Topband: FAR Circuits K9AY Loop Preamp/Bandpass filter

[Joe Subich, W4TV]

Pete, 

You are looking at the schematic backward.  The input goes to 
the filter then through the preamp and back out the relay. 

The four series caps plus the shunt LC sections are a high pass 
filter with a corner frequency about 1600-1700 KHz.  The four 
shunt caps and three series inductors are a lowpass filter with 
an upper corner frequency just above 4000 KHz.  The preamp is 
a standard "feedback" design (see the W7IUV data or look at the 
ON4UN "Low Band Bible").  

In general the Far Circuits design is not too bad for something 
that can be easily duplicated although I suspect the Q of the 
inductors are not particularly high which results in greater 
insertion loss and poorer stopband response than might be found 
in a higher quality filter. 

> I have a separate ARR wideband preamp here, which I have used 
> with K9AY loops in the past with some success, but wonder if 
> the FAR Circuits design would be better for 160 and maybe 
> 80. 

If you're happy with the ARR preamp and don't have BC overload 
issues, you can probably bypass the Far Circuits filter/preamp 
("main board") without a problem. 

73, 

   ... Joe, W4TV 
 



> -----Original Message-----
> From: topband-bounces at contesting.com 
> [mailto:topband-bounces at contesting.com] On Behalf Of Pete Smith
> Sent: Wednesday, January 06, 2010 6:10 AM
> To: topband at contesting.com
> Subject: Topband: FAR Circuits K9AY Loop Preamp/Bandpass filter
> 
> 
> I picked up a FAR Circuits kit for a complete K9AY loop system at 
> Dayton, and am building it now. I have a separate ARR wideband preamp 
> here, which I have used with K9AY loops in the past with some 
> success, 
> but wonder if the FAR Circuits design would be better for 160 
> and maybe 
> 80.  If not, I'll just bypass that part of the circuit.
> 
> Typical FAR, I guess - next to no information about the circuit is 
> available.  The bandpass filter uses three inductors to ground with 
> series caps and three in series with caps to ground.  The 
> preamp has a 
> center-tapped bi-filar matching transformer on the input, with the 
> output signal fed from the collector through half of the 
> transformer to 
> the output jack, and an RC network from that center tap back 
> to the base 
> (negative feedback?).
> 
> Does this sound familiar to anyone?  Does the BPF cover both 
> 160 and 80? 
>   If you need a schematic to comment, I'll be glad to put one 
> up on the web.
> 
> -- 
> 73, Pete N4ZR
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