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17
votes
2answers
2k 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 ...
13
votes
4answers
805 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, ...
12
votes
2answers
2k 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" ...
12
votes
3answers
6k 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,...
10
votes
2answers
690 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 ...
7
votes
1answer
300 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 ...
7
votes
2answers
434 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 ...
6
votes
3answers
994 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 ...
6
votes
2answers
456 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 ...
6
votes
2answers
554 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-...
5
votes
1answer
623 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 ...
5
votes
2answers
1k 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 ...
4
votes
2answers
831 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 ...
4
votes
2answers
182 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 ...
4
votes
2answers
352 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 ...
4
votes
1answer
1k views

What frequency to set trap on?

I plan to build trapped multiband dipole so I read lots of documentation I found. There is one thing that confuses me: various authors recommend setting resonant trap frequency differently: some say ...
3
votes
1answer
63 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 ...
3
votes
1answer
206 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 ...
3
votes
1answer
342 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 ...
3
votes
1answer
565 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 ...
3
votes
0answers
75 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. ...
2
votes
3answers
114 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: ...
2
votes
2answers
67 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 ...
2
votes
4answers
321 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 ...
2
votes
2answers
363 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 ...
2
votes
2answers
54 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 ...
2
votes
1answer
246 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 ...
2
votes
2answers
107 views

Filtering VHF FM on my DTV signal

I've noticed that when I transmit on my HT when I'm watching TV (OTA), the signal drops and the screen goes blank (or digital "static") and then returns when I stop. I've replicated the issue by ...
1
vote
2answers
91 views

If I parallel two crystal filters together, will that double the bandwidth? [closed]

Will the bandwidth be doubled ? And will the impedance be halved ?
1
vote
1answer
58 views

Diplexer termination?

When using a 2M/70cm diplexer for low-pass filtering (or with only one antenna on one of the two "outputs"), should the unused connection be left open, or terminated? (using a suitable impedance, ...
1
vote
1answer
85 views

Can I filter after modulation when using a square wave oscillator? [closed]

Most designs appear to filter most of the harmonics out of the oscillator before modulation, then apply additional filtering as needed for the specific modulation they are using after modulation. ...
1
vote
1answer
120 views

Raspberry Pi rpitx for SSB with filter?

Using rpitx, we can transmit SSB using a single GPIO port from the Raspberry Pi. This of course uses square waves so creates a lot of harmonics. If I were to use a ...
1
vote
1answer
73 views

How can I improve my Es'Hail 2 reception?

I'm using an RTL-SDR to receive Es'Hail 2. My chain is as follows in order: 60 cm TV Satellite dish pointing 2 degrees off Es'Hail 2 Universal LNB (from TV company, not really a stable frequency). ...
1
vote
1answer
97 views

What dimensions to choose when winding your own inductor

Suppose you want to make a wilkinson 3-way splitter with lumped components Given the following circuit, you need a 31.5nH inductor The inductor can be wound various ways: 3 wdg, 1 cm diameter, 2.36 ...
1
vote
1answer
66 views

How to match crystals using NWT?

I want to build an ladder SSB filter using 8 MHz quartz resonators. I don't have frequency meter but I have NWT-70. How can I match crystals to build a good filter? What kind of parameters should I ...
1
vote
1answer
78 views

ICOM IC-910h How much bandwidth does it listen when receiving USB and FM?

CD-audio has a bandwidth of 22kHz. CW usually has a bandwidth between 3 to 5 kHz. But I am wondering what is the bandwidth my Icom IC-910H can listen to in both, the SSB and the FM mode? Also, what ...
1
vote
1answer
160 views

Toroidal inductor and transformer basics

In basic terms, using an Ultimate3S Beacon (above) as an example, what do the various toroidal components do exactly? How do they accomplish this? In construction terms, does it matter if the wound ...
0
votes
1answer
188 views

Cell Phone Filter / Interference?

How do cell phone filters filter out all the extraneous EM waves and noise (especially noise) when you get a call? I would expect a phone to have a bandpass filter to get rid of most noise but even ...
0
votes
2answers
71 views

What's the practical significance of the source impedance in an RF filter?

I'm reading through a tutorial on filter design and Bartlett's bisection theorem. It gives an example of a filter with 2.909k terminations: Then, the right side of the filter is scaled by a factor of ...