2m Bandpass Filter for Cheap Quansheng K5 Handheld Radio Connected to External Antenna

I built my first 2m bandpass filter for Quansheng k5(8), and during testing noticed that it receives weak off band (air-band) signal with filter better than without filter even when this filter attenuates air-band signals by -14db (filter was designed to pass 142-148MHz band and filter out everything else).
Probably FM broadcast stations overloads frontend so much, that even bad filter makes radio work better.
When I tuned my IC-7100 to that air-band station it receives better without filter (s5 vs s1) as it should.
To calculate filter parameters I used next calculators:
To start I use bandpass filter calculator markimicrowave.com/technical-... to select a schema and calculate filter parameters.
To calculate coil dimensions I use online calculator like this: coil32.net/online-calculators...
I use all of those calculators as a staring point, and after I built it I use a nanoVNA s.click.aliexpress.com/e/_oDj... to fine-tune a filter.

[@TrimeshSZ]

The front-end on those things is basically wide-open - there are two switchable RF paths with provision for filtering components, but the VHF one has been setup to pass between about 130-175MHz, so you get lots of potential for interference with stuff like high power PMR. So you have about 40Mhz of "stuff" hitting the LNA and lots of intermod products. If you have a microscope, lots of time and a willingness to swear a lot then you can build the filters inside the radio - I took off the old filter components and build a 4th order Cauer filter with a passband between 144-148MHz in the VHF path instead - it now works much better on 2m - but it's obviously as deaf as a post anywhere else.

[@user-lc2gz9tq5l]

Great

[@johnemmons9087]

Awesome! Can you make something so we can successfully transmit on 11 meter CB band? Everyone is looking for this! 👌😊👍

[@yurishbox]

We can filter out spurious emissions using bandpass filter, but this way transmission power on CB band will remain milliwatts. Also stock radio has pretty bad signal/noise ratio on HF. HF mod should help, but for receive only. However it’s amazing radio for the price and I hope some interesting HF solution will be found soon.

[@johnemmons9087]

Yes. I agree with you. However, could you not build a mini board to mount on the main board to solve the power issues with CB? I know hundreds of people around the world that would purchase something like this! You could make much money! Every one around the world are talking about using the Quansheng for CB! Please consider it! Thank you!

[@marceloeduardoluiz]

alguém viu diferença?

[@EWSRadio]

Which filter did you use? Butterworth?

[@yurishbox]

Yes

[@gilvanzb]

Quais seriam as medidas dessa bobina e diametro

[@yurishbox]

Coil parameters depends on capacitors are used. To start I use bandpass filter calculator markimicrowave.com/technical-resources/tools/lc-filter-design-tool/ to select a schema and calculate filter parameters. To calculate coil dimensions I use online calculator like this: coil32.net/online-calculators/one-layer-coil-calculator.html I use all of those calculators as a staring point, and after I built it I use a nanoVNA to fine-tune a filter.

[@gilvanzb]

@@yurishbox ok muito obrigado

[@AdamosDad]

Yes, but why?

[@yurishbox]

I think that radio does not have good filters and powerful FM broadcast stations located about 108MHz might overload radio receiver even when radio tuned to 135MHz. When I add a bandpass filter it filters broadcast stations for over -40db, and air band receiving benefits from that even if filter decreases airband signal for -10db. I would expect if I use air band bandpass filter it would work even better for air band. It was just an experiment with 2m filter I had. When I tried to connect the same bandpass filter to Icom-7100 it didn't provide any benefits, because Icom already has good filters and does not suffer from off band strong signals as this UV K5 radio.

[@AdamosDad]

@@yurishbox I was just kidding, I like to experiment to see how things can be made better myself. I have currently built a small high power(500watt) 40-15meter mag-loop antenna. I learned somethings from your comment, thank you for responding to a cranky old man. 73's🎙KD9OAM🎧

[@SP6QKX]