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The malfunctioning STL antenna

Right after Tropical Storm Irene, it was noted that the STL signal strength at the WHUD transmitter site was low. Normally it was 300+ µV, now reading around 100 µV, which is a problem. Upon further investigation, it was revealed that the STL transmitter on the intermediate hop had higher than normal reflected power.

Time to call the tower crew.

The STL transmit antenna for WHUD’s STL (WPOU464) hop is a Scala Paraflector (PR-950), mounted at the 280 foot level on this tower:

Scala PR-950 on a guyed tower

Scala PR-950 on a guyed tower

The fact that it happened after a major storm and the transmitter was showing higher than normal reflected power indicates a problem with either the antenna or the jumper between the 7/8″ Cablewave coax and the N connector on the antenna.  A measurement with a spectrum analyzer shows very high return loss:

WHUD STL antenna return loss

WHUD STL antenna return loss

This shows distance to fault 413 feet, with a return loss of -7.4 dB.  That distance is either near or at the antenna and -7.4 dB indicates a lot of reflected power.  We had the tower climber take apart the jumper connections and terminate the jumper with a known good 50 ohm load.  The return loss did not change.  We then had him swap out jumpers and reconnect to the antenna.  That did the trick:

WHUD STL antenna with new jumper

WHUD STL antenna with new jumper

Much better, most of the power is now being radiated by the antenna, the VSWR is 1.02:1.  The impedance bump at 51 feet is a sharp bend in the coax where it is attached to an ice bridge.  Reconnecting the transmission line to the transmitter and turning it on confirms that all is normal again.  The problem with the jumper was found in one of the connectors, it was full of water.

Water contaminated Andrew flexwell connector

Water contaminated Andrew flexwell connector

I cut away the boot, water had entered the connector from the back because waterproofing and tape was not applied all the way to the coax.  This was installed in 1998 when the station moved from Peekskill to their current location in the town of Fishkill.  The fact that it happened now in the nice weather when Mt. Beacon is still accessible and not in the middle of winter means the radio gods are smiling on us.

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4 comments to The malfunctioning STL antenna

  • Zane

    I’m curious as to how visible the water was in the connector?
    Dripping wet or just not dry? I would think a really wet connector would be much worse than -7.4 dB RL.

    When you mentioned a “sharp bend” in the coax, I expected the root cause to be an aggressive bend that exceeded the minimum bend radius of the cable. The use of foam dielectric cables allows for low loss, but a kink in a jumper can be a pretty big impedance mismatch.

  • Paul Thurst

    After I cut the boot off, it was moist, I could see some water and it was moist inside where the center conductor mates to the connector. Nothing was dripping wet, or as you note, the return loss would have been higher.

    Often times at 950 MHz, water and moisture can look resistive and not show higher SWR but still attenuate the signal before it gets to the antenna. In those cases, it can be harder to find the problem.

    The sharp bend is a result of an over zealous tower crew attaching the cable to an ice bridge. It doesn’t seem to effect the signal too much.

  • Zane

    Thanks Paul – I enjoy your blog.

  • Neil Schubert

    Old thread, but sounds like what I used to do at the Cable Company all the time. Always wrench tighten those, use silicone based dilectric grease in the connector, and on towers, it never hurts to use silicone sealant on the threads before tightening. Always treat a coax like plumbing.

    Usually, the loss is caused not even by the water, but by corrosion on the center conductor that acts like a diode, resistor and sometimes a capacitor…

    When I worked for a cable company in the Milwaukee area, I had to fix house coax wiring in Greenfield WI. I checked signal at the tap with a meter and check for leakage, which was non-existant, and was amazed at the very high quality of the connection. It was an early high quality crimp type connector with a boot on it, and was indeed the original! (they used RG6 “quad” which was double braid double foil) The customer had said that they had cable since 1984 and never had any problems until they moved their TV.

    The key to its remaining corrosion free was the use of a clear or corn starch colored dielectric grease that the connector was dipped in prior to connecting to the tap. I would say that this grease is similar to what the phone company always put in old bell telephone transformers, it was very thick and did not wash off with water or anything else.

    Nevertheless, that connection was as good as new 20 years later, and I had suggested that the cable company go back to using that stuff. Of course, I found out that it was cheaper in 1984, and is now like $50 a tube.

    Anyway, after seeing this, I had to laugh as I had to deal with that exact problem for 2 years. Even if the coax is starting to leak at the seal to the fitting/connector, the dielectric grease will keep water out for a very long time, and will help keep water from going down the coax itself.

    Anyway, I used to see this often on KU band dishes for DMX music. Only having 18 volts for the LNB made the water eat right through the center conductor of the coax.

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