Oops. Forgot to include the link. Here is the link. http://sites.google.com/site/lwmwbandrejectfilter/
73, Don (wd8dsb) On Sun, Oct 11, 2015 at 9:03 PM, Don Kirk <wd8...@gmail.com> wrote: > 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 <mikew...@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 <w...@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. >> > >> _________________ >> Topband Reflector Archives - http://www.contesting.com/_topband >> > > _________________ Topband Reflector Archives - http://www.contesting.com/_topband