Re: NEC-LIST: Hi-Q inductors

 Latest solenoid inductor tests and test of the K6STI coil calculator at
2.05 MHz.

K6STI Q calculation of the Air Dux style inductor: 409
Measured: 244

K6STI Q calculation of the PVC coil form inductor: 394
Measured: 164

Note that the K6STI calculator does not account for the wire insulation.
In this case there an additional 4 pF or so across the inductor. I
calculated the dissipation factor (DF) of the PVC form plus wire (PVC
insulation?) as 0.1. I found the link below that states the DF of 0.072
for PVC wire insulation. So I am able to account for the insulation
losses.

http://www.dupont.com/industrial-polymers/elvaloy/loy_pvc3.html

At this point it appears that the K6STI program over estimates coil Q
and certainly doesn't account for the wire insulation.

The problem with the measurements yesterday turned out to be the series
resonating capacitor. It is a ceramic capacitor with a specified Q of
>1000 but it sure doesn't meet this spec. The capacitor today is a 470
pF, 500 V silver mica.

     Dave Cuthbert WX7G
     Micron Technology

-----Original Message-----
From: drcuthbert
Sent: Monday, February 14, 2005 1:56 PM
To: drcuthbert; nec-list_at_robomod.net
Subject: RE: NEC-LIST: Hi-Q inductors

K6STI results:

I built a solenoid inductor wound on a PVC Pipe form. The measured
results are close to the K6STI results except for the exceptionally low
measured Q.

Diameter = 2.75"
#14 AWG
8 TPI
13 turns
18" leads

L = 12.3 uH
Q @ 2 MHz = 390
SRF = 17.9 MHz
Distributed C = 6.74 pF

Measured:
L = 13 uH
Q @ 2 MHz = 15
SRF = 12.5 MHz
Distributed C = 12.5 pF

I made a SPICE model of the inductor and the distributed capacitance Q
is 1/3. Not good. The wire is some garden variety low voltage hookup
wire. I am thinking the extra capacitance is due to the wire insulation.
I will rewind the coil using magnet wire and see what happens.

     Dave Cuthbert
 

-----Original Message-----
From: nec-list-bounces+drcuthbert=micron.com_at_robomod.net
[mailto:nec-list-bounces+drcuthbert=micron.com_at_robomod.net] On Behalf Of
drcuthbert
Sent: Monday, February 14, 2005 8:53 AM
To: nec-list_at_robomod.net
Subject: Re: NEC-LIST: Hi-Q inductors

Frank,

PVC:
Here is a link to Schedule 40 PVC pipe specifications:
http://home.carolina.rr.com/microhydro/pipe_properties.html

It has a UL V-0 flammability rating which means it will not support
combustion after the source of heat is removed. It is
self-extinguishing. I use this type of PVC pipe for building coils. I
will hit some PVC with a flame and see what happens.

My fun fire:
I once had a 160 meter vertical catch on fire and the Polycarbonate
tubing continued to burn and was dripping flaming plastic onto the roof
of my house. The source of ignition was a wire arcing to a screw
securing the coil to the house. The RMS voltage on the coil was 8 kV.

K6STI calculator:
On the subject of the K6STI calculator. I have that and it does a good
job and runs on Windows XP. I have not been able to get a hold of Brian
Beezley to see if he will allow this to be shared or not. I am trying to
find out if the Q calculated by his software is correct. He appears to
take into account skin effect and the shunt capacitance. I will build a
PVC coil today and measure it at work and report my findings.

    Dave Cuthbert WX7G
 

-----Original Message-----
From: nec-list-bounces+drcuthbert=micron.com_at_robomod.net
[mailto:nec-list-bounces+drcuthbert=micron.com_at_robomod.net] On Behalf Of
Frank Donovan
Sent: Sunday, February 13, 2005 5:58 PM
To: nec-list_at_robomod.net
Subject: Re: NEC-LIST: Hi-Q inductors

Eric,

The only issue I take with your approach is your use of
PVC in high power RF applications. PVC supports fire
if ignited by any source, such as an arc from any
component in a high power RF amplifier.

PVC is a particularly bad choice for any use in a high
power amplifier. I recommend that you rewind your
coil on a form that cannot support fire, such as
ceramic or porcelean.

Another example of bad RF engineering is the use of
combustible materials on antennas, particularly at
high power. There have been many cases where
cheap plastic insulators and traps with plastic housings
or coil forms have ignited. Can you imagine flaming
plastic dropping onto a wood roof or into a tinder
dry forest during a drought?

There's more to good RF engineering than high Q...

73!
Frank
W3LPL
donovanf_at_erols.com

<EVonvaltie_at_aol.com> writes:
> I have personally built and tested many inductors, usually aimed at
Hi-Q
for
> RF applications.I have found it difficult to get above 250-300 using
the
best
> materials and winding techniques.
>
> For applications to antenna loading coils and PI-Network coils in
xmitters,
> I would like to pass along a technique which I have developed within
just
the
> past year, which is easy and works consistently well.
>
> I utilize a coil form of ordinary PVC pipe, with diameter of 1-4
inches,
> which is a home depot items. An additional, but not necessary,
addition
is the
> PVC end caps, which make a nice device for retaining the wire. Also,
this
> material is very easy to work with and allows you to fabricate
mounting
hardware,
> holes for exiting the wire, etc.
>
> The winding is done with #12 or 14 TFE stranded teflon hookup wire.
This
wire
> is silver plated and the insulation is thick enough to provide almost
all
of
> the benefit that comes from spaced winding. To eliminate the
"proximity
> effect" from close wound turns, it is only necessary to allow a gap of
several
> skin-depths between turns. The 20 or 30 mil teflon insulation does
this
quite
> nicely and the teflon has minimal loss in the small electric field of
the
winding.
>
> I have wound many coils, and for the last year I have used this
technique
> exclusively. The results are coils that CONSISTENTLY measure within
10-15%
of the
> common inductance formula, which I will repeat here for reference, and
> measured Q's in the range of 200-300. As further proof, I offer the
example of the
> tank coil in my 160M. amplifier. It is wound on 2.5" PVC with about 25
turns
> of #12 TFE, and when run at 1.5KW OUTPUT, the coil is just slightly
warm
to the
> touch. A little calculation shows that this implies a very high
Q(unloaded)
> of the coil.
>
> A convenient form of the inductance formula is as follows, where
N=#turns,
> d=diameter (inch) and l=length(inch):
>
> L(uH)=N(squared)xd/(18+40l/d)
>
> 73
> Eric von Valtier K8LV
>
> --
> The NEC-List mailing list
> NEC-List_at_robomod.net
> http://www.robomod.net/mailman/listinfo/nec-list

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