Topband: Inverted-L Summary

[Tom Rauch]

<<Seal the end(s) of any coaxial capacitors used and tie and coil them
up.  Do not just lay them out on the ground.  (I have NO idea how to
"estimate" the capacitance of a variable capacitor!!  Guess I will cut and
try here.)>>>

Of course I'm not saying it won't work, but everyone should remember a chunk
of coaxial cable makes a pretty low-Q capacitor and also has higher voltage
across the open end than the voltage at the connection end.

A 33ft piece of RG8 used as a capacitor looks like a series 1.3-ohm resistor
and -j75. The Q of this "capacitor" is only about 58, while a regular air
variable would have Q in the 1000's.

The working capacitance is NOT the capacitance per foot of the cable unless
the cable is only a few degrees long. In this example 10 meters of RG-8
cable is the equivalent of 1100pF on 160 meters, but using the
capacitance-per-foot might make us think it is only 970pF.

In the case of a 33ft line on 160 meters, the open-end voltage is about 25%
more than the connection-end voltage so that is where you have to be most
careful with insulation.

At the same time it does all the above, it also reduces SWR bandwidth
because the equivalent capacitance value changes with frequency!

The loss is often pretty low (in this case it would be about 40 watts power
loss with a 50+j75 load an 1500 watts). Most of the heat will be at the
connection end, and would be spread over a fairly large area. Voltage is
probably the larger issue.

Coaxial cables work, but they generally are poor capacitors compared to
other choices. They are really open transmission lines operating with
near-infinite SWR, not capacitors.

This is why some commercial amateur antennas that use coaxial cables as
capacitors have failure problems in the coax. I'm sure some people have
experienced this effect. They should never be used in high current or high
voltage applications, but probably are OK with an inverted L.

73 Tom