Hi Mike,
Here is a link to a simple website I created that describes a high pass
filter I built earlier this year that rejects both the LW & MW band. It
was originally described in a Feb 1978 QST article. I use it in front of an
SDR receiver, and really happy with its performance.
Don (wd8dsb)
On Sun, Oct 11, 2015 at 8:35 PM, Mike Waters <mikewate@gmail.com> wrote:
> Where can I find a schematic for a good high pass filter that rolls off at
> 1700 kHz?
>
> Yesterday, I installed a preamp --that I made over 20 years ago-- right at
> one of my 2-wire Beverage antennas. Even in the reverse direction (NW)
> where losses are the greatest, it easily overloads from out-of-band
> signals. Today, I even noticed it oscillating.
>
> It uses an MRF5812 NPN bipolar. I built it over 20 years ago, before I knew
> some things I've learned since then (such as being more careful about
> separating the input and output).
>
> I just took it out of line and brought it back to the house. It's the one
> on the right near the bottom of
> www.w0btu.com/W0BTU-broadband-preamps.html on the Radio Shack proto board.
> Not shown in the photo is an RF choke between the input and output that
> passes the 12 VDC through the coax, so it can power the relay that switches
> from SE to NW. Also, the back-to-back diodes and the input circuit are
> slightly different.
>
> There are some other things I want to do to it, but I think a HP filter
> should be near the top of the agenda. Any suggestions would be appreciated.
>
> 73, Mike
> www.w0btu.com
>
>
> On Mon, Aug 31, 2015 at 1:43 PM, Tom W8JI <w8ji@w8ji.com> wrote:
>
> > Bandpass filters do nothing for in-band signals -- their only function is
> >> to reject OUT OF BAND signals. The primary reason for using bandpass
> >> filters IS for RX. A good 160M bandpass filter would be effective at
> >> reducing overload from AM broadcast stations. So would a high pass
> filter.
> >> There is a survey of bandpass filters for contesting at this link.
> >> http://k9yc.com/BandpassFilterSurvey.pdf
> >>
> >
> > I use a high pass filter for rejection of the AM BCB band. Between 70 and
> > 80% of the net power (or voltage) into my RX system comes from distant AM
> > BCB signals.
> >
> > Without a small 5-pole highpass that starts to roll off at 1700 kHz, I
> can
> > connect a miniature 12V 50 mA incandescent lamp (like the MFJ 1025 uses
> as
> > a fuse) and it illuminates a dull red.
> >
> > This is with no attempt at matching power to the filament cold
> resistance.
> >
> > My system can be bothered by the sum of all those thousands of signals, I
> > add a BCB high pass, and then I can run 1500 watts and not bother my own
> RX
> > when transmitting on 80 or 40 while receiving on 160. Of course I have
> > 500-2000 ft separation on antennas, but this still shows how a bunch of
> > small signals can add up to disaster if they hit something non-linear
> > before being filtered.
> >
> > Always remember there are two problems. One is the absolute limit of
> > in-band signal a receiver system can take. The other is the absolute
> limit
> > of the sum of all the signals entering an overload sensitive point in the
> > system.
> >
> > Less than one volt peak line voltage is not enough headroom to prevent IM
> > products in a reasonably good system. Back-to-back parallel diodes are
> fine
> > for Sky Buddy receivers and FT101's. A single diode opposing another
> diode
> > in parallel will clamp at about 6 dBm if your receiver looks like 75
> ohms.
> > Almost all receivers will conservatively take 15-20 dBm, or 2-4 volts
> peak,
> > at the antenna port in band.
> >
> > If you have a good system, you'll want something other than back-to-back
> > diodes.
> >
> _________________
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