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Author Topic: Link Antenna Coupler - Ladder Line Shunt Inductor  (Read 27445 times)
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aa5wg
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« Reply #25 on: December 02, 2011, 12:42:03 PM »

Don:

If you have time please draw the schematics of post number 16 of this thread please.

Chuck
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k4kyv
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Don
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« Reply #26 on: December 02, 2011, 06:07:34 PM »

See Fig. C

http://www.somis.org/bbat.f1.jpg

L1A and L1B can be a single coil, split at the mid-point, instead of two separate coils.

C1 can be a split stator, with each section twice the capacitance of C1 as shown in the diagram.

The choke-balun is not needed if the transmitter uses a balanced link-coupled output.

Mine is without the balun, a single split coil, and split-stator capacitor.
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Don, K4KYV                                       AMI#5
Licensed since 1959 and not happy to be back on AM...    Never got off AM in the first place.

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Steve - K4HX
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« Reply #27 on: December 02, 2011, 06:46:39 PM »

Or you can use one coil and a single section cap and skip the mechanical complication.   Wink
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aa5wg
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« Reply #28 on: December 02, 2011, 10:48:07 PM »

When you say split you mean cut the coil in half electrically?
Chuck
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k4kyv
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Don
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« Reply #29 on: December 03, 2011, 09:24:35 AM »

When you say split you mean cut the coil in half electrically?
Chuck

Instead of two separate coils, use only one coil, with the same total inductance as the two separate coils in series.

Electrically and physically split it into two identical sections without widening the turns spacing between the sections at the split. Actually, slightly widening, or even doubling the spacing at the split point won't hurt if you have more than a just a few turns, but it is best to maintain uniform spacing between all the turns if possible.

Theoretically, you will need only 70.7% of the total number of turns (1.414 times the number on one of the separate coils), although practically, the number count will likely vary, due to such factors as turns spacing, coil diameter, length vs diameter, etc.  See the coil formulae in the handbook, or use this handy calculator at http://hamwaves.com/antennas/inductance.html . Over the years I have developed enough of a feel for the dimensions of a coil for a particular application that I can usually hit the number of turns by trial-and-error in one or two attempts, more quickly even than using the calculator, when building from existing coil stock on hand with some means of temporarily attaching taps to the turns to the coil and moving those taps around.

Once you have determined the number of turns and the dimensions, and constructed or tapped the coil, count turns from each end and determine the exact mid-point of the coil. Think of each turn in terms of 360°, and roughly count degrees if the start and finish points of the coil are not exactly in line with the axis of the coil. (If you plan to use an existing high quality commercially built edge-wound coil, I would suggest throwing together a mock-up as a prototype, using heavy-gauge scrap copper wire or small diameter tubing to make sure your estimate is close to correct before you cut and damage a good coil only to find it won't work.)

When the exact mid-point is determined, cut out a section of the conductor to form a gap in the winding at that exact point. Make the width of the gap wide enough to withstand the peak rf voltage on the line at the transmitter output, with ample safety margin. Connect the wire leads from the transmitter (or balun if you use one) to the cut ends of the coil stock at the gap, and mechanically secure the cut ends.  With a homebrew coil, it is merely a matter of winding it in two identical, symmetrical sections. Make sure that the spiral of both coils continues in the same direction.   The split in the coil serves as the low-Z input, and the outer ends serve as the hi-Z output where the variable capacitor goes and where the output line is attached.
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Don, K4KYV                                       AMI#5
Licensed since 1959 and not happy to be back on AM...    Never got off AM in the first place.

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aa5wg
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« Reply #30 on: December 05, 2011, 08:20:09 AM »

Your description is very good in how to make the coil and apply the connections for the capacitor.  Have you experimented with a tapered line from your rig to the balanced L coil vs. the traditional balanced line?   I have a home brew balanced L on the shelf that I could experiment with.  Or, I believe the home brew link/tank coils would work here if needed.
Chuck 
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k4kyv
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Don
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« Reply #31 on: December 05, 2011, 11:15:51 AM »

From my notes on the prototype:

Transmitter output circuit is link coupled, with 4 turns on the links of both transmitters, although the HF-300 rig has a larger diameter link than the one on the 8005 rig. Data was collected mostly using the 8005 rig.

The leads between the output link and the split coil must be as short as possible.  With longer leads (approx. 4 ft) the physical placement of the leads had a drastic effect on tuning; apparently the series inductance of the leads was a substantial percentage of the total inductance in the high-C/ low-L tuned circuit; lead inductance is strongly affected by spacing between the leads. With short leads, the link inductance is in series with the gap in the coil, so the link + each side of the split coil (plus any inductance contributed by the leads) make up the total inductance in the tuned circuit of the tuner.

160M: 8⅔ turns on split coil (4⅓ turns each side of gap).  Variable capacitor is 900/900 pf split stator. Each stator section connected to one side of the coil with frame/rotor left floating, so maximum total variable capacitance is 450 pf. 840 pf fixed capacitance (large transmitting micas) shunted across variable cap/ far ends of the split coil/ 450Ω dummy load. A short section of 438Ω OWL runs between the tuner output and the DL.

80M: As above, 8 turns total (4 turns each side of gap).  No fixed shunt capacitance.

40M: Works with either 1 or 2 turns each side of gap (2 or 4 turns total), no fixed shunt capacitance.

This is essentially the same circuit as the "alternative for small links" shown in the 1957 ARRL Handbook, as mentioned earlier in the thread, except that a single split coil is used instead of two separate isolated coils for the series inductance.

I believe this circuit could be thought of either (1) as a balanced L network matching the link to the load while cancelling the inductive reactance of the link, or (2) a high C parallel tuned circuit, feeding the load as an ATU running parallel tuning, with the split coil in series with the link to form the total inductance of the parallel tuned circuit.
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Don, K4KYV                                       AMI#5
Licensed since 1959 and not happy to be back on AM...    Never got off AM in the first place.

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This message was typed using the DVORAK keyboard layout.
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k4kyv
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Don
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« Reply #32 on: December 05, 2011, 11:17:05 AM »

From my notes on the prototype:

Transmitter output circuit is link coupled, with 4 turns on the links of both transmitters, although the HF-300 rig has a larger diameter link than the one on the 8005 rig. Data was collected mostly using the 8005 rig.

The leads between the output link on the transmitter, and the split coil in the tuner, must be as short as possible.  With longer leads (approx. 4 ft) the physical placement of the leads had a drastic effect on tuning; apparently the series inductance of the leads was a substantial percentage of the total inductance in the high-C/ low-L tuned circuit; lead inductance is strongly affected by spacing between the leads. With short leads, the link inductance is in series with the gap in the coil, so the link + each side of the split coil (plus any inductance contributed by the leads) make up the total inductance in the tuned circuit of the tuner.

160M: 8⅔ turns on the split coil (4⅓ turns each side of the gap).  Variable capacitor is 900/900 pf split stator. Each stator section is connected to one side of the coil with frame/rotor left floating, so maximum total variable capacitance is 450 pf.  840 pf fixed capacitance (large transmitting micas) is shunted across the variable cap/ far ends of the split coil/ 450Ω dummy load. A short section of 438Ω OWL runs between the tuner output and the DL.

80M: As above, 8 turns total (4 turns each side of gap).  No fixed shunt capacitance.

40M: Works with either 1 or 2 turns each side of gap (2 or 4 turns total), no fixed shunt capacitance.

This is essentially the same circuit as the "alternative for small links" shown in the 1957 ARRL Handbook, as mentioned earlier in the thread, except that a single split coil is used instead of the two separate isolated coils shown in the Handbook (one wired to each side of the link) for the series inductance.

I believe this circuit could be thought of either (1) as a balanced L network matching the link to the load while cancelling the inductive reactance of the link, or (2) a high C parallel tuned circuit, feeding the load as an ATU running parallel tuning, with the split coil in series with the link to form the total inductance of the parallel tuned circuit.
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Don, K4KYV                                       AMI#5
Licensed since 1959 and not happy to be back on AM...    Never got off AM in the first place.

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This message was typed using the DVORAK keyboard layout.
http://www.mwbrooks.com/dvorak
aa5wg
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« Reply #33 on: December 06, 2011, 12:13:36 AM »

I found the "Alternative For Use With Small Links" information on the bottom of page 333, 1957 ARRL Handbook.  I am going to read this again.  I recognize this material but it has been a while.

Chuck
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