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[AMPS] Coil Forms

To: <amps@contesting.com>
Subject: [AMPS] Coil Forms
From: W8JI@contesting.com (Tom Rauch)
Date: Tue, 13 Nov 2001 09:39:41 -0500
Although I don't have the K6STI program, I have many other 
inductor Q programs and none of them hit close. One way to test 
the program is to raise the frequency and watch Q. 

1.) Q is almost always maximum between 1/5th and 1/7th the self-
resonant frequency of a single layer inductor having a non-ferrous 
core. Q decreases above and below that region.

2.) Q is almost always maximum when turns are spaced one turn 
diameter, unless the wire is covered with a dielectric. If the 
conductor is insulated, the Q peak generally peaks with slightly 
increased turn spacing.

3.) Any dielectric or anything that increases turn-to-turn 
capacitance causes a noticeable reduction in Q.

4.) Optimum form factor (L/D) ranges from around 1:1 to 5:1 or 
more depending on application. The higher the reactance required 
for a given frequency, the longer the optimum form factor becomes. 
That's why long slender mobile loading coils are so good, and why 
tank coils (with lower reactance) are short and almost a square 
(1:1) form factor.

5.) The highest Q I have ever measured is in the high hundreds, 
around 800 or so. That was with a moderate reactance silver plated 
copper tubing inductor in a large shielded fixture. Many edge-
wound inductors are very good, in the Q 500-700 range. Yet most 
inductor model programs I have tried can indicate Q's in the 
thousands, while the actual inductor is in the hundreds or less!!! 

Most modelling programs I have tried fail to follow these well-
established real-world measured criteria. The above criteria is a 
good test to see how good a modeling program is.

> I ran a program written by K6STI, which computes coil losses and the
> results are interesting.

The results are also misleading, if anyone takes them as directly 
related to loss change in a normal inductor. The biggest problem, 
assuming the program is reliable, is you almost made a strip line 
or transmission line rather than a good inductor. Transmission 
lines, like linear loading system touted as "low loss" by antenna 
manufacturers, make notoriously poor inductors and capacitors!   

I measured the Q of a very similar inductor on 28Mhz, and I get 
Q=105 with air dielectric, Q=100 with .05" thick teflon formed into a 
form inside the coil, and Q=102 with Nylon .05" thick. All are within 
tolerance of measurement error.

The less electric field in the dielectric, the less change the 
dielectric will cause. For example suspending a length of 1" PVC 
conduit inside the 3" coil caused no measurable change in Q. If the 
PVC coated the turns, the change would be dramatic.

The amount of electric flux in the dielectric is a major factor.

If Q was important I'd follow the guidelines for selecting proper form 
factors, and suspend the coil away from the form with narrow 
spacers. I'd worry much more about HOW I built the inductor rather 
than what type of material gave the mechanical support. Of course 
I wouldn't use wood-coated wire or a grooved wood form that the 
wire lays in for a number of reasons.   

> 
> Using 28 mhz, the highest frequency of the normal amp, I looked at a 4
> turn coil made of #14 wire with 4 turns on a 3" diameter form and 2"
> long.
> 
> Form loss was calculated as:
> 
> Dielectric                     Loss in %
> Air                                   0
> Air Dux                               .1
> Polystyrene                        .2
> Teflon                                 .3
> Steatite                            3.0
> Glass                               3.8
> Polycarbonite               18.9
> PVC                              17.0
> Porcelain                      21.2
> Bakelite                        24.7
> Nylon                             28.2
> Douglas Fir                  31.3
> 
> QST in September, 1931 reports on the new National SW-3 receiver. 
> James Millen notes that bakelite is often made with wood flour
> (perhaps Douglas Fir) and has very high losses - especially on 10 and
> 20 meters.  He recommends the use of National R-39 material.
> 
> Colin  K7FM
> 
> 
> 
> 
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73, Tom W8JI
W8JI@contesting.com 

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