The AM Forum

Hello AMfone group,

  I"m working on building an LCT, Link Coupled Tuner.  Have made balanced couplers in the past but am not experienced with LCT construction and the different configurations.  Have read up on the "JJ" LCT as well as the Annecke and Johnson Matchbox.  I like the idea of using a capacitive voltage divider like the Annecke and Johnson Matchbox but also like the idea of being able to reconfigure for series or parallel with the JJ design that is very prevalent with members of this thread.  Have attached a picture of a proposed "compromise" that may use the best of both but not certain how it will work in the series configuration.  C2 is nothing more than the 4 section 100pf/section differential capacitor seen in the Johnson Matchbox and tied to ground in the center the same way.  C1 can be jumpered to be in parallel to C2 as in the Annecke and Matchbox but also can be jumpered into the series configuration in the middle of the larger/secondary coil while leaving C2 between the balanced feedline.  That is where I'm uncertain how it may work. Why make it more complicated?? I'd like to be able to maximize the tuning range on 160 where it is electrically short and still get full 80/75 coverage where the antenna is electrically long.  Maybe someone has already discused this or done it but I cannot find it having been done anywhere on the internet.  I'm not sure how the circuit will work with a series tune capacitor and a parallel voltage divider when the antenna is electrically short so am asking the experienced LCT folks on AMfone for some guidance.  Crude drawing done in paint and saved as a .JPG but it should get the point across for a schematic representation with the dots being tap points for jumpers (besides the jumpers on the coil as well).  Also, I've read W8JI comments stating the two center sections of C2 are not needed (although I don't agree necessarily) and also read on this thread that W2DU weighed in on the subject of a differential capacitor but cannot find any of his writings on the subject. Comments regarding C2 regarding W8JI's perspective or W2DU's persepective are also welcome but my main question is regarding the series/parallel jumpering of C1 while keeping C2 in the traditional location.
73, 
Mike KE6CVH

Here is the link to the W2DU explanation why the differential capacitor does not add any value to the tuner:

http://amfone.net/Amforum/index.php?topic=26452.0

Designing your tuner to configure either series or parallel circuits will provide the most versatile operation and match a much greater range of impedance and reactance.  Use a heavy inductor, large copper wire or copper tubing, and allow for many tap combinations.  Clip on tap method is more versatile and efficient than a rotary swich, and enables the series or parallel connection with no difficulty whatsoever.

Rick,

  Thanks for that link on the W2DU comments!  It will give me something to read and re-read many times until I can properly push the "I believe button" on the subject.  I'll give you some more insight.  I love to make antennas and have dreamed about making tuners.  I love 1/4" tubing most in any coupler and QRO components even when used with low power.  In this pursuit I bought a 3D printer, a Taig DSL-3000 desktop milling machine, a 6 cubic inch Morgan Press so I can eventually make a mold for open wire spreaders/end insulators/feed insulators...and any other needed part to be molded from plastic as well as a CNC converted lathe, some software, and some test equipment.  I'm getting ready to invest in some molds for stamping aluminum rotor/stators for butterfly caps and have already researched the subject.  Got some really neat ideas for tapping also.
  So, I was/am working on a different coupler but decided to step it back and go back to the basics of a balanced L and an LCT for now so that I may hone skills on CNC equipment and CAD software that are completely lacking.  I'd like to half the price of what MFJ is charging and make it better at the same time. 
  This is a big dream but if anyone is prepared to do it I may just be that person as I've got the drive also.  I used to own some fiberglass boat molds and am prepared to make cases out of fiberglass.  All that said I've seen 50' rolls of 1/4" copper tubing going for $12.50 and if I make my own plastic parts, case, capacitors, and jumpers the cost is really quite low and the rest is just a labor issue for me personally. 
  If you want to email me I can be reached at michael.kendall@ymail.com and can discuss it a bit more.  I think the time is right to make a professional looking but down and dirty rugged QRO LCT and balanced L.  Plug in coils have also been considered but I like the current tap configuration that I'm looking at.

73,
Mike

Mike, it sounds like you are well prepared to do home brewing with professional results.  I have always enjoyed home brewing, and making do with what is available whenever possible.  I built my 4-400A linear in 1968 in my barracks room at Davis Monthan AFB.  I remember I did not have a plate tuning capacitor with the right capacity and spacing, and I converted a split stator cap to a higher voltage single cap using washers a spacers between most of the plates.

My brother in law built a 3d printer from a kit and used the arduino platform and open source software to get it operational.  Most of the parts that hold the aluminum frame together can be duplicated with that same 3d printer.  While I visited with him over the holidays I helped him refurb a large industrial milling machine, and he put stepper motors on it with timing belt drive, and he now has that fully computer controlled. 

And to go back further in history, I can recall as a kid in the late 50's helping my dad construct antennas for the first five TIROS weather satellites in our basement in NJ, using a lathe and drill press that my grandfather owned.

Good luck on the tuner design.  Touch base with Tom, K1JJ, and some of the others here that have made strapping efficient and versatile tuners.

Hi Rick,

 Ran across this article about a home brew knife switch:
http://w5jgv.com/knife-switch/knife_switch.htm
  I've got a nice mod to that concept that will allow me to use it for tapping and allow high current.  I think it is a winner for sure and it will be a game changer.  The general consensus is that the hardest part will be the band switch, if used, and the idea I'm working on will allow a case and electrical safety to be part of the solution.
  So, if I eliminate the two center sections of C2 do you think I could have C1 as series or parallel while leaving the two section, vice 4 section, C2 and get an improved low impedence matching range.  It would still be a modification of the JJ design if it works.

73,
Mike

OK, thanks again Rick for that link and now I think I'm understanding what needs to be done for the schematic to achieve maximum tuning range (after reading it again very closely).  If I remove the two inner capacitors sections of C2 that is no the solution it appears after reading W2DU's comments.  If the circuit was viewed with only having C1 (tapped in series configuration) and the outer sections only of C2 it appears to be a "T" network from reading Walt's comments. That is because the C1/inductor make up two balanced L circuits and the addition of the outer two sections of C2 would make it two T networks.  If I only kept the inner sections and kept them as tuned individually and eliminated the two outer sections than it would be two parallel L networks (when in series) and using the two inner sections of C2 to electrically do the equivalent of varying the transmission line length.  Am I understanding this correctly?  Has anyone tried a series/parallel tap of C1 with only using the two inner sections of C2 while eliminating the two outer sections? 

73,
Mike

If the switch is the problem, consider plug-in coils. Make your own jack bar. Band changes will be a snap. If you can find a B&W butterfly cap, you can make a very compact tuner with short leads and excellent symmetry.

Hi Steve,  Yes I've considered making plug in coils and am considering all possibilities.  What I'd ultimately like to do is make the best possible LCT and start cranking them out at a low cost at some point.  Before 911 I made/sold about 1k wire antennas that were sold on ebay and the internet.  The principle was rugged but low cost.  This can be applied to LCT's as well and especially now that I've got equipment to make stuff besides hand tools and my hands.  CNC and plastic injection should bring another dimension to it.  The concern with plug in coils is driving up overall cost by having so many plug in coils.  Ultimately, the plug in coils will have the best performance.  I'm probably going to pay for the dies (or molds I can't remember what they are properly called) for making rotor/stators for butterfly caps then turn that part of the manufacturing over to an aluminum stamping company in USA.  I've already called around and it is the only way to go.  The water jet is insanely expensive in any quantity it seems.  What I'm trying to figure out is what configuration of LCT will give me the highest impedance matching range and suspect it is one that combines series/parallel with the "proper" differential capacitor that W2DU discusses.  The plug in coil sets will not do low impedance unless it is a plug in that allows switching from parallel to serial and that should be possible.  Also, I've been reading a lot about W2DU's comments and it makes sense but it also brings up a ton of questions at the same time.  For example, a common L coupler has 400pF on it's output portion and on the input portion it is not uncommon to see 3200pF of capacitance.  Doing a lot of modeling of L couplers 400pF does seem to be the magic number.  I'm oversimplifying I'm sure but that means with a double L like W2DU discusses then each half should need 200pF.  The Johnson has 400pF but it is differential so it's really not 400pF probably because it is for higher impedances and only above 80mtrs.   To have a series/parallel plug in would be awesome to maximize inductors but then the question still for me is what is the very best way to do the capacitors in series/parallel to squeeze the most out of impedance matching.  I even considered a plug in that uses a switchable 4:1 balun to a bypass of 1:1 to increase the range on the lowest tuning impedance if switched from 1:1 to 4:1.  Doing that would almost certainly cause LCT purists to groan but maybe it is a really good solution for people with reduced size 160mtr antennas.  A lot of people believe that if the antenna cannot get high enough just to use a 43ft vertical and it will work DX better anyways but I used to have a 160mtr loop that was not too high and I loved it and fed it with a homebrew balanced L that could have just as easily been an LCT.  I'm hoping to enlist some help for the ultimate design and also get some input where I do not have any experience before homebrewing this and potentially making more of them.

73,
Mike

AMfone,

  Just found something interesting.  Pat Buller, W7RQT (SK) , made a 160-10 meter link coupled tuner.  I've been able to find one web page where a ham owns one but have not yet been able to contact him.  Anyone know anything about these?  On a Ten-Tec post they are listed as "quite well made".  It would be interesting to find out details on it.

73,
Mike

OK. I thought that you were building one for yourself. Yes, plug-in coils would not be the best choice for production.

So, since we are helping you design the tuner, do we get a cut of the profits?  ;)

Kidding aside, the PA0FRI design seems like it would be the least expensive to produce.

http://pa0fri.home.xs4all.nl/ATU/Smatch/smatcheng.htm

there's a guy, http://www.qrz.com/db/VA6POP , who offers air variable capacitor kits.

Hello Mike,  I am out of town with a family member in the hospital.  Sorry for delay, I will resume discussion when I return.  Many of the folks on this forum will very capably assist with your questions.  73, Rick  (second harmonic of W2DU)

Hi Steve,  Yes I'd looked at the S-match as well as ferrites and toroids.  The BALUN in the S-match is not a transmission line transformer (TLT) and will likely have high voltages on it under high power as it is inductors interacting with each other.  Correct me if I'm wrong on this assumption. There is a HAM in Germany that will be doing testing of it.  There are several HAM's in Europe that have built and used it as well.   It will be interesting to find out if the efficiency is high enough to be able to successfully be considered as a replacement for the LCT that is obviously more complicated to build.  I suspect that there will be impedance transformations in some areas that will be very efficient and others that will have heat in the unit if there is an appreciable voltage drop on it.  The page states there was an air core version of it but I've not seen any detailed pictures of an air core version.  With an air core version of an S-match it begins to get closer to a link coupled tuner as it is using link coupling when in an air core correct?  If that is the case why not use replace the link with a torroid in a link coupled tuner?  I think the answer why a torroid is not replacing a link coupling in an LCT is because of the voltage drops on the ferrite or iron powder transformer in some configurations.  If I was going to go for a replacement of the LCT it would likely be a balanced L vice an S match with any ferrites or torroids only on the input of the network and only being TLT's (transmission line transformers) for simplicity IMHO. The balanced L tuner would have 1/4" copper tubing.  I've made two of these that were manually taped and they were crazy cheap to build.  The only problem is when tapping positions are at the first few turns it is better done with some type of roller inductor or just exposing the tapped coil.  With all that said, I'm still looking into possibilities with the LCT.  

73,
Mike
 

I own one of his high-power 160m-20m models.  In addition, he made a 500W model as well as similar versions for the high bands.  Pat resisted building an all-band matchbox as he had trouble attaining high efficiency in one unit.  

The design is a modified Johnson Matchbox that allowed for variable link input coupling.  The voltage divider is identical to the the classic Matchbox.  Pat also included a high-quality Bruene coupler into the design.  Craftsmanship is excellent.  You can see his tuner toward the bottom of my QRZ.com page.

The other tuner seen near the bottom of the page is a massive balanced type, designed and constructed by Pete, K4EWG.  This is the tuner that Pete used with his V-beam.  I wanted that tuner for a long time.  It uses the jumbo size B&W jack bar and the extra large coils used in a high-power amp that matched a BC-610.  These coils are substantially larger than the BC-610 type, again they use the jumbo banana jacks.  The 160M coil is home-brew and is a whopping 12-inches in diameter and about the same length.  

Paul, W9AC  

Hi Paul,  Wow great web page!  That is a great looking LCT.  I like the low/high band switching and agree it would be very hard to escape resonant frequencies if it has a 160 meter capability.  I looked at the Rockwell-Collins filter page you linked and sadly it stated that there are no guarantees on the mechanical filters and no new designs due to declining supply.  I'll google the grounding you mentioned on your page.  We just had a multi-hundreds of thousand dollar iDirect HUB go kaput upon initial turn on.  The power supply went and took out the 24v bus on it causing the backplane to burn out on an unused slot.  Repair bill is over 100k and they don't want to honor the warranty and looking for excuses like grounding to not honor it :-(  (sorry about the off topic but your web page is awesome and I'll have to look at that amazing motor driven balanced coupler more.  Ribbon inductors...nice
73,
Mike

Good info in this topic.  Frank, AHE has a very nice, beefy looking tuna with copper tubing that I'd like to copy if I make up a link coupled tuner.  I think it's about 6 in. Dia. Wound with at least 1/4 or 3/8 inch.

Hey other Rick,

"the folks on this forum will very capably assist with your questions.  73, Rick  (second harmonic of W2DU)"

Actually your only a half wave second harmonic if W2DU is your only progenitor.
Someone else may have been involved.  ;D

Seriously, I can see why your proud to have him as Dad.   I'll bet you never had a chance.  Probably came home from hospital nursery clutching your first J-38 hanging key mobile.

Sounds like a conjugate match to me.

Well you know what they say...
For every harmony there's a melody.

Rick, thanks for your kind comments.  You are correct, I did not stand a chance.  But I had not only my father as a mentor, but also my grandfather (w8YNG), who taught me to solder at 4, and when I was 8 he taught me the techniques of arc and oxyacetylene cutting and welding.  I built my first superregen at 5 and a two band superhet at 7.  At ten I was welding mini bikes and a gokart with a 50 hp onan welder engine and 3-speed plymouth transmission.  Mom was also very astute technically and mastered the morse code early.  More about the importance of morse code here:  http://amfone.net/Amforum/index.php?topic=31854.0

Hi Rick(s) and AMfone crew,

  After reading the W4RNL web page regarding the link coupled tuner he stated it would be best to have a roller inductor but the problem I see with 160 meter coverage is SRF, Self Resonant Frequency, even if it only goes 160 through 20.  The screwdriver antennas use the copper pipe so SRF is impossible but the finger stock does not seem to be a reliable way to transfer QRO energy (but maybe I'm wrong and please correct me where I am).  6" OD copper pipe is way expensive but at 1.8mhz skin effect X three skins is about 7 mil.  I've purchased copper from this website before:
http://basiccopper.com/16-mil-016-inch-.html
  That page is for 16mil but they have a variety of thickness.  The thought was to buy some copper blind rivets, use some concrete cardboard tube (maybe in the 6 to 8 inch range for a test) and put a little fiberglass around it with the copper sheet riveted in place around the outside.  Then make a kind of screwdriver antenna arrangement so there is no possibility of SRF.  Still thinking of how just to do that but an 8 inch OD coil should fit in a 19" rack barely with the shields coming off each side.  What I'm currently wondering about is if there is a copper "pipe" coming off each end will it somehow destroy the fields needed for the link coupling after it is adjusted?   I'm guessing the answer is no but trying to think of all angles.  Roller vs finger stock is another dimension to consider but a 1/4" copper tube roller inductor on each side with the copper "pipe" like a screwdriver in possibly an 8" diameter would be pretty cool just not sure how much better it would work.
  Sorry....just modified the post to give an example link for the concrete tube in 8" diameter (7 bucks for 4ft of it).  Really low cost and that cardboard should soak up a low cost epoxy, vinyl ester, or polyester resin without any problems.  The copper sheet should be no problem adding around it and could even solder the joint lengthwise carefully.

http://www.amazon.com/Quikrete-Company-Concrete-Quik-Tube-6922-01/dp/B000UG7MI2/ref=sr_1_1?ie=UTF8&qid=1390853112&sr=8-1&keywords=concrete+cardboard+tube

  Sorry...another modification to the post so that I can link the coil data from that awesome/massive tuna link coupled project.  On 160 they are using an 8" diameter coil with an overall length of 9.414 inches.  It may just be possible to squeeze in the coil with the two pseudo 8" diameter copper pipes coming off the end and fit into a 19" rack case enclosure.  The copper pipes would have to be a little shorter to allow the coil to have it's minimum length on 20 meters and the link to fit around it. The link for 160 is shown as 1.92" long (and would need to be tapped with a series capacitor).  Maybe a S.W.A.G. on the length for each side on the copper pipes would be approximately 3.5" for each side but that is a big guess.  If the coils were reversed from side to side that should not work as the fields won't be correct is that right or not an issue?  To have counter wound rollers would allow a single motor but if that is not allowed maybe a reverse threaded rod of some type going down one side and a normal threaded rod down the other side and then using the finger stock scheme.  Also, looking at the concrete cardboard tube it would be 8" for winding and there is another "Amazon" tube that is 10" diameter but reading the reviews a little less in actuality.  Not sure if that 1" space on each side of the coil will do it's job to eliminate losses due to eddy current and eliminate the SRF.

http://www.wz5q.net/index/shack_data/inductors.htm

73,
Mike

Rick and AMfone crew,

  I did find a web page where people have used two screwdriver antennas as a portable dipole but nobody seems to have made a coupler using the idea.  Years ago, I was working with a Harris employee while I was active duty military during an exercise.  He very briefly made a statement that I'm now paraphrasing where Harris used to have a coupler that used a coil with some type of shield that covered the coil similar to the screwdriver concept.  I've never seen this anywhere looking at various Harris couplers.  The Collins ribbon inductors over the drum are the only ones that even come close to that concept in a coupler from what I've personally seen.  Not sure how feasible it is but parts cost would not be all that high.  If it works in a LCT and it can be produced reliably and rugged it may be worth considering. 
  Off topic but I just had a chance to read the stories you linked to from your dad.  Great stuff, I especially liked the one about the meters counting when you were 4.  Our triplets are in the Philippines.  I am currently working in the middle east where my solar/PV powered makerspace is while my wife and our children are there.  We've got two girls and a boy.  They are at the stage where every day on skype I either count to 10 or 5 and they play peek-a-boo.  alot of counting of various items they run across. I've often wondered if they all don't pack up the bags and stand in line at the airport to go to USA sooner than later if I'll take advantage of the extremely low cost labor (about 12 dollars a day) to get some more labor intensive stuff made then complete the final portions in USA.  My wife, Glenn, has looked into exporting costs and it is actually quite simple.  The lady she spoke to quoted volume based cost (I think it was cubic meter) so potentially a single part that is very labor intensive could be made then sent where the majority remaining of it is made in USA.  For me that would be a last resort as I'd prefer just to make 100 percent in the USA with the children there as well.  I have to admit the family is better off in the Philippines while I'm working in the middle east.  You mentioned a couple stations, one of the more unusual was when I tried operating portable in the Philippines with a hand crank generator, 80aH of NiMH battery packs with smart chargers, and fold out solar panels out of a grass hut, called a kubo, on our property.  We've got a large house now and it will become a good operating location in the future while on vacation with a pool close by for relaxation after a little on-air time. 
 

73,
Mike

Hello AMfone group,

  OK I've been making progress in different areas.  Waiting on responses for the dies to get the butterfly cap rotor/stators before I push the button on who gets to stamp them.
  Was looking into good tap methods.  The way I did it before on my balanced L tapped couplers was with the good ole Mueller copper clips.  Was under the assumption they only made the serrated edge alligator clips as I'd read how people flatten them out.  Was on digikey looking at various ones and this one was originally made for clipping on to seismograph rings.  Question is if anyone has used this particular clip on the 1/4" copper tubing for their link coupled tuner?  It is round and smooth but rather large and looks ideal unless someone knows a better one.

http://www.digikey.com/product-detail/en/BU-27CGW/314-1069-ND/2687997
http://www.muellerelectric.com/PartSearch/pdf/DS-BU-27CG.pdf

  One of the thoughts was to use a piece of clear plastic with slots and attach it to plastic tool to open the jaws on the Meuller for re-tapping.  Seems a bit sissy to me based on my experience of just leaving it out in the open but electrical safety is a big deal nowadays.  Saw some fiber re-inforced 100% plastic pliers on amazon and also thought about the low cost insulated fuse pullers attached to the 2.25" long copper clips that could run down the slot.  Got a lot of other ideas but figured I'd run this one across the group.

73,
Mike

Something that can make contact with as much of the surface of the coil is better.

These are good for smaller conductor size but aren't real good for quick changes.

http://www.bwantennas.com/coils/coilclip.htm

Hi Steve!

  Yup, I agree that the more contact the better......maybe a short piece of copper tubing cut in half lengthwise and soldered to the already curved surface would give a heck of a lot of contact area.  In that specification sheet it has ridges on the curved surface to give it strength.  Certainly hard copper not soft on that clip.  I'm still looking at the copper tube that can slide up and down the length of the coil made from resin impregnated concrete tube with the copper 16 mil sheet attached with copper rivets and seam soldered as well as soldering the rivets.  If I attached that Meuller clip to the end of the tube and let it slide freely I'm hoping that a simple slide of the copper tube and a re-clip and goodby to circulating currents and SRF's !  Just not sure if finger stock would be needed to keep the inductive field from sneaking in under the copper tube onto the coil.  That area is black magic to me but my thoughts are that will not be an issue with the "skin effects" of the tube outside of it and the tube being attached to both the clip on the end and the OWL.  Maybe a way to get 160-20 meters FB.

73,
Mike

AMfone group,

  Have a question regarding capacitors in the K1JJ design as well as general link coupled tuners.  Specifically I'm concerned about the input capacitor.  I'm seeing 1700pF for input network capacitance on LB Cebik's write up and it appears 1kv is sufficient from what I understand.  However, I'm seeing 2kv in the K1JJ.  From what I understand the voltage rating depends on the Q and the type of tuner.  Looking at the MFJ sight I can see a plate load capacitor at 800pf rated at 1.1kv with a minimum capacitance of 25pf at $57.  
http://www.mfjenterprises.com/Product.php?productid=282-2573-1
  They also have an 800v unit at only $26 that goes 20-823pf.  The 1.5kw unit is considerable more expensive and not worth it.  
  So, I'm wondering if anyone is using 800 to 1.1kv caps to tune the input of an LCT with full legal limit?  
  On another note, I've recieved a quote to get the butterfly capacitors rotor/stator dies made along with production cost with tumbling/polishing.  For the output/balanced section this is going to be the way I go.  I'm going to invest in the dies and buy in quantity of 1,000 or more rotor/stator sets at a time and will be buying soon enough.  The higher value 1700pf units are not currently in the realm of something I can make as these seem to be soldered together like a broadcast capacitor.
  
  Just added a modification to this reply for something that is a bit puzzling on the input cap.  The LB Cebik (1960ARRL book) value for 80 meters is showing 900pf.  The modification page for the Johnson Matchbox to make it similar to the Annecke at 1kw output is a 1.5kv 330pf for 80mtrs.  I'm not getting the huge difference in values for C1 yet.   Any advice on C1 would be greatly appreciated. 

73,
Mike

Usually the inductance of the transmitter side of the link is to be equal to 50 Ohms reactance. The series capacitor must resonate with the inductance at the lowest freq for the given band and provide any needed additional capacitance transferred from the antenna side of the link.

The voltage requirement is driven by whether tuning will be done at high power. If not, the spacing need be no larger than the typical loading cap for the given power. I've used a BC receiver tuning cap in this application with no problems at 1500 PEP. The assumption is that the impedance with be rather close to 50 Ohms once tuned.

Hope this helps.

Also an interesting way to do the coil tap connections at the link below.

http://www.pituch.net/Steve's%20Page/Radio/500%20Watt%20Link%20Coupled%20Tuner/500%20Watt%20tuner1.html

Howdy Mike,
These are some of the things I learned (mainly from Mr. Cebik's grand site) when I researched and built my LCT...


I would think that the difference in C1 series capacitor values is due to the difference in Inductance of the input link coils of the 2ea LCT designs. Up to a point, you can increase the Inductance value of the coil to then thereby reduce the amount of capacitance needed to resonate it at the lowest frequency of interest.



The voltage rating for the input series capacitor depends upon the capacitor reactance and the power level. For the power level, you have to go by the instantaneous peak voltages that will appear across the cap plates remembering that off resonance voltage peaks are considerably higher. If you use the recommended values of capacitance and inductance from LB Cebik's site, then the voltage seen across the input series cap plates will be around 400v at 1500 watts. You need to put a safety reserve into this and would generally use a cap rated at least double in voltage which would be around 1Kv. If you reduce the recommended input series capacitance by increasing the inductance of the Primary input coil, then the capacitor reactance increases as well as the peak voltage across the cap plates and a higher voltage cap will be needed.

Basically what all this means to is to use the biggest voltage rating cap you can find and then you won't have to worry about it.  ;D

I decided to go the brute force direction in my LCT build. I could not find the split stator or dual differential caps at the high voltage ratings I wanted, so I decided to use huge Vacuum Caps for the input and output circuits. I also opted to use plug-in Inductor Coil sets so I would not have to band switch and could optimize each coil set for my particular antenna system.

Take Care,

Hi Mike,

  I've seen your LCT web page many many times and it is awesome, thanks for such inspiring work like so many others who contribute to the hobby!!  I've got a quote on dies for the butterfly cap and will get another one from a different USA manufacturer as a sanity check before buying anything but will be producing my own butterfly caps soon enough.  Will be using 6061-T6 instead of that 7075 stuff because better electrical conductivity for a small bit of extra money.  What is the problem is making a butterfly cap with only 1kv voltage. The spacers would be more like washers and any slight bend in the rotor would be a disaster.  That said, I've been looking for a good source for the caps on the input of the network.  Was on Epay last night and saw there is a gent in Tawain selling caps real cheap and it looks to be the same as the MFJ units.  I'm thinking they are selling to MFJ and whoever else they can.  They had more than 10 pieces in two different styles, here are the links I saw:

http://www.ebay.com/itm/High-Voltage-Air-Variable-Capacitors-40-445pF-Amplifier-Tuner-REPAIR-DIY-/161086324757?pt=LH_DefaultDomain_0&hash=item25817e4015

http://www.ebay.com/itm/High-Voltage-Air-Variable-Capacitors-30-250pF-Amplifier-Turner-REPAIR-DIY-/160999183137?pt=LH_DefaultDomain_0&hash=item257c4c9321

http://www.ebay.com/itm/High-Voltage-Air-Variable-Capacitors-22-360pF-Amplifier-Turner-REPAIR-DIY-/151216034189?pt=LH_DefaultDomain_0&hash=item23352d918d

  That last link for the cap going for $20 is just asking to be modified.  It is 1kv rating and 360pF while only 1.56" long.  Not sure what size that all-thread is on it but if I bought 5 of them for a total of $100 then I could potentially get an 1800pF 1kv cap that is going to be under 8 inches long.  I cannot get plates/rotors stamped that cheap in USA so likely will only do the butterfly in the USA and this one overseas.  It will also need the last of the 5 sections switched in/out seperate from the other 4 sections like the Palstar does giving a minimum capacitance of 22 with a max of 1800pF.  The corners are a bit sharp on it but the voltage is not going to be high enough to make a difference I think.  Any ideas to make the 1800pF cap in a better way (in the USA) I'm open to.  I'll let you know what the seller says about a modification possibility on this cap and what size all-thread they are using for it.
  There are alot of different angles to this project that I'm working on still but thanks for the input on the capacitors. 

73,
Mike

Reference

http://amfone.net/Amforum/index.php?topic=5931.0

Can I adapt the classic K1JJ design to match my inverted L? I realize the OUT POOT is balanced. If circulating currents are an issue, then I can just use a current balun on the feedline. What say you?

Tnx

Philip
KA4KOE

Does the cap on the transmitter side of the link need to be a butterfly type?

Hi Phillip and Steve,

  Sorry about the delayed response been crazy busy the last couple days.  I know the Johnson Matchbox could tune but to a different impedance range if I understand by strapping one of the outputs to ground making it unbalanced.  The reason why I'm looking into using a butterfly is because there will not be a wiper causing the Q to be very high and it to be less mechanically complicated.  That is why the magloop folks love the butterfly caps so much (when a vacuum variable cannot be found which is the best). 
  Learned a little about aluminum stamping.  They use sheets that come off a roll causing there to be sometimes 3 to 5 mils per inch of a bow.  Not sure how that can be alleviated but know that traditionally aluminum stamping was the only option for the stators/rotors before water jet and laser was around. I'll ask that question on a machinist thread I belong to for my CNC mill.  It would be nice to have one size in the 5 inch range and another in the 10 inch range but if the rotor/stator was 10 inches that would give a bow of 30-50 mils.  If the spacing between plate and stator is 1/8" (roughly) using 1/4" aluminum spacings and giving almost 7kv working voltage then I think there is enough to be able to accept that bow in the stamping.  This is likely the reason why I've only seen the giant capacitors in the Soviet transmitters like the one I see on Ebay right now.  I've seen some ex-Soviet variables that had plates that almost looked like they were cast aluminum.  Anyone seen some really large, 10-12 inch, plates on air variables that have the standard .040 inch thickness aluminum plates?

Best regards,
Mike

Non-butterfly type caps are used as loading caps in Pi-nets all the time without concerns of Q and loss. Why would it be any different in a link tuner?

Hi Steve,
 
  The matchbox uses a differential capacitor with a wiper so it definitely will work.  The butterfly capacitors are used in magloops due to the high current and low ohmic value so any resistance in a wiper becomes a significant amount of overall resistance.  The only thing achieved from a butterfly capacitor is less loss and mechanically a simpler design having no need for a wiper contact.  One other advantage, if a stepper motor was used to control the cap so the tuner could be placed directly below the feed point of the doublet it has virtually no turning resistance compared to a split stator with a wiper and only the inertia due to the weight of the rotors is of any concern with another trade-off is only 90 degrees from min-max vice 180 degrees less resolution per step of the motor but quicker time to the tuning solution. Also, if a LCT was used to tune an electrically short antenna the resistance of a wiper would start to become more of an issue than if it was tuning a doublet just over 1/2 wavelength.  Example, if I was trying to use an LCT with a 150ft doublet for 160-20 meters  due to space limitations of the property a house was on and was in series tune for the 160 portion I'd personally prefer a butterfly (or even better a vacuum variable) over a regular cap. The drawback it appears is a smaller spacing between rotor and stators that is 1/2 for a given voltage rating compared to a regular variable capacitor so tolerance on how flat the stator and rotor becomes more important.  This is what I did not take into account initially.  For a 5 inch butterfly cap at 400pf and 7kv (if the aluminum was perfectly flat) it seems that it is a good approach.  For a 10 inch butterfly air variable I'm not certain this is a good approach without a way to ensure the bow of the aluminum is a lot better than 3-5 mils per inch.  I suspect that all stamped rotor/stators have this same issue but could be wrong and sure would like to know how manufacturers controlled this.  Even if water jet or laser is used if the sheet came off a large roll and has any bow to it the same problem will be introduced.  I'll try to call around on Monday night but being overseas it is a real hassle.  Will also do some emails to water jet servicing companies to see how they ensure everything is nice and flat for their products.

73,
Mike

The matchbox does not have a cap on the transmitter side of the link. I thought you were planning on placing a cap on the transmitter side. But I think you are talking about the antenna side now.  :P Sorry for the confusion.

Hi Steve,

  You are absolutely correct, I'm planning on putting a cap on the transmitter side and have considered a split stator on the transmitter side and a butterfly on the antenna side.  Was hoping that the same stator/rotor set that is used for the antenna side could also be used for the transmitter side but not sure if that is possible as an 1800pF butterfly cap has the stator and rotor really close together for 2kv rating.  The old rule of "you don't get something for nothing" is definitely kicking me as the advantages of the butterfly are good but only in some applications (antenna side) and not necessarily in all (transmitter side).  What would be really nice is to be able to make all the parts using USA labor and make it really affordable.  Certainly is a hard puzzle to get all straightened out on.

73,
Mike
  Just found this site on allowable tolerances on bending of stock aluminum:
http://www.trident-metals.com/alumtolls.shtml
  And this web site on how to flatten sheet metal.  It seems to be near impossible and certainly the reason why capacitor rotor/stator plates just don't get very large:
http://bbs.homeshopmachinist.net/archive/index.php/t-40775.html
  Looks to me that around 5 inches is about as large as anyone would want to go with aluminum stator/rotors.  Has anyone seen ones that are larger than that?

73,
Mike

This is from the Collins 32V manual. The last two diagrams indicate how to feed an unbalanced antenna with a LCT.

Philip

Hi Philip,

  Thanks for the schematic!!!  It appears that Art Collins had a different way of coupling than the series/parallel way the K1JJ does it.  Lots to think about on this project and it is really getting interesting.

73,
Mike


 

.

Here's MMANA GAL runs for my 150' inverted L.

Top line should read 299 pF Cseries, 7 uH Lshunt.

AMfone crew,

  OK, as I'm working on the Link Coupled Tuner I've got a question for all the collective brain power and more importantly vast experience.  OK, from this first line you can tell I'm desperate to get some straight answers and/or debate on this question.
  As I prepare to wind the coil I want to do it the best way for Q.  W8JI states on his web pages that some of the best Q inductors he has seen are edge wound.  Not having any just sitting around to check I have to wonder "what about the distributed inter winding capacitance going up"?  So then the next thought is why not just use some 1/2" wide 20 mil copper strip?  I've got lots of copper tubing and a couple 100ft rolls of the copper strip to boot.  If it was edge wound it surely would be hard to bend but then I thought about all that counterfeit stuff showing up that is copper clad aluminum.  Actually, if advertised as such there really isn't anything wrong with it and it typically will have 15% or higher copper content.  One can buy everything from copper clad aluminum bus bar to flat wire and the aluminum would certainly make it easier (and cheaper) to wind edge wound. I've bent aluminum wire before in large gauges and it is ridiculously easy to bend so I'm thinking something with 15% or 20% copper cladding over aluminum won't be much harder.  
  But the nagging question still remains....what is better edge wound flat copper tape or copper tube for the ultimate Q on the ultimate link coupled tuner.  The matchbox has edge wound.  I've seen flat wound (or barrel wound I've even seen it called) in everything from a small inductor after the roller inductor of the tuner to inside amps.  Honestly, 20 mils is going to have a heck of a low inter winding capacitance and give a great (and easy to buff/clean) flat surface to any contact from a roller to something else.  
  Reading up on proximity effect makes me think the edge wound is the choice but then what would be the optimum width/depth?
  There has got to be an engineer out there that can shed light on the subject as it seems to be a complete mystery after scouring the net over and over and over.
  Oh, and then there is the flat wound strip in the Collins 180 couplers.  Certainly that was a mechanical choice but was it also a better Q to boot?
  I'm thinking with a 20 mil thick copper strip wound flat vice edge wound then the SRF will be extra-ordinarily high for a large link inductor (let's say one for 160 meters).  It would be the equivalent of 20mil wires side by side as far as the inter winding capacitance goes but of course much wider. 
  I've got some answers from that 3more company out of Taiwan who will sell individual rotor/stators or any other parts.  The butterfly capacitor dies and rotor stators from USA mfg are probably a "go" but am waiting for more answers from this gent in Taiwan regarding a 1700pF 2kv input capacitor for pricing with the butterfly on the antenna side of the LCT.  

Very best regards,
Mike