Originally Posted By Mach:
no, I don't think so, but I didn't even know there were bluetooth multimeters available. The resolution of 0.01 volts is already finicky because it is a 1:1 ratio with the turning cap. There is only about 7.6  milliamps of current going through the 3 pots that make up the voltage divider so when the primary pot moves from 300 to 800 ohms, the resolution for that 180 degrees of turn from full mesh to full unmesh on the cap is only 3.8 volts. or about 0.02 volts ( 20 millivolts ) per angular degree which is a very small voltage so it would have to be a fairly expensive multimeter with an accurate resolution in the microvolts and my primary DVM and these cheap voltmeters will display to the 10 microvolt resolution but they both vary about +- 10 miliivolts with no movement.  I think the limiting factor is the pot construction limiting the repeatability at the same position. It appears to be +- 0.01 volts.

I think the tuning resolution fix may be a digital angle sensor, (but they are magnetic and sitting within inches of a 100 watt mag loop may not be ideal )  to a microprocessor where I can dial in the freq I want on a display and the microprocessor moves the stepper motor to the correct stored position. But I am not even sure that would be better. But for now this will work until I figure out the digital stuff. I will probably try the digital stuff. It has the potential to greatly simplify the task of tuning the loop while driving and require a lot less attention on my part. Having presets and auto start and stop with the push of a button and a dial for dialling in a frequency with autostop when it gets there would be awesome. I have someone that can help in that regard since digital shit is not my thing, but I can probably figure it out.

Originally Posted By WillieTangoFox:


Thank you for taking the time to provide so many details. I would very much like to build a mag loop, and I have seen parts that "look" correct at hamfests in the piles of junk, but need to get a better idea of how to identify the correct variable capacitor.

Originally Posted By smeeg:
He's claiming he's getting NVIS with the hustler RM-40 because of high take off angles. Said he's tested with significant mountain ranges inbetween

https://www.youtube.com/watch?v=j3umxmwXuOc

https://www.youtube.com/watch?v=t73KHtCwGWk

Originally Posted By Mach:
Thanks, I watched the videos.

He is making it very difficult with the QRP radios and very small mobile antennas.

I have no doubt he can sometimes get regional comm up to a couple hundred miles out with a very short mobile vertical. I did too on occasion. A short vertical has a very high takeoff angle especially with a marginal ground on a vehicle. Using digital modes that are weak signal modes makes it happen more often, but it is not going to be very repeatable because not only does he need NVIS conditions, he needs VERY good NVIS conditions due to the extreme loss of signal being radiated at the take off angles needed. Real time testing between my vertical ( 11 foot radiator that starts under the fiberglass cap ) and the Mag loop completely under the fiberglass cap and coupled to the bed resulting in much lower Q that an isolated mag loop, using the S meter on my IC-7300 which we know is not accurate at lower S readings S meter vs real signal dbm readings ) , I have shown about  36 db difference is displayed signal strength, ie. 6 S units. ( the big 3 radio manufacturers purposely use different dbm calibrations than standard for their S meters on the lower S meter readings to falsely show a quieter radio. For example, in a low RF noise location, my IC-7300 will read zero S meter and I can still hear and copy phone signals that do not register on the meter. It is impossible to to built a radio that has an internal S noise floor of zero, A typical internal noise floor of a well built radio is typically S5.  So when I did the testing I used a day when ECARS net control was coming in at S5 on the vertical  and when I switched over to the mag loop he was repeatedly coming in at 10 - 15 db over S9 which I equated to about 36 db increase. But it could be more or less. When the meter was showing net control at S0 on the vertical but being heard and I switched over to the mag loop, I was getting S 9, but it was not really an S0 signal and not really 9 S points more, so I assumed S5 on the 7300 was probably closer to a standard S5 in dbm signal strength and used the better propagation days as the comparison. I wish i could do it with an accurate dbm reading but I don't have the equipment and realistically the numbers don't really matter much, just the ability to make phone QSOs inside 200 miles regionally which is what i was after by building this thing.

There are many days that ECARS net control is simply not there on the vertical but comes in S5 on the mag loop and I make a phone QSO with him, so it seems like it really is an increased capability that I did not have before while mobile.

However, I have stopped working on making it better or improved. Local new gun laws mean i need to move and I am spending my time on that instead. I hope I can make this happen, but i have my doubts.

Originally Posted By Mach:
So I had this idea to attach a linear taper potentiometer to the air variable capacitor to the axle of the capacitor and use a pot above the main pot and a pot below the main pot and use a voltage divider to calibrate the voltage reading of the divider to the position of the air variable capacitor.

I thought the whole thing would be linear.

The voltage readout at 3.5 Mhz is 3.5 volts. The voltage readout at 7.2 Mhz is 7.2 volts, and is perfectly repeatable through the 180 degrees of the capacitor.

I assumed between the points would be linear, but it is not. At 3.9 mhz the voltage reads 4.1, at 5.3 mhz the voltage reads 6.3. It is not linear. The frequency that the loop is tuned to raises slower than the rotary position of the capacitor. Something is not linear.

I am pretty sure the linear taper pot is linear using DC.
I am pretty sure the voltage divider is linear as the voltage changes.
I assumed the capacitance value changes linearly with the area of overlap of the plates but maybe it doesn't. After all when completely unmeshed, their is no overlap but there is still a capacitance. But when almost fully meshed, it is not linear. at 3.6 Mhz, the voltage reads 3.6, almost fully meshed at 3.9 mhz, is slightly less fully meshed and the voltage reads 4.1 instead of 3.9, so in only 0.3 mhz increase in freq, the voltage divider is already off 0.2 volts.

I thought capacitance varying with frequency was linear, but maybe not.
I though inductance varying with frequency was also linear, the loop is an inductor, but again maybe not.

I suspect  the non-linear effect is from the tuned frequency coupling differently with the surrounding metal of the truck..  But I am just guessing.

Anybody have any ideas why the tuned frequency is not linear to the movement of the variable capacitor