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Old 26th Feb 2023, 1:34 pm   #1
Malcolm T
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Default Link coupled Z match

Nearing the end of a build here of a Link coupled z match from bits and bobs.
Question , will it work with just a single VC across the balanced line o/p ?.

An amateur on you tube posted his build and it appears too work very well.
Or does it really require a double butterfly cap and a center vane earth.

Looking to feed an antenna for 10 mt band as i have heard conditions are picking up.
I haver never built nor used one before so should be an interesting experiment with different feeders and aerial combinations.
There are no toroids nor coax involved in feeding and z matching the aerial, that was one of my caveats. Trying to rule out coax, PL259 connectors and toroids !.

Any suggestions / ideas welcome
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Old 26th Feb 2023, 7:06 pm   #2
Graham G3ZVT
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Default Link Coupled ATU

I'm sure I saw a post about an hour ago with a similar title to this one
Was it in a different part of the forum?

Anyway, it addressed the same dilemma that I had when I built this experimental ATU i.e. the difficulty in obtaining a differential "butterfly" VC
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Old 26th Feb 2023, 7:24 pm   #3
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Default Re: Link Coupled ATU

Hi link to other post https://vintage-radio.net/forum/showthread.php?t=198628
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Old 26th Feb 2023, 7:28 pm   #4
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Default Re: Link Coupled ATU

Hi i built my own from parts https://www.etsy.com/uk/listing/8888...MaAnvhEALw_wcB just found the link Mick
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Old 26th Feb 2023, 7:31 pm   #5
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Default Re: Link Coupled ATU

Thanks Mick, I'll pull the emergency call cord, and ask the mods to merge the threads where they deem appropriate.
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Old 26th Feb 2023, 7:49 pm   #6
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Default Re: Link Coupled ATU

Quote:
Originally Posted by rambo1152 View Post
I'm sure I saw a post about an hour ago with a similar title to this one
Was it in a different part of the forum?

Anyway, it addressed the same dilemma that I had when I built this experimental ATU i.e. the difficulty in obtaining a differential "butterfly" VC
What I understand by "differential" VC, is one where one section increases capacitance while the other half decreases. This would have the effect of moving the earthed "tapping point" on the tuned circuit - and that is not what the circuit requires. That will alter the balance of the circuit.

The purpose of the variable capacitors - as I understand it - is to alter the resonant frequency of the tuned circuit. That requires a standard variable ganged capacitor, where each section has the same capacitance value at each point in the rotation.
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Old 26th Feb 2023, 8:04 pm   #7
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Default Re: Link Coupled ATU

Quote:
Originally Posted by trh01uk View Post

What I understand by "differential" VC, is one where one section increases capacitance while the other half decreases. This would have the effect of moving the earthed "tapping point" on the tuned circuit - and that is not what the circuit requires. That will alter the balance of the circuit.

The purpose of the variable capacitors - as I understand it - is to alter the resonant frequency of the tuned circuit. That requires a standard variable ganged capacitor, where each section has the same capacitance value at each point in the rotation.
Yes, that's the dilemma.
On the other hand a normal VC is asymmetric, with one side connected to its frame and the control spindle. This could be isolated from ground but seems less than ideal in the circuit I posted.

Is there a different VC that is ideally suited?
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Old 26th Feb 2023, 8:20 pm   #8
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Default Re: Link Coupled ATU

Quote:
Originally Posted by rambo1152 View Post

Yes, that's the dilemma.
On the other hand a normal VC is asymmetric, with one side connected to its frame and the control spindle. This could be isolated from ground but seems less than ideal in the circuit I posted.

Is there a different VC that is ideally suited?

Sorry - I don't really understand what the problem is here. In the original circuit, we have the centre point of the two capacitors C1 grounded. So a totally standard variable capacitor, with the stator grounded, should meet the requirement. Not difficult to find at all! The rotors of these two capacitors go to the outer tapping points on L1, as indicated on the schematic.

The only difficulty that you might have with the circuit is that of power levels, if you are using the ATU for transmitting. In which case you will have to watch the plate spacing on C1 is appropriate to whatever power you want to pass through the ATU. If its receive only, then any standard ganged variable capacitor should work just fine.
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Old 26th Feb 2023, 8:28 pm   #9
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Default Re: Link coupled Z match

The old KW E-Z match type circuit should meet your requirements. That's described on various websites, but here's a typical one: https://vk6ysf.com/kw_ezee_match.htm

I don't think it requires any "exotic" variable capacitors at all. The ones pictured appear to be pretty standard.
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Old 26th Feb 2023, 8:35 pm   #10
Cruisin Marine
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Default Re: Link Coupled ATU

You could use just one capacitor on the primary side of this (with no earth centre tap) as long as you realise both sides of the cap will be RF "Hot"
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Old 26th Feb 2023, 9:04 pm   #11
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Default Re: Link coupled Z match

Have been using this design for many years. Much easier than the original Z match.

http://users.tpg.com.au/users/ldbutl...CoilZMatch.htm
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Old 26th Feb 2023, 9:05 pm   #12
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Default Re: Link Coupled ATU

Quote:
Originally Posted by Cruisin Marine View Post
You could use just one capacitor on the primary side of this (with no earth centre tap) as long as you realise both sides of the cap will be RF "Hot"

Quite. I suspect the only reason for having a dual gang capacitor at all is to go for easy construction by allowing the centre point - the common stators - to be grounded.
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Old 27th Feb 2023, 10:31 am   #13
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Default Re: Link Coupled ATU

As well as maintaining symmetry for feeding balanced-lines, the dual-capacitor approach also has the benefit that since both halves are wired in series, the RF voltage appearing across each half is halved.

If using old broadcast-capacitors with a beefy transmitter, the voltage issue becomes significant under certain (mis)matched conditions!
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Old 27th Feb 2023, 11:51 pm   #14
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Default Re: Link Coupled ATU

I thought that it was the spindle, chassis and rotors that were grounded in a 'standard' dual gang variable capacitor.

The insulated 'stators' are then connected to the inductors.
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Old 28th Feb 2023, 3:26 pm   #15
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Default Re: Link Coupled ATU

Quote:
Originally Posted by trh01uk View Post
In the original circuit, we have the centre point of the two capacitors C1 grounded. So a totally standard variable capacitor, with the stator grounded, should meet the requirement. Not difficult to find at all! The rotors of these two capacitors go to the outer tapping points on L1, as indicated on the schematic.

Quite right. I've been using this arrangement for years to match a doublet to any frequency and keep it balanced to cancel noise.
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Old 28th Feb 2023, 6:36 pm   #16
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Default Re: Link Coupled ATU

Quote:
Originally Posted by Silicon View Post
I thought that it was the spindle, chassis and rotors that were grounded in a 'standard' dual gang variable capacitor.

The insulated 'stators' are then connected to the inductors.
Yes - quite right. My mistake. Its the common rotors that are grounded, and the stators go to the inductor taps.

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