW filter?

Does anyone know if it's possible to shift the center frequency of a SAW filter? I'm only talking about 0.1% shift, say from 433MHz to 437MHz. Can it be done or is the center frequency baked into the ...
Sam's user avatar
  • 161
3 votes
0 answers
91 views

Where did the LP to BP transformation $ \frac{\omega ^{2}-\omega_0^{2}}{\omega\cdot BW} $ come from?

I'm not sure this question is appropriate for this site, but I feel that if anyone knows about a filter, a radio ham knows! And as a ham myself I feel I need to understand this important area better. ...
Buck8pe's user avatar
  • 989
2 votes
2 answers
1k views

What is the current state of the art in CW filters?

In the 1970s I made a CW “filter” based on a phase locked loop that detected a specific audio frequency, and this drove an NE555 oscillator: the result seemed at first like a “zero bandwidth” CW ...
RiskyScientist's user avatar
2 votes
2 answers
129 views

What function do high-pass filters in a direct conversion receiver perform?

Use case: I'm considering hacking into my SoftRock RXTX to bypass the filters, then add external filters. I'm wondering if I can use just the low-pass filters I've already built, or if there's a ...
Phil Frost - W8II's user avatar
2 votes
3 answers
249 views

Maximum frequency for discrete components bandpass filter

What is the maximum frequency that can be practical to build a filter with discrete components? I need a band pass filter for 1090Mhz.
mpradio's user avatar
  • 75
2 votes
2 answers
298 views

What's the difference between LPF and BPF in context of bandwidth setting?

I'm used to simple RX/TX bandwidth configurations, on my VHF FM rig it's either narrow, normal, or wide. It seems to be much more complex on my new HF SDR rig, and the instructions don't say anything ...
rcx935's user avatar
  • 411
2 votes
2 answers
154 views

Second filter is not working on my mcHF kit

I'm building my first kit, it's the mcHF SDR radio. I've started with the low pass filters. The radio has four of them: I tested the first filter with a NanoVNA and got a result that resembled what ...
rcx935's user avatar
  • 411
2 votes
1 answer
158 views

How to select capacitor for diplexer use

Follow-up on this question: How to prevent cracking the capacitors in my diplexer? Short story: 2m/70cm diplexer, to be used up to 50W. Schematic from Elsie: S21/S11: I don't know exactly what to ...
Dieter Vansteenwegen's user avatar
2 votes
2 answers
100 views

Daisy Chaining AM filters to help with IMD

I live near an AM transmitter, and the input of my RTL-SDR becomes overloaded. I've added an AM filter and it's really helped, however I still get a very strong AM signal with some IMD all over ...
user14615's user avatar
  • 157
2 votes
1 answer
94 views

In which cases do SAW filters improve performance?

I am looking for ways to improve reception between two of my devices. The communicate on the 868 MHz ISM band with a 2-GFSK modulation. The problem is that one of them is heavily shielded by metal (...
Karsten's user avatar
  • 123
2 votes
1 answer
292 views

Coupling Capacitor of an Antenna

I have a simple question about general meaning & importance of the coupling capacitor in following part of a simple receiver circuit: Is it true that the proper reason why this coupling capacitor ...
user267839's user avatar
2 votes
1 answer
591 views

What is shape factor of a resonant circuit?

I have seen the shape factor of a resonant circuit defined in different places as both "the ratio of the -3dB point to the -60dB point", and elsewhere as "the ratio of the -6dB point to the -60dB ...
Harry Weston's user avatar
2 votes
1 answer
69 views

Identify filter on schematic

I am analyzing the Baofeng UV5R schematic. And trying to identify which filter was used in input UHF/VHF RF stage on RDA1846. It seems to me that it is a 3rd order Butterworth bandpass filter. But i ...
Adi Deljanin's user avatar
2 votes
1 answer
315 views

How do I choose between T37-2 vs T37-6 for this High-Pass Filter?

My RTL-SDR V3 is being overloaded by an AM broadcast so I'd like to build a high-pass filter to block the AM broadcast band. I thought I'd start simple and try making the one shown in Fig 3 of "...
Bezewy's user avatar
  • 153