Quick question re coil damping resistors

[Martin G7MRV]

Hopefully this is a quick question - Ive built two round 'bundled' coils for my Surf PI - a 10" and a 6", both measure a fraction under 300uH (270uH and 280uH resp.) and about 1.4 ohm. Ive then found what I think is about right for the damping resistor using a potentiometer. The 10" coil measures 427 ohm and the 6" 446 ohm,

How accurate do I really need to be? The reason I ask is that 430 ohm is a standard 5% series value, the next up being 470 ohm. Will standard series values be ok or should I make up a more accurate resistance from a few selected standard values?

[Qiaozhi]

A standard value resistor will be ok. If you examine the RX signal at the preamp output with an oscilloscope, this will tell you if the signal is critically damped or not. Sometimes it's possible to improve sensitivity by making the signal slightly underdamped. Again, examination with an oscilloscope will tell you if 430R or 470R is the most suitable value.

[Sacchetti]

Hopefully this is a quick question - Ive built two round 'bundled' coils for my Surf PI - a 10" and a 6", both measure a fraction under 300uH (270uH and 280uH resp.) and about 1.4 ohm. Ive then found what I think is about right for the damping resistor using a potentiometer. The 10" coil measures 427 ohm and the 6" 446 ohm,

How accurate do I really need to be? The reason I ask is that 430 ohm is a standard 5% series value, the next up being 470 ohm. Will standard series values be ok or should I make up a more accurate resistance from a few selected standard values?

Hi Martin!

You can use a potenziometer to found the correct value as first step.

After you can use a resistance of not wire type, just get one of lesser value and filing (polishing) it in some point of the body to raise the value!
Measure it continuosly using a multimeter and stop to file (polish) it when you get the right value.

I do not know english very well, sorry.
Can be the "file" or "polish" as word is not correct.
However I want to tell you to remove gradually a bit of material from resistor itself.

[baum7154]

I would not recommend putting just a potentiometer across the coil to determine the damping resistor value. This is because of the possibility of going to the zero end of the pot putting a short across the coil and burning the wiper contact of the pot. It is much better to put a fixed resistance of about 400 to 600 ohms in series with the pot. Then parallel that pot and the series resistor with another resistor of about 1200 ohms. This is the method documented by a BB Sailor in his paper on Building a fast PI Monocoil. This method insures that the damping will not go to zero while still allowing the pot to be used to find the best damping resistance by measuring resistance across the pot and resistor network.

Regards,

Dan

[Sacchetti]

I would not recommend putting just a potentiometer across the coil to determine the damping resistor value. This is because of the possibility of going to the zero end of the pot putting a short across the coil and burning the wiper contact of the pot. It is much better to put a fixed resistance of about 400 to 600 ohms in series with the pot. Then parallel that pot and the series resistor with another resistor of about 1200 ohms. This is the method documented by a BB Sailor in his paper on Building a fast PI Monocoil. This method insures that the damping will not go to zero while still allowing the pot to be used to find the best damping resistance by measuring resistance across the pot and resistor network.

Regards,

Dan

Dan,
I agree.

However when I play with potenziometers I start always with maximum value.
It is normal for me to add a resistor to avoid shorts.
But you are right, can be it is not the normal procedure for all.
It is better to describe the procedure correctly as you and BB Sailor has done.

And the method to increase resistor value work fine. Do you have done sometime?

About dumping resistor many people use wire resistor.
It is wrong because they add extra uH to the existing coil and modify the result.
Always I check technology of resistor before to buy it.
RS and other similar sellers give you always the datasheet where this information is present.
A very good alternative is to measure it using an LCR meter!

Have a nice day!