HFU HF Underground
Technical Topics => Equipment => Topic started by: ChrisSmolinski on April 03, 2023, 1915 UTC
-
This past winter, my 58 ft T2FD (which is actually nearly vertical and not tilted, but that's another story) developed very high noise/RFI levels. Around S9 above the MW band in the 1700 kHz area. Not exactly optimal for DX.
Mid winter, I lowered the antenna to check that no wires were cut/damaged, verify the Cyclops ( https://www.blackcatsystems.com/rf-products/cyclops_rf_ham_shortwave_radio_matching_transformer.html ) transformer was OK, etc. But it was too cold to do anything serious.
Now that we've got warmer weather, I had the opportunity to do some troubleshooting.
The coax cables for all my antenna run down to buried conduit runs that go to the house, with grounding blocks on each end of the conduit runs. I brought my AOR AR8200 outside, so I could progressively connect it to each section of the coax runs, starting at the antenna, to determine where I started to get the massive RFI levels. That turned out to be at the house. I put a networking terminator on one end of the coax and checked the other end... open circuit. So something's wrong with the coax. I still get signals, my guess is there's an open in the shield. I'll find out when I pull a new coax cable.
In the meantime I'm using a spare coax cable in the conduit, and all is well. Once again I can hear WEDG on 1620.
-
Now, if you had a TDR or VNA there is a very good chance you would see that fault on the instrument. It would tell you this right down to how many feet / inches away it is. My bet is on one of the connectors. I have seen water seep into connectors and corrode the wires away regardless of how good of a seal job you do. Ice is funny that way.
-
Do you doing anything to deal with condensation and water ingress in the buried conduit? Direct-bury coax tends to fare better in underground conduit, but regular coax could be susceptible to moisture ingress or even outright jacked rot if encased in water for months to years.
I have been (slowly) migrating to Belden PPC Perfect Flex RG-11 flooded direct-bury coax for outside coaxial runs to receiving antennas. The center conductor is CCS, but I will take mid-grade Belden over most of the typical generic branded offerings. An 1000' rool often goes for under $200 shipped on eBay.
-
This topic reminded me that after winter you need to check if there is water in the pipes?
Last spring I moved the power point of my Sky Loop antenna from the wall of my apartment to a tree 6m away from it. The box with the transformer is placed at a height of about 6m above the ground on a massive PVC pipe. The RG6 coaxial cable that feeds the antenna is routed to the house in a PVC pipe buried about 15 cm underground. In parallel, a second pipe is buried for future experiments.
After about 2 months, pulling the auxiliary wire through this pipe, I was surprised to find that it is partially filled with water, despite the plugging of the inlet and wrapping it with insulating tape.
The soil is dry and permeable (no standing water), I decided that the best solution would be to drill holes in the pipe to drain the water that collects there. Every 1m or so I dug the pipes up to their top surface and drilled holes through with a 4mm drill bit. I sealed the top hole with a solid adhesive repair tape (aluminum would be best) and buried it back.
In general, after this modification of the distance from the building, I did not find a decrease in RFI and noise.
-
Humidity and condensation from temperature gradients. Often happens even if the conduit is well sealed. Drilling drain holes is the typical fix, as it at least gives the water somewhere else to go.... eventually. Adding a rock layer below the drain holes can help if needed. Note the drain holes will work in reverse if there is standing water, so direct bury coax might still be advisable in poor drainage situations. YMMV.
The situation is similar to why cable pressurization systems are available for air-dielectric coax and waveguides.
Conduit is good for protection and replacing failed drops, but otherwise decent direct-bury flooded coax really does not need it, at least from a water intrusion standpoint.
-
Now I realized the importance of gelled coaxial cables. Particularly in the case of buried cables,
the gel sheath under the sheath provides an osmotic barrier to the penetration of moisture into the cable.
Gel cables are recommended for outdoor installations, but I didn't appreciate it.
When I cut the insulation of a classic cable that had been working outdoors for several years, instead of aluminum foil, white powder of aluminum oxides (AL2O3) spilled out.
The copper mesh of the braid survived
-
That is a big part of why I have been migrating to flooded direct-bury RG-11 for outside drops.
1000' of Belden PPC RG-11 is about $200 shipped. 1000' of Belden PPC RG-6 is about $100 shipped.
https://www.ppc-online.com/products/coaxial/perfectflex-p11et77efrf
https://www.ppc-online.com/products/coaxial/perfectflex-p6et77efrf
They have copper-covered steel (CCS) center conductors, but that is fine for our receiving purposes IMO.
-
When did you switch the SDR that used to be fed by the 58 ft. T2FD to the 400 ft. South beverage, Chris? I first noticed it a couple of weeks back. Are you dx'ing Argentine taxi cab dispatchers?
-
When did you switch the SDR that used to be fed by the 58 ft. T2FD to the 400 ft. South beverage, Chris? I first noticed it a couple of weeks back. Are you dx'ing Argentine taxi cab dispatchers?
About a week ago, the 58 ft T2FD is misbehaving again, so I switched over to another antenna. The south beverage should actually be handy for picking up the various Latin American pirates, in addition to the Buenos Aires taxi cab dispatchers. As a bonus, it does a good job with local pirate 1620 WEDG.
I've got more antennas than KiwiSDRs, so this works until I can sort out the T2FD. I might keep it on this antenna for a while anyway, and see what folks think.
-
Around here they (isp/cable systems) use more or less two cables, one is orange and about the size of rg11, the other is black with green stripe about an inch or so thick and has a support wire built in. The neat thing is they're both direct bury, and the fat line with the green stripe also has something added to the jacket that discourages animals from tasting it. Get the fat cable with the green stripe if you can. I asked a isp service tech what he'd charge for 75ft of orange stuff and he gave it to me, along with the connectors. The fat cable would be best for long runs and where high power is used. I'm only sending 100w down the line so orange will do.