Johnson Matchbox - Extending The Tuning Range

[N1BCG]

It's well known that these venerable tuners can match most antennas, particularly when open wire feeder is used, but there are limits. I've been told "If you can't tune your antenna with one of these then fix your antenna!"

I was faced with this very situation using a 40M dipole with a carefully measured radiator and feed line to produce a low impedance at the feed point. This ill-conceived strategy created an unwieldy situation on 20 and 10 that the Matchbox was unable to wrangle.

The topic came up in discussion with Tim "The Radio Godfather" (WA1HLR) who suggested placing either an inductor or "condenture" (capacitor) across the feed point to tune out the excessive reactance.

This worked magnificently, and by adding a switch to select either a 20 meter or 10 meter inductor, these bands became easily tunable and many contacts have proven the concept.

BTW, the output of the Matchbox has no DC connection, or even high resistance, to ground. I've added RF chokes between each leg to ground to bleed off any static buildup and lower noise.

[aa5wg]

Sir,

It's great to hear that your Matchbox antenna system is working well.

What was the length of your antenna plus feed line that was
giving you trouble?

Did Tim, WA1HLR, recommend a rule-of-thumb for the antenna plus feed line
length?

I read in an old handbook that the preferred antenna system length
(feed line length plus one half of dipole length) should be measured
in multiple quarter wavelengths.  The last quarter wavelength (input to
feed line at antenna tuner) is minus 10% of quarter wavelength (at frequency in use)
to help or give the antenna tuner something to work with. 

It was implied that a "perfect" low and purely resistive impedance
(current loop) connected to the tuner could be difficult to tune
because there is effectively no reactance to tune out.   

73,
Chuck

[N1BCG]

The antenna tunes quite well on 40M and 15M even with a balun in place of the tuner. Since the tank settings weren't all that different from feeding a 75 Ohm dummy load, it didn't seem as though the reactance was significant and the Matchbox was able to handle it. This was certainly not the case on 20M and 10M.

Generally speaking, when using open wire feeder, the feed point impedance varies either from high to low or low to high as the feedline is increased in length by 0.25 wavelengths. The antenna is a common 40M dipole with a 0.5 wavelength feedline. This put a low impedance at the antenna feed point and at the feedline feed point at 40M.

However, at 20M, the antenna feed point impedance became high, and since the feedline is 1 wavelength, the feedline feed point also became high. Same phenomenon at 10M.

Some hams vary the length of their open wire feeders using switches to add any combination of 16', 8',4', and 2' of looped feedline to match their antenna. Pretty cool concept if you have the room in your shack for those loops!

Clark

[aa5wg]

Hi Clark,

I am getting a better picture of your antenna system.
Did you try shorting you feed line by 3 feet and 4 inches
to see if it would work on 20, 10 and the rest of the bands?

How do you know if your feed point, at the tuner, is capacitive
or inductive reactive?

Thank you.

Chuck

[N1BCG]

Changing the feedline length is certainly a solution, but I wanted to find an alternative for cases where it needed to remain as-is.

[W2VW]

Works every time just like Colt 45.

http://www.somis.org/bbat.html

When constructed properly this design delivers most of the power to loads the Johnson Flashbox simply cannot load. Low impedance loads are a problem for the Johnson. It was not designed for those.

Side by side comparisons can make one want to make the Kilowatt Matchbox into a wheel chock.

The Matchbox is a great tuner but..................

There are always plenty of ways to skin a cat.

[Steve - K4HX]

And it has a design has a flaw.

http://amfone.net/Amforum/index.php?topic=26452.msg199740#msg199740

[k7iou]

It doesn't have a design flaw, it was designed for tube rigs that had a different impedance matching than the newer solid state rigs prior to warc bands. It works flawlessly with my Johnson Five Hundred and Valiant tuning bands it was designed to tune.
Read about the design and a modification for newer solid state 50 ohm rigs and warc bands.
http://www.dj0ip.de/antenna-matchboxes/symmetrical-matchboxes/viking-vs-annecke/

My KW is museum quality and tunes my full wave 80M loop through all bands it was designed to tune. It's a balanced tuner.
I would be happy to do a burn-in test with your commercial tuner for you with OWL connected directly to the tuner with my Johnson Five Hundred. ⚡️⚡️⚡️😊

[Steve - K4HX]

Your Johnson 500 has an output impedance of something other than 50 Ohms? Maybe you are thinking of the 300 Ohm receiver tap on the MB.

The design flaw is the so-called capacitive voltage divider. I tends to work against itself and makes the tuning far more touchy and difficult. Short out the inner capacitors and this goes away. Using more components than needed and making operation more difficult is a design flaw in my book.

None of this is to say that it cannot tune many antennas. I've used it to do just that. It was just far easier with other tuners.

[k7iou]

Did you read the article in the link I sent? Very informative dispels all rumors, misconceptions and explains the shortcomings. I highlighted and underlined key points.
73

Many people still believe that the old E.F. Johnson Viking Matchbox is the best matchbox that was ever built; especially when we speak of link-coupled symmetrical tuners.  
 
Although this is definitely wrong, I think it is fair to say that the Viking was the father of the Annecke Symmetrical Coupler.
 
The Annecke "Symmetrische Koppler" was the logical extension to the Johnson Viking design, but modified to accomodate transistor amplifiers and the WARC bands.
 
In fact "accomodating the WARC bands" is probably an exageration of the design.  At the time Alfred Annecke designed his Symmetrical Coupler, the WARC bands were not on the horizen.  It just happens that his design lended itself better to supporting frequencies outside of the original 5 HF ham bands (excluding 160m) than the Viking did.
 
WHY THE ANNECKE CIRCUIT IS BETTER:

Tube finals encorporated a Pi-Filter in their output that enabled them to compensate for loads which were not exactly 50 Ohms.  Matching 3:1 or even 5:1 SWR was no problem for transmitters of the 1960's or earlier.  
 
Transistor finals which came many years later, required a load close to 50 Ohms.  The Viking was not always capable of presenting this load to the transmitters.  
 
Realizing this, Alfred Annecke made two fundamental changes to the Viking design which enabled matching a much broader impedance range to 50 Ohms.  This was accomplished by adding a 3rd variable capacitor to the design; a 270 pF capacitor in series with the input side of the matchbox, and by adding taps to the input side of the tank circuit.
 
In the following schematic, you can clearly see why the Annecke circuit is superior to the original Johnson circuit:

 Annecke Vs. Johnson Viking
Although the output side (on the right) of both matchboxes is identical, the input side has two subtle differences.
 
The Annecke circuit is on the far left; the Johnson Viking is in the middle.
 
Whereas the Johnson input side was simply a link coil, the Annecke had a tapped-coil and a series (variable) capacitor.  This enabled the Annecke under certain circumstances to present a much better load to the transmitter than the Viking.
 
The Annecke is the undisputed winner from a technology standpoint.  Both matchboxes had very robust components.  Beauty is in the eyes of the beholder, but for me, the Viking was the better looking matchbox.
 
If you have an old Johnson and feel like I just burst your bubble, don't dispair.  It is not difficult to add the Annecke changes to your Viking, and bring it up to the standards of the Annecke.  Since the changes were on the low impedance (50 Ohms) side of the link, the components are not subject to high voltage.  

http://www.dj0ip.de/antenna-matchboxes/symmetrical-matchboxes/viking-vs-annecke/

[N1BCG]

Another consideration is when the matching circuit is separated from the transmitter by a coax. I have that unfortunate situation, so it's important for the Matchbox to provide a 50Z load, and it *does*, but sometimes L or C needs to go across the output terminals to address excessive reactance.

And no drilling or blasting...

[Steve - K4HX]

So Walt, W2DU was dealing in rumors?   

I read that article in years past. It's a nice mod for the MB. It does not address the design flaw pointed out by W2DU.

Did you read the article in the link I sent? Very informative dispels all rumors, misconceptions and explains the shortcomings. I highlighted and underlined key points.
73

[k7iou]

Your Johnson 500 has an output impedance of something other than 50 Ohms? Maybe you are thinking of the 300 Ohm receiver tap on the MB.

no I wasn't thinking of the 300 ohm receiver tap. I will refer you to the Annecke article.  

The design flaw is the so-called capacitive voltage divider. I tends to work against itself and makes the tuning far more touchy and difficult. Short out the inner capacitors and this goes away. Using more components than needed and making operation more difficult is a design flaw in my book.

not in my case it tunes very easily

None of this is to say that it cannot tune many antennas. I've used it to do just that. It was just far easier with other tuners.

tunes much quicker and easier than my Palstar tuner

[k7iou]

Well, I will eat some crow today. Some say it tastes like chicken. 🐓
I was wrong, as a friend had this same issue described in this thread. He is using a dipole 130' with 30' of 450 ohm ladder line connected to his KW Matchbox. He had to add capacitance to tune his KW Matchbox on 40M.
See picture.
73
de k7iou

[NA3CW]

Just passing along a recent mod to my KW Matchbox.  Taking a tip from the Annecke coupler, I added a heavy-duty surplus switch on the rear panel of the box.  The usual link coil tap on the KW Matchbox is turn 2.  To this I added a tap at turn 1.  Both taps connect to the switch which feeds the coax connector.  My antenna is a 80/40 inverted vee fan fed with homebrew OWL.  Formerly I could not match this antenna across the 80 meter band.  With this turn 1 link tap I can now match the antenna easily from 3.5 to 4.0 MHz.  The rest of the bands, except 30 meters, are matched using turn 2. 

I realize I could have done things with feedline length but given my situation it would be very unhandy to do.  This simple added tap did what I needed done, all within the box.

For your consideration,

Chuck
NA3CW

[KD6VXI]

I've found a stub can be used too.

Run it open or shorted,  depending on band.

Yes,  it took.some cutting and trying to get it right,  but it did work FB.

Then I changed feedline routing and took another 12 feet out,  and can now tune everywhere almost.   20, I can only get a 1.5 to 1.  May add a stub.

--Shane
KD6VXI