GM All,
Can someone recomend me a simple high pass filter that will reject strong AM
signal trasmitted at 225 khz?
I've just looked at Petr OK1RR website and there is nice filter, but rejects
400khz-1600khz band, so will not work in my case...
This signal at 225 Khz comes from Polish Radio I AM station, located abt
120kms away, but it puts 9+60db with ATT-30db turned on (IC 765) and it's
quite spourius since abt 3 years (I know they changed something in the audio
to make it "more readable"...).
In practice, I have QRM and harmonics on 160m (specially in daylight) and it
makes impossible measuring any beverage antenna in my QTH, using MFJ or VNA
analysers as you can see on this video:
https://www.youtube.com/watch?v=pEWTYrRPgjo
TNX for help and CU soon on the band
73's
Mac SP2XF (SN2M)
----- Original Message -----
From: "Tom W8JI" <w8ji@w8ji.com>
To: <topband@contesting.com>
Sent: Monday, August 31, 2015 8:43 PM
Subject: Re: Topband: Bandpass filters for receive antennas
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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