Survivalism & Self Sufficiency Topics > Amateur Radio How-To's

best practices for making coax patch cables?

<< < (2/4) > >>

TexasGirl:
Hey Carl,

Well....  You certainly have me beat on rambling.   Can't relate on the med subject though, I have a no-med philosophy.

I wasn't trying to get too deep into RF and hardware philosophy, or make these new ham's heads spin, but I will attempt to answer your questions.

Basically, vinyl jacket and foam are great for flexibility but not weather durability.  Vinyl microscopically pinholes over time (especially with UV exposure) allowing moisture to infiltrate the braid and dielectric.  There's a reason outside plant telephone cables are made with polyethylene (PE, HDPE,) and polypropylene, not vinyl.  So, it would have been better for me to have said use polyethylene jacket and dielectric outside.  Technically, a vacuum is the best dielectric, followed by air, foam, teflon, then PE.  Each cable is a compromise of properties needed for a particular purpose.  Foam dielectric works in hardline because the shield is basically solid copper tubing. 

Coax breaks down electronically in two ways.  As moisture begins to corrode a braided shield (or in a connector) it's capacitive properties change and become reactive.  Each place two corroded conductors cross becomes a reasonant point (how many points are in braid shield?)  This will cause an imbalance in SWR or simply a place to facilitate intermodulation.  Moisture in the dielectric changes the velocity of propagation, effecting the efficiency, and can even allow a hi impedance short between center conductor and shield.  Much like static on a phone line after a rain on bad spot of cable.

30 ohm coax inherently handles more power, 77 ohm is the lowest loss.  That's why CATV systems use 75 ohm, they are running long distances.  The RF industry chose 50 ohm as a compromise between power and low loss. 

Antenna impedance changes with its environment, especially its relation to ground.  Take that halfwave dipole with a free space impedance of 75 ohms, as it approaches the ground it's impedance will drop.  The same 1/4 wave radiating element, vertically at ground, or with radials making a ground plane, will be close to 50 ohms.  The two-way industry revolves (or evolved) around that magic 50 ohm match.

Remember, braided shield on cables leaks, it is not solid, but is generally made to a 91% or 88% standard, although some cheap cables can be as poor as 60% shield.  Sure, it appears more or less solid to many RF frequencies, much like a faraday screen cage, but it still leaks.  For permanent cables, I like dual braid where flexibility is needed, and "quad shield" (which is a foil under braid construction) where flexing will not break down the foil over time.  Why worry about braid leak?  One of the most overlooked hazards is leaky RF radiation.  It's good to check your ham shack periodically with a "S" meter to see how much RF is inside the shack when gear is keyed at various frequencies.

~TG

Smurf Hunter:
Lots more brain dumping for me to digest...

Some more tactical questions.

What is the best balance of performance and economy for the following uses:

1) cable running outside my house up to a roof mounted antenna.  I live in the Seattle area, and we get rain, some wind and occasional sub-zero temps in the winter
2) cable inside the HAM shack used as short length patch cables
3) cable buried underground
4) cable run inside walls of a house

Is it reasonable to buy a single big spool of coax that meets all these needs well?

TexasGirl:

--- Quote from: Smurf Hunter on May 22, 2014, 09:59:27 AM ---Lots more brain dumping for me to digest...

Some more tactical questions.

What is the best balance of performance and economy for the following uses:

1) cable running outside my house up to a roof mounted antenna.  I live in the Seattle area, and we get rain, some wind and occasional sub-zero temps in the winter
2) cable inside the HAM shack used as short length patch cables
3) cable buried underground
4) cable run inside walls of a house

Is it reasonable to buy a single big spool of coax that meets all these needs well?

--- End quote ---

LMR 400 UHF, and LMR 600 UHF, are kind of the industry standard for outdoor use.  If you are planning to be there awhile, and you can afford quality, Andrew is the Mercedes of cables.  I'd consider getting a roll of either their Cinta CNT-400 or CNT-600.  400 flexes better, 600 is better in rigid environments.  For patch cables in your shack, any double shield cable will work fine.

~TG

Carl:
I have to agree with TG on LMR,just a bit out of my budget...But do it RIGHT and SEAL the connections and you will not have to do it again. Mini 8 is a budget helper,if the COO agrees with the cost ,go for it . Though it is good ,sub zero will cause degradation in mini 8..I just do not know how much as even 25 degrees is rarely seen here in Louisiana.

Smurf Hunter:
My area probably has about 6 weeks where it drops below freezing at night, but thaws during daylight hours.  Nothing near as cold as northern midwest states.  It's quite rare that the temp remains below 32F for more than a few days.  (this is actually rather annoying as each morning there's a fresh sheet of ice on the steep roads).

If I were to permanently mount an antenna on my 2nd story roof line, and fish the cable down the side of the house, and into the HAM shack, etc. it might be worth the investment of premium cables.  Climbing ladders is not my favorite past time and I just assume do it right once :)

That said, as I'm experimenting with DIY antenna designs at a picnic table, I certainly don't need the best cables.

Thanks for all the knowledge transfer folks!

Navigation

[0] Message Index

[#] Next page

[*] Previous page

Go to full version