[Elecraft] K3 + LP100A + Dummy Load

[Don Wilhelm]

Phil,

All dummy loads are not created equal.  Connect yours directly (no coax) 
to the LP-100 or an antenna analyzer to determine its characteristics 
with frequency (I assume Larry calibrated your LP-100 to NIST 
standards).  A common ham dummy load is something to tune your amplifier 
into and use for general purposes.  If you are going to do measurements, 
you will need something more like lab quality dummy loads.

Both your dummy load and the 6 feet of RG8X will show a frequency 
dependency since you do not have a precision 50 ohm dummy load.  A 
transmission line that is not terminated in its characteristic impedance 
acts like an impedance transformer, and that transformation ratio is 
frequency dependent.

If you want a dummy load that is flat WRT frequency and suitable for 
instrumentation use, take a look at the Caddock 50 ohm Thick Film 
Resistors - they are available up to 100 watt rating.  You must use them 
on a heat sink, and keep the leads as short as possible, and you will 
have a precision dummy load.  Caution - even with the heat sink, do not 
exceed their power ratings, they will go open circuit in an instant.
I use several of those as instrumentation precision loads.

Other sources of precision dummy loads is Ridge Equipment - 
https://www.ridgeequipment.com/store/dummyloads.html.  They have one 
rated at 75 watts for $12 and one rated at 15 watts for $7.50 - those 
are quite good for instrumentation purposes.

73,
Don W3FPR

On 12/22/2011 11:37 AM, Phil Hystad wrote:
> Is this normal or can someone explain?  I was looking at the R+iX values reported by the LP100A with my K3 feeding a dummy load.  The SWR reported was always small but never super close to 1:1.  Most of the SWR in this setup was 1.1:1 up to 1.5:1.  The R values ranged from 34 ohms to up to 59 ohms and X was ranging over a number of smallish values<  20 for all the bands.
>
> Is this what most of you would call normal?
>
> Does the K3 output R+iX vary from band to band (I assume so) and is that what I am seeing?
>
> I had thought my dummy load was mostly frequency independent so I am not sure if I am seeing mostly the K3 here or maybe the K3 plus the 6 feet of coax (RG8X) + dummy load.
>
> 73, phil, K7PEH
>