TheRadioBoard

What would be an easy way to measure the capacitor loss?

I could make a stand-alone version of the homebrew capacitor and compare its loss with that of an air-spaced cap I have on hand, but what should be measured?

If you're thinking about the MFJ butterfly caps the loss comes from connecting to the cap. Typically, you'd use a thick material like copper tubing. Then the losses are ohmic, from the connections. Given the very low radiation resistance of the antenna itself, even a few tenths of an ohm of dc resistance would be significant. But it might be a fair trade-off for tuning simplicity. You'd need a milli-ohmmeter to measure it. Try this search for pictures of how MFJ incorporates the cap in a loop tuner. https://duckduckgo.com/?q=MFJ-933

73,

_________________

http://kr1s.kearman.com/
http://qrp.kearman.com/

Actually I was thinking of how to compare the total losses in the homebrew capacitor vs. the total losses with an air-spaced cap. (I have a 1 kV air-spaced variable capacitor but it has a rotary wiper contact, a no-no for high- magnetic loops. It would be interesting to compare its loss with the homebrew capacitor loss.)

How about measuring the overall tuned circuit Q as a measure of loss? Proposed procedure:

* Define fixed positions of antenna and a sepaate oscillator signal source.

* Tune oscillator to 21 MHz.

* Peak the antenna tuning capacitor for maximum signal at 21 MHz.

* Adjust oscillator output level so that receiver S-meter lies just barely above the S8-S9 threshold. Tuning just slightly down or up in frequency should yield an off-peak reduction in signal strength to the S8 level.

* Tune receiver downwards until S-meter reading lies just barely above the S7-S8 threshold. Note the frequency reading.

* Tune receiver upwards until S-meter reading lies just barely above the S7-S8 threshold. Note the frequency reading.

* The -6dB bandwidth can be calculated from the two frequencies noted above.

Perform the above steps for both the homebrew solderless capacitor and a soldered-in air variable capacitor to determine comparative bandwidth (and hence, tuned circuit Q) values.

Would the above procedure be able to accurately detect tiny (but significant, for a magloop) differences in loss resistance or other capacitor losses?

Your idea needs one change to give you a comparison between the two caps.
I think you are making the frequency adjustment in the wrong place.
You need to adjust the frequency of your frequency generator and watch your S meter until it changes by *usually 3 db.
Then subtract the low 3db point from the high 3db point for the difference, and divide the peak frequency by the difference.
Example; Peak frequency 1,000,000 hz lower 3db frequency is
997,600 and your high 3db frequency is 1,002,600. The difference is 5000, soo... 1,000,000/5000= 200. A Q of 200.
You'll need to find a place on your S meter to get the best accuracy and stay there for both caps.
I suspect you will need to adjust the output level between caps. I used BCB , but the same for higher frequencies.

*getting 3 db on an S meter i hard unless you run a calibration test.
But for comparison one S unit could be used.
If you have a scope, I will post a method I have used dozens of times.
Mikek

I"ll let one who can do the math answer that.
If you have a cap with a Q of 6000 and a cap with a Q of 4000 then connect each to a coil with a Q of 150. What will the total Q be with each cap?
I might spend the next hour doing that or someone that knows math can do it in 5 minutes.
Then you can decide if you can measure that frequency change accurately with your S meter.

Mikek

I took me just under an hour I love Google!
I used the following Q's, inductor Q = 200 C1, Q=4000 C2, Q=6000
I don't know if these are realistic at 21 Mhz.
With a 6000 Q cap and a 200 Q inductor, Total Q is 193.4762.
With a 4000 Q cap and a 200 Q inductor, Total Q is 190.5485.
I don't think you will be able to measure that difference.
But if your inductor Q is higher and your cap Qs are lower,
Say,
With a 2000 Q cap and a 400 Q inductor, Total Q is 333.333.
With a 1000 Q cap and a 400 Q inductor, Total Q is 285.714.
That, you should be able to discern.
Formula:
Q at resonance = QL * QC / QL + QC
Mikek

What's wrong with tuning the rig to find the 3dB (or 6dB=1 S-unit) points?

No scope, but plenty of curiosity. What's your method?

If you tune the rig, I think it tells you more about the IF filter in your radio
than the Q of your Loop.

It's late, I copied this from another thread I posted in 2006.
Read it and if you have questions I'll review it tomorrow and see if I can clarify.
Oscilloscope Method,
You need to lightly couple some energy from your signal generator into the
resonate circuit. This can be done by placing the generator wires near the
resonate circuit close enough to get the scope signal level you need but as
far
away as possible so you don't load the circuit. You can couple it with high
value resistors if desired, but this increases loading on the inductor.

Then I adjust coupling and signal generator output to get 7 units
on the scope. Why 7 units? I'm glad you ask! You want to move the signal
generator frequency up until the voltage on the scope drops to 5 units.

Some explanation; We want to measure the upper and lower frequency points where the voltage drop is 3db or .707 times the resonate voltage.
So, back to the 7 units, .707 times 7 units equals 4.949 units or 5 units
when I'm looking at my scope.

So we adjust the signal generator frequency to peak the waveform on the
scope.
Let's say the waveform peaks at 1,596,200hz.
To get the 7 units sometimes I adjust the generator drive and sometimes I
change the scope variable attenuator.

I move the frequency up until the scope reads 5 units, Record this
frequency.
Lets say it's 1,600,200hz
Now move the frequency down until the scope reads 5 units, Record this
frequency.
Lets say this is 1,592,219hz
Do the math 1,600,200 - 1,592,219 = 7981
then using the resonate frequency of 1,596,200 / 7981 = 200
The Q of your inductor is 200

I think I got this all correct, it has been a few years since I did this. I
used it a lot when I was
trying methods to improve the Q of some potcore inductors.
Let me know if you have questions.
Mike

If I'm interpreting the AA5TB loop calculator properly, at 21MHz with a conductor diameter of 20mm and a loop circumference of 0.3 wavelength (giving over 90% efficiency), the inductor Q is a modest 147, which means measuring tuned circuit Q won't be able to detect changes in capacitor loss. Oh well.

In spite of possible losses, probably I will go ahead and try use my homebrew capacitor on the air after I figure out the mechanics.

Got another question about Q. Say I take the approach of this receiving loop antenna:

http://users.erols.com/k3mt/hla/hla.htm


My loop will be vertical but the strip ends will overlap in a manner similar to the above diagram. In the above antenna, the strip ends overlap and are separated by a newspaper dielectric. A book adjusts the amount of strip overlap with the excess strip just hanging loose over the book.

In my case I would support the strip ends on top of a cardboard box (part of a stealth antenna arrangement). The newspaper dielectric would be replaced with 1mm-thick polyethylene. The book would be replaced with one or two thick polyethylene cutting boards, 1cm-2cm thick.

What do you think? Will the Q be usable or miserable?

Right, makes sense. So it seems you need to tune both the signal generator and the rig (keeping the signal generator's beat note exactly the same so the generated signal is still at the same point within the IF passband). That way the only variable signal attenuation (hopefully) will come from the off-resonance response of the loop.

Anyway, as mentioned above, it seems the inductor Q is too low for a magloop to allow S-meter measurements, but the procedure is good to know. Thanks.

Well I've got the strip magnetic loop antenna set up on my balcony in semi-stealth mode (designed not to look obviously like an antenna). I'd like to ask if there are any obviously horribly lossy aspects that need to be improved.

Pictorally, it is laid out as follows:

Elements in the diagram are as follows:
A: 60mm-wide, 1mm-thick polyethylene.
B: plastic barrier tape, 60mm-wide, sticky-side-in
C: copper foil tape, 50mm-wide, 5m-long, sticky-side-out
D: cardboard boxes
E: coupling loop taped to inside of cardboard box, with coax cable leading to rig

Here's a photo:


A close-up of the capacitor:

It's held together with plastic clothespins. The photo shows the capacitor resonated with the loop at 7 MHz. For higher bands it is workable just to slide the plates apart in a scissors-like fashion, though this isn't mechanically stable.

A couple of questions:

* The loop element is directly resting on the ground surface of the 2nd-floor balcony. Is this lossy? Should the loop element be elevated off the ground, or does it not matter here since the balcony "ground surface" is already ~3m above the earth's surface?

* The space of the loop is filled with the (empty) cardboard boxes. Lossy?

* The capacitor plates are resting on top of the cardboard boxes. Lossy? Should the plates perhaps be elevated off the surface of the cardboard with some kind of scaffold structure made of PVC tubing (or polyethylene blocks)?

One other thing that may cause losses, but about which I can't do much, is the proximity of the aluminum railing.

Any comments appreciated!

Glad to see your pandas are lined up. There is probably steel reinforcing bar (rebar) in the slab, so another row of pandas under the loop would help. There isn't much you can do about the railing. You can judge its effects by having the pandas take one step forward toward it, and see how that affects tuning.

73,

_________________

http://kr1s.kearman.com/
http://qrp.kearman.com/

The aluminum frame in the sliding patio doors appear to be roughly the same dimensions as your antenna. So, it may also have some effect.

Having now gained some experience (on receive only for now) with the copper strip loop, I'm considering taking the experiment to the next stage and using ordinary household aluminum foil - affixed to long strips of tape for a non-tearable backing surface - as the loop conductor. Advantages are the wide width (20 or 30cm) and long unbroken length of 20m or more available cheaply. Greater conductor width would allow making a smaller-diameter loop while still maintaining efficiency. Disadvantages are lower conductivity compared to copper (though greater width might compensate for this), possible over-thinness of the foil (less than the RF skin depth?), and increased susceptibility to wrinkling (sharp edges may impede RF current flow and/or cause coronal discharge - the copper strip loop has the same potential problem).

Nonetheless, the ease and low cost of making a continuous and quite wide loop are attractive; equivalently wide and long unbroken lengths of copper foil aren't available easily.

A small and very portable 1m diameter loop at 30cm wide should have around 25 percent efficiency at 7 MHz, with increasingly acceptable efficiencies at the higher bands. Since the aluminum can't be soldered to, the capacitor would have to be some variant of plate overlap already discussed earlier.