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THE AM BULLETIN BOARD => Technical Forum => Topic started by: Opcom on February 18, 2018, 12:21:49 PM

Title: antenna "tuner" question
Post by: Opcom on February 18, 2018, 12:21:49 PM
OK, lots of tuners are the high pass type, but that's not the best idea.

For what reasons would reversing the scheme not work? Because the sections of the inductor can't be separately adjusted?  Or, the caps would not have the desired pF range and still be practical? Just thoughts about it.

Title: Re: antenna "tuner" question
Post by: W1ITT on February 18, 2018, 02:02:29 PM
The first job of an antenna tuner is to match impedances, and sometimes to provide a transition between balanced and unbalanced ports.  Once you have that accomplished, harmonic suppression can be a nice bonus, and that's done with a low pass configuration, obviously. 
I suspect that many tuner designs are driven by the availability of variable capacitors which are common and relatively inexpensive.  But if you have good rotary inductors, or at least fixed coils with a lot of switch taps available, you can go that route.  It just costs more.  There are numerous configurations (or "topologies" as smart people like to call them) that will match two impedances.  For we who don't operate on a generous project budget, we make do with what we can procure.  Most modern commercially available transmitters meet the harmonic requirements all by themselves, so a high-pass "topology" wouldn't be a deal breaker.  Go with what you have or can get for components.  There's more than one way to skin this cat. 

Title: Re: antenna "tuner" question
Post by: WA4WAX on February 18, 2018, 03:27:37 PM
High pass is not a deal breaker, but I do not like "hot rotors".

Are you running into a balanced or unbalanced load?


Title: Re: antenna "tuner" question
Post by: WA4WAX on February 19, 2018, 04:14:26 PM
I might also add that the series caps will need wider low pass T and Pi circuits, not so much.  Plus, rotors are cold............neutral and floating in the balanced circuits.  Put a high performance current balun (1:1 DX Engineering, or MyAntennas Common mode beast.  Full disclosure: No connection to either party.)

OK.........more on balanced.  Get a 40 or 20 meter territory BC-610 coil.  Probably not one of the lower frequency coils, as they have formvar on the wire if memory serves.

Now, Mount the 610 coil in parallel with a good 150 to 250 pF range split stator cap.  Short, direct, and heavy connections here.  Make a 90 degree sheet aluminum L bracket, and put a hole in it for an N or SO-239 bulk head female.  Center conductor of said jack goes short and direct to one end of BC-610 coil link.   The other end of the link goes to the stator a 500V or so variable, say 300 to 500 pF.  The rotor goes short and direct to the aluminum bracket.  No need to ground bracket to center point of 610 coil.

OK........Attach feeders on either side of main coil with respect to neutral.  Resonate coil on desired frequency. GDO useful here.  Now, put a bridge on the input port of the jack, and mesh the low voltage variable.  If you cannot hit 50  ohms, move the tap points on the main coil out a bit or in a bit.  As a last resort, change the line length a bit.

If you you your work well, phase will be 180 spot on, or very close.  Amplitude balance as measured by a VTVM (410B, etc) will be within 2% or less!

Have fun!  If it works, wind some low band coils using G10 or polystyrene tubing, say 2.5 to 3 inches OD.  Do not use PVC!

Use calibrated dials with 0 to 100 markers.  Make a little book for dial and tap settings.

Find best taps by terminating balanced line with copper gator clips.  When you find the right place, attach Dx Engineering coil clips.  No solder, and great connections.


Title: Re: antenna "tuner" question
Post by: Opcom on February 20, 2018, 12:30:47 AM
It's a balanced load. The diagrams show unbalanced circuits for simplicity of the question. I'd prefer the low pass design - I have no idea how clean my amp under construction will be, and the Tucker transmitter is not so clean by today standards -an old 1952 design, and always used with a LPF. The BC610 coil setup is a fine one but may be be underpowered for later. I never likes floating both sides of the capacitor. My main curiosity is as to whether the lower diagram had been used, and what it might take in the way of 'upgraded' parts over the trandional T matcher that is so popular.

Title: Re: antenna "tuner" question
Post by: W3RSW on February 20, 2018, 07:27:12 AM
Your lower network is essentially a common L network where the shunt capacitor is normally switchable from one end of the coil to the other. Works well in most situations without requiring a variable coil. 

I think Ive heard that Upper circuit T matches may have more phantom bad matches into opens and shorts.

Whats with the emphasis on low and high pass nomenclature? Frequencies arent affected If impedance matches are realized all across a required spectrum.

Title: Re: antenna "tuner" question
Post by: Opcom on February 21, 2018, 10:56:53 PM
The upper circuit T match can definitely be set to give a fake match with no load on it! That is how they burn.

Low and high pass, maybe accusations? It is very commonly said that the upper circuit allows harmonics through. Could be a fallacy?  The lower kind of series coil arrangement is supposed to reduce harmonics.

I plan to use the mentioned ' common L network where the shunt capacitor is normally switchable from one end of the coil to the other'. The lower diagram seems to me a simplification assuming a large enough coil is around.

If a coil may be too large inductance, the variable cap may not be able to tune out any extra reactance and at the same time give the wanted match, so maybe the idea of my question is too simplistic for a tuner to cover a wide range.
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