[Amps] 50 Ohm Loads

[jeremy-ca]

Jim, how about backing into Elsie with a .01 cap as that is what I believe 
Gates used in that load.
With large diameter nichrome elements arranged in an array that minimized 
coupling I would have to assume that the inductance was quite low.

I just measured three 50 Ohm wirewound resistors. The flat 25W Clarostat 
is110 uh, a flat 95W Dale is 280 uh and the 200W round Ohmite is 880 uh.

Carl
KM1H


----- Original Message ----- 
From: "Jim Tonne" <tonne at comcast.net>
To: "jeremy-ca" <km1h at jeremy.mv.com>; "Jim Brown" 
<jim at audiosystemsgroup.com>; <Amps at contesting.com>
Sent: Monday, November 12, 2007 1:29 PM
Subject: Re: [Amps] 50 Ohm Loads


>
> On this business of resonating an inductive dummy
> load, I decided to put some numbers into the thing
> so we can sort the wheat from the chaff.
>
> First, I am sure that we are all in agreement that if
> the inductance of the dummy load is really low then
> the bandwidth of it will be really great.  Duh!
>
> I got out my network design program (Elsie) and
> set up the discussion, using a frequency of 1 MHz
> and a system impedance of 50 ohms.
>
> Then I added in 1 microhenry in series with the 50 ohm load and added a 
> capacitor in series with it
> to bring the thing to resonance.  With 1 uH it took
> .02533 or so microfarads to resonate.  And the
> return loss was 20 dB or better from 500 kHz
> to 2000 kHz.
> In a 50 kW load I wouldn't be surprised to see
> one microhenry of inductance with all the interconnecting
> wires and so on.  For a ham rig (smaller physical size
> load) it might very well be less.
>
> BUT - if I run the inductance on up to 10 microhenries
> and so ran the capacitance down to 2533 pF, then
> the bandwidth shrunk a whole lot:  20 dB return loss
> from 920 to 1080 kHz.  And using wirewound resistors might easily produce 
> 10 uH inductors.
>
> Bottom line in my mind is that we reallly have to
> find out what that load resistor's inductance is in
> order to calmly and rationally talk about bandwidth.
>
> - Jim WB6BLD
>
>
>