TSSP: List Archives

From: "Terrell W. Fritz"
Date: Fri, 16 Feb 2001 20:02:15 -0700
Subject: Re: [TSSP] Top voltage testing

At 03:03 PM 2/16/2001 +0000, you wrote:
>Hi Terry,
>
>The situation is looking a little better with the raised signal level,
>is this once again the HP reading in error with small signals?

Maybe, I will have to run some calibrations...

>
>> The reason the current is dropping with the probed coil is due to
>> the Q of the coil dropping with the probe in place.
>
>Ah, I haven't been bothering to take account of the resistive part of
>the probe, since other losses are not well quantified - I assumed it
>wouldn't make much difference. In fact, I put in a 10 Meg load resist-
>ance alongside the probe capacitance, and the probed coil Q fell to 102
>compared with 195 for the bare coil. Previously, without the allowance
>for the 10 Meg, I got a Q around 170 for the probed coil. Your base
>current readings have highlighted the fact that I must take the probe
>resistance into account too. 
>
>Adjusting the model's ctop to match your new f1, we now get:
>
>      measured      modeled     error
>  f1:  310.9 kHz    310.9 kHz    0.0%  Ctop adjusted to match: 3.145 pF
>  f3:  847.8 kHz    828.4 kHz   -2.3%
> 
>  Zft: 34450 ohms  34877 ohms   +1.2%
>  Les: 17.64 mH    17.85 mH     +1.2%
>
> Lfac: 79.8%       80.8%
>
>You mention that the coil is not 'perfectly well known', and we have
>a 2% discrepancy in the Ldc, with 2.3% in f3, so maybe more refined 
>dimensions would resolve the remaining error.  
> 
>Having got a reasonable match on Zft, perhaps we should have a quick 
>look at some of the Q-dependent measurements.   Looking at your sweep 
>charts, the bandwidth of the probed coil f1 looks to be around the 
>3 kHz region, so maybe the Q predicted at 102 is not too far from 
>reality.  For the probed coil, the model predicts:
>
>     Q: 102
>   Rin: 293 ohms
> Vgain: 119 

With the signal generator cranked up, I get using the transformer/resistor box.

Ibase = 1.175mA
Vbase = 0.48885V
Vtop = 40.80
Qtopv = 72.5
QIbase = 70.98

So Rin = 0.48885/1.175 = 416.0
Vgain = 40.8/0.48885 = 83.46

This coil uses white PVC for the form that is 1/16 inch thick.

The diameter is 4.250.
The length is 26.125
The turns are 1175.5
Rdc = 33.40 ohm

I tried hooking the generator directly to the base of the coil and got over
1kV peak to peak on the top but the Q was still around 70.
I tried using the amplifier but that only raised the Q a small amount.
However, I raised the coil about 2.5 inches off the ground plane and the Q
did go up significantly.  I will have to clean up the setup to know how
much but perhaps eddy currents in the foil are causing to low Q.

If I we to make a radial counterpoise out of #24 wire, do you have any idea
how many radials I would need and their length for an effective plane?
That would eliminate a lot of ground plane effects.  I could also raise it
off the floor to hide test stuff under it.  I could probably only build one
big enough for the 26 inch coil.

Cheers,

	Terry


Maintainer Paul Nicholson, paul@abelian.demon.co.uk.