[Elecraft] KX3 6m receiver sensitivity (was K3 6/10M preamp)

[Edward R Cole]

Joe,

Thanks and a good question so I made a quick test at 50.125-MHz:

KX3+ATU -> 6m amp* -> yagi  (*amp off)
SWR = 1.2

KX3+ATU(bypass) -> 6m amp* -> yagi

SWR = 2.5

Looks to me that the ATU in the KX3 is switched out in bypass else 
the SWR would have not changed.
Why no change in MDS whether in ATU or in bypass may just mean there 
was no effect on the XG3 level.
------------------------
Not germane to the topic but with the 6m turned on (KX3 driving the amp):
ATU:  SWR=1.7  (obviously the ATU is tuned for input into the antenna)
bypass:  SWR=1.5

side note on Mirage amps:  My experience is they affect impedance 
when radios are connected even in bypass:
Tests with 6m amp removed:
ATU: SWR=2.6 without retuning the ATU
bypass: SWR = 1.8

This made me suspicious of findings so I did more extensive tests by 
including my Bird 43 meter to confirm loads.
I connected my Bird 43 directly to a 25w Bird Termaline Load which 
shows zero reflected power when used at 1296-MHz and Z=51+j0 on my 
antenna analyzer at 144-MHz.

I swapped out cables and adapters to check if any were not good.  I 
even used another coax load that I tested at Z=54+j3.  Results were 
that when ATU is in bypass the load shows SWR=1.0 to 1.1 on all 
frequencies thru 10m but shows SWR= 1.6 on all 6m frequencies.  The 
Bird shows absolutely no reflected power on any freq. with the 
load.  Tests were made at 5w as shown on the Bird with 5A element.

Conclusion is that the impedance of the ATU bypass is not 50-ohms on 
6m.  This also questions the earlier readings made in ATU bypass.  I 
re-tested the 6m antenna SWR = 1.5 with the ATU in-line and with the 
Bird in-line.  The ATU had been pre-tuned to 1.0 into the 50-ohm 
load.  The Bird shows .25w reflected with 5w forward (SWR from the 
chart is 1.57); good agreement between SWR meters.

Simple solution is always use the ATU on 6m.  KX3 with ATU is tuned 
to 1.1 looking thru the 6m amp and 1.3 when driving the amp.  Happy 
KX3.  Mirage amp is happy with SWR=1.57 into the antenna.

73, Ed - KL7UW
--------------------
From: "Joe Subich, W4TV" <lists at subich.com>
To: elecraft at mailman.qth.net
Subject: Re: [Elecraft] K3 6/10M pre-amp
Message-ID: <51DDA3E0.2080407 at subich.com>
Content-Type: text/plain; charset=ISO-8859-1; format=flowed


Ed,

  >> Bypassing the ATU did not change the level so I guess the ATU was
  >> already adjusted to 50-ohms.

I have not checked the KX3 schematic but based on the K2 and K3
"bypassing" the ATU only adjusts it to a stored condition that the CPU
'thinks' represents 50 Ohms.  I've seen several of the other rigs be
very far off when the ATU is bypassed and known 50 Ohm load attached.

I would always "tune" into a known 50 Ohm load before switching the
ATU to bypass - or in the case of the K3 use the RX IN - before making
any measurements if the radio under test has an ATU installed.

73,

     ... Joe, W4TV

73, Ed - KL7UW
http://www.kl7uw.com
dubususa at gmail.com
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