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Author Topic: Suggestions for tuner?  (Read 4459 times)
W1SFR
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Posts: 21




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« on: February 29, 2012, 09:08:12 AM »

I recently fired up my homebuilt OHR 100A on 40m. I'm using a homebrew Mag loop antenna that has an internal 350pf variable cap. The antenna doesn't have an swr indicator built in so I need to be able to fine tune it. I can get it down to about a 6:1 SWR according to my Wattmeter, but that's it. I built a BLT+ tuner, but it doesn't work at this point and I'm looking for off the shelf tuner suggestions.

Steve
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OHR 100a QRP XCVR at 5W on 40M
TenTec Argosy 525
40M LNR ParEndFedz Antenna
AA4PB
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Posts: 12345




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« Reply #1 on: February 29, 2012, 10:12:17 AM »

MFJ makes at least one QRP level tuner.
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KJ4FUU
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Posts: 158




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« Reply #2 on: February 29, 2012, 10:14:18 AM »

I use an Emtech ZM-2 tuner. You can get them preassembled, full kit, or kit with toroids pre-wound. It will tune just about anything.

Depending on how you build your loop , I think you can get "hand capacitance" issues that will make finding a proper match tricky.

-- Tom
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WB6BYU
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Posts: 12772




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« Reply #3 on: February 29, 2012, 11:12:12 AM »

How is your loop designed? 

If you can tune it through a dip in SWR but the minimum doesn't get below 6 : 1, then the
impedance matching on your loop needs improvement.  It may be 8 ohms or so, which would
give you an SWR of 6 : 1.  That is rather low for many tuners to match, so you may be
better off improving the matching in your loop.  Try putting 1000pf across the feedpoint
and see if that helps.

If the problem is that you can't set the variable capacitor close enough because the tuning
rate is too fast, then a larger knob or a smaller capacitor (perhaps in parallel with a fixed cap)
will make it easier to tune.  A reduction drive may also help.

If hand capacitance is the problem (you tune it for minimum SWR, then the value shifts
as you move your hand away) then a long insulated shaft on the tuning capacitor will
help, or driving it with a remote motor.  Sometimes you can learn how to adjust so that
it gives good match when you remove your hand.


Any of these is likely to improve performance at less cost than adding an external tuner
between the rig and the loop.
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K5BJS
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Posts: 50




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« Reply #4 on: February 29, 2012, 12:04:04 PM »

What are the dimensions of your loop?  I use an MFJ-936B loop tuner with a 6.5' x 6.5' loop using 4" aluminum flashing.  The current meter is extremely useful (fine-tune for maximum loop current).
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W1SFR
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Posts: 21




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« Reply #5 on: February 29, 2012, 12:07:38 PM »

http://www.g4ilo.com/wonder-loop.html

Here's the web page that has the info and the loop design. I pretty much built it to the spec as shown. I think the outer loop is maybe 12" longer. I'm using it for 40m which I realize isn't ideal. The current cap is 500PF. This plan called for a 250pf so I figured that the 500 would do the trick. The tuning spot is so sharp that I agree, a reduction gear would really help. I'm finding with my current set up reception is very good, but crappy on the TX side...most likely due to my poor SWR.

Steve
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OHR 100a QRP XCVR at 5W on 40M
TenTec Argosy 525
40M LNR ParEndFedz Antenna
K5BJS
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Posts: 50




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« Reply #6 on: February 29, 2012, 01:47:43 PM »

So your loop has a perimeter of about 9.2 feet?  It will be more efficient and easier to tune if you make it larger (mine has a perimeter of 26 feet).  Don't be afraid to try a square or even a rectangle if necessary.
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W1SFR
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Posts: 21




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« Reply #7 on: February 29, 2012, 04:17:20 PM »

I guess I need to make the loop larger. I'll have to experiment and see what I can come up with.
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OHR 100a QRP XCVR at 5W on 40M
TenTec Argosy 525
40M LNR ParEndFedz Antenna
WB6BYU
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Posts: 12772




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« Reply #8 on: March 01, 2012, 06:55:16 AM »

Quote from: MILLERHILL
I guess I need to make the loop larger. I'll have to experiment and see what I can come up with.

A larger loop generally would improve efficiency, but that alone doesn't solve your current
problem.  If you want to use the loop on the higher bands, you can't make it too large.
But for just 40m, making it larger is a good starting point.

The variable capacitor needs to be sized for the appropriate tuning range.  You might look
at the setting of the current variable and estimate how much capacitance you really need,
then use perhaps a 50 or 100pf with some fixed capacitance across it (or even just a small
variable for fine tuning across the current one.)  The larger the capacitor the sharper the
tuning because the ratio of pf / degree of revolution is higher.  Ideally you would restrict
the tuning range to, say, 6 to 8 MHz.  (The larger capacitor will also limit the high
frequency coverage because of the higher minimum capacitance.)

The SWR at resonance is determined by the shape, size and placement of the coupling loop.
This is what transforms the low impedance of the loop to something that can be matched
to the feedline.  The common rule of thumb is 1/5 the diameter of the main loop, which is
a good starting point, but needs some experimentation.  Try varying the length of the wire
and making the loop more oblong in one dimension or the other to see what effect that
has.  The exact requirements depend somewhat on the local environment and losses, but
if you made the loop itself longer than the original you may need to make the coupling loop
larger as well.
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N5RWJ
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Posts: 461




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« Reply #9 on: March 01, 2012, 12:35:41 PM »

I use an Emtech ZM-2 tuner. You can get them preassembled, full kit, or kit with toroids pre-wound. It will tune just about anything.

Depending on how you build your loop , I think you can get "hand capacitance" issues that will make finding a proper match tricky.

-- Tom
I agree, the ZM-2 best for QRP and ZM-4 is best QRO , they are magic ? the ZM-4 is a UK kit
« Last Edit: March 01, 2012, 12:38:14 PM by N5RWJ » Logged
W1SFR
Member

Posts: 21




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« Reply #10 on: March 01, 2012, 01:42:21 PM »

I'm new at this (my 3rd day as a ham Grin) so can someone explain to me the relationship between the frequency, the size of the loop and the size of the capacitor used to tune the frequency?
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OHR 100a QRP XCVR at 5W on 40M
TenTec Argosy 525
40M LNR ParEndFedz Antenna
WB6BYU
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Posts: 12772




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« Reply #11 on: March 01, 2012, 02:41:37 PM »

The loop forms a coil - it has a value of inductance that is a function the diameter
(and some other factors.)

You want to resonate that coil with a capacitor to form a resonant circuit.  To do so
requires more capacitance on lower frequencies and less capacitance on higher ones.
This is what your variable capacitor does.  Actually the resonant frequency depends
on the product of L (inductance) and C (capacitance), so either a smaller loop or
a smaller capacitor will raise the resonant frequency.

The resonant frequency varies with the square root of the capacitance.  To tune a
4 : 1 frequency range you need a variable capacitor with a 16 : 1 range, such as
15 - 240pf.  Your loop also have to be sized appropriately so that it resonates at
the top of the range with 15pf.  Let's assume that your loop requires 200pf to tune
the low end of 40m.

If you replaced the 240pf in the preceding example with a 500pf variable, the minimum
capacitance will probably be around 30pf, so you can't tune the loop as high in frequency.
The tuning rate, assuming 180 degrees of rotation, is now 500/180 = 2.8 pf / degree of
rotation.  With the old capacitor it was 1.3 pf/degree.  So to cover the whole 40m band
takes more than twice as many degrees of rotation with the smaller capacitor, making
the loop easier to tune.  If you used a 5 - 60pf variable in parallel with 180pf of fixed
capacitance to get the same 200pf then the tuning rate is 0.33 pf / degree, and it
is much easier to tune the loop to resonance, but the total tuning range is greatly
limited.

The maximum loop circumference for the usual mode of operation is around 1/4 or 1/5
wavelength.  Beyond that the current distribution changes.  That usually sets the upper
frequency range for a loop.  It can be used on lower frequencies by adding more capacitance,
but the loop efficiency drops off as the loop becomes electrically shorter on the lower bands.

Making the loop larger increases the efficiency (up to a point) and requires a smaller tuning
capacitor for the same operating frequency.
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W1SFR
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Posts: 21




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« Reply #12 on: March 01, 2012, 04:55:46 PM »

Thanks WB6BYU for that great response! I took a reading of my variable cap at the place where the loop seemed most efficient and it was around 350pf. The large loop is 2.5m and the control loop is 50cm. Given that, what would you suggest I do to make this rig more efficient. From what I've seen here, I need a loop that is about 25' and a control loop of about 5'. Does that sound kosher to you?

Steve
KB1WUD
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OHR 100a QRP XCVR at 5W on 40M
TenTec Argosy 525
40M LNR ParEndFedz Antenna
WX7G
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Posts: 5691




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« Reply #13 on: March 01, 2012, 05:19:05 PM »

Take a look at the MFJ-932 MINI LOOP TUNER.
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WB6BYU
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Posts: 12772




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« Reply #14 on: March 01, 2012, 07:01:52 PM »

Quote from: MILLERHILL
...From what I've seen here, I need a loop that is about 25' and a control loop of about 5'...

That would be more efficient if you have the room (roughly 8' in diameter).  But use whatever
size loop will fit - indoors you might want to try 5' or 6' in diameter to give some clearance
between it and the ceiling and floor.  You can still get reasonable efficiency and better
bandwidth compared to your current loop.

You should hear the background noise increase significantly and signals come way up when
you tune the loop through resonance.

With the larger loop you'll need less capacitance.  If you don't have another variable capacitor
handy you can put a fixed capacitor in series with it and limit the tuning range - this will take
some experimentation (or calculation.)


I have a loop that is about 12" in diameter that will tune down to 40m, but I don't think it
is very efficient there.  I use it for receiving instead.
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