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I'm putting up antennas on a tower, and have a couple of 7/8" cables style on hand. One is Hj5-50 air dielectric cable and the other AVA5-50 "Virtual Air" cable. I notice that the HJ5 air dielectric cables are slightly more lossy that the AVA5 cable, but not enough to make a difference. I'd like to use the HJ5 cables since I have them on hand, but I'm wondering if there's something that makes them unsuitable for this application. I see that they can be pressurized, but that is not a requirement. So in what application would you use an air dielectric cable? Is there any real drawback to using the air dielectric on a standard installation and not pressurizing them?

So in what application would you use an air dielectric cable? Is there any real drawback to using the air dielectric on a standard installation and not pressurizing them?

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    $\begingroup$ Pressurising the coax eliminates moisture ingress and allows higher power, though for amateurs this probably won't matter. Without pressure, you'll still need a breather somewhere (indoors) to prevent moisture being sucked in. $\endgroup$
    – tomnexus
    Commented Jun 14, 2021 at 20:21

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The problem with air dielectric cables is moisture can get in them. Once water gets in the efficiency of the cable will decrease due to high dielectric loss from the water and increased resistive losses from corrosion. Water's dielectric constant is also very different from air, and so will change the characteristic impedance of the cable.

You might think if the cables are properly sealed it's no problem. However sealing is much harder than it would seem. As the temperature of the cable changes due to weather so does the pressure inside it. When the temperature decreases the cable will literally suck in air.

Consider a cable that's been baking in the sun, and then a thunderstom rolls in. The temperature decreases, and the cable sucks in humid air from the storm. Water vapor can change the impedance of the cable. That vapor can also condense into water, causing bigger and irreversible problems.

Some air dielectric cables may be specified for unpressurized use in indoor environments, but I am not aware of any that recommend unpressurized use outdoors. A couple excerpts from the Heliax installation manual make the point that your HJ5-50 should be pressurized:

[page 1] Cover all cut cable ends that are not immediately terminated to protect them from the weather and the entry of foreign matter. Seal and repressurize air-dielectric cable cable to prevent moisture from developing inside the cable. Use the cable end caps that were supplied with the cable shipment to cover the cable ends.

[page 7] Pressurization is needed in air-dielectric cables because changes in temperature can cause condensation of moisture from outside air that enters the cable. This moisture can seriously impair the efficiency of system operation. Connecting a pressure source of dry air or nitrogen (dehydrator) to the cable at slightly more than atmospheric pressure will correct this condition since the moisture will be removed and air will then leak from instead of into the cable.

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    $\begingroup$ Very good explanation! But, then what is the purpose or advantage of the air dielectric to begin with? The loss is slighlty greater that then the regular 7/8" cable. Also the regular cable can be used outside without pressurization. So when would you want to choose air dielectric over standard 7/8" cable? $\endgroup$
    – Frank
    Commented Jun 14, 2021 at 20:41
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    $\begingroup$ @Frank Good question. I would have thought air dielectric would have lower loss, but perhaps foam dielectrics have gotten so good that's no longer really true. You might consider leaving the question open, or asking a new question that's more focused on that particular point. $\endgroup$
    – Phil Frost - W8II
    Commented Jun 16, 2021 at 15:24
  • $\begingroup$ I wonder if air dielectric cable is used in some cases because it's less expensive? The foam dielectric costs something, and air is free (with a spacer disk at intervals). But then there's the cost of the tank and regulator. A TV station transmitter we visited years ago used dry nitrogen. $\endgroup$
    – Mike Waters
    Commented Jun 17, 2021 at 14:45

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