As much to do with frequency as power level. I've recently completed the design of a Ka band transceiver for satellite (30GHz uplink, 20GHz down). It is a domestic product for delivery of broadband to rural areas. All of the internal RF filtering is done in waveguide, as is the external combining of the transmit and receive signals into a single horn. The transmitter power is just 3W.
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How do you get that to the antenna without waveguide? Coax losses are much higher than waveguide, and is less likely to have problems since there is no dielectric to break down.
Andrew's Heliax is pretty low loss, and good for VHF and UHF runs. Dielectric amounts to nothing more than a thin spiral spacer - the rest is all air.
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Heliax for 10 KW? Ever had the filter fail on the compressor and get water in Heliax? I had a stupid SOB for a boss 30 years ago who was too sheap to replace filtes on schedule and ruined a piece of 3" Heliax used at 4 GHz. Waveguide is better at high power, and better than Heliax. I had over 1700 feet of it at one TV transmitter. It carried about 195 KW of RF to the top of the tower. We had to maintain a set pressure of dry nitrogen on the waveguide to keep from compressing the sync pulses.
No, not at 10kW. At 100W or so it is fine. On a long run I would always opt for waveguide - tower dimensions permitting.
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You can't use waveguide for digital TV transmission-something about the joints making unsuitable anomalies. Air dielectric co-ax is what's used.
Cites? You can't use existing waveguide on a new channel. You have to calculate the location of the nodes and they are related to the channel frequency. I can see usinig coax for the pre-transistion phase, but not after.
Have you ever designed a UHF broadcast plant?
A tower that will support a high power antenna will have have room for waveguide, if the tower isn't overloaded with other antennas. A tower that won't support a high power antenna definately won't have room for waveguide. :)
Designed and built close to 2 dozen.
Hardline! In essence it's a solid shield with a center conductor. That's enogh for RF into the 900MHz and 1.2GHz range.
what exciting things happens when you get moisture ingress?
how hot can coax or waveguides run at high powers? I've never been by large transmitters, so the concept of anything but a power cord running warm is just strange to me.
There are many types of hardline, and more in use by CATV systems than anything else. I engineered in that field for four years and designed extensions and upgrades. I also designed community loop systems.
Specify a brand, type length, frequency and power level. There is a reason they have so damn many trunk amplifiers in a system.
I found a link to some 5.5" heilical air dielectric that doesn't look too bad, but the specs stop before a GHz. look at the power handling spec at .5 MHz, and at 894 MHz. It drops from 1890 KW to 43 KW. The insetion loss goes from .0045 dB to .215 dB over that range.
Corrosion, which increases the attenuation. If it gets bad enough, the conductor will get a hot spot and burn through leading to spectacular fireworks. The mositure tends to collect in low spots, which are at regular intervals in helical cable. vertical runs will let it run down to the first horizontal section and pool.
It also changes the impedance of the air dielectric, which increases attenuation.
IR losses turn current into heat, the frequency doesn't matter. RF power costs a lot more than power from the utility company, and using a bigger transmitter to turn that RF into heat costs more for the hardware & air conditioning.
The reason we use 6" line for 20 KW at 500 MHz is the skin effect, which is quite frequency "dependent".
No, it's because of losses caused by the corrugated conductors. The conductors aren't that thick to start with that the depth of the skin effect cause all the attenuation. I've had to scrap thousands of dollars worth of internally oxidized Heliax.
Dielectric losses go up with frequency, as well. Compare the loss similarly constructed teflon & foam coax at 1.2 GHz. The silver plating helps reduce the resistance of the braid, but the main purpose is to prevent the copper from oxidizing due to trapped air and ingression of oxygen through the plastic jacket.
That's why the rigid 6.125" O.D. coax isn't corrugated. Neither can withstand a hard vacuum, so they need to be properly pressurized with dry gas and Nitrogen gas is not only cheap, but it won't oxidize the conductor surfaces inside the coax.
Look at the losses in braided VS foil 75 Ohm coax as frequency increases. The rough surface of the braid has higher losses because the path is longer and the resistance is slightly higher.
how does one terminate such cables? What sort of wiring goes on inside a giant transmitter or the antenna end of such a monster?
A matching network to get it as close to the characteristic impedance of the cable. The antenna is designed for the same impedance to get the maximum signal into the airwaves. 50 ohms is the nominal impedance for these systems
The transmitter end would have a fancy bandpass filter and the antenna end may or may not have a matching section. In high power RF 50 Ohms is standard for almost everything that needs interconnecting. Get too far away and stuff starts snapping. Zzzt!
Why is there less loss with bigger air dielectric line at UHF?
Longer leakage path. BTW, Fused ceramic disk 75 ohm hardline for CATV was in use in the early '80s. All else being equal, the air dielectric is a good choice as long as the RF voltage doesn't arc over. Standing waves from a mismatch can exceed the breakdown voltage. They have come a long way from the original dielectrics in Coax in over a century of development. Still, a dry gas pressurized system is the most stable. Even the telco industry injects dry nitrogen into cables to reduce crosstalk & losses to prevent moisture. They sometimes chain a tank & regulator to a pole as a temporary measure when they have a leaky cable to replace the escaping gas.
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