2 MHZ low pass filter for MW and LW reception

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DaveB
Posts: 129
Joined: Sun Feb 01, 2015 6:04 pm

2 MHZ low pass filter for MW and LW reception

Postby DaveB » Thu Dec 01, 2016 11:33 pm

Medium wave reception suffers break through from the LF shortwave bands. Based on an article found on the internet for a filter for LW radio I've built a filter for MW using cheap axial inductors and polyester capacitors. I've documented it in the link below and hope it will be useful.

As a quick summary - anything above 3 MHz is killed almost stone dead with -60 dB attenuation and it also takes care of FM and DAB signals.

I hope this is useful - use a slightly bigger box to make assembly easier.

https://app.box.com/s/f9od2ikdtklcajyfuznynddgcknzwcau

David

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g1hbe
Posts: 216
Joined: Sat Jan 17, 2015 3:28 pm
Location: Cheshire

Re: 2 MHZ low pass filter for MW and LW reception

Postby g1hbe » Fri Dec 02, 2016 9:40 am

Thanks for posting that, Dave. I've built myself a switchable filter box. It's an ABS box with suitable connectors and inside are:

400KHz LPF for beacon & LW listening.
2MHz LPF for MW.
1.7 to 2.2 MHz BPF for the 160m band.
2MHz HPF for general HF.

All 5 pole type filters built from junk-box components. Each filter can be selected or bypassed with a 2-pole changeover toggle switch. There are four switches, one for each filter obviously. This filter goes between my Wellbrook loop aerial and the Hi-z input of the RSP2. I've put a 50r resistor across the receiver input to terminate the filters correctly. Filters were designed with the aid of RF Sim99, a freeware package from Ye Days of Yore.

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Andy

DaveB
Posts: 129
Joined: Sun Feb 01, 2015 6:04 pm

Re: 2 MHZ low pass filter for MW and LW reception

Postby DaveB » Fri Dec 02, 2016 10:48 am

Hi Andy,

My main interest is FM DX in the 65 - 108 MHz range - the MW interest is secondary but might develop once I get some transatlantic reception. I did get Xingyang on 1377 kHz a few evenings ago. The only aerial I have is a 70 MHZ dipole fed directly to the RSP (now through the filter)

As far as aerial switching is concerned once you get to VHF switches and relays have to much capacitance and just leak the signals through - I managed 45 dB isolation with two supposed RF relays in series which is nowhere near good enough. So my solution was to build a patch box. This consists of a row of chassis mounted f-sockets on a diecast box directly wired to corresponding phono sockets. The centre socket feeds the SDRPlay and a short flexible jumper from that to any of the other sockets gives the aerial path. That gives an isolation of about 55 to 60 dB. This box handles a total of six inputs. There might not be a constant impedance - but I doubt I'll notice the difference. I've had it a couple of years now and have had to change out one phono socket which got a bit worn which is not bad considering how much use it gets.
I built a 2nd box using 3.5 mm jack sockets which automatically terminates any un-used inputs with a 75 ohm resistor but because of the stray capacitance it only has about 50 dB isolation.

David

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