[CQ-Contest] band-pass filtering -- a new approach?

Jerry Flanders jflanders2 at home.com
Fri Nov 9 11:49:42 EST 2001

I would try some measurements at low power levels and see how much unwanted 
RF actually shows up before I started building filters to reduce it.

I am not sure how much RF a modern rig would tolerate before damaging the 
front end, but I found that I had less than a quarter volt rf at the rig on 
the "other" antenna when using only simple stubs, even with high power on 
20, 15, and 10 M. 40 M was higher, but is FB with 150 W or so..

I rigged up a measuring circuit with a 50 ohm resistor, signal diode, and a 
cap. I measured the voltage across the cap with a DMM. Started at QRP level 
and worked my way up so there could be no surprise smoke.

My SO2R antennas are a vertical dipole for 10 and 20, which has a 15/40 
rejection stub, and a 40/15 horizontal dipole which has a 10/20 rejection 
stub. Separate feedlines, of course. I have operated this setup in one 
contest at low power to check everything out and then another at high power 
RTTY (600W on 10m, 500W on 15m, 1000W on 20m, and 150W on 40m) for 40 hours 
without incident. I operated either 15/40 along with either 20/10 
simultaneously. One 756PRO +AL-1200 (10/20) and one FT-1000D + FL7000. No 
antenna tuner - resonant antennas

My antennas are cross-polarized, and are separated by 150 feet, which 
probably helps a lot. No audible cross-talk unless I actually tune to the 
tx's second harmonic, which I remember was 40 over 9 the one time I looked. 
I could operate 5-6 kHz away from the second harmonic.

I may add filters later, mainly so I don't have to lose sleep over the 
higher rf level when I tx on 40 M at higher power. On the other hand, I 
don't really know what level of RF the rigs can tolerate - maybe much 
higher than I could generate on 40M. Sure would be nice of some of you guys 
would sacrifice a FT-1000D or two and let us know at what level it smoked. 
I hate flying blind!

Jerry W4UK

At 10:12 AM 11/9/2001 -0500, Pete Smith wrote:

>I've been researching band-pass filtering for SO2R, and it has occurred to
>me to try a different approach if the peer review sounds favorable.
>What I'm planning is to build a box for each of my transceivers, containing
>a receiver protection circuit (diodes and a resistor), receive-level
>bandpass filters and a relay switching matrix to select the filters AND
>bypass them on transmit.  This would seem to offer several advantages --
>I can use W3LPL's 3-pole receive filter circuit, which uses standard cap
>values and delivers performance that is probably superior to any commercial
>alternative except for W3NQN's expensive transmit filters.
>I avoid issues with burning out the filter components from transmit power,
>particularly with a messy amplifier input SWR on some bands.
>I avoid the reduction in drive to my SB-220, which really needs its 120 watts.
>I can be fairly relaxed about the physical layout, since SWR is not the
>issue it is on the transmit side.
>I realize that having a bandpass filter on the transmit side offers
>advantages too, in particular reducing harmonic output and wideband noise.
>But on balance, this seems like a good idea, plus it'll give me a project
>for the winter.  Comments?
>73, Pete N4ZR
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