I've been following the thread here with interest, combining RF amplifier
pallets for 12 kW total. And I would agree that rigid line is the safest
design, and is what I would use if it were my project to decide.
If a flexible solution must be found, the options are few if any, to be
reliable and withstand any sort of reflected power. There are some ancient
coaxial varieties such as RG218, 220,
that can handle tremendous voltage and current, but are not readily
'connectorized', typically used for medium wave and lower, and HV applications.
Heliax and Heliflex/Cellflex can do it,
with EIA flanges or DIN 7/16 connectors on the input side, but they are not
very flexible when going to 7/8 inch and large sizes. I am not very familiar
with the LMR cables to comment.
> Steve Thompson wrote:
> I thought 35ohm was max power rating, 75ohm was minimum loss?
> Hence 75ohms being the standard for CATV.
Yes, if one takes the attenuation calculation for coaxial line,
and differentiates it with respect to the ratio of diameters, setting the
result to zero,
the optimal geometry is a conductor diameter ratio of 3.592, which is 76.6 Ohms
in air spaced line. It is a broad minimum
so the ratio can be from 3.2 to 4.1 with only 0.5% variation.
For highest power rating, the calculations result in a ratio of 1.649, or 30
Ohms. This is for optimal voltage breakdown withstanding.
For the least power dissipation along the inner conductor, with lowest
temperature rise, optimum is 60 Ohms.
Therefore, 50 Ohm transmission line is a good compromise for high power
applications.
Ron, KA4INM wrote:
> Actually, with everything being the same the lowest loss is from the
> highest impedance possible
> for the form factor, the highest power handling capacity is from the lowest
> impedance.
See above, not lowest, but around 30 ohms char. Z.
> 75 Ohms coax has lowest loss because for a given power rating because it has
> the highest Voltage and
> the lowest current which fits into the I*R = power loss formula.
> With 250 to 1,000 Ohms surge impedance the form factor "open wire feed
> line" (and shielded open
> wire feed line called "waveguide") posses the lowest loss and it really makes
> a difference when
> there are reflections, as in when you are operating off of resonance, such as
> the other end of the band.
This does ignore radiation loss.
I prefer closed coaxial line for QRO++ high power as it can always have
pressurization added to increase voltage or air blown through to assist in
cooling. Also I can put my hands on it while it is operating,
not recommended with open wire line.
73
John
K5PRO
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