[AMRadio] Need 75/80M Final Tank Coil for BC610E


John Lyles jtml at losalamos.com
Tue Jan 28 01:59:55 EST 2014


Don and all,
Polystyrene is still available for high performance insulation for 
microwaves and RF voltages. It is known as cross-linked polystyrene, 
used to be made by the Polymer Corporation as material Q200.5, It is 
also known generically in the microwave and radar industries as 
Rexolite, used for radomes for antennas in nosecones and as insulators 
in very large coax. At work we use it as a support spider in 14 inch 
diameter coax that carries 3 Megawatts of peak power at 200 MHz (for 
particle accelerator RF systems). We also use it as a window in the 
coax, where it goes from air insulated to vacuum in the linear 
accelerator. At this point it holds off about 22,000 volts of RF at VHF. 
This stuff has very high Q, but not as high as pure Teflon. It is 
mechanically a lot stiffer though.

Today, you can still buy it, from C-Lec Plastics in New Jersey. They may 
sell small quantities, but generally it is priced like gold. We buy it 
in 1 inch thick sheets and cut and machine it into shape. They also make 
it in rod stock. Many of the plastic distributors like Reid, Westlake, 
Cadillac, Plastic Supply, Bodeker Plastics, all can order it.
If I could get my hands on just the scraps that are left from machining, 
I could supply many BC610 coil rebuilds!
73
John
K5PRO


> Message: 4
> Date: Mon, 27 Jan 2014 10:57:37 -0600
> From: "Donald Chester" <>
>
> Those coils used a plastic substance called cellulose acetate for the
> insulative strips. I believe it's close to the same stuff as celluloid, and
> shares a common  characteristic: it self-destructs over time, turning brown,
> shrinking and finally decomposing into a powdery substance. Just like the
> classic movie films that have self-destructed while in storage. B&W
> continued to use this material to make their air-core coils, while EF
> Johnson, Bud and National used polystyrene. The latter is much more stable
> and does not deteriorate with age if not exposed to UV.  I often see Bud,
> National and Johnson coils at hamfests still in perfect condition.
> Unfortunately, all the BC-610 coils I have seen were made by B&W, and now
> most of those are falling apart. Sometime the shrinkage of the plastic is
> enough to break the ceramic bar that holds the banana plugs. The
> deterioration also causes a degradation of the dielectric properties of the
> plastic. I once  had the plastic strips on an HDVL coil literally burst into
> flames while I was transmitting.
>
> The most convenient way to fix the coils is to cut out replacement strips
> from plexiglass sheet, and use plexiglass glue to secure them in place on
> the wire. Plexiglass, an acrylic plastic is not the best insulator in the
> world, however. Polystyrene would be superior, but it appears  that solid
> polystyrene sheet has become unobtanium. I checked several vendors of
> plastic material, and the only polystyrene I could find was the foam
> variety, like what is used to make plastic coffee cups and packing material,
> not solid transparent sheets or rods. Years ago, solid polystyrene was
> readily available at electronic suppliers. I recall buying mine from
> Berstein-Applebee in Kansas City. You could also buy polystyrene glue,
> similar to plexiglass glue for bonding the pieces together.
>
> Polystyrene is not for every application, however. I once made the mistake
> of using polystyrene rods for open wire spreaders. It deteriorates very
> rapidly when exposed to UV. In about a year, the rods had turned to a brown
> powder, similar to what happens to the cellulose acetate. Plexiglass works
> much better for spreaders because it is highly resistant to UV damage. So,
> best to use plexiglass outdoors, and polystyrene in localities not exposed
> to too  much daylight, if only one could find the stuff.
>
> If anyone knows of a present-day source of solid polystyrene, please let me
> know.
>
>


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