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Examining the Johnson Matchbox ATU




 
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Steve - K4HX
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« on: January 17, 2011, 12:55:48 PM »

Another reflection from Walt:


Examining the Johnson Matchbox ATU

The Johnson Matchbox comprises an unbalanced input section, link coupled to the inductor, thus providing an excellent conversion from the unbalanced-to-balanced balun function; two balanced L networks that perform the impedance matching function; a main tuning capacitor in parallel with the inductor, forming an LC tank circuit that is tuned to resonance at the operating frequency; a second capacitor having four sections, all on the same shaft. The two outer sections of the second capacitor are the output capacitors of the balanced L networks that provide the balanced output terminals. The two inner sections are simply two capacitors in series, connected in parallel across the output terminals, and thus are also connected in parallel with the input of the balanced feed line.

The Matchbox is an outstanding antenna tuner (ATU), and has an excellent decades-old reputation as a low-loss tuner.  

However, because the designer(s) didn’t fully understand the function of the inner two sections of the four-section capacitor, they misled the users concerning the function of those two sections. The instruction manual for the Matchbox tells us that the four-section capacitor is a voltage divider, and thus performs as an impedance divider, which is why they included it in the design. Big mistake, because it in no way performs as an impedance divider. The inner two sections are unnecessary, perform no useful function, and are in fact superfluous, and can be disconnected with no harmful results.

We’ll now examine why the inner sections perform no useful function, but actually detract from the impedance-matching function. We’ll also see why they would perform a useful function if they were on a shaft separate from the two outer sections, allowing adjustment independently from the output capacitors of the L networks.

Because the two inner sections of the capacitor are connected in parallel across the input terminals of the feed line, the output capacitors of the L networks never see the true input impedance of the feed line—they see only the line-input impedance as modified by the capacitance appearing in parallel with line-input impedance. The function of this circuitry is disturbing because the modified impedance the L network capacitors see is constantly changing during the tune-up procedure, because the inner sections of the capacitor are turning simultaneously with the turning of the L network capacitors. Thus, for every change of the L network capacitors during adjustment to obtain the match, the impedance we’re matching to is also changing. It’s similar to a monkey chasing his tail. A match is ultimately achieved, but with no help from the two inner sections of the capacitor.

We’ll now explain why the two inner sections of the capacitor could assist in the matching procedure if they were adjustable independently from the outer sections, the output capacitors of the L networks. We know that occasionally the feed-line length that reaches from the antenna to the tuner presents an input impedance that is outside the range of the tuner. This situation usually leads the operator to change the length of the line so that it does present an impedance within the range of the tuner. However, there is an alternative to changing the physical length of the line to modify the input impedance—changing the electrical length of the line by adding either series or parallel inductors (or capacitors) at the line input terminals. Whether to use capacitors or inductors depends on whether the line is too short or too long. The two inner sections of the four-section capacitor are already in parallel with input of the feed line, so if these sections of the capacitor were adjustable independently of the L network capacitors, some additional impedance range of the ATU would be accomplished by adjusting the inner capacitors to change the electrical length of the line.

I hope the comments above help in understanding the function of the Johnson Matchbox ATU.

Walt, W2DU
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« Reply #1 on: February 01, 2011, 09:09:05 PM »

Would it be possible to post a schematic diagram? That would make the explanation a lot easier to follow and make it easier to visualise the balanced L network.
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« Reply #2 on: February 01, 2011, 09:57:56 PM »

Matchbox Schematic


* match-5.jpg (262.37 KB, 1650x1275 - viewed 3812 times.)
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« Reply #3 on: February 02, 2011, 10:39:15 AM »

Hello,

First, thanks Terry for putting up the schematic.   

I assume "inner sections" of the 4 section cap are the two sections that each have a side connected to the ground lead.

Thanks for your comments Walt.   It would be interesting to experiment with the Matchbox with the inner sections disconnected from the feedline input and see what happens.   So I gather that the inner sections are superfluous in their current configuration, but would be useful if they could be adjusted independently?   The only thing about that, that bothers me is more of a operator issue--I like tuners with a minimum no. of variables.  This is why I like L networks  and have never owned one of those T networks.   In this case though, the inner sections could be left out of the picture and only switched in if a match cannot otherwise be achieved? 

Maybe it is semantics but I do not see much difference between an impedance divider and an impedance modifier.   Maybe technically and electrically the terms refer to two different things but the result, changing the Z seems to be the same.

Another question:   You say the inner sections are in parallel with the feedline, but it seems to me that if they were pressed into service to handle a feedline that is not of a "matchable" length, they would have to be switched in in series as additional capacitance?  I see the outer sections of the four section cap as already being in series with the balanced feed and inductor sides and the two section capacitor as being parallel with the feed.

73
Rob
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« Reply #4 on: February 02, 2011, 12:58:07 PM »

Hi Rob,

I've performed the experiment, Rob. Disconnecting the inner caps changes the impedance at the terminals of the feed line, but only slightly. The match is again achieved with a small change of the caps (now with only the outer caps performing, the caps portion of the balanced L).

If the inner caps were adjustable independently of the outer caps, the inner caps alone would change the input impedance of the feed line, but only with those caps in parallel with the feed line. If the outer caps were in series with the feed line, the resulting impedances would simply be different than with them in parallel.

And yes Rob, the outer caps are in series with the feed line, but as the caps in the L network formed along with the inductors.

Let's assume for a moment that the input impedance of the feed line is outside the range of whatever tuna is being used. One way to bring the impedance within range is to place equal caps either in parallel or in series with the line, which ever is found to achieve a match experimentally. Or, on the other hand, inductors either in series or parallel with the feed line--one of these four choices can convert the line input impedance to one that is within range of the tuna's capability. Thus, using this procedure, a match can be obtained without requiring a change in length of the feed line.

As the inner caps in the Matchbox appear in the circuit, while adjusting the four caps to obtain a match, the inner caps serve only to change the feed line input impedance in a direction that is not necessarily the correct direction--it depends!!!

 Hope this helps,

Walt
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« Reply #5 on: February 02, 2011, 02:41:44 PM »

IIRC, capacitor C2 is built so that the inner capacitors are fully meshed when the outer ones are fully unmeshed and vice versa. Each half of C2 serves as a differential capacitor that the manufacturer intended to be a variable capacitive voltage divider, serving the same function as tapping the load down on the coil, while the primary resonance is determined by C1.

I tried tapping down on the coil of a balanced link-coupled tuner in order to feed my 80m dipole as a short dipole for 160.  The flat top is about 135', and the feed line is close to the same length.  The line-input impedance on 80m is low and mostly resistive, so the 80m tuner uses series feed formed by splitting the coil at the mid point and attaching the feeders across the gap.  But on 160, the feed point is highly reactive, since each leg (feed line + 1 leg of the dipole) is 3/8λ, placing the line input point exactly midway between a voltage loop and a current loop.  I threw together a tuner using a BC-610 plate  capacitor, 150/150pf @ 7 kv, and a link coupled coil.  I tapped the feed line  down on the coil until I  could get close to a 1:1 match between the link and the coax line back to the transmitter. When I tried modulating the rig, I couldn't run over about 100 watts before the tuning cap would arc over.  I tried  several LC combinations with other capacitors and adjusting  the number of turns of the coil, always with the same result.  I finally just added an extra 60 ft. section of OWL that I automatically switch in with that tuner, and it has worked perfectly for nearly 30 years, arcing over only when bugs or trash get between the plates of the tuning cap.  I suppose I could have added either a series capacitor or inductor to each side of the OWL making it appear to the tuner as purely voltage or current feed, but I decided the extra section of OWL would be simpler and there would be no series variable capacitors or inductors to adjust when moving from one end of the band to the other.

Regarding the Johnson Matchbox, as I see it, wouldn't the coil and C1 form the balanced L network, and the combination of C2 inner and outer caps merely alter the feed point impedance of the OWL?  Actually, instead of simply removing the inner caps, it would appear to me that re-wiring the inner caps so they are disconnected from ground and connected in parallel with the outer ones, and the rotor plates re-positioned so that both sections would mesh unmesh together, would give the tuner a greater range, since you would now have an increased range of variable capacitance in series with the feed line.  When C2 is rotated so that the outer section has maximum capacitance while the inner section has minimum, the inner caps are in effect nearly disconnected from the circuit. Placing those sections in parallel with the outer sections would allow a higher series capacitance, something you would want if the feed point of the OWL were at a voltage loop with the tuner looking into a high-Z with minimal  reactance.

Also, it looks to me like using the stock tuner to feed a low impedance, such as the case of feeding the OWL at a current loop would be inefficient, since you would have a large circulating current between the coil and C1, while simultaneously trying to force-feed a high rf current to the OWL through a small capacitance at the outer section of C2 (since, to form the capacitive voltage divider that gives lower voltage/higher current to the OWL, the differential cap  would have to be adjusted towards minimum capacitance at the outer section/maximum capacitance at the inner section).  Add to that, any reactance,  capacitive or inductive, appearing at the feed point would drastically alter the match, probably unpredictably.

Frankly, I don't see how the thing works at all when attempting to feed anywhere near a current loop.
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« Reply #6 on: February 02, 2011, 08:33:07 PM »

<<<
Frankly, I don't see how the thing works at all when attempting to feed anywhere near a current loop.>>>

Right.  I think I slightly understand you Don; (most of it over my head) but I know the Matchbox wasn't intended to match low Z loads where there's a lot of current like very small << 1/2 w. dipoles.   this is something Matchbox bashing hammies whine about--the crowd who want an ATU to handle every possible Z on every frequency both balanced and unbalanced. 

Let's see...I have to think about what you posted Walt, thanks.   Also, I did not know the inner sections were 180 degrees opposite the outer sections on C2.  Yes, putting inner and outer in parallel is an interesting idea.   I'm trying to imagine the reasoning for the stock MB scheme.   For the first time I began to think that the cap arrows opposing each other in direction on the schematic might mean something (duh).   but if they do I'd think the outers would point in the same direction and ditto for the inners but opposite.  And C1's would also point in the same direction.

If you are right about C2 Don, what if we think of the inner sections of C2 as being parallel with C1 (maybe you are already doing that, what do I know) so if we run out of C1, can C2's inner sections augment it.  by increasing the inner sections of C2 the series capacitance of C2's outer sections would proportionately decrease but would the overall effect of C1 increase?  What does that mean/do?   to illustrate by going to the logical conclusion, imagine just one side for simplicity and C1 is maximized.  between its hot side and the feedline you have a C2 section in series.  If it is minimum it is almost out of the circuit and the other C2 section (the inner one) is maximized and parallel with C1 to ground.  C1's capacitance is augmented?   As we decrease C1 by lowering the value of the C2 inner section the series side of C2 goes up.  I'm not an engineer so I don't know why this would be desirable (but mine works so I'm not complaining hi hi).   I don't know if I contributed anything or not, but this sure beats cleaning out the basement  Grin

Rob
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« Reply #7 on: February 02, 2011, 08:53:50 PM »



   Don,

   I'm here to tell you, it don't match low Z, high I loads!  

   In that case you must add a section of OWL to bring the feed Z to a
   reasonable value or (like I do) use a piece of coil stock as a balanced
   auto xformer... This effectively increases the matching range of the unit.

   Another weak point on the 275 Watt box is the fiber coupler between
   the sections of the differential caps.  If one hits the box with over a
   couple watts of power in any unmatched settings of the tune/match
   controls it WILL flash over, causing a carbon path making the box useless
   till repairs are made...  Ask me how I know!  

   The big boy has an isolantite coupler in there so it isn't prone to this problem.



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« Reply #8 on: February 04, 2011, 03:21:06 PM »

Looking at the Johnson schematic, I see the link is grounded at one end, and what looks like two taps and an unused portion of the coil, but it doesn't show a switch to select the taps or to connect anything to the top end of the coil.  Also, there is no variable cap in series with the link.

How do you change the taps on the link coil?  With no resonating capacitor, it would seem to me that you would have to change the number of turns on the link every time you changed bands if it is fed directly with coax, and the coax is to work into a matched load.  Also, it shows  the receiver connected to a different tap on the link from the transmitter.  Is that always the case?

I have found that to feed coax directly into an untuned link and achieve a 1:1 SWR, the coupling has to be extremely tight, and the number of turns is critical.

On my link coupled tuners, I use a series resonating capacitor, and it takes just the right combination of main tuning cap and link resonating cap adjustments to achieve a perfect match to a 50Ω coax line.  If the adjustment is made at the middle of a band, I can usually cover the entire band by simply rotating the main tuning cap without further adjustment of the link resonating cap.  The SWR may creep up to something like 1.3:1 at the extreme band edges, but it's not worth the extra step in the tuning procedure to keep adjusting it back down to a flat 1:1 all across the band, so I just leave it as is.
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« Reply #9 on: February 04, 2011, 06:50:00 PM »

The tap on the link that gives more turns is the one for a receiver.  The TR relay switches between the taps.  The idea was that originally an old rx would have 300 ohm antenna input terminal strip on the back instead of a UHF jack for 50 ohms.  I think you'd connect the rx twin lead to the back of the Matchbox at a terminal barrier strip at the bottom of the rear of the cabinet.  This is the stock wiring.   A lot of guys have removed the TR relay and rx tap and all that extra stuff and just run the tx line from the UHF jack on the back of the MB right to the link tap and use that for tx and rx.

I do that and run mine with that one tx tap on the link, no cap in series to ground or any other link taps mine seems to work okay, but I had to add about 10 feet of ladder line to get it to match the low band dipole.  to get the high band dipole to work with it I had to make it a full 1/2 wave on 20 m.    Maybe that means its range is limited but it is hard for me to tell because my situation is complicated by antennas interacting with each other.  If I can't get a match I find that futzing with another antenna like grounding the verticals makes the difference. 

I don't really know how much adjusting I have to do at N kc away from where it is tuned because when I was QRV I pretty much parked on one frequency for the evening and did not move so it was not something I bothered to notice.

rob
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« Reply #10 on: February 04, 2011, 07:23:59 PM »

Maybe that means its range is limited but it is hard for me to tell because my situation is complicated by antennas interacting with each other.  If I can't get a match I find that futzing with another antenna like grounding the verticals makes the difference. 

That's why I prefer to run one dipole fed with OWL and use it on multiple bands, instead of having a giant spider web of separate antennas for each band. Less interaction, less maintenance and looks less cluttered, too.

What about the unused turns shown on the link in the schematic?

And the 50Ω match holds when you change bands without making any adjustments to the link?
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« Reply #11 on: February 04, 2011, 07:43:40 PM »

I believe the brief manual for the Johnson Matchbox states the outer link taps are as follows. TX is at 50 ohms and the RX tap is at 300 ohms, from factory.

Still using the relay in my KW Matchbox and is the tunna for the Desk KW. But I moved the tap for the RX!! I soldered it next to the TX tap, reason being my receivers are aligned for a 50 ohm antenna inpoot.

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« Reply #12 on: February 05, 2011, 02:22:24 PM »


What about the unused turns shown on the link in the schematic?

I don't use them.


And the 50Ω match holds when you change bands without making any adjustments to the link?

Well it depends on what you mean by "holds."  I make no adjustments to the link; the link is static always here; I don't change the link tap or have a variable cap in series with it or anything like that but when I change bands I have to change C1 and C2. 

I can use the 80 meter dipole on 40 and 20 with my MB; I don't think I have tried it higher up in frequency, because I have the 20 meter half wave dipole for that and with it I can operate up through 10 meters.   Another gripe about the MB is it "doesn't cover WARC bands" but that's another hammy complaint.  Mine finds a 50 ohm match on 17 meters (I don't think I have ever actually operated there with it though; I was just experimenting).   Some of this is moot because with the exception of 10 m. there isn't enough AM above 40 m. to matter.  For now about the only use I get out of the high band dipole is a net I check into occasionally on 20 m.

It might reveal something if I describe my installation.   I have the MB in front of a basement window where the feedline comes in through the middle of a window pane.  The feed plugs into the MB rear balanced lugs (I have banana jacks attached to them).  I have a ground rod outside and about 4 feet of 3 inch wide copper strap from the rod, in around the window, to the MB ground lug.  A RG213 jumper goes from the MB UHF jack to a Bird coax switch with a watt meter in the middle of the jumper (so there are actually two jumpers).  To tune the MB I put a swr analyzer on it at one position with the coax switch and adjust it to 50 ohms then switch it over to a line to the rig.  I have a hammy hambone Daiwa cross needle watt meter in the line to the matchbox that shows forward and reflected power.  the accuracy of it isn't important, only that I see forward go up while reflected says at zero while I fine tune the match.  I do this during "tune up time" at the start of the Saturday morning Classic Radio Net on 3885 after I've tuned the rig up into an old Bird dummy load I have.  Over time I have noticed that the rig's output increases by about 30 watts if the Z into the MB as measured by the analyzer is around 58 ohms J0.  I sweep the Matchbox up and down 5 kc from zero beat frequency and see no reactance but resistance that's a few ohms up and down from 58.  So I have learned to tune the MB to get this.  It is probably anal but since I can do it and don't QSY often, then I do it.    Anyway, that was probably a lot of hot air and y'all can build these wonderful tuners, but since I don't have room for a balanced antenna on 160 and the Matchbox seems to handle all the power I'll probably ever come up with, it seems to be all I need.

The thing with C2 is still a mystery though, but I believe there is a real clue in the inner and outer sections being 180 out of alignment if that is in fact the case.   If I had time I'd open mine up and study C2 to see how it works.   Next time I remove the thousand screws for some reason I'll do that.

rob
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« Reply #13 on: February 05, 2011, 05:03:59 PM »

That's why I prefer to run one dipole fed with OWL and use it on multiple bands, instead of having a giant spider web of separate antennas for each band. Less interaction, less maintenance and looks less cluttered, too.

If you want gain and directivity, especially on the lower bands, then we have no choice but use separate antennas for each band optimized for particular jobs.  The exception would be a log periodic, which is mostly limited to the higher bands and itself is a compromise.

Interaction can be minimized by planning antenna placement with back to back reflectors or at right angles with a certain spacing minimum.   Yes, higher maintenance when using separate antennas is the downside, no doubt.

That said, for all-around hi hi FB OM general use, then an OWL-fed dipole is hard to beat for all band operation. Personally, I would choose separate coax-fed dipoles that are each placed at the optimum height for a particular job on each band. (high angle  or low angle to suit our operational tastes)  

T
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« Reply #14 on: October 16, 2014, 12:15:13 AM »

for all-around hi hi FB OM general use, then an OWL-fed dipole is hard to beat for all band operation. Personally, I would choose separate coax-fed dipoles that are each placed at the optimum height for a particular job on each band. (high angle  or low angle to suit our operational tastes)  
Most of us do not have the real estate, heighth, or enough coax for a dipole for each band.

The center fed 80 mtr dipole fed with open window line worked the best for me for years.  Those antennas used to be sold at every hamfest but I have not seen them at hamfests for years.  There is a shortened version of that still sold but I prefer the full half wave dipole myself.
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« Reply #15 on: October 16, 2014, 12:44:07 PM »

The center fed 80 mtr dipole fed with open window line worked the best for me for years.  Those antennas used to be sold at every hamfest but I have not seen them at hamfests for years.  There is a shortened version of that still sold but I prefer the full half wave dipole myself.


Welcome to the forum with your first post, Jimmy!


Some ideas:

Many AMers build their own dipole and open wire line.  A 500' spool of black #12 or #10 stranded copper insulated wire from Home Depot, a strip of 3/8" Lexan rod cut up for spacers - and you're golden.  It is stronger, more efficient and will last a lifetime.  Homebrew open wire will not have rain sheeting swr problems like the common plastic covered OWL most commercial ants use.  HB OWL is more environmentally stable.

Why do I recommend heavy #10 or #12 wire?  Two reasons:  Less stretching in the wind and ice loading conditions. I have #10 up at 190' for 8 years now used as a reflector and it still is resonant where I tuned it.  Also, when tuned up on the higher bands, there will be multiple low impedance/ high current nodes along the feeder and dipole flat top that will rob power.  And skin effect.  When running a KW, thin wire will heat up more.

T
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« Reply #16 on: October 21, 2014, 07:00:42 PM »

An open wire line antenna has gain as you go up in frequency.  For instance a half wave on 160M is two half waves in phase on 75 and if I remember right has about 2.1 db of gain on 80M.  The thing you can't do is rotate it to put the gain where you want it. 

I have been using an open wire line antenna which I built with 12 ga wire including the feeders for over 30 years now with enough good success that I will continue to use it.  I like the versatility that the antenna provides along with some modest gain.

Not saying anything is wrong with resonant dipoles for each band coax fed, but when you want just one wire antenna up its a good solution.   

73,
Joe, W3GMS     
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« Reply #17 on: October 21, 2014, 07:47:25 PM »

Only hams worry about resonant antennas.
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« Reply #18 on: October 22, 2014, 08:25:00 AM »

Only hams worry about resonant antennas.

Me worry, never Wink

Hope all is well Dave!

Joe, GMS
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« Reply #19 on: January 07, 2015, 09:37:37 PM »

FYI, an interesting read relating to the Johnson design.
http://www.dj0ip.de/antenna-matchboxes/symmetrical-matchboxes/viking-vs-annecke/
73
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« Reply #20 on: January 17, 2015, 02:35:01 AM »

Nice read.  Looks like Annecke's modification is taking care of Don's K4KYV previous concerns.  Nice page all about matchboxes and "antenna tuners".  Thanks for posting!
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« Reply #21 on: June 23, 2015, 04:50:06 PM »

Lately I have been reading again Reflections III and also Jerry Sevick's book on transmission line transformers. I need to study things more. Each time I do, I see more clearly the reasons why things work well or badly.
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« Reply #22 on: November 09, 2017, 02:59:04 PM »

After building most of my own ATUs I finally wound up with a Johnson Match Box (Junkston Flash box). Of any commercially made and sold antenna matching devices, in my opinion, it is the best one out there. HOWEVER,there are a number of loads that it won't couple into. Particularly reactive loads. Capacitive reactive loads seem to be the most problematical. The tuner will tolerate a certain amount of inductive reactance. The resitive component may be within matching range of the tuner but the reactive component says "No, I Don't Think So."

I will cite two scenarios. Enough space to put up a 40 meter dipole but want to use it on 75. A 40 meter dipole fed with open wire line of a particular length say 33 feet or so. Being that this is a quarter wavelength on 40 the system feed impedance is very high from 40 meters on up. Taking into account the total amount of wire used in the antenna system,(radiating portion and feed line wire) a condition may be reached that the total is a 80 meter/75 meter half wavelength resonant antenna. The bandwidth at resonance is sharp The impedance is in the order of 10-15 ohms. The stock Johnson Match Box will not match into that impedance. If the tuner is opened up and the 15 meter taps are brought out to the rear panel to feed through insulators to connect to the 40 meter dipole and feed system, it is possible to get a match.

There are two other ends of the equation. A friend of mine had enough space for a 40 meter dipole on a city lot. He did have two monster trees at the right distance. The antenna was constructed with #10 copper clad wire and #10 hard drawn copper @ 4 inches for the feed line. Problem being the length of the feed line was about 45 feet or so. The impedance was much higher than 10 ohms but very inductively reactive. I frigged and fragged around with a number of tap combinations on the main inductor to effect a match. No Dice! The magic Bullet came in the form of a Centralab type 857 100pF transmitting condensher. I placed it across the outpoot terminals of the Match Box. A wondrous thing happened. I was able to get a flat match over most of the 75 meter band. Due to the height of the antenna above ground 80 feet or so, the signal reports were comparable to many station with full sized 75 meter dipoles. The MFJ SWR analyzer is a very important tool in setting thing up.

On the other side of the equation I have a 40 meter dipole up here with somewhat shorter feed line. No matter what I did playing around with taps I could not get the antenna to accept power. Using the SWR analyzer, I found system resonance to be @ 4.2Mhz. At 75 meter frequencies this antenna system is quite capacitive reactive. I took a hefty tank inductor, #4 solid copper wire, I connected one end to the end of the feed line to one leg. The other leg to the case ground of the MFJ SWR analyzer. I shoved a 8-32 bolt into the SO 239 connector on the analyzer, took a short Radio Shack clip led, set the analyzed tuning to 3870 and dragged the clip lead down the inductor until I hit resonance. I marked the tap point, and connected the inductor across the antenna feeders. I then clip leaded the outpoot of the match box to the juncture of the inductor and feed line and put RF to it. I was able to get a flat match with the stock configuration of the Match Box. It worked! Flat match over most of the 75 meter band.

This story is to demonstrate  that with some external reactance canceling, the Johnson Match Box will deal with many different antenna system scenarios. It would be easy to build a match Box range extend-O-peener. Simply take a roller inductor and a dual section air variable condensher. Mount them on a suitable base. A two pole 3 position ceramic rotary switch is used to select either the inductor or the capacitor to get rid of those reactance woes.

Next time, A discussion on 160 meter modifications.

Tim, WA1HnyLR
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