The AM Forum

About 4 months ago I asked Robert to build me a tuner for the big rig.  Well.. You guys can guess how that went. paid him and still no tuner.  Time to build my own.

I dug around and found three coils. This is the smaller of the three.  It says 200-206 johnson on it. Will this work for the k1jj style tuner?  The other one is close to this but twice as many turns and the last is 3 ft. The latter two are used at the base of BC verticals I am told.  Massive and really to big.. They must weigh 30 lbs. I guess I could use them mounted on the wall. 

I also found:

1. - 1000PF 5kv jennings.
1.  340 PF bread slicer. says .100 gap.
1.  350pf at 9kv cap.
1.  HUGE air cap out of a gates BC rig. Unkown value but huge 1/4 thick plates.

Any help would be great here..  I have 14 gauge wire here to make the link coil and some connectors. I need a turns counter for the vac cap If I have to use it.  I sent two to robert.

Any idea how many UH I need for the coil? knowing that will help my search.
C

50uh total should be enough on 160.
So who is Robert, so all will know...

This one measures 20uh with the MFJ. Not sure I measured it right. I used a 2 ft piece of RG mini 8. 

C

You ought to use shortest possible leads for measuring there, probably not coax, and as low a frequency as reasonable.

I tried to measure another coil. It also reads 20 UH on the Mfj. I dont think I am doing this correctly. I have to read the manual. How about this coil?  It says 43UH on it.  Its about 3 ft tall.. LOL  I could mount it on the wall. I could put rings on it tapped for the bands, then use a smaller banna plug or clip lead to attach it.



C

That coil should work just fine for 160/75M.  I have one in the cellar parts bin too.  However, on 40M you may find the coils are so large that for certain matches you may need to move only 1/2 turn to find 1:1  - the tap will be on the opposite side of the coil, thus not easy access. On 20-10 it would be more difficult. But other than that you will be OK.

That's why many guys wind a long coil about 5" in diameter or so - to get the finer coil selection taps.  Looks like Dave used 1/4" copper tubing from any plumbing house or Home Depot.   If he pre-labels the coil taps for each band, he will be able to switch 160 thru 10M in seconds.

Here's the nice neat layout built by by Dave, KA2J - simplicity plus:

T

I wish I had those coil forms. I dont own a Drill press. Would be nearly impossilbe to drill without a press. I will ask around to see if anyone has one.

Tom, are you talking about the smaller of the two coils?

C

Clark, I use a coil very similar to that one in my HB toona, out of a broadcast rig.  My input link is 1 turn of 3/8"X1/8" thick copper STRAP.  Taps to the cap via alligator clips as are the taps to the ant.

You may or may not need much inductance at all.  On 75 the cap taps are about 3 turns either side in from the end, and the ant tap is 3 turns either side from the center.  On 160 the ant taps are as close to the center as I can get 'em and the cap taps are about 5 turns either side from the center.

I use a 350 pF or so vacuuuuuuum variable with a fixed 100 pF vacuuuuuum cap strapped in parallel for 160.   My coil gets slightly warm to the touch on 160 and the vac variable as a bit of talk-back  ;D  Lotsa circulating current there.  On 75 it's stone cold running the GPT-750.

It all depends on your feedline length, and yer ant, where your taps will end up.  It's a royal PITA figuring it out the first time - I never did find a 40 meter setting til I hooked the damn thing to my HP3577A network analyzer which made short work of the job.

Ok...  I might try this small coil.  Now to settle on which caps to use.

C

Clark,

I'm talking about using the smaller coil, pic #1, edge wound, silver plated -


T

Wow John 1 turn link. I'm surprised it couples much RF. I wonder if you tried more turns.
I use a pair of 22uh inductors in the fugly at max inductance on 160. 125 feet on a leg with about 85 feet of 450 open wire line.
The easiest way to set up the tuner to get close is put a 6 or 10 dB 50 ohm pad between the RX and tuner. Then tune for max noise. This will get you quite close and doesn't stress the finals.
I would try the larger inductor.

Clark,

I am not sure which MFJ you are using to measure inductance but for RF work you are probably better off using a new purchase low value dipped mica cap in parallel with the unknown coil and dip it with a decent grid dip meter.  The MFJ antenna analyzer with its dip coils does NOT qualify as a good dip meter :)  You can determine the actual dip meter frequency by listening to it in a nearby receiver.  Then a simple calculation will give you the inductor value.

Using a reasonably large value dipped mica (in the 300 to 500 pf range) will minimize the problem of stray capacitance.  Your accuracy then is a function of the capacitor tolerance (should be very good with new (not NOS) dipped micas), avoiding excess stray capacitance, and adjusting accurately for the maximum dip.

 My Tektronix 130 L/C meter does agree closely with calculations made using the dip meter method and with my Boontoon bridge and is the fastest method.  My results with a modern digital meter were not so good.

Thanks for the tips guys.

Ok. I will try the small coil for now. I have a 1600 PF 2kv cap here ready for the input and I have a 500pf 10KV jennings. I need to order a turns counter as I sent both to robert and probably will not get them back.  If the coil does not work, I will wind my own. But since I have this coil, Why not try it?

THanks for the tip on the MFJ. Its readings are not working out. The one coil is 43 when dipped and the other was 38. So I have to trust the Grid Dipper. The MFJ shows 20 for both the coils.

C

Has anyone ever fabricated custom forms for this type of coil?  I have several coils on hand, but the turns are either too close together or too far apart, and I need to use only part of the coil stock.  I have a couple that I could combine into a single coil that would be exactly perfect for my 40m tuner, if only I could fabricate a form for them.

I am thinking of cutting strips out of 1/4" polycarbonate stock, but it would be very tricky to accurately gauge the slots, cutting with a hack saw.  A band saw (which I don't have) might be easier, but I would need one with a tiltable table, since the slots need to be at the proper angle to follow the pitch of the coil, not exactly perpendicular to the insulating material.

Don,

There are some simple shop built jigs used in woodworking for cutting finger and similar joints with specific spacing between slots and one of these may give you some ideas how to build something for your needs.  Some are built for table saw use while others are meant to be used with routers and I have used my Leigh dovetail jig and router to make spacing forms for large coils.

Here is one site to give you an idea whether or not this method might work for you:  http://sawdustmaking.com/Box%20Joint/boxjointjig.html

There is a simple formula in any handbook  used to determine the inductance of a single layer inductor.

There are a number of nice online calculators too. One even give skin depth and loss.

I need some more books.  My handbooks have a very small section on tuners and how to build one. So not much to learn with the books I have.  I probably wont have time until after the Tday holliday but I am going to mock this up on a plywood board and start playing around with it.

C

Rodger, I need help understanding the exact procedure.   Do I take the cap and put it across the entire coil and dip the GDO to get the coil's resonance or is the purpose of the capacitor to find tap points for ham bands by putting the cap's leads on the same number of turns working in from the ends and dipping and measuring the GDO each time to find where the taps should be for each ham band?  What if anything is done with the ends of the coil in this case? 

Frank, I was surprised John uses a one turn link also but he's running the 750  :o  ;)

Rob

Rob,

Put the cap across the entire coil and then dip it for resonance.  It helps to have a general idea of the inductance (figure it from one of the many coil winding programs) to save time.

For maximum accuracy the standard dip rule applies, use the minimum coupling necessary for a readable dip and back the dip coil away from the test coil for the final check.  Of course this level of accuracy isn't necessary for most of what we do.

could you use the GDO in the procedure above but find band tap points by shorting the ends to some pair inner points, one on each end, and putting the cap across the unshorted part of the coil?   

The Cebik site has some very good stuff on tuners. It's online.

Frank said:
Yepper, and everytime I used it, I compared it to my old Boonton and it was +/- 10%. That should get you in the ballpark.

Looked up the Johnson coil in the old "Radio-Electronic Master" catalog. It lists the 200-206 as 50uh.

Rob,

If I correctly understood what you were asking this was probably the most typical use in the old days.  It allowed you to set up the tuned circuits in a transmitter during construction and/or make sure your tuned circuit would actually resonate at the desired frequency.  In a typical circuit you would short the unused turns from only one end. 

The MFJ analyzer does work well for testing the proposed output network settings with the operating voltages removed.  You should leave the tube(s) in place so that it(they) presents the normal capacitance and hook the MFJ to the output connector.  Then connect a 1/2 watt (non-inductive) resistor equal to the typical operating load of the tube from the anode to ground.  If your taps are properly chosen the analyzer will indicate unity SWR with reasonable settings of the tune and load capacitors. 

Don't forget to disconnect the resistor and analyzer or lots of smoke will result :)

Thank you!  Someone wrote 43 UH on it and that is probably where it is tapped for with the single tap that was on the coil. This will work if in fact, I have enough space to tap for the bands.

C

Okay right; thanks--what I was searching for is an easy way to get in the ballpark finding places to tap a coil for a balanced tuner with a link in the middle.  I was wondering if you could take a coil of decent size, and set the GDO in the middle of 80 m. say, (to cover the band with one pair of taps) then start shorting the coil coming in from each end equally, until you have shorted enough turns on each end to achieve a dip on the meter.  Then repeat coming in farther shorting more turns for 40 m. etc.  I wondered if this might be a way to mess around with just the coil, marking the points on it so that once the whole thing is built it would make it faster to find the actual tap points.  John (or someone) wrote that finding the tap points the first time is a pain.  I can believe it.

If you are not dipping it when it's connected to the antenna, it probably won't be close. Remember, with the tuner, you are resonating the entire antenna system - tuner, wires and feedline.

Okay thanks Steve; oh well, you win some you loose some  :D  I guess there's no easy way here.

Hi Clark I think if you tackle the famous K1JJ tuner you will be impressed with the ease of tuning and simplicity.
I wrestled with 3/8 copper tubing and wound it on a 6 inch PVC pipe. I found a ceramic coil form 3 inches dia and wound 6 turns on that. That's the input coil. I carefully grabbed the coax connected to the input coil while transmitting and found a better match by moving it around within the bigger coil. First I made sure that my antenna taps and cap were equal distances and then played with the input coil,,,,THEN I used clear RTV sealant to permanently fix the input coil in its spot.

Tom or Steve posted mods to enable 160-10 use .
FRED

The easy way is to beg borrow or steal a HP 3577A, or a service monitor with a tracking generator and a directional coupler.

You forgot rent.

I haven't bothered to calculate μHs in a coil for decades.  Years of working with transmitters gives me a ball-park idea of the tuning range of a coil just by eyeballing it.  I figure out the amount of capacitance I need, then find a bread slicer within that range, attach the coil with clip leads, and search for resonance.  I then add or subtract turns by moving the clip around on the coil until it resonates with the appropriate amount of capacitance. Then I trim the coil to the required number of turns and fabricate some kind of physical mounting.

I have several GDOs, but usually I just tune my transmitter to frequency and run the variac way down to only a few watts output, and couple to the coil with a JS link, maybe a couple of turns of clip lead or hookup wire, and hold a neon lamp near the  coil, using minimum coupling to the transmitter that allows the lamp to light up at resonance.

I have tried the mathematical method of calculating the inductance needed, then calculating the dimensions of the coil, but find the trial-and-error method as described above much faster, and the results usually come out right the first time.  When doing it by calculations and then constructing the coil, on the first try I would be lucky to get it within +/- 40% anyway, so I still ended up doing trial-and-error, regardless.

I don't even think about μHs; I just go by whatever resonates with the appropriate capacitance.

Clark
Tuning my K1JJ tuner on its virgin voyage took about an hour to find the happy spots for 160-40M using the famous MFJ 259 analyzer.
I have an A3S for the "upperbands"

Fred

Some of those old Agilent CDMA analyzers have an analog service monitor included, with a 400 KHz- 1 GHZ tracking genny/spec an.  I use the 8935 and it can be had for about a grand.

A hamfest coughed up a couple of MiniCircuits directional couplers for ten bux.