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Re: Topband: Bandpass filters for receive antennas

To: <topband@contesting.com>
Subject: Re: Topband: Bandpass filters for receive antennas
From: "Tom W8JI" <w8ji@w8ji.com>
Reply-to: Tom W8JI <w8ji@w8ji.com>
Date: Mon, 31 Aug 2015 14:43:26 -0400
List-post: <topband@contesting.com">mailto:topband@contesting.com>
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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