[Elecraft] KX3 AM breakthrough (&KX2)

[Wayne Burdick]

Wunder,

The KX3 does have high-pass filtering integrated into the T/R switch, like the KX2. But it starts rolling off below the 160 m band. The KX2’s rolloff starts somewhat higher, as you noted.

73,
Wayne
N6KR



> On Aug 11, 2017, at 11:53 AM, Walter Underwood <wunder at wunderwood.org> wrote:
> 
> The KX2 has internal filtering to reject frequencies below 3 MHz. The filter is 30 dB down at 1.5 MHz, and 56 dB down at 1 MHz. The KX3 does not have that filtering, probably because it would interfere with performance on 160 m.
> 
> There was some discussion of this a few years ago, including suggestions for commercial filters.
> 
> http://elecraft.365791.n2.nabble.com/Broadcast-band-interference-td7568427.html <http://elecraft.365791.n2.nabble.com/Broadcast-band-interference-td7568427.html>
> http://www.digitalhams.net/ForumMsgs/msgsDisp.php?forum=KX3&subject=Broadcast+Band+Interference <http://www.digitalhams.net/ForumMsgs/msgsDisp.php?forum=KX3&subject=Broadcast+Band+Interference>
> 
> Contact Elecraft support and they should send you a schematic for a high pass filter that rejects the AM broadcast band.
> 
> wunder
> K6WRU
> Walter Underwood
> CM87wj
> http://observer.wunderwood.org/ (my blog)
> 
>> On Aug 11, 2017, at 11:21 AM, Philip Anderson <phil.anderson at invades.net> wrote:
>> 
>> Hi everyone,
>> 
>> I have been using a KX2 as my main rig for most of this year, it works very well although the receiver sometimes struggles a bit from my favorite portable location right next to the sea, where there's often a large amount of very strong signals.
>> 
>> Because of this, recent review comparisons of the KX2 with the KX3 tempted me to buy a KX3 mostly for the extra receive performance. Unfortunately, when I tried the KX3 from my favorite portable location next to the sea yesterday, I had a problem with breakthrough of an AM broadcast station on 20m, the only way I could completely eliminate the problem was by turning on RX SHIFT, which unfortunately disables the roofing filter (a big reason for buying the KX3) and dual watch.
>> 
>> I understand from comments I have read that this issue is an inherent weakness of the direct conversion architecture, however - my KX2 never suffers from this problem.
>> 
>> So before surrendering to the possibility that I've spent $$$ on a larger KX2 with disabled dual watch ;) I thought I'd explore alternatives to using the RX SHIFT feature.
>> 
>> Since I've never had this problem with my KX2, and since that radio does not have the RX SHIFT feature, I'm assuming that for some hardware reason, the problem of AM broadcast breakthrough is not expected on the KX2.
>> 
>> Does anyone know what the difference between the two is? I suspect that the KX2 has a more severe high pass filter, in which case a 3Mhz high pass filter in the KX3 aerial lead might help sort out everything above top band.
>> 
>> Has anyone tried that?
>> 
>> Has anyone found any other solutions?
>> 
>> Phil G4PWO
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