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Thank you Mike!
I will keep that in mind, as I build this later. I'm just in the planning stages now, while waiting for parts. I hadn't noticed that post - but I did look up the capacitance per unit length of various coax, and it is quit high ( about 25pf / foot ), and somewhat lower for high-Z coax (75 or 300 ohm hardline).
As for low "capacitance coils", I ran into some posts that told about the dielectric constant (DC) of the coil form, and wire insulation, etc. That caused me to look into some low DC materials. And I remember old high-Q / low-capacitance coil winding tricks from long ago, and the coil forms like basket weave, etc. Anyway, here's what I found about the low DC materials.
Air....... The best dielectric - which is 1.00.
Plastics........
Polypropylene: 1.25 - 1.8. [ Widely available in the US, at Walmart, etc ]
LDPE: 2.3 [ have not found any yet ]
NOTE: Polypropylene is identified in the US, by the triangle recycle symbol, with a number "5". LDPE should have a number "4" in the triangle. Neither of these are the BPA containing plastics, which are being blamed for health problems (those are 3, 6, and 7).
Teflon.... -> 2.0
Aerogel.... probably slightly above 1.00..?
It is almost entirely made of air, which should have a very low DC.
I'm not aware of any easily available for low prices.
The Polypropylene is used in most of the hazy translucent Rubbermaid containers, and other storage containers at Walmart, etc. Look for the "5", or "PP". They would be far better to use than compressed particle board for low DC coil forms.
Kevco
I will keep that in mind, as I build this later. I'm just in the planning stages now, while waiting for parts. I hadn't noticed that post - but I did look up the capacitance per unit length of various coax, and it is quit high ( about 25pf / foot ), and somewhat lower for high-Z coax (75 or 300 ohm hardline).
As for low "capacitance coils", I ran into some posts that told about the dielectric constant (DC) of the coil form, and wire insulation, etc. That caused me to look into some low DC materials. And I remember old high-Q / low-capacitance coil winding tricks from long ago, and the coil forms like basket weave, etc. Anyway, here's what I found about the low DC materials.
Air....... The best dielectric - which is 1.00.
Plastics........
Polypropylene: 1.25 - 1.8. [ Widely available in the US, at Walmart, etc ]
LDPE: 2.3 [ have not found any yet ]
NOTE: Polypropylene is identified in the US, by the triangle recycle symbol, with a number "5". LDPE should have a number "4" in the triangle. Neither of these are the BPA containing plastics, which are being blamed for health problems (those are 3, 6, and 7).
Teflon.... -> 2.0
Aerogel.... probably slightly above 1.00..?
It is almost entirely made of air, which should have a very low DC.
I'm not aware of any easily available for low prices.
The Polypropylene is used in most of the hazy translucent Rubbermaid containers, and other storage containers at Walmart, etc. Look for the "5", or "PP". They would be far better to use than compressed particle board for low DC coil forms.
Kevco
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