Looking at the AT1 manual, the switches and traces are certainly
not 50 Ohm transmission lines. As such, one gets all kinds of
stray behavior. While you do not say at what frequency or range
of frequencies you made your test, I would suspect you will see
higher SWR as one goes up in frequency and that the departure
from 50 + j0 will be higher on 144 MHz than on 1.8 MHz.
As a general matter, one may be able to "flatten" the SWR somewhat
by placing (and "tuning") shunt capacitors across the input/output
BNC connectors to compensate for the inductance of the traces and
switches.
73,
... Joe, W4TV
On 9/22/2023 10:14 AM, Steve Gray wrote:
I have purchased a second user AT1 recently, largely due to the ruinous
TransAtlantic logistics costs, but also the ongoing absence of any support here
in The UK.
Although purely from observation of the circuit topology it seems reasonable to
assume that it is not a constant impedance array of resistors, I was quite
surprised to see the high SWR readings recorded after connecting a good dummy
load and an antenna analyser- encroaching as it does well into double figure
readings. My received wisdom being that these devices need to look into a 50
Ohms load to preserve accuracy.
I have checked all the resistors and they are extraordinarily accurate leading
me to believe that it has not been cooked by the/a previous owner, and
confirming the quality of Elecraft dedigns. I also noted that the greater the
level of attenuation, the lower the SWR is with the lowest attenuation
displaying the worst result, and the highest displaying a near perfect match,
but I guess that is not surprising because it is attenuating the reflected RF
too. What I would like is some reassurance that such elevated SWR readings are
to be expected for a nominally 50 Ohms stepped attenuator of this nature set to
the lower levels of attenuation.
Steve, G4VRR.
e&oe ...
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