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 <wd8dsb@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 <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.
>> >
>> _________________
>> Topband Reflector Archives - http://www.contesting.com/_topband
>>
>
>
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