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