[SixClub] Recommended Coax?

[Mike Pappas]

There was a big debate in broadcasting about using nitrogen in transmission lines many years back.  The power levels involved are obviously much higher than those found in Ham radio but for those of you interested in the minutia of what happens when you have an arc over in a pressurized feed line filled with nitrogen here you go.

Courtesy Radio Magazine:

Jampro Investigates Nitrogen vs. Dry Air Debate
Sacramento, CA - Jan 21, 2004 - The use of desiccated air vs. the use of nitrogen to pressurize transmission lines is seldom given much attention beyond the ongoing costs of the desiccating equipment or the nitrogen bottle rental. But following some research by Dupont at the urging of Technical Broadcast Associates, Jampro has released information that endorses the use of dry air for chemical reasons.

Pressurized transmission lines use Teflon insulators to maintain the spacing between the outer and inner conductors. When Teflon is heated to 500° C, the carbon and fluorine atoms, normally bonded into CF2 molecules, will become a double-bonded CF2 molecule. This double bonded CF2 becomes CF3(a gas) and C (carbon, which is visible as soot).

The chemical result of CF2 + CF2 when heated with nitrogen yields HF + CF2O (carbonate fluoride) + C (carbon). Carbon inside the antenna and coax is not a favorable condition.

However, when CF2 + CF2 and dry air are heated, the result is HF + CF2O + CO2, a less damaging gas.

While dry air contains some nitrogen, the additional gases, notably oxygen, modify the chemical reaction.

High heat in a coaxial cable can be caused by an electrical arc or a lightning strike. During the arc, the heated air expands quickly and may open the pop-off valve, which allows more air to move through the system. When dry air is used, more oxygen is provided to bond with the CF2, creating more CO2. Once the oxygen is depleted, the resulting byproduct is carbon.

Depending on the duration of the flash-over or arc some carbon will still form in the presence of dry air, but not nearly as much as in the presence of nitrogen. Some carbon will likely result whether nitrogen or dry air is used, but using dry air should significantly reduce the amount of carbon during the reaction.

Jampro recommends that stations likely to experience lightning or that use transmitters capable of sustaining an arc for more than few seconds should only use dry air, not nitrogen. Nitrogen can be used to test pressurization or to purge a line, but the nitrogen should also be purged when the coaxial cable is put into regular service.



We migrated to dry air at all of our transmitter sites back in 2000 after a feed line arc over/fire caused by a nearby lightning strike.  We were running nitrogen at the time and it was an expensive mess to clean up and fix. 

Mike Pappas
advanceddigital at mac.com



On Jun 16, 2010, at 4:50 PM, Howard Bingham wrote:

> ATT uses Nitrogen to purge air from wire cables (That's becoming 
> rarer these days though with use of fiber optic phone lines.).
> 
> My apartment complex is on OLD wires & every so often when there is a 
> lot of static in the phone, a call to ATT (name your local phone 
> provider here) will get the issue fixed, that's the purpose of 
> nitrogen cylinders one occasionally see's on phone poles the end og a 
> street with copper wire lines.
> 
> After Hurricane IKE crippled much of the Houston/Galveston area, the 
> familiar dial tone was still heard on most hard-wire phone lines, as 
> cell users ran around 2+ weeks later with dead batteries (In cases 
> where they didn't have a cigarette lighter adapter to charge their 
> phones & the local cell towers were fortunate enough to have a 
> working generator).
> 
> Howard Bingham
> KE5APJ