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.
73 Tom
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