Multiple runs of Coax in conduit

[andy101202]

Hi Guys,

Have small a query about this set up I've some large conduit was thinking of running 10 feeds through and onto a plate with so239s.
Idea is one plate on the wall with say 10 antennas covering hf mostly and some vhf with some for rx only.
These will feed to an enclosure on the wall outside where I can connect different antennas as required or disconnect for lightning protection etc.

Will crosstalk happen with the coax I haven't decided on a brand yet. And will the plate be an issue with combining everything on a shared earth etc.

Thanks

 

[Lauri-CoyoteFrostbite]

In theory Andy, it should work fine. I have seen many installations where antennas feeds pass through a common bulkhead block like you describe without anything messing up each other. That said, I have also seen this arrangement induce some strange cross over effects... and 10 feeds thru a common ground point..... others maybe able to advise you better-- but my suggestion is to try this- one cable at a time and see what the addition of each new attachment has.... don't be wed'd to the system... you may need to have separate cable runs.

When I read your question it brought back a memory of one installation we did with a similar set up. It involved UHF antenna runs thru a common block like yours. Try as we would, whether it was an induced common mode or just some fluke, (faulty component ?)- we simply could not separate them. In the end the feeds were dealt with individually.

emptor caveat !

Good luck !

Lauri

 

[GlobalNorth]

I concur with the above. Proceed slowly and test, but as with LED lighting in a house, make certain.

 

[prcguy]

I've run a least a hundred coax cables in conduit and the cross talk potential depends on cable type, shielding, distance they are together and frequency range. Some single shield cables might have leakage through the shield rated around 45dB down at some moderate frequencies like VHF. That will get worse as you go higher in frequency and two cables run close together side by side would have around double their shielding spec, so for two cables rated at 45dB at some frequency, you would expect a signal on one cable to show up in the other about 90dB down.

A bunch of cheap poorly shielded cables carrying HF signals that are only side by side over a few feet will probably never be a problem but above 1GHz you might notice some pickup between cables. Make the run a few hundred feet long with cables tightly bundled and now leakage at HF could creep in.

The problems I've encountered with coupling between cables were at higher frequencies than most amateurs would use like the 950-2150MHz range used for satellite cables from the LNBF into a building. Using long runs of single shield RG-6 will have enough leakage to see on a spectrum analyzer. Do the same thing at HF frequencies and it might work just fine. This is why double and quad shield cables are made and mostly for the satellite industry.

If you consider a number of antennas all on the same roof or property, there will probably be much more pickup between the HF and VHF antennas than through leakage in the individual coax.

 

[iMONITOR]

pcrguy, this would occur when only using receivers, no transmitters?

 

[ladn]

All of the above are great answers with which I concur.

Other factors I'd take into consideration that weren't mentioned are transmitter power applied to the cables along with good antenna matching. With that many antennas so close together, you may have to add some filterion/isolation.

I'd be more inclined to use high quality "N" connectors rather than SP-239 UHF connectors and space them liberally on the termination wall plate(s).

 

[prcguy]

pcrguy, this would occur when only using receivers, no transmitters?

Coupling between cables is the same on transmit and receive. If you stick 1kW (60dBm) into some coax with 45dB shielding at your operating frequency with the right length of cables bundled together to reach that 45dB number then I would expect a signal about 90dB down or about -30dBm to show up in the other cable. -30dBm would be a huge signal for a receiver and about 43dB over S9.

If you consider many useable receive signals are in the -120 to -20dBm range, with 90dB of coupling between two cables that would put these signals in the -210 to -110dBm range. Your receiver might be able to discern a signal down to -125dBm or so meaning only the strongest signals above -35dBm would be receivable in this case.

I don't have specs for LMR400 at the moment but I believe the leakage is way better than 45dB at any frequency below 1GHz, so to me its a non issue unless your using old Radio Shack RG-8 where you can see right through the cheap shielding.

 

[mmckenna]

I have a site where we did this with a bunch of Iridium modems for a research project. I think at its peak, there were 20 modems running at the same time.
All coax was 1/2" Heliax, so 100% shield. All the coax came through a "dog house" type entry on the roof. It all terminated on a large grounding plate on the wall with lightning protectors.

No issues, but Iridium modems are fairly low power.

However, you'll see the same setup at most legacy cell sites. 9 or more runs of large Heliax coming through the ports into the hut. That's more power, higher frequencies, and it all works fine.

 

[andy101202]

Thank you guys for the answers it's not something that's written about often.

Yes wont be stuck with the one system but it would be a tidy set up and I think 10 antennas is more than enough sure I can only talk one at a time lol!

Antennas will be spaced over an acre between 2 towers and poles.

But if I do go for this set up I'll have to stick with it which is why I'm trying to figure it all out now.
With the right coax I think it will be fine all my freqs will be under 1 ghz.
I'll do the one at a time test and if the second radio opens I'll go from there etc.
Thanks again guys