开云体育

Hi

Maybe some of you have used the FA-VA5 for measuring coil "inductance" using the LCR meter function. Maybe you made a trap coil or a loading coil and want to measure it.?

If you have used the LCR function, then maybe you have encountered what may look like anomalies or mysteries.
Perhaps you noticed that for some coils the inductance value changes with frequency. In standard circuit theory, however, the inductance "L" of an ideal inductance is a constant with the unit Henries (mH, uH, nH).
Practical inductances possess a so-called winding capacitance. At higher frequencies they may even exhibit higher order properties.
Winding capacitance acts like a small lumped capacitor in parallel connection with the coil.
While at low frequencies the effect of the coil capacitance may be hardly or not noticeable, it rises slowly and becomes very significant when we move towards the parallel resonance frequency of the coil inductance and the winding capacitance.
We first notice this when we measure a coil and increase the FA-VA5 frequency. The displayed inductance value "L" increases slowly with frequency as a consequence of the winding capacitance.?

If this subject is of some general interest and you would like to learn more, I am happy to elaborate more. For the time being, I end here.

vy 73

Dieter, VK3FFB

Dieter - Would it be correct to state that we will have a similar experience measuring capacitance, due to the small value of inductance which is present in the capacitor leads, body etc.?

I ask because I am about to test some air-dielectric variable capacitors. I need to know their minimum and maximum capacitance. The goal is to build a receiving loop antenna (~3.5m circumference) and I expect that I will need somewhere between 20 and 200pf to tune it to resonance.

--
73 de Gary, AF8A
Cleveland, Ohio, USA

Hello Gary

Yes, lead inductance creates a positive reactance in series with the negative capacitor reactance. The "effective" measured capacitance is then necessarily larger than the capacitor's capacitance and this larger effective capacitance is frequency dependent.

Xc_eff = Xl - Xc

and

C_eff = 1 / ( 2 x PI x f x Xc_eff)

So, if you notice a change in measured capacitance when you change the frequency of the FA-VA5, then this is because of your lead inductances. Towards lower frequencies you get closer to the actual capacitor's capacitance. The lead inductance can be measured by disconnecting the capacitor and connecting the leads together, keeping the leads in the same or similar shape as when they were connected to the capacitor.?
You can measure your loop inductance at different frequencies with the FA-VA5. It will also vary with frequency because of its large size which gives it the character of an end-shorted transmission line to some extent. Your 3.5m loop is not a "lumped element" because it is not very small compared to the wavelength.

73
Dieter, VK3FFB

Hi Dieter,

When designing a coil for a 40m trap, would it be advisable to set the frequency to 7MHz?

Or would it be better near 1MHz for instance?

Thanks!
73'
Matt -WA0V
?

Hi Matt,

It is advisable to measure the coil at its operational frequency of 7 MHz. We never get pure coils. First and foremost there is a winding capacitance does affect the effective inductance. If you measure L at 1 MHz, the effects of that capacitance are not strong. You may find that if you adjust your coil for the desired inductance value at 1 MHz, it may show a higher inductance at 7MHz. This is caused by the winding capacitance which is in parallel to the inductance forming a parallel tuned circuit. The resonance depends on the coil design and is usually much higher than the frequency of operation

73
Dieter

PS
I edited the first version of this reply.

Dieter,

Thank you, that was the conclusion that I came to as well. I'm making traps for the OZ1CX version of the W3DZZ 5-band dipole.



Using the LCR function, with a single frequency OSL cal at 7MHz, the inductor measures 8.4uH and the capacitor measures 61pF,? resonance of the traps is noted as 7.050 MHz. My measured values are at 7.35 MHz.

Calculated values are 7.175 MHz for 8.2uH and 60pf.

Edit: I found the original article on ARRL's website. The original author states to adjust the capacitance for 60pf, then adjust the coil as needed for resonance at approx. 7.20 MHz.

I'm uncertain where the 7.050MHz frequency listed on the diagram above originated from.

The LCR function of the FA-VA5 seems to be pretty close. A marked 57pf doorknob cap measures 56pf.

Thanks again!

73'
Matt - WA0V