Topband: Bandpass filters for receive antennas

[Mike Waters]

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 at 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.
>