K3: SWR meter readings on 10m...

[VK7JB]

Hello All,

I've just noticed that the SWR meter on my K3 is reading higher than expected on 10m.

I have 2 different 50 ohm dummy loads. Both have <1.1:1 as measured by my external LP-100A meter and  my MFJ 259b AA.  

If I connect either dummy load directly into the ANT1 connector of the K3 (no connector lead at all or a very short lead between the DL and the K3 ANT1) and read SWR on 10m, it is measured by the K3 as 1.5:1.   On 15m, SWR reads 1.4:1 but on 20m and lower bands, the SWR displayed by the K3 accords with the LP-100A external meter (1.1:1).

Is this frequency-related phenomenon common to all K3's, or just mine?  Is any adjustment possible or am I sweating over nothing.  Perhaps this SWR difference is within spec @ 28MHz??


73,

John
VK7JB

[VK7JB]

I've solved my mystery.

I read in the archives that the KAT3 ATU is not completely out of circuit in "bypass mode".  If I understand correctly, small values of L and C are used to remove it functionally, but there is stray reactance that might contribute to the higher indicated SWR on some bands.

So then I wondered if the KAT3 might need to be tuned at least once into a 50 ohm load for the "bypass" state to be correctly determined.  Made sense in my imagination, and thinking on it,  I realised I'd never tuned the ATU into a 50 ohm load before.

So I did that - tuned the KAT3 to the dummy load on the bands where it previously indicated higher than expected in ATU bypass mode.  I let it do its stuff and find a match for the dummy load, then bypassed the ATU.  

Now, in ATU "Bypass" mode, the K3 reads 1.0:1 SWR into the same dummy load that it previously showed as 1.5:1.   Issue gone.

73,
John
VK7JB

[Dave Perry N4QS]

John,

I noticed the same high SWR in bypass mode on 10M the other day.  I have the
KPA500 and thus was wondering why the K3 was seeing up to a 1.7 to 1 SWR
into the KPA500.  So I got home from work today and tried your solution.  I
turned on the ATU and tuned into a 50 ohm dummy load.  When I went back to
normal operation in Bypass mode, the SWR into the amp was back to 1.1 to 1.
Thanks for the tip.

Dave, N4QS

----- Original Message -----
From: "VK7JB" <[hidden email]>
To: <[hidden email]>
Sent: Monday, June 27, 2011 7:39 AM
Subject: Re: [Elecraft] K3: SWR meter readings on 10m..mystery solved...

> I've solved my mystery.
>
> I read in the archives that the KAT3 ATU is not completely out of circuit
> in
> "bypass mode".  If I understand correctly, small values of L and C are
> used
> to remove it functionally, but there is stray reactance that might
> contribute to the higher indicated SWR on some bands.
>
> So then I wondered if the KAT3 might need to be tuned at least once into a
> 50 ohm load for the "bypass" state to be correctly determined.  Made sense
> in my imagination, and thinking on it,  I realised I'd never tuned the ATU
> into a 50 ohm load before.
>
> So I did that - tuned the KAT3 to the dummy load on the bands where it
> previously indicated higher than expected in ATU bypass mode.  I let it do
> its stuff and find a match for the dummy load, then bypassed the ATU.
>
> Now, in ATU "Bypass" mode, the K3 reads 1.0:1 SWR into the same dummy load
> that it previously showed as 1.5:1.   Issue gone.
>
> 73,
> John
> VK7JB
>
>
>
> --
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> Sent from the Elecraft mailing list archive at Nabble.com.
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