Different twist on the link antenna coupler circuit

[aa5wg]

Saw these link antenna circuits on a European ham radio forum.  

Here are three attachments showing the circuits.  Has anyone used these?

Chuck

[w3jn]

Your link is to your local C drive, none of us will be able to see it either 

[aa5wg]

W3JN:
You are right.

Thank you.
Chuck

[k4kyv]

Saw these link antenna circuits on a European ham radio forum.  

Here are three attachments showing the circuits.  Has anyone used these?

I have used the circuits in Fig.3 for series and parallel tuning for many years. That way, you can use a regular split stator tuning capacitor for C3 instead of two separate capacitors for series tuning.  Back when I used the same tuner with plug-in coils for multiple bands, I arranged the jack/plug bars so that changing coils automatically switched between series and parallel tuning according to which coil was plugged in.

I never tried using C2 at the mid point of the coil.  Not sure exactly what that is supposed to accomplish, given the variable capacitors C1 and C3. Usually, if the coupling is made physically variable by swinging the link in and out of the coil, C1 can be eliminated. I never tried feeding very far away from a current loop or node, so series or parallel tuning was always easy.

[aa5wg]

Hi Don:
I will have to try circuit 1 and 2.  I also use circuit 3.  I once saw C2 at the midpoint of the coil in an G5RV article.  I am going to try and find that article.  I have it some where on the ham radio book shelf here.
Chuck

[k4kyv]

Do you know what C2 is supposed to do?  C3 would be for resonating the main coil.  C1  would be for adjusting the coupling.  So, what is C2 for?

[aa5wg]

Don:
I don't know what C2 is for.  I am dog sitting for a friend and I do not have access to my shelf of ham materials.  It seems I read, in that GR5V article, that it replaces link taps.  When I get back home in a few days I will look for that paper.

Can you test this circuit with your current antenna systems?

Chuck

[W2DU]

Don, I used the circuit with C1. As I understood it, C1 cancels the inductive reactance in the loop.

Walt

[aa5wg]

Hi Walt, Stu and all:
Dose anyone know what C2 in figure 1 is suppose to do?
Chuck

[W0BTU]

This is my balanced tuner, used to feed an 80 meter inverted-V on all HF bands from 80-10 meters. The ladder line clips directly to the coil either side of center. It has the 20-15-10 meter coil plugged in.

It may be clunky, but it's efficient. Even when operating a contest at the full legal limit, the coil temperature rise is barely perceptible. And I have yet to find a frequency or feedline length that I can't match. Try accomplishing all that with a balun. :-)

The big, long bread slicer nearly hidden behind the plug-in coil is a 100-pf-per-section differential capacitor with a grounded rotor which is in parallel with the coil. The other two capacitors are in series with the input link, switched with the SPDT ceramic knife switch along with a mica padder.

Since I'm not usually a band hopper, having to switch coils to change bands has never been an inconvenience.

[aa5wg]

Mike:
Your link coupler looks very nice. 
Chuck

[AB2EZ]

This post has been replaced by the post below (by AB2EZ)

[k4kyv]

Don, I used the circuit with C1. As I understood it, C1 cancels the inductive reactance in the loop.

That's pretty much my take on it.  I use C1 in my present 80m and 40m tuners.  C3 tunes the main coil, with feeders connected, to resonance (series tuning for 80m and parallel tuning for 40m). C1 is adjusted for the best match to the transmission line back to the transmitter.  I can easily get 1:1 SWR to a 50-ohm line by juggling the settings of the two capacitors.  I set C1 to get a perfect match at the middle of the band, and then leave it be.  I am able to tune from one end of the band to the other using C2.  The SWR might creep up to 1.1:1 or 1.2:1 at the extreme ends of the band, but that's not enough to worry about.

On my 160m tuner, I use parallel tuning.  But instead of C1, I adjusted the number of turns on the  link and fine tuned by adjusting the number of turns on the main coil, until I got close to a perfect match.

C1 may be used as a variable loading control.

My original tuner with plug-in coils fed my dipole when it was strung between two trees. The links were  fixed on all the  coils, and were all 4 turns. No C1 was included. The transmitter was (is) link-coupled using a swinging link at the final tank coil.  I tied the two links directly together with a few feet of heavy duty zip cord, with the two conductors pulled apart. No SWR reading to worry about. I adjusted the loading using the variable link in the transmitter, and used C3 to tune the ATU for resonance. 

I tuned up by first dipping the final, with the link nearly fully out of the coil.  Then I increased the coupling a little, and adjusted C3 for a peak in DC plate current.  Then I increased coupling until I got about  half normal plate current and re-adjusted the plate tank capacitor to maintain the dip, then adjusted C3 for maximum plate  current. After that I increased the coupling using the swinging link until I got close to normal plate current. I always re-dipped the final as the  last step to make sure it was still at resonance, and fine-tuned the  loading  control for the precisely correct plate current.  The three adjustments - plate tuning, link coupling and ATU resonating capacitor C3, were always somewhat interactive, so I had to juggle all three.  But once one gets the hang of it, the final can be coupled and tuned to the antenna in a few  seconds.

[aa5wg]

I found the article titled "ATU or ASSTU?" by G5RV, Radio Communications, August 1983, pagees 702, 704 and 705?  The copy I have did not have the page number for the last page.

In the article the feeders are connected to each side of C2 not C3.  It says "Circuit diagram of unbalanced-to-balanced atu for all hf bands 3.5 to 28 MHz usig variable feeder coupling capacitor    C2 in place of coil taps."  C2 is describes as a 3 x 500pf receiver type variable (sections connected in parallel) shaft insulated from earth.  

I wonder if this really works?

Chuck

[k4kyv]

That looks like a balanced pi-network.

The BC-339, a military transmitter using a pair of 833As in push-pull in the final (no modulator included), used a balanced pi-net output for the tank circuit. The main plate capacitor was a 200/200 pf split stator capacitor, and the loading cap was 900/900 pf.  It was designed to feed a balanced open wire transmission line directly without a separate ATU. Its frequency range was something like 4 to 20 mc/s, CW or RTTY.

[AB2EZ]

Post deleted... I am still not happy with my model/analysis of this circuit.

Stu
AB2EZ

[AB2EZ]

I'm still thinking about how one can best analyze this network.

Below are:

Attachment 1: An equivalent circuit, where the coupled coils are replaced by the combination of: an ideal transformer with one primary and two secondaries, an inductor L_m across the primary winding to represent the magnetizing inductance of the coupled coils, and a pair of inductors, L/2, to represent the leakage inductance of the coupled coils.

Attachment 2: Since C2 and the pair of secondary transformer windings are in series, the current that flows through any of them is the same (Kirchhoff's current law)... unless one adds some additional parasitic capacitances to the circuit. Therefore, moving C2, as shown, will have no effect on the behavior of the circuit. Likewise, the two inductors of value L/2 can be combined.

Attachment 3: The two secondaries of the ideal transformer, which are in series, can be combined

Stu

[AB2EZ]

Continuing my post directly above:

One can now move C1 and L_m to the right side of the ideal transformer, taking into account that the turns ratio is not (except as a special case) 1:1. C1 becomes C1a, and L_m becomes L_ma. Note: the ideal transformer is not shown.

In this configuration, C3 looks a lot like a loading control, and C2 looks a lot like a tuning control. However, the presence of L_ma complicates the picture (at least for me)... and perhaps that is why C1 and C2 are used, together, to simplifty the tuning process (as Don pointed out in an earlier post).

Stu