Re: [softrock40] Using the MFJ269 to measure inductance.
One would hope the cores are not counterfeits from another mfr when looking up Al to plug into formula.
From: firstname.lastname@example.org <email@example.com>
To: firstname.lastname@example.org <email@example.com>
Sent: Tue Dec 01 20:55:24 2009
Subject: Re: [softrock40] Using the MFJ269 to measure inductance.
If you are measuring toroidal coils and you know the material the inductance is very close to the number of turns through the coil using the formula for that material. I have found that while inductance values are given in the kits, the most important thing is resonant frequency inside the circuit.
As a side note for inductors used in most all of the kits you can just wind according to the specs - number of turns through the toroid. If you want to be critical, once you get it in circuit you can use a sig gen on the input (ant for RX) and a scope on the output (I or Q) to find the resonant frequency peak. Then adjust turns to put it where you want it.
Again none of this is really necessary unless you are a perfectionist! I guess I am and for the lite 80 meter it peaked at about 2.7mhz - removing turns (pri and sec) brought it up to the 80 meter band. Remember on the scope I am looking at this on a linear scale. In dB difference it is probably negligible.
The other point here is that inductance meter can be all over the place depending on so many factors. A calibrated sig gen and scope would tell you much more.
I use a B&K LCR meter that works well for me. One of my most used lab instruments mainly for checking unknown C & L.
--- On Tue, 12/1/09, aisea6 <aisea@connect. com.fj> wrote:
From: aisea6 <aisea@connect. com.fj>
Subject: [softrock40] Using the MFJ269 to measure inductance.
To: softrock40@yahoogro ups.com
Date: Tuesday, December 1, 2009, 8:29 PM
I had my kit version 6.2 trx for sometime.
Today I was using the MFJ269 to measure the inductance on the coils and amazing and more frustrating as the values can be easily way off.
Is there any sure way to ensure that you are not in the wrong direction.
I am using short hook up soldered lead to the earth post and poke the other end using a short lead to the inner of the N socket.
I noted it's very easy to change the inductance value just by compressing or spreading the windings around cores.
I guess it's very easy to take a false reading.
Is there any pitfalls to watch out for?
Does the number above the X1 on the MFJ269 reading matter or just concentrate on the inductance value?
- Hi there,
One other point to take into account is that any leads you add to connect to the coil or inductor to measure it will add to the overall inductance reading.
I am not sure how the MFJ is set up to make the measurement, but on my simple homebrew PIC L/C meter, you 'calibrate' the meter with the leads shorted to start off with to take them into account, therefore in theory you are only measuring the actual inductor.
As previously said, on coils with very few turns, the spacing around the core can alter the value significantly. That's an easy way of 'adjusting' them closer to the required value.
As has been said many times before, unless you have a suitable means of measuring inductance and are confident of doing so, just wind them as per the instructions. There are so many variables in the designs that without access to considerable amounts of test equipment it is very unlikely you would be able to detect any difference in performance.
Have fun, 73, Bob G8VOI
--- In firstname.lastname@example.org, aa9wqham@... wrote:
> The inductance will very as you expand and contract the windings. That's
> normal. You can use that to your advantage when trying set the value. A few
> 1: Set the frequency on the MFJ to the middle of the frequency range the
> inductor use used at. Example if the range of the inductor is 4 to 8 mhz set
> the analyzer to 6 mhz.
> 2: Keep your fingers away from the windings. I have a N connector with a
> PC board soldered to it and I tack solder the inductor to pads on the board.
> Screw it on top of the MFJ. If you are going to use the just touch method
> touch just the ground side with your finger and let the other side touch the
> center pin. It is a balancing act but doable.
> 3: The windings need to be evenly spaced as best you can if there is only
> one layer. used as many turns as you can to achieve the value. Don't just
> spread the windings to hit the right value if you can add a winding to hit
> the value. Good rule of thumb is wind to many. Remove turns till you hit the
> value I use 1 to 2 inch leads so I can put a winding back on if I over
> shoot. A few inches aren't going to skew your result enough to worry about.
> Pushing the windings together has the effect of adding windings. Spreading
> looks like removing.
> 4: Believe it or not there is some room for error. If you need 5.0uh and
> you end up at 4.8 don't worry. Most people don't have a analyzer and just
> follow the builders notes and do just fine.
> 5: Have fun and wind neatly. I use tally marks on paper to keep track of
> turns mark every 5 turns. them add a few turns and usually end up removing
> 2-5 turns to get close enough.
> 6:The uh and the frequency are the only values to look at on the display.
> The rest doesn't matter.
> 7: I hope I have explained this well. Others please comment and feel free
> to add to this.
> Larry W9AMR
> In a message dated 12/1/2009 8:31:14 P.M. Eastern Standard Time,
> aisea@... writes:
> I had my kit version 6.2 trx for sometime.
> Today I was using the MFJ269 to measure the inductance on the coils and
> amazing and more frustrating as the values can be easily way off.
> Is there any sure way to ensure that you are not in the wrong direction.
> I am using short hook up soldered lead to the earth post and poke the
> other end using a short lead to the inner of the N socket.
> I noted it's very easy to change the inductance value just by compressing
> or spreading the windings around cores.
> I guess it's very easy to take a false reading.
> Is there any pitfalls to watch out for?
> Does the number above the X1 on the MFJ269 reading matter or just
> concentrate on the inductance value?
> Aisea (3d2aa)
- Hi all,I don't think that measurement error is what you should be concerned with. The MFJ can measure the inductors with good enough accuracy. Yes, you do need to be aware of the accuracy of your test equipment, but more important in this case is the fact that the filters are broad and it isn't necessary to use exact values.I asked a similar question a few weeks ago about capacitor values, and got similar answers that said basically just build it and don't worry about it!What I did to satisfy my curiosity was to use LTSpice to model the filter. The LTSpice schematics of the filters can be found in the Files. I duplicated the filter schematic except for the source voltage and then changed the capacitor that I was concerned with to the value I measured. That is, I had two similar filters driven from the same source voltage so I could display the two responses at once. One had ideal values for L and C as a reference (I calculated the series resistance of the coils for a Q of 200 at the mid band frequency), and in the other filter I played with values to see what difference it made in the response.In my case I was worried about a capacitor that was 10 or 15 percent high, but the same test could be applied to inductors.Long story short, there is very little difference in the overall response caused by varying the values over quite a large range.I love LTSpice! Its free, easy to use, and quite useful.Dan K9WEK