What makes Heliax good?

[W7TFO]

The earliest coaxial lines made by Western Electric were 66-2/3 Ohm impedance.

The center conductor was supported by porcelain spheres, which were located about a foot apart, and held in place by a small crimp on both sides of the sphere.

The 90 degree elbows and terminations were soldered on, and had little glass inspection ports to make sure it was OK.

Nitrogen or dry air ruled as well.

Typical Bell Laboratories overkill, in the pre-standard days.

I pulled a lot of it when redoing an AM directional array a few years back.

73DG

[W0BTU]

Why would the lowest loss not also equate to maximum power handling capability? After all, lowest loss = lowest heating of the coax.

The power rating of coaxial line is related to the amount of r-f current that it can carry without exceeding the permissible temperature rise along that transmission line including the effects of load SWR, and the environment (local ambient air temperature, solar heating, and the thermal characteristics of the insulating media within the line, itself).

For example, 75-ohm coax requires less r-f current, and hence has lower loss to convey the same amount of power for a given length and frequency, but is not capable of carrying the same maximum power as its 50-ohm version.

Andrew LDF4-75A, 1/2" OD, 75-ohm Heliax is rated for 0.273 dB loss per 100 feet and 3.8 kW maximum average power at 20 MHz into a load SWR of 1:1.

Andrew LDF4-50A, 1/2" OD, 50-ohm Heliax is rated for 0.3 dB loss per 100 feet and 7.75 kW maximum average power for the same conditions.

Amazing. What is the reason for this, the thermal conductivity of the dielectric? I wonder what the difference in power handling capability is for gas insulated lines of similar dimensions?

[R. Fry SWL]

I wonder what the difference in power handling capability is for gas insulated lines of similar [1/2" OD] dimensions?

Below from Andrew Catalog 36 are those ratings for standard conditions:

   * Andrew HJ4-50, 1/2" OD, gas-insulated 50 Ω Heliax is rated for 0.368 dB loss per 100 feet and 5.36 kW maximum average power at 20 MHz.

   * Andrew LDF4-50A, 1/2" OD, foam-insulated 50 Ω Heliax is rated for 0.300 dB loss per 100 feet and 7.75 kW maximum average power at 20 MHz.
//

[W0BTU]

Thanks, Rich. This seems to me to indicate that the foam plastic dielectric is carrying heat away from the center conductor. What do you (or does anyone else) think about that theory?

[R. Fry SWL]

This seems to me to indicate that the foam plastic dielectric is carrying heat away from the center conductor. What do you (or does anyone else) think about that theory?

Below is an extract of four pages from the Andrew catalog, including their comments on the subject.

R. Fry

[W0BTU]

That was really interesting. The higher power rating does seem to be due to better heat transfer away from the center conductor through the foam dielectric.

"Average power ratings for semiflexible cables are determined by the maximum permissible inner conductor temperature. This maximum temperature depends on the type of dielectric... "

[KM1H]

What makes Heliax good?

Its cheaper than all the RG and LMR stuff since its being sold at or below scrap prices as the 2way biz drys up and towers come down.

Carl

[K1JJ]

Finally got some time to take another look at this question.  I'm curious about something...


I would like to see the difference in loss between say, 100' of RG-59U at 100 mhz and openwire line using the SAME diameter wire as used in the RG-59/U inner conductor. (.032")


100' of RG-59/U shows about 3.39 db loss at 100 mhz. (1:1 swr)

Using this feedline calculator:
http://vk1od.net/calc/tl/tllc.php


100' of .032" diameter openwire spaced at 1" apart has an impedance of about 412 ohms. However, I didn't find an openwire calculator on the web that shows loss.  Does anyone have a way to calculate the loss for 100' at 100mhz using only air as the dielectric? (and .032" wire)


Actually, one of the the RG-59/U's conductors is fatter because it is shield, so should show a better IR loss.  The openwire uses .032 wires for both leads.

If dielectric has little effect on the loss at 100 mhz, then the loss between the two feedlines should be close, no?  If not, then maybe we are dealing with something else besides dielectric, like distributed capacitance causing circulating "eddy" currents?  The coax obviously has more capacitance between leads per foot than the openwire.

Watsa anyone?

T