John says we should be conservative in design of the rf deck. He probably is
conservative in the power supply, too.
Well, as we get older and older, the amplifier is perhaps the last arena we can
generate our own fireworks. Building an amp with questionable components can
keep us in the risk taking category.
As for me, I have found other things to take risks on, so I think I will follow
his advice.
Colin K7FM
-----Original Message-----
>From: "John T. M. Lyles" <jtml@lanl.gov>
>Sent: Apr 14, 2006 10:03 AM
>To: amps@contesting.com
>Subject: [Amps] Chimney materials - Loss or not TSPA
>
>Air cooled tube chimneys seem to be a big topic these days, as much
>as discussion of EPAY and that other service. I want to suggest that
>people consider that these chimneys are typically situated at the
>highest impedance of an RF amplifier, across the anode to ground
>space. With 2-3 kV of peak RF swing there, at, say 28.25 MHz, one can
>expect a minimal but measureable dielectric heating effect. With a
>thin cross section, like transformer paper that Will suggests, this
>may not cause appreciable heating in such a narrow piece of
>dielectric. As long as moisture doesn't ingress into the paper, it
>should be OK. Vinyl chloride-containing polymers (like PVC) are just
>the opposite of what you want for a low loss dielectric. According to
>Arthur Von Hipple's classic compendium of dielectric data from MIT in
>the 1950s, (Dielectric Materials and Applications, available in
>reprint from Artech house now), PVC formulations have typically 100 x
>the loss tangent of polyethylene at 1 - 10 MHz range. RF sealing
>process is used to stamp PVC articles together, like the patches on
>auto carpets under the pedals, like inflatable rafts, like door
>panels for autos, packs and raincoats. The harder polymers such as
>used in pipe may be a bit more resistant than the flexible variety,
>but they still have a lot of ionic content. We don't use PVC pipe
>fittings in commercial Deionized water systems as they tend to
>release ions into the solution.
>
>As an RF engineer I try to stick by rules of things which are
>acceptable and not, and apply them universally. I spent 6 years doing
>just dielectric heating experiments for applications at E. I. DuPont
>Co, so I got very familiar with what plastics I could trust or not.
>My overly conservative stance (go ahead AG6K, fire away!) often gets
>criticism from those who say it will work fine at lower voltage, or
>for low duty operation like amateur transmissions. It true that cost
>is no object in many high powered systems where you don't want to run
>the risk of a burn-out. In low Q applications like pi-networks, the
>voltage is well defined. In cavity and line circuits, beware, as the
>Q can be higher, the voltage rises significant. I still suggest that
>we don't tempt Mr. Murphy too far, as one may be sitting up on 10
>meters (or worst yet, 6) key down and wonder why output power seems
>to be dropping while smoke begins to rise from the PA compartment
>(and that hideous smell of melted PVC). Think of it this way, would
>you use the same material as a standoff insulator in an RF network
>with high voltage? Do you mind the lowering of Q that lossy
>dielectrics can introduce? Unfortunately, the old microwave oven test
>doesn't work well enough to apply the results, as constant fields are
>not produced, they purposely move them around to cook a chicken
>uniformly. Applying a fixed electric field in one direction through a
>thick dielectric, with minimal spaces (air gaps) between it and the
>metal, is the exact method used to get temperature rise in dielectric
>heating industry. Be careful; check your homeowner insurance policy
>if you care to experiment and take materials close to their limits.
>
>John
>K5PRO
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