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Re: Coil form

>In building my variable TC capacitors, I use high density 40 mil 
>polyethylene HDPE sheet glued with low density polyethylene LDPE.  Hot 
>gun glue is LDPE.  The melting temperature of LDPE is lower than HDPE. 
>I use two methods of gluing.  In the first, I glue two pieces of HDPE 
>together with hot glue.  I then role them together (an ink bray works 
>nicely) and apply heat with a model airplane monokoat iron.  The heat 
>is adjusted to melt the LDPE and not the HDPE, since I don't want to 
>distort the thickness of the HDPE sheet in the capacitor.  The bond is 
>excellent.  In fact, if you pull the two pieces of HDPE apart after 
>cooling the LDPE remains attached to both pieces of HDPE and tears 
>down the center of itself.  In the second method, I am now 
>experimenting on larger pieces using a 6 mil LDPE sheet between the 
>two 40 mil HDPE sheets and an electric iron.  The temperature is 
>easier to control with the iron. 
>
>I am also experimenting using this method to glue HDPE to other 
>materials such as aluminum foils.  
>
>RWW
>
>

8/23/96

Dear R. DelBueno,

I have built two variable caps.  The plates are roughly about 6" x 6" 
30 mil aluminum and there are 25 plates.  The first is two sets of 
semicircular plates like in old variable radio air caps.  The plates 
are covered on both sides with 40 mil HDPE.  They are in mineral oil 
and housed in an old metal military field telephone housing which is 
lined on the interior by three layers of 40 mil HDPE.  Capacitance is  
~0.55 to 4.5 nF.  I run it in parallel with my main 10 nF commercial 
cap on a 1 kW coil.  Results are excellent.  The TC can be tuned on the 
fly with a long wooden pole.  I find peak tuning is more accurate than 
moving a primary tap.  Also toroids and top capacitance can be rapidly 
changed and the coil tuned almost immediately.

The second coil has a rotating bank of center butterfly shaped plates 
which interlace with quarter circle banks of side plates on each side.  
All plates are covered with 40 mil HDPE and in mineral oil.  The 
housing is a large circular polyethylene cannister.  Capacitance is    
~0.62 - 3.6 nF.  It was harder to build, but works equally well. 

Although he is not interested in building one, R. Hull says about a 
1000 pF variable cap is about right for a small to medium TC.

My plans are to simplify things and use heavy duty Al foil.  Believe it 
or not, hot glue (LDPE) sticks very well to Al foil.  Probably why they 
call it glue.  The side coverings are still 40 mil HDPE and form quite 
a rigid "plate".  The semicircular or two rectangular plate knife 
configuration are easiest to construct.  I am considering making them 
smaller as R. Hull suggested.  Also, I may change to transformer oil 
and have even considered SF6.

I strongly recommend R. Hull's excellent educational series video on TC 
capacitors.


RWW