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Coil is finished, installed and resonated…

– Posted in: 2200 Meters

Over the last few days I have tried to spend some of what has become limited free time getting the antenna modifications in place for 2200-meters. Shorter days and colder weather are rapidly approaching so no time like the present.

For starters, winding the coil just about killed me.  #10 AWG solid wire is challenging to work with anyway but its especially challenging to use when one is working alone and the decision is made to not spend time building a winding jig.  Needless to say, I got a nice upper body work out and my shoulders are paying for it today.  And I feel like I have been playing the guitar for days – callouses abound!

finished coil

12-inch diameter by 32-inches wound #10 AWG coil for 2200-meters

I started the coil with two anchor holes for the wire to loop through.  The keeps the wire somewhat in place while the initial turns are made.  I did my best to make tight turns which would help prevent them from slipping while being wound.  This is easier said than done given that the winder is dragging wire off of the spool while trying to prevent kinks in the wire being wound.  Initial winding was done horizontally and I switched to vertical winding once I was near the end of the spool and it was easy to handle.  As you can see at the bottom of the coil, keeping the wire from kinking as the spool got lighter was a major challenge.  Similar anchor holes were drilled at the bottom end of the coil and a few turns of vinyl tape were placed around the outer turns to keep everything in place.    That’s a lot of wire – 500-foot to be exact!

The coil was placed into the new box and the cables were routed through the conduits.

coil in ATU - first try

2200-meter coil in situ in the the new ATU box

 

In the main 630-meter ATU, the unused top connections for the knife switch were fitted with copper mechanical lugs used to attach wiring to power busses in AC service.  This worked well and the stiff wire was not problematic in the least.

coil leads and switching

Connection from the 2200-meter coil to the knife switch that controls RF from the main 630-meter variometer / loading coil

All set, right?  Not exactly.  I did not expect to hit it on the first try, even with my good luck.  What I found by scanning with the analyzer was that the resonant frequency was at 156 kHz.  Resistance was around 40 ohms.  Ok great.  Something to work with.  I had room on the coil for for about 36 more turns of #10 AWG but I used all of this wire so it was time to hunt more down.

I called a local electrical contractor who gave me a price for 500-foot of AWG #10.  They do not sell wire by the foot and I estimated that I would need about 115-foot.  The price was not as bad as I thought it might be and I figured I would find a use for the additional wire in the future.  But first I wanted to check Home Depot because I knew that they sold wire by the foot.  I learned today that Home Depot only sells certain varieties of wire by the foot and while #10 AWG was available, it was only available in stranded.  OK, now I am getting picky, but for the price per foot that the stranded was listed, I might as well buy the full 500-foot spool.

I came back home to re-evaluate just how much wire I might need using some existing coils as stand-ins.  I didn’t need as much wire as I originally thought to achieve resonance at 136 kHz and I found a partial spool of #12 AWG silver plater copper with Teflon insulation.  This is the same stuff that I use on the main 630-meter coil.  I argued back and forth with myself for a few minutes and decided to splice this stuff in with the #10 AWG.  After all – it was silver plated wire so even though it might be slightly smaller, maybe it will be OK.

I began winding and finally ran out of silver wire.  I put the coil back together and checked it with the analyzer.  Resonance was at 118 kHz  and resistance was up to about 75 ohms.  Yikes!  But not unexpected.  This was good because I had passed the resonance point and could begin removing turns.

Amended coil with extra turns to reach 136 kHz

2200-meter coil with added turns to resonate the system at 136 kHz

I removed a few turns at a time until success – X was close to zero.  Note that my analyzer is not normalized to 0 ohms reactance.  Its more like 3-6 ohms and I was at 7 ohms of reactance.  I was happy.  The resistance was even 50 ohms and I had not even done anything to try to match it – loss loss loss!

Analyzer says its ok

Even with a loss dominated resistive portion of the complex impedance, I will take it!

I made no changes to the matching circuit used for 630-meters.  It currently works out to 50-ohms on 2200-meters.  Who can argue?  I realize that the losses are very high but this radial system has nearly 15000 feet of wire in it and 26 ground rods.  I have trees and houses and other obstructions that are just part of life and reality when living in the suburbs on a lot that was never designed to launch RF, particularly MF and LF!   Its just not going to get any better.  I accept my fate on 2200-meters, whatever it might be in the end.

I’ve run some tests to make sure that the 630-meter network is not too upset with the presence of this new coil and I use a copper shorting bar on the knife switch to minimize current flow in the new coil while on 630-meters, for what that’s worth.  Base current on 630-meters looks normal so I think I may “skate” on this one.  One of the goals was to get the match close enough on 2200-meters so that I would not need to move the variometer (much) to find the match on 136 kHz.  I also note that large changes of the variometer result in small changes in 2200-meter match.  This fact may be a blessing in disguise as I was fearing the rate of travel of the variometer to be too quick for me to dynamically find resonance.  As it turns out, its just the opposite.  The bandwidth is quite narrow, as expected, perhaps 500Hz – 1 kHz 2:1 SWR but until I can actually put some RF on the system, its anyone’s guess.

I’m happy to have completed this part of the project.  I can relax and just observe the system stability as the seasons and weather changes.  Its my expectation that the system won’t be very stable and I may have to engineer some additional flexibility in the match.

I took just a moment to listen at 136 kHz with the resonated vertical and as expected it was a mess.  That’s life in the suburbs.  Its just something else for me to consider as I progress on this journey.

As far as where I go next, I’m not in a big hurry.  I’ve been evaluating transmitter designs and considering low pass filter options.  Either way, things look pretty good here for the new 2200-meter band administered under Part-97.