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Shielding RF Cables - RF Cafe Forums

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Amateur Radio | Antennas | Circuits & Components | Systems | Test & Measurement


antenna_ant
 Post subject: Shielding RF cables
Posted: Tue Jun 21, 2005 3:53 pm 
Hi guys,

I need some suggestions on shielding RF cables that are running from the amp to the transmitter.
The signnal being transmitted is a 2.85mhz modulated by a 2khz carrier wave.
There are 8 transmitters, al the 8 cables run together in a bunch. I know its bad. But i wasnt here when they did it.
Whats the best way? run them seperately in a copper/aluminum conduit? or just run them seperately away from each other?
We are looking for a cost effective solution.
Also, what is ground shielding?
Thanks in advance.


 
  
 
Kirt Blattenberger
 Post subject:
Posted: Fri Jun 24, 2005 10:47 am 
 
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Joined: Sun Aug 03, 2003 2:02 pm
Posts: 308
Location: Erie, PA
Greetings antenna_ant:

Transmit/receive cables are run in bundles all the time without a problem. If you are having problems, the it is probably due to poor shielding or poor grounding or both. There could be a breach in one or more of the cables that is(are) allowing power to leak out.

You didn't mention the power levels through the cables. Are you experiencing cross-talk between the cables? Is the problem that TX power is leaking into the RX cables, or between TX cables? How long is the cable run? What kind of cable is being used?

Some easy checks would be:

* Inspect all the connectors in the system for damage, filth, or not being seated properly. Bad connections can be a major source of problems.

* Visually inspect the cable for damage or a bend radius smaller than the manufacturer's recommend. Reflections in a cable cane cause all kinds of wierd phenomena.

* If physical properties are OK, then do as you mentioned and try separating the cables by some distance. Maybe only one cable is causing problems.

* Anyone else have other suggestion?


- Kirt Blattenberger :smt024


 
   
 
Graham
 Post subject:
Posted: Sun Sep 04, 2005 8:33 pm 
 
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Joined: Fri Sep 02, 2005 7:25 pm
Posts: 34
Location: Hampshire UK
The first thing is to ask how long the cables are. There are some lengths just unfortunate for the frequency in use. Anyways - work through this.

When you say "from the amp to the transmitter", is that from the RF power amplifier to the transmitting radiators (antennas)? Do they come from a power splitter, or are there 8 "amps"?

Realise that the "screen" or "shield" braid on a coaxial cable is misnamed. It is actually one half of an unbalanced transmission line structure. If the cable sees anything other than a resistive load equal to its characteristic impedance, you will have a mismatch at the end of the cable, and standing waves in it. If this happens, then antenna currents will run on the OUTSIDE of the braid. They will radiate freely, and couple strongly to the rest of the cables in the bunch. The radiation pattern is likely to be wierd.

2.85 MHz has a wavelength of 105.26 metres. The phase velocity of the wave in most cable is about 66% x velocity of light. In low-loss foam dielectric cable, it gets faster, and in spiral supported conductor types, it can get to more than 90%. Whatever, it lets you know cable lengths that can be troublesome if you have a significant VSWR, these being quarter and half wavelengths.

The way out of this is to power them each, and check the VSWR. Seek to match the loads on the cables to get it to 1:1. By now your troubles have gone away. You can allow the cables to operate with some standing wave in them by preventing RF currents from flowing on the outside braid, by placing high permeability ferrite toroids along them at intervals. If the cables are thin enough, you can coil them up with say 8 to 12 turns about the size of a dinner plate, taped up, to make an in-line inductance that will stop the outer braid current.

Finally, the approach to the antennas is critical. The one place where there IS supposed to be a standing wave is on the antenna conductors themselves, and unless they are fed properly, such that their near field is not using any part of the arriving cable outers as their counterpoise, they will make troublesome currents on the cables. With a wavelength as long as 105 metres, I would be surprised if you could avoid it unless you took care to keep the radiating bits to themselves.

I know am guessing a lot, because your scenario sounds unusual. I cannot think what it is for? Yes you can use triax with the outer grounded, or put them in conduits, and try not to turn the conduits into antenns, or you can bury them, etc. You don't have to do any of that if you get them matched to a VSWR of less than 1.2 to 1. If you are running kilowatts, then you need better than 1.05 to 1.




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