Re: [K3] SWR - Numerical Indication

Posted by John Oppenheimer on Dec 01, 2018; 1:04pm
URL: http://elecraft.85.s1.nabble.com/K3-SWR-Numerical-Indication-tp7643839p7646698.html

Hi Dick,

Some numbers: using the K3 Schematic, "K3S RF Board" Sheet 2, Section
B1. The power meter uses a series Schottky diode detector. The low
reasonable resolution is about 0.3 VDC, or 0.3 * .707 = 0.21 Vrms out of
the directional coupler.

The Power for 0.21 Vrms is:
0.21^2 / 50 = .88 mW
Assuming a 20 dB coupler 1:10 turns ratio, 0.88mW * 100 = 0.88W
reflected power.

Assuming forward power of 100 watts and 0.88 watt reflected power the
return loss is:
10 * log(100 / 0.88) = 20.6 dB

Using the calculator:
https://www.allaboutcircuits.com/tools/vswr-return-loss-calculator/
20.6 dB Return Loss computes to VSWR = 1.222

Therefore, the best lowest estimated K3 VSWR, assuming a 20 dB coupler,
is about 1.2.

Check my numbers.

John KN5L


On 7/27/18 9:58 AM, Dick Dickinson wrote:
> I've noticed that I'm not showing a reading of less than 1.1:1 SWR on my
> antennas per K3 SWR Numerical Readout.  K3EZ will record 1.0:1 SWR in band
> sweeps.
>
> Can the K3(S) Numerical Display show 1.0:1 SWR?  If so, is there a likely
> reason why K3EZ will show 1.0:1 while my K3 will only go as far down as
> 1.1:1?
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