Thanks to all your replies. There is a lot of valuable information. This is my first toroid and I want to make sure I have it right.

I'm winding T3 which is an FT37-61 with 23 turns of #28 and 6 turns of #26.

Looking at:

... suggests that the 23 turns should give me 29uH and I'm measuring 28.4uH on my L/C meter. The calculator on that page assumes #28 wire. I haven't added the secondary yet. I just wanted to make sure I was on the right track and that the 0.6uH is within tolerance.
I'm using one of these:


Thanks to all.
73,
Paul VA3ZC

Hello all,

I’m working on a prototype for a low cost magloop. ?Cost of ALL parts including WiFi/low energy Bluetooth “arduino-based” electronics and mechanical assembly for driving the capacitor is about $100 (no joke). ?Attached is a pic of the current prototype. ?It is mounted on some form board as a temporary support for testing, but I have plans to make this thing portable with a quick deployment system. ?The loop itself is 10mil 1 inch wide copper. ?Still working on the software, but It will self-tune and be manually controllable from an iPhone.

The capacitor is 3D printed from my own CAD design and still a work in progress. ?My current testing is confined to 20m, but I should be able to modify this thing to cover 10-30m at least...maybe 40...but I would rather have a 2nd loop for 40 thru 80.

?

The capacitor dialectic should handle at least 100W, but since I don't have a non-qrp radio in the house, I don’t know how stable it would be at that power...I plan to stress test this thing in the near future with the help of a non-qrp ham friend. ?It works well at qrp power.

I put it on 0.2 watts of WSPR today. ?My results:



Anyways...thought I would share. ?

73,

Kurt-AE6UJ

?

?

Correction, at 9v the 1n4001 graph shows 10pf, not 30pf:
? wrong:? ?Using the midrange 9v:30pf pair to compute? ?K = 10*sqrt(9) = 30,
? right:? ? ?Using the midrange 9v:10pf pair to compute? ?K = 10*sqrt(9) = 30,
Just in case anybody was trying to make sense of this stuff.


> Perhaps performance at high frequencies, where the 1n4001 might show a bit of PIN behavior?

I usually blame poor performance at high frequencies on capacitance, but not the case here.
So how would a 1SV322 be better than a 1n4001?
To show "PIN behavior" it would have to occasionally be forward biased, that's not the case here.
But is there a similar behavior that might cause trouble at high frequencies?

No diode used as a variable capacitor will have a linear response to voltage, but some are better than others.
This is significant when trying to tune a VFO, don't want all the action to be happening at the bottom end of pot rotation.
How close to linear is not a characteristic of varactor diodes I've ever seen discussed.
Most analysis is a quick look at min and max capacitance.

I think I'm done here, will go back to fiddling with si5351's.
We now return you to your regularly scheduled programming.

Jerry, KE7ER

Sorry, this time copied correctly.

On Sat, Dec 21, 2019 at 04:22 AM, n3fel@... wrote:

Al Dutcher, EDN April 12, 2007

https://www.edn.com/.../test...measurement/.../Cheap-and-easy-inductance- tester-uses-few-components

Howard, n3fel

It's a good idea to wind and measure the inductance of the toriod before installation.? In my experience not all cores are alike especially the five for a dime variety found on Ebay.? Testing of magnetic material is steeped in theory and the art is well practiced.? Check the litergy on test methods.?

I used a multivibrator design by Al Dutcher, EDN April 12, 2007 that formed a charge/ discharge oscilator and ran at a few Mhz which I viewed on a scope and measured on a frequency counter.? The frequency of oscillation related to the inductance.? A general coverage receiver quite loosely coupled to the large signal of the multivibrator might also work if the oscillation is within range of its rx coverage.? I keep an ancient RS DX302 general coverage receiver on the test bench just for that purpose

Buy the cores from Kits and Parts.? You'll get what you want within a few days of your order rather than waiting 6 weeks or more.? ?Howard, n3fel

The 1n4001 seems to follow the? ?Cj = K/sqrt(V)? ?formula rather closely.
From the graph of fig 4 on page 2 of the 1n4001 datasheet at? ?
? ??
I read the following sample of values:??1v:30pf? 9v:10pf? 30v:5pf? 80v:3pf

Using the midrange 9v:30pf pair to compute? ?K = 10*sqrt(9) = 30,
we get the following results from Cj = K/sqrt(V) = 30/sqrt(V):
? ? 1v: 30.0pf? ? 9v 10.0pf? ? 30v: 5.48pf? ?80v: 3.35pf
The figures computed from? Cj=K/sqrt(V)? follow the graph quite well.


Some diodes don't follow our formula so closely.
Take a look at the figure on the left side of page 2 for the 1SV322:
? ??
Using that figure, I read:? ? 6v:5pf 4.5v:6pf 2.8v:10pf 1.6v:20pf? 0.5v:38pf
Using the 2.8v:10pf center value pair, I find that K=16.73? and Cj=K/sqrt(V) gives:
? ?6v: 6.83pf? ? 4.5v: 7.88pf? ? 2.8v: 10.0pf? ? 1.6v 13.23pf? ? 0.5v: 23.66pf

Note that values from the figure have higher capacitance at low voltages than our computed values.
So this specially designed 1SV322 varactor diode is even more non-linear than the 1n4001.
Do I have something wrong here?
Why would we not use a 1n4001 instead?
Perhaps performance at high frequencies, where the 1n4001 might show a bit of PIN behavior?
Curious.

Jerry, KE7ER
?

Hi

The phase noise numbers for the RFzero was recorded subject to a 50 ohms load, i.e. no additional 74xx buffer stage:

100 Hz -103 dBc/Hz
1 kHz -128 dBc/Hz
10 kHz -136 dBc/Hz
100 kHz -138 dBc/Hz
1 MHz -144 dBc/Hz

at 9 MHz. From

Denis, G0OLX, uses an RFzero generating ~100 MHz driving the GB3SEE 24 GHz beacon. The frequency shifting, not the CW-FSK, comes from Denis walking around the lab (Doppler), and the fact that the FT-817 is xtal controlled. Given the x240 multiplication the phase noise is not bad. OK not tuning to the sides, but still good audio. Like any other RF circuits also the Si5351A needs a good home to do its best.

Bo

A minor error in my previous post where I said:? "assume our 10v power supply has 0.1v of pk-pk ripple on it"
My subsequent calculations for the variation in capacitance assume noise on the power supply of 0.2v pk-pk, not 0.1v pk-pk.

Chuck wrote:
> **I did this stuff about 60 years ago.? Kinda hazy now.

My college days were back in the 1970's.
So I've got a 15 year advantage on you.
It's scary to revisit this stuff and realize how much I forgot.


For those who forgot high school algebra, this might be confusing:

If we lump all those constants into a single value I'll call K, we get:
  Cj = K/sqrt(V)

Assume we have a varactor diode that measures 100pf when we put 1 volt across it.
What is the capacitance when we put 10 volts across it?
  At one volt:   K = Cj*sqrt(v) = 100*sqrt(1) = 100
  At 10 volts:   Cj = 100/sqrt(10) = 31.62 pf

Cj * sqrt(V) = K * (1/sqrt(V)? * sqrt(V)? ? ?# regroup
Cj * sqrt(V) = K? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ?#? sqrt(V) divided by itself is equal to 1
K = Cj * sqrt(V)? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ?#? switch sides

So if we know the capacitance Cj is 100pf at a bias of 1v, we can calculate the value of K:
K = Cj * sqrt(V)? = 100 * sqrt(1) =? 100?

The two different instances of "100" are an unfortunate coincidence, and thus a source of confusion.
The first 100 is 100pf, the second 100 is the computed value of the constant K for this particular diode.

Now that we know the value of K for this diode, we can calculate the capacitance Cj for a bias of 10v:
Cj = K/sqrt(V)? = 100/sqrt(10) = 31.62 pf

Jerry,? ?KE7ER

I have built a lot of QRP radios over the years, both kits and from scratch.? Including the NC40A from the old group's project.? Have probably wound 100s of toroids in the process and have never measured the inductance let alone the frequency response of any of them.? Don't have the test equipment for the latter anyway.

All of the radios including the NC40A have worked well.? Most, including the NC40A are still in use here? The only problem ever encountered due to toroids was getting 43 and 61 mix cores "mixed up" in couple of instances.? Worst case is sometimes especially in PA output networks the toroid windings need to be spread out or bunched a little to hit the sweet spot.

Not criticizing measurement for those with the equipment and inclination. Personally haven't found it necessary to building a working project.? Recently, I find using a cheap multitester to check for counterfeited parts a worthwhile effort.? Avoided installing a lot of bad capacitors and active components recently, all provided in radio kits.

Curt KB5JO

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I think the curves on Diz's page are in the best format for varactor capacitance. We normally consider the voltage input linearly. If I'm graphing one, I automatically mark off the V (x) axis in evenly spaced volt increments. But I'm normally looking for something different on the output. Marking the C (y) axis linearly makes no sense. We are rarely looking at the linear response of a varactor because we aren't really concerned with capacitance. We're normally concerned with the resulting resonant frequency response in a tuned circuit. Log/linear can help us with that.

Most of the time, I want a frequency readout to be linear. A varactor capacitance with a non-linear response to voltage is going to give me that. On Diz's graphs, I see a linear voltage input. If I see a fairly straight curve for C, I'm probably going to get a fairly linear frequency response. So log/linear makes sense. If he used linear/linear, you would see a deceptively "hooked" curve that truly shows how C responds, but it wouldn't be as useful for predicting frequency response. Log/linear provides more insight that you can use.

Some of the newer hams may not have noticed the shape of the plates on larger "tuning" capacitors. They weren't symmetrical. They were very oddly shaped to make the dial calibration more linear.

There. Marketing guys owe me one.

BTW, I used to follow your website pretty closely in the Rock Mite days. Fun stuff.

Eric KE6US

Chuck,

It took me awhile to figure out how to make sense of it.
Could be errors, but this is what I am currently thinking:

Here's that equation again, a power of (1/2) means we take the square root:
  Cj =   A/2 * ((2*q*e/(V0-V))*(Nd*Na/(Nd+Na)))**(1/2)

V0 is very close to zero volts so we can ignore it, and V is a negative voltage.
A is the cross sectional area of the cap, Nd and Na have to do with doping levels.
Those all get determined when the diode is fabricated, and won't change for us.
e (epsilon) is the permitivity, should be about the same for all silicon diodes.
q is the charge of an electron, determined back when the universe was born.
If we lump all those constants into a single value I'll call K, we get:
  Cj = K/sqrt(V)

Assume we have a varactor diode that measures 100pf when we put 1 volt across it.
What is the capacitance when we put 10 volts across it?
  At one volt:   K = Cj*sqrt(v) = 100*sqrt(1) = 100
  At 10 volts:   Cj = 100/sqrt(10) = 31.62 pf

Getting back to our original conversation, assume our 10v power supply 
has 0.1v of pk-pk ripple on it.  How much variation in capacitance do we get
with this particular diode for a bias at 1v versus a bias at 10v?

Here's the difference between max and min capacitance at a nominal 10v of bias:
  delta-C = 100/sqrt(10-0.1) - 100/sqrt(10+0.1) = 0.316 pf 

When we adjust the pot to divide down the power supply to give 1v of bias,
we also divide down the noise by a factor of ten.
Here's the difference between max and min capacitance at a nominal 1v of bias:
  delta-C = 100/sqrt(1-0.01) - 100/sqrt(1+0.01) = 1.000 pf 

Conclusion:
Our oscillator will be significantly more sensitive to power supply noise
at low bias voltages across the varactor diode.  This is in spite of the fact
that the voltage fluctuation on a bias of 1 volt is 1/10 what it would be for a 10 volt bias.

Looking closer at the chart posted previously?from Diz's?diodes page



SMD/SMT Varactor 1SV322.?would be a good choice.??

The V/C curve is about the same as the thru-hole?MV-209 or?BB910.

Note that the curves for these two are plotted differently:? SMT part is linear/log and the Thru part is log/linear.? It gives the? plot a different look even though they are much the same.? Depends on what the marketing folks want to emphasize...8^)

Plot them linear/linear if you're curious?about?the "real" shape of the?V/C?curve.
--
Chuck, W5USJ (ex K2OFN)
Point, Rains Co, TX? EM22cv

Jerry,? That's a fun equation!

The half-power Sqrt sez is not linear. I can't quite visualize** how the parallel capacitor calculation works out.? I may just have to get really curious and check out the constants and variables and work some examples from the back of the book.

In the meantime, I'll just use my measuring gadget...8^)

Fun Stuff

**I did this stuff about 60 years ago.? Kinda hazy now.
--
Chuck, W5USJ (ex K2OFN)
Point, Rains Co, TX? EM22cv

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yea..... we have another term here ...goes way back before BAOFENG showed up here......

Two decades ago we had a local radio club sub-group who gathered once a month for a fox hunt...direction finding ..beacon chasing.

All you needed was an HT to participate so it was a mixed bag of old and new hams and the new guys had no HF gear

so they were looked down upon.... and called "shack on the belt techs"

meaning they were nothing more than HT operators.

I was one of those guys for 2 years.

It was very demeaning.

It really just boils down to respect.

The HF guys totally disrespected me because I couldn't pass a 13WPM CW exam and if I couldn't get a general license I had no incentive to

chase HF.

While at the same time.....? I wrote satellite tracking ..azimuth / elevation.. rotor interfaces ... in assembler... on DOS laptop computers......

so I didn't care about dickheads looking down on me because I couldn't do HF....? I was too busy running circles around them with computers

attached to radios chasing satellites with home brewed hardware...... which was definitely in the spirit of the hobby.

It has been my experience in the hobby of ham radio that IF WE DO NOT MAKE THE EFFORT TO SHARE the hobby many facets then

newbies just have ZERO footing in those facets of the hobby.

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Hi Paul,
Yes, you can measure your torroids.

What I do is put the filter together on the PCB and sweep it using my PHSNA (poor ham's scaler network analyzer).
Did my xtal filter too.
See attached (I hope).
73,
Gary
WB6OGD

I'm building a Norcal 40a and wondering if the toroids can be measured and verified prior to installation. I have an LC meter sitting here on the bench.
Thanks.
Paul VA3ZC

Gents,

Absolutely great!

Thanks for these suggestions and links! If this does not drag them out of Baofeng "Tech-hood" nothing will.
--
William, k6whp
--------------------
"Cheer up, things could get worse. So I cheered up and things got worse."

--

I second what Dale says, however (since measuring traps is the subject),? a simple noise generator along with a receiver can be employed to measure traps.? Example:???>.

73.......Steve Smith WB6TNL
? ? ? ? ? ? ? ? "Snort Rosin"

?On Thu, Dec 19, 2019 at 02:07 PM, Dale Hardin wrote: