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KX3 AM breakthrough (&KX2)

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Philip Anderson

KX3 AM breakthrough (&KX2)

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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Vic Rosenthal

Re: KX3 AM breakthrough (&KX2)

I live in line of sight of a 50 kW AM BC station and I had interesting problems with my K3 when I was using a vertical antenna. I made a 3 MHz highpass filter which fixed it. Note also that the common T network style antenna tuner also is a highpass filter, so that could be a simple solution. Switching to a horizontally polarized antenna will also help if practical.

Vic 4X6GP

> On 11 Aug 2017, at 21:21, Philip Anderson <[hidden email]> 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
> ______________________________________________________________
> Elecraft mailing list
> Home: http://mailman.qth.net/mailman/listinfo/elecraft
> Help: http://mailman.qth.net/mmfaq.htm
> Post: mailto:[hidden email]
>
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> Please help support this email list: http://www.qsl.net/donate.html
> Message delivered to [hidden email]

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k6dgw

Re: KX3 AM breakthrough (&KX2)

An open 1/4 wave stub at the BC station frequency might knock it down
enough to eliminate the problem. That's only 71.3 m [234 ft] of coax if
the station is at 1 MHz. [:-) More realistically, a series resonant L-C
pair across the antenna connector might do the job with less weight. We
used these in the "Olden Days" when the BC stn signal was driving the RF
amp non-linear. Of course, we didn't also transmit out of that
connector then either.

73,

Fred ("Skip") K6DGW
Sparks NV USA
Washoe County DM09dn

On 8/11/2017 12:26 PM, Vic Rosenthal wrote:
> I live in line of sight of a 50 kW AM BC station and I had interesting problems with my K3 when I was using a vertical antenna. I made a 3 MHz highpass filter which fixed it. Note also that the common T network style antenna tuner also is a highpass filter, so that could be a simple solution. Switching to a horizontally polarized antenna will also help if practical.
>
> Vic 4X6GP
>

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Philip Anderson

Re: KX3 AM breakthrough (&KX2)

Many thanks for all the thoughts and information people have offered on
this issue! If in addition to the extra high pass filtering the KX2 has
RX SHIFT permanently enabled, that certainly explains why it's immune to
AM breakthrough.

It sounds like it might be worth trying a 3Mhz low pass filter to sort
out 80m and above and accepting that RX SHIFT will need to be used on
top band (which I rarely use anyway). Because the local broadcast
station is quite close to top band (around 1.6Mhz), I doubt a high pass
filter that allowed top band through would reject the broadcast
transmitter sufficiently to sort the problem.

It seems like there might be a few commercial products which could work
- or I could just get out my soldering iron!

Phil G4PWO

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Philip Anderson

Re: KX3 AM breakthrough (&KX2)

opps that should be a 3Mhz HIGH pass filter!

On 12/08/2017 08:02, Philip Anderson wrote:

> Many thanks for all the thoughts and information people have offered
> on this issue! If in addition to the extra high pass filtering the KX2
> has RX SHIFT permanently enabled, that certainly explains why it's
> immune to AM breakthrough.
>
> It sounds like it might be worth trying a 3Mhz low pass filter to sort
> out 80m and above and accepting that RX SHIFT will need to be used on
> top band (which I rarely use anyway). Because the local broadcast
> station is quite close to top band (around 1.6Mhz), I doubt a high
> pass filter that allowed top band through would reject the broadcast
> transmitter sufficiently to sort the problem.
>
> It seems like there might be a few commercial products which could
> work - or I could just get out my soldering iron!
>
> Phil G4PWO
>
>
> ______________________________________________________________
> Elecraft mailing list
> Home: http://mailman.qth.net/mailman/listinfo/elecraft
> Help: http://mailman.qth.net/mmfaq.htm
> Post: mailto:[hidden email]
>
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wayne burdick

Re: KX3 AM breakthrough (&KX2)

Administrator

In reply to this post by Philip Anderson

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 <[hidden email]> 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 <[hidden email]> 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
>> ______________________________________________________________
>> Elecraft mailing list
>> Home: http://mailman.qth.net/mailman/listinfo/elecraft
>> Help: http://mailman.qth.net/mmfaq.htm
>> Post: mailto:[hidden email]
>>
>> This list hosted by: http://www.qsl.net
>> Please help support this email list: http://www.qsl.net/donate.html
>> Message delivered to [hidden email]
>
> ______________________________________________________________
> Elecraft mailing list
> Home: http://mailman.qth.net/mailman/listinfo/elecraft
> Help: http://mailman.qth.net/mmfaq.htm
> Post: mailto:[hidden email]
>
> This list hosted by: http://www.qsl.net
> Please help support this email list: http://www.qsl.net/donate.html
> Message delivered to [hidden email]