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Author Topic: Magnetic loop antenna question  (Read 2403 times)
RSHIRE22
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« on: November 07, 2013, 07:09:54 PM »

I am building a magnet loop antenna for 20 meters. I want an antenna that handle 20 to 30 watts of power. I am using the KI6GD magnetic loop calculator. I have an 8-200 pf air variable capacitor but it is only rated at 1 KV which would limit my power to about 5 watts to avoid arcing.

In order to avoid purchasing a high voltage air variable or a vacuum variable I am considering doing the following:

The loop has a circumference of 10 feet so tuning to 14 mhz requires a capacitance of 29pf. I calculated I can get 29pf by putting 50pf and 90pf capacitances in series. If I put the 1 KV air variable (8-200pf) in series with 3 feet of RG58A (90pf) should I be able to tune the loop running higher power? RG58A has a breakdown voltage of 1900 volts rms according to specs.

Will this work and has anyone tried this before? Would two capacitances in series have higher combined breakdown voltages or less?

Thanks in advance to all replies.

Ron
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WB6BYU
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« Reply #1 on: November 07, 2013, 08:27:43 PM »

With two capacitors in series the RF voltage divides between them in inverse ratio to the
reactance.  So to reduce the voltage across your variable capacitor you would want it
to have low reactance, that is, high capacitance.

In the case of 50 and 90 pF capacitors in series, the reactances on 20m are about
240 ohms and 130 ohms.  The voltage drop across the 50pf capacitor will be
240 / ( 240 + 130 ) = 65% of the total, and the 90pf will have 35% of the voltage.

Assuming that your fixed capacitor has a high enough voltage rating, I'd choose one
that allows you to use close to the maximum capacitance of your variable.  For example,
for a total of 29pf a 33pf fixed capacitor isn't quite big enough, but a 39pf unit (the
next standard size up) would allow your variable capacitor to tune up to 32.5pf when
the variable is set to 200pf, with most of the voltage across the fixed capacitor instead
of the variable.


There can be an additional issue with standard variable capacitors in this application
due to high current flowing through the bearing that connects the rotor shaft to
the frame of the capacitor:  that's why "butterfly" capacitors are often used, because
there is no wiper contact in the RF path, resulting in lower losses.  But I'd suggest
trying it first with the capacitor you have to see how it works.
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RSHIRE22
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« Reply #2 on: November 08, 2013, 08:27:51 AM »

WB6BYU,

Thank you for your reply. You suggest using the air variable set to 200pf which in series with a coax capacitance (35 ohms - my calculation) combines for a 29pf capacitance. This will result in less voltage across the air variable.

So you suggest this is doable? I suspect it will affect the Q of the loop but hopefully not too bad. I read also if connecting the air variable by stator points only and not rotor points will work better. Correct? I'm also thinking about using RG9 for the fixed capacitor (20pf per ft). It has an impedance of 70 ohms but probably not relevant in this use. It has a much higher break down voltage - about 5KV.

If I can run 25-30 watts at 30% efficiency I will be a happy camper.  I'll build soon and post my progress.

Ron
« Last Edit: November 08, 2013, 08:40:46 AM by RSHIRE22 » Logged
WB6BYU
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« Reply #3 on: November 08, 2013, 09:36:47 AM »

Well, I might choose the series fixed capacitor initially based on ~150pf for the variable,
to provide a bit of margin in case it requires just a little more capacitance to tune the
loop than you expect.  Once you get it tuned, you can decrease the fixed capacitor a
bit if you need to lower the capacitance a bit across the variable.

A couple things to keep in mind:

1) the Q of coaxial capacitors isn't as high as many fixed units, so you'll have a little
more loss using RG-59.

2) The same current flows through both capacitors, and that is going to be
the most significant source of loss.  Anything you can do to reduce the effective
resistance of the capacitors will help to improve efficiency.  This includes the rotary
joint in the standard variable capacitor:  you need a dual variable (two sections
on the same shaft connected in series) to avoid the rotor joints.  Similarly, the
braided shield of the coax has higher losses than a solid copper tube, and the
center conductor of RG-59 is rather thin for the heavy current. Something like
RG-8 or RG-11 would be a better choice to reduce current loss in the center
conductor, and a capacitor with fixed plates (perhaps with glass or Teflon
dielectric) will be even better, as long as the joints are well soldered.

Overall I'd say it is practical, at least for a test.  My guess is that you won't
see the expected efficiency at your first try, especially using RG-59, but it should
give you an idea of whether it is worth pursuing the idea.  But from your first
try you can begin improving the capacitors to see if you get an antenna that
meets your needs.


I tried a loop antenna back when I was first licensed, and did just about everything
wrong.  I used iron wire for the loop, tuned it with a standard receiver variable capacitor
and some standard fixed mica capacitors, and leaned it against my closet door.  The loop
was about 6' in diameter for 75m SSB, and I was still able to make contacts on it - at
least the locals.  But to get reasonable efficiency you need to pay attention to all the
sources of loss.
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W0BTU
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« Reply #4 on: November 08, 2013, 04:53:21 PM »

I am building a magnet loop antenna for 20 meters. I want an antenna that handle 20 to 30 watts of power. I am using the KI6GD magnetic loop calculator. I have an 8-200 pf air variable capacitor ...

I've never seen (or heard of) the KI6GD loop calculator and so cannot comment on it. But I'd like to ask, What are you making the loop from? Most STLs of reasonable efficiency use large copper or aluminum pipe and a butterfly capacitor as a minimum. Wire --even large wire-- is out of the question for transmitting because the radiation resistance is often in the milliohms, and that means you have to pay careful attention to minimizing the resistance everywhere. That includes the short connections between the loop itself and the capacitor.

As Dale says, that includes the wipers on an ordinary air variable, which can turn red hot at even modest power levels due to the enormous current flowing through the loop. I second his suggestion on the butterfly, which has no wiper, and has been discussed here before in this context.

My own STL project is on hold, but I think you will find some very useful info on a page I created at http://www.w0btu.com/magnetic_loops.html .  There's a link there to AA5TB's loop calculator, which I used, and a screenshot of it with the STL I want to build (someday).

Using a section of coax as a capacitor has also been discussed here at length. I forget the details, but the butterfly is quite a bit superior, though you cannot beat a proper vacuum variable. One person made a variable capacitor using paralleled sections of homemade hardline which move in-and-out trombone style. The link to it is on my page.

Anyway, I wouldn't be quick to rule out a vacuum variable. I bought a very nice NOS Russian surplus vacuum variable very cheap. EBay is your friend.

I heard a guy on 17 meters today with a small loop who always has a good signal.

I wish you well on your project, Ron.

EDIT: copy the line below into google.com for a partial list of previous discussions here on this subject:
site:eham.net transmitting loop capacitor efficient
« Last Edit: November 08, 2013, 05:04:15 PM by W0BTU » Logged

RSHIRE22
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« Reply #5 on: November 10, 2013, 05:55:33 AM »

I am building the loop out of ten foot RG213 with a braid outer diameter of .30 inches. I am using a T140-43 toroid transformer to feed. Right now I have a single gang 8-200 air variable capacitor rated at 1KV. Loop calculator: http://www.gobookee.org/ki6gd-magnetic-loop-calculator/

I have built a transmitting loop previously with similar materials and it was usable up to about 10 watts then arcing began. I'm exploring the possibilities of using coaxial capacitance in parallel or series combinations with the air variable to increase the wattage.

Mike, I will check out your website. Any new information is appreciated.

Ron
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