[TowerTalk] Field Day

jimlux jimlux at earthlink.net
Mon Jul 3 17:26:32 EDT 2017

On 7/3/17 11:03 AM, Richard (Rick) Karlquist wrote:
> On 7/3/2017 9:13 AM, Jim Thomson wrote:
>> From: Grant Saviers <grants2 at pacbell.net>
>> We experience lot's of buzz from cw into ssb, need to get as far up the
>> band as possible.  Reverse also true.  About the same level of trouble
>> Grant KZ1W
>> ##  band pass filters, each being 7000-7300, probably are not going to
>> do much good,
>> if 2 x xcvrs used on the same band, say 40m cw..and 40m ssb.   Heres a
>> thought though.
>> I see ICE and others make band pass filters for the warc bands, like
>> 100 khz wide for 17+12M,
>> and only 50 khz wide for 30M band.    If they can make a 50 khz wide
>> band pass filter for 30M
>> band, they, or somebody should be able to make any BW  filter you
>> want.    IE:  say 7000-7050,
>> Jim  VE7RF
> I am extremely skeptical that any off the shelf filter for 30 meters is
> only 50 kHz wide, simply based on the fact that the band is 50 kHz wide.
> That is only 1/2%.
> It is worth reading "Field Day Filters" (April 1973 QST, page 18).
> The authors made a fairly serious attempt at building helical resonator
> filters.  Even with this technology, the authors state that the
> filters they built are not sufficiently selective to do much good
> in terms of separating the phone and CW sub-bands.
> A better strategy is to use separate receive antennas (as opposed
> to receiving on the transmit antennas).  That gives a lot of flexibility
> to combat interference in its own right.  We had a lot of success with
> this at K6AO ten years ago.

These days, with inexpensive microcontrollers and vector mods, how about 
adaptive cancellers - feed a Tx sample in, adjust the phase and 
amplitude with the vector mod driven by a pair of DACs, and notch that 
transmit signal right out.

I do it at 3GHz and get a >30-40 dB null in 3 seconds using a not very 
sophisticated algorithm.

The nice thing is that it's all low power stuff - you'd collect a low 
power sample of the tX signal (<1 mW would be a lot), whether from a 
pickup antenna, or some sort of resistive T coupler.  It would be single 
band: you'd need a canceller for each Tx band; but theyre low power, so 
band select could be simple LC lumped filters.

It would be a bit of a rats nest of cables: N*(N-1) cables for N 
stations.  OTOH, maybe you could do it with some clever processing and 
two pickup antennas for each receiver?

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