I have been tasked with installing one of these systems for a sixteen channel bi-directional STL. This system was first mentioned here: The 16 channel bi-directional STL system. As some of you pointed out, the unlicensed 5.8 GHz IP WLAN extension was the weak link in this system. It was not an interference issue, however, which was creating the problems. The problem was with layer two transparency in the TCP/IP stack. Something about those Cambium PTP-250s that the Wheatstone Blade hardware did not like and that created all sorts of noise issues in the audio. We installed the Wheatstone Edge Routers, which took care of the noise issue at the cost of latency. It was decided to go ahead and install a licensed link instead of the license free stuff as a permanent solution.
Thus, a Cambium PTP-820S point-to-point microwave system was purchased and licensed. The coordination and licensing took about three months to complete. We also had to make several changes to our network architecture to accommodate the new system. The PTP-820 series has a mast mounted radio head, which is the same as the PTP-250 gear. However, for the new system, we used three different ports on the radio to interface with our other equipment instead of the single port PTP-250 system. The first is the power port, which takes 48 VDC via a separate power cable instead of POE. Then there is the traffic port, which which uses Multi-Mode fiber. Finally, there is the management port, which is 1GB Ethernet and the only way to get into the web interface. The traffic port creates a completely transparent Ethernet bridge, thus eliminating all of the layer two problems previously encountered. We needed to install fiber tranceivers in the Cisco 2900 series switches and get those turned up by the IT wizards in the corporate IT department.
Andrew VHLP-2-11W 11 GHz microwave antenna
The radios mount directly to the back of the 24 inch 11 GHz Andrew antenna (VHLP2-11) with a UBR100 interface. The wave guide from the radios is a little bit deceptive looking, but I tried not to over think this too much. I was careful to use the O ring grease and conductive paste exactly where and when specified. In the end, it all seemed to be right.
Cambium PTP-820S mounted on Andrew antenna
Not wanting to waste time and money, I decided to do a back to back test in the conference room to make sure everything worked right and I had adequately familiarized myself with the ins and outs of the web interface on the Cambium PTP-820 radios. Once that was done, it was time to call the tower company.
Cambium PTP-820S on studio roof
One side of these are mounted on the studio building roof, which is a leased space. I posted RF warning signs around the antennas because the system ERP is 57.7 dBm, which translates to 590 watts at 11 GHz. I don’t want to fry anybody’s insides, that would be bad. The roof top installation involved pulling the MM fiber and power cable through a 1 1/4 inch EMT conduit to the roof. Some running back and forth, but not terrible work. I used the existing Ethernet cable for the management port. This will be left disconnected from the switch most of the time.
Cambium PTP-280S 11 GHz licensed microwave mounted on a skirted AM tower
The other side is mounted at about 85 feet AGL on a hot AM tower. I like the use of fiber here, even though the tower is skirted, the AM station runs 5,000 watts during the daytime. We made sure the power cables and Ethernet cables had lighting protectors at the top of the run near the dish and at the bottom of the tower as well as in the transmitter room rack. I know this tower gets struck by lightning often as it is the highest point around for miles.
PTP-820S RSL during aiming process
Aligning the two dishes was a degree of difficulty greater than the 5.8 GHz units. The path tolerances are very tight, so the dishes on each end needed to be adjusted in small increments until the best signal level was achieved. The tower crew was experienced with this and they started by panning the dish to the side until the first side lobe was found. This ensured that the dish was on the main lobe and we were not chasing our tails. In the end we achieved a -38 dBm RSL, the path predicted RSL was -36 dBm so close enough. This means the system has a 25 dB fade margin, which should be more than adequate. While were were aligning the transmitter site dish, a brief snow squall blew through causing a white out and the signal to drop by about 2 dB. It was kind of cool seeing this happen in real time, however, strangely enough, the tower crew was not impressed by this at all. Odd fellows, those are.
Currently brushing up on FCC part 101 rules, part C and H. It is always good to know the regulatory requirements of any system I am responsible for. As AOIP equipment becomes more main stream, I see many of these type installations happening for various clients.
In my spare time (lol!) I have been fooling around with one of those RTL 2832U dongles and a bit of software. For those that don’t know, the RTL 2832U is a COFDM demodulator chip designed to work with a USB dongle. When coupled with an R 820T tuner a broadband RF receiver is created There are many very inexpensive versions of these devices available on Amazon, eBay and other such places. The beauty of these things is that for around $12-30 and a bit of free software, one can have a very versatile 10 KHz to 1.7 GHz receiver. There are several good software packages for Windoze, Linux and OSX.
The one I recommend for beginners is called SDR-Sharp or SDR#. It has a very easy learning curve and there is lots of documentation available on line. There are also several worth while plugins for scanning, trunking, decoding, etc. At a minimum, the SDR software should have a spectrum analyzer, water fall display and ability to record audio and baseband PCM from the IF stage of the radio.
Some fun things to do; look at the output of my reverse registering smart (electric) meter (or my neighbor’s meter), ACARS data for the various aircraft flying overhead, a few trips through the EZPass toll lanes, some poking around on the VHF hi-band, etc. I also began to think of Broadcast Engineering applications and a surprising number of things came to mind:
- Using the scanner to look for open 950 MHz STL frequencies
- Inexpensive portable FM receiver with RDS output for radio stations
- Inexpensive Radio Direction Finder with a directional antenna
- Inexpensive Satellite Aiming tool
Using SDR sharp and a NooElec NESDR Mini+ dongle, I made several scans of the 945-952 STL band in a few of our markets. Using the scanner and frequency search plugin, the SDR software very quickly identified all of the in use frequencies. One can also look at the frequency span in the spectrum analyzer, but this takes a lot of processing power. The scanner plugin makes this easier and can be automated.
Analog and digital 950 MHz STL frequencies, Albany, NY
I also listened to the analog STLs in FM Wideband mode. Several stations are injecting their RDS data at the studio. There is one that appears to be -1500 Hz off frequency. I’ll let them know.
Next, I have found it beneficial just to keep the dongle and a small antenna in my laptop bag. Setting up a new RDS subcarrier; with the dongle and SDR# one can quickly and easily check for errors. Tracking down one of those nasty pirates; a laptop with a directional antenna will make quick work.
Something that I found interesting is the water fall display for the PPM encoded stations:
WPDH using RTL 2832U and SDR Sharp
Not only can you see the water marking on the main channel, you can also see the HD Radio carriers +/- 200 KHz from the carrier frequency. That is pretty much twice the bandwidth allotment for an FM station.
WDPA using RTL 2831U and SDR Sharp
Those two stations are simulcasting. WPDA is not using Nielson PPM nor HD Radio technology. There is all sorts of interesting information that can be gleaned from one of these units.
Aiming a satellite dish at AMC-8 can be a bit challenging. That part of the sky is pretty crowed, as it turns out. Dish pointer is a good general reference (www.dishpointer.com) and the Dish Align app for iOS works well. But for peaking a dish, the RTL 2832 dongle makes it easy to find the correct satellite and optimize the transponder polarization. Each satellite has Horizontal and Vertical beacons. These vary slightly in frequency, thus, but tuning to the correct beacon frequency, you can be assured that you are on the right satellite. All of the radio network programming on AMC-8 is on vertically polarized transponders, therefore, the vertical beacons are of interest. Here are the vertical beacons for satellites in that part of the sky:
||C band Vertical beacon (MHz)
||L band (LNB) Vertcial beacon (MHz)
For those in the continental United States, there is not much else past 139W, so AMC-8 will be the western most satellite your dish can see. Of course, this can be used in other parts of the world as well, with the correct information. Bringing a laptop or Windows tablet to the satellite dish might be easier than trying to drag a XDS satellite receiver out.
AMC8 vertical beacon output from LNB
In order to use the RTL-2832U, simply split the output of a powered LNB, install a 20-30 dB pad in between the splitter and the dongle. Using the vertical beacon on 949.25 MHz, adjust for maximum signal.
Some other uses; look for the nearest and best NOAA Weather radio station. Several times the local NOAA weather station has been off the air for an extended period of time. Sometimes, another station can be found in the same forecast area. Heck, couple these things to a Raspberry Pi or Beaglebone black and a really nifty EAS receiver is created for NOAA and broadcast FM. One that perhaps, can issue an alarm if the RSL drops below a certain threshold.
I am sure there are plenty of other uses that I am not thinking of right now…
As a part of our studio build out in Walton, we had to install a high capacity STL system between the studio and transmitter site. Basically, there are five radio stations associated with this studio and the satellite dish and receivers are going to be located at the transmitter site.
The audio over IP gear is getting really sophisticated and better yet, more reliable. For this application, we are using a Cambium networks (Motorola Canopy) PTP-250 radio set and a pair of Wheatstone IP88 blades on either site. Since there is quite a bit of networked gear at the transmitter site, the IP88’s will live on their own VLAN. The PTP-250’s will pass spanning tree protocol, rapid spanning tree protocol, 802.1Q and other layer two traffic.
The Wheatsone IP88A blades are the heart of the system. Not only do they pass 16 channels of audio, we can also pass 8 logic closures bi-directionally. This is key because we are shipping satellite audio and contact closures back from the transmitter site. The IP88A set up is fairly easy, once the IP address is entered. The web GUI is used for the rest of the configurations including making the connections between units.
Pair of Wheatstone IP88A AoIP interfaces
The switches are managed units. The switchports need to be set up via command line to pass VLAN traffic. There is an appendix in the IP88 manual that outlines how to do this with various manages switches. This is the most important step for drop out free audio. The switchports that connect to the two radios are set up as trunk ports using either VTP or 802.1Q.
Cambium PTP-250 5.8 GHz out door units
The PTP-250 radios were already on hand, new in box. They are built really well and look like they should not break in a year or so. These particular units are connectorized, therefore an external antenna was needed. There are many such antennas, this system ended up with a RF Engineering & Energy 5150-5850 MHz dual polarized parabolic dish with RADOMES. RADOMES are necessary to prevent ice or snow build up in the winter.
RF Engineering & Energy 5150-5850 MHz dual polarized parabolic dish with LMR400 jumpers
STL link dish installed
1 1/2 inch EMT going from TOC to roof
Since the path is only 3.37 miles (5.43 kilometers), I set them up with a 40 MHz wide channel. This is a rural, small town setting. When I looked at the 5.8 GHz band on a spectrum analyser, it looks fairly uncongested. These are MIMO single or dual payload selectable. I will try them as single payload units, since the path is short and the band uncongested. This should keep the throughput high.
Studio to transmitter site LAN extension
The PTP-250’s use POE injectors in mounted in the rack rooms. CAT5e shielded cable with the proper connectors properly applied is a must for lighting protection. The PTP-250 units came with Cambium PTP-LPU lightning protectors. I also installed Polyphaser AL-L8XM-MA type N surge suppressors on each RF port of each PTP-250.
In the progression from Circuit Switched Data to Packet Switched Data, I can think of many different applications for something like this:
FMC01 MPX to IP CODEC
The FMC01 MPX to IP encoder can be used for multi-point distribution (multi frequency or same frequency network) of FM Composite audio, or as a backup solution over a LAN bridge, LAN extension, or public network. I can think of several advantages of using this for a backup when composite analog STL’s are in use. There are many compelling reasons to extend the LAN to the transmitter site these days; Transmitter control and monitoring, security cameras, office phone system extensions, internet access, backup audio, etc. I would think, any type of critical infrastructure (e.g. STL) over a wireless IP LAN extension should be over a licensed system. In the United States, the 3.6 GHz WLAN (802.11y) requires coordination and licensing, however, the way the rules are set up, the license process is greatly simplified over FCC Part 74 or 101 applications.
Another similar CODEC is the Sigmacom Broadcast EtherMPX.
Sigmacom Broadcast EtherMPX CODEC
• Transparent Analog or Digital MPX (MPX over AES), or two discrete L/R channels (analog or AES).
• Built-in MPX SFN support with PTP sync (up to 6.000km in basic version). No GPS receivers!
• Unicast or Multicast operation to feed unlimited number of FM transmitters with MPX from one encoder.
• Linear uncompressed PCM 24-bit audio.
• Very low audio latency: 2,5mS in MPX mode.
• Perfect match with Sigmacom DDS-30 Exciter with Digital MPX input.
• Can be used with high quality 802.11a/n Ethernet links.
• DC coupled, balanced Analog inputs & outputs with -130dBc noise floor.
• No modulation overshoots due compression or AC capacitor coupling.
• Decoder provides simultaneously Analog & Digital output for transmitter redundancy.
• Aux RS232 serial transparent link, Studio to Transmitter.
• Auto switchover to Analog input when Digital signal is lost.
• Centralized remote control & management software
One last thought; separating the CODEC from the radio seems to be a good idea. It allows for greater flexibility and redundancy. Using an MPX type STL allows sensitive air chain processing equipment to be installed at the studio instead of the transmitter site.
Like all data carrying technology, WLAN, or WiFi, continues to evolve into a better, faster and more robust platform. The IEEE wireless ethernet specification 802.11ac combines all of the past developments, plus some added features, into one specification. Here are some of the highlights:
- Operation on 5 GHz only. Many more available channels in this spectrum than in 2.4 GHz
- Increased channel bonding making wider channels carrying more data. In the 5 GHz spectrum channels are 20 MHz wide and do not overlap. 802.11ac allows for 40, 60, 80 or even 160 MHz channels. This is great for short distances, longer distances will be prone to greater interference over wider channels
- Modulation schemes that allow up to 256 QAM. A 256 QAM constellation is going to look pretty crowded unless it is on a wide channel. Again, this would be good for short distances.
- Increased MIMO. Up to 8×8 MIMO (Multi In Multi Out) which can greatly improve throughput. MIMO means multiple transmitters and antennas in the same unit. The first number is the transmitter count the second number is the antenna count. Thus an 8X8 system will have eight transmitters and eight antennas. This allowed beam forming by use of phased antenna arrays, which can greatly reduce multi-path
- MU-MIMO (Multi-User MIMO). Basically, the access point sends the data frame only to the desired host, thus instead of acting like an ethernet hub sending the frame to every connected host, the AP is acting more like an ethernet switch.
Comparison of 802.11n to 802.11ac
The goal of all of these modifications is to get gigabit transfer rates over WLAN.
What does all of this have to do with radio broadcast, one might ask. That is a good question.
There are several applications that have to do with remote broadcasting. Many sports areas, night clubs, or other likely places to be broadcasting from have WIFI installed. Using a laptop with an AoIP client installed not only can connect to the studio for audio delivery, the same laptop can use RDP or VNC to control the station’s automation computer as well. This means easier integration of the remote into voice tracked or syndicated programming.
Secondly, wireless LAN bridges between studio and transmitter site can act as a STL, a backup STL, a remote control return link, bridge for a network connected transmitter, VoIP phone link, IP security camera back haul or almost anything else that can send ethernet data. I have found it useful to simply have a computer available at the transmitter site, even if it is only to download manuals and what not. We have taken several old Windows XP machines and reloaded them with a Linux variant and installed them at various transmitter sites. It saves the trouble of having to download a manual on the smart phone then page back and forth across a really small screen to read it. As for using unlicensed WiFi to link to a transmitter site; the link between the WICC studio and transmitter site runs a 78 Mbps most days. This is a two mile link over mostly water. I will say, when there is fog, the link rate drops to 32 Mbps, which is still pretty good, all things considered.
Of course, office network applications; laptop, tablet, smartphone and other personal devices.
Finally, Broadcast Engineers really need to keep abreast of networking technology. There are many, many applications for WiFi units in the broadcast industry.
Had a problem with this Kintronic FMC-0.1 isocoupler the other morning.
Kintronic FMC-1.0 STL isocoupler
After an overnight drenching heavy rain and very high wind, the STL transmitter associated with this unit was having high VSWR faults. This isocoupler crosses a base insulator of an AM 50 KW directional antenna. This particular tower has negative impedance, which is to say, it sucks power out of the pattern and feeds it back to the phasor. An interesting discussion for another time, perhaps.
Using a dummy load, we isolated the problem to the isocoupler by first connecting the load to the output on top of the unit (problem still exists) then to the transmission line prior the unit (problem went away). Of course, the AM station had to be taken off the air to do this work.
Once the issue was confirmed as the isocoupler, I opened the unit up and found that water had entered and pooled in the top of the bottom half of the isolation transformer.
Kintronic isocoupler transformer
The isolation transformer consists of two loops to ground capacitively coupled through air dielectric. The issue is with the opening around the top of the unit, under the lip of metal lid. Apparently, this allowed water in.
Kintronic isocoupler isolation transformer
It is difficult to tell with the lighting in this photograph, however, the bottom part of this isolation transformer has water pooled around the center insulator. Using a rag, I cleaned out the water and dirt from the center insulator. After reconnecting the antenna and transmitter transmission line, a quick check revealed the problem was much better, but still not completely gone. I suspect water seeped further down into the bottom half of this unit. The repair work was good enough, however, to return both stations to the air.
Glad to get that bit of work done while it was still relatively warm out.
After a bit of delay, we were able to return to the WICC transmitter site to install the Wireless LAN link. The installation was pretty straight forward. The studio unit was mounted on an existing STL tower on the top of the elevator room, the transmitter unit was mounted on an existing pipe on the roof of the transmitter building.
M5 Nanobridge mounted on transmitter building with RADOME
I included RADOMEs for a couple of reasons; first, there is a lot of critters around of the two legged and winged kind. The upright two legged critters may be attracted to the signal strength lights at night. This unwanted attention could invite the
juvenile delinquents bored teenagers to throw various objects found laying around on the ground at the antenna, damaging it. The winged type critter may be inclined to view the feed horn as a good nesting location. The other reason is this site gets a lot of rain, wind, ice and snow, therefore the RADOMEs afford some protection against the weather.
Aiming the antennas was pretty straight forward, but requires at least two people. Using landmarks, we aligned the dishes in the general direction of each other. Both ends of the system were turned on and we had a -89 dBm signal path, and somewhat surprisingly, the radios linked up and my laptop grabbed an IP address via DHCP. Using the signal strength meter on the side of the antenna, each dish was peaked in turn:
M5 Nanobridge Antenna signal strength meter
Then, somebody on either end went below and looked at the signal strength screen on the web interface while the other end was peaked. In the end, we had about -65 dBm signal strength, which is somewhat less than the -58 dBm predicted. I think we can do better, so on the next clear day, I am going to peak the signal again.
The data rate initially reported was over 100 MBPS, however, once I started transferring files back and forth, that dropped to about 50 MBPS. If it is raining, that rate drops to about 35 MBPS, which is still far above what we need this link to do. As a test, I streamed a youtube video, downloaded a windows update, loaded several web pages and checked my email simultaneously. There where no issues with the data rate while those tasks were being preformed.
It is quite amazing to me that these little inexpensive radios can work so well. My boss thinks that they will be blown up by lightning during the first thunderstorm of the season. I don’t know. There are several of these units that have been installed at mountain top tower sites and have been working for several years without issue.
Next step, installing the IP cameras and warning signs on the fence, setting up the monitoring software, etc.
Transmitter site security cameras
Cameras mounted on old chimney platform. This is the first set of cameras covering the south, north and west approaches. A fourth camera will be mounted on the back of the building covering the east approach. Then, under the eves cameras will be mounted on all four corners of the building and the generator shed. If anything moves, it will be recorded.
I am in the process of installing a pair of the Nanobridge M5 units as an IP network link between a transmitter site and the studio location. The path is relatively short, about 1.5 miles over mostly water. The main reason for this is to replace the analog phone lines used for remote control data and backup programming delivery to the transmitter site. One added benefit, we are also installing several IP cameras to keep an eye on the place. We purchased the Nanobridge system for $80.00 per side. The price is pretty good, but the configuration and testing is a bit intensive.
There are many versions of these spread spectrum radios, some are licensed, some are license free. These are inexpensive, license free links that I would count on for short paths or use in non-congested areas. In congested areas, licensed (Part 101) links should be used, especially for critical infrastructure like STLs.
Since I dreamed up this idea, I figured I should make sure it is going to work before recommending it to the powers that be. I have learned the hard way, almost nothing is worse than a failed project with your name on it. Better to over study something than to go off half cocked, spend a bunch of money, then realize the idea was flawed from the start. See also: Success has a thousand mothers but failure is an orphan.
Nanobrige path study, 5.8 GHz, moderate noise floor, 1.5 miles
Looks pretty good. 300 MB/s bi-directional which is faster than the Ethernet port on the unit. This will be set up in bridge mode with pretty robust encryption. The transmitter site side is configured in the router mode, creating a second class A network at the remote site.
Nanobridge M5 22 dBi antenna
Next step, configuring the units. The Nanobridge units were set up in a back to back configuration in the engineering room. Each end comes with a default IP address of 192.168.1.20. The units were several steps behind the latest firmware version, therefore the firmware was upgraded first. The default admin user, password, and IP addresses were changed. There is no greater security risk than default user and password. The wireless security feature is enabled using WPA2-AES PSK and a greater than 192 bit access code. The unit allows for any access code length up to 256 bits. With a key of between 192 and 256 bits, the number of possible solutions is between 6.2771 E 57 and 1.1579 E 77, which should be pretty hard to crack. By way of reference, a 192 bit password has 24 ASCII characters and a 256 bit password has 32 ACSII characters.
Air OS main screen
The system requires an access point, which is configured for the studio side making the transmitter site stub network the station side. The access point is configured not to advertize its SSID, thus it should be transparent to anyone sniffing around. The WLAN is configured as a layer two bridge, which will cut down on the data overhead, as layer three framing will not need to be opened between the two units. The transmitter site network is set up with SOHO router function built into the Nanobridge. One static route is needed to get to the main network. Once the security cameras are installed, PAT may need to be used to access individual camera units via the public network.
Ubiquity air os signal strength screen
Next step, deploy the units and aligning antennas. These are 22 dBi gain antennas, which have a pretty tight beam width. Maximum transmit power is 23 dBm, or 200 mW. The transceiver/antenna unit has a handy signal strength meter on the side of the unit, which is good for rough in. The web interface has a more precise meter. In addition to that, there is a java based spectrum analyzer, which is very handy for finding open channels in congested areas. These units can also be used on UNii frequencies with special requirements.
According to the manufacture, UV resistant shielded Category 5e cable should be used for outdoor installations. We have several spools of Belden 1300A, which fits the bill. The shielded Cat 5 is necessary for lightning protection as the cable shield offers a ground path for the antenna unit. The antenna mounting structure is also grounded. I did not take the equipment apart to examine, but I believe the POE injector and antenna have 15KV TVSS diodes across all conductors. It will be interesting to see how these units do at the transmitter site, where there are two 300 foot towers which likely get struck by lightning often.
More pictures of the installation when it is completed.
Next step, put the system into service and monitor the link. At the transmitter site, a re-purposed 10/100 Ethernet switch will be installed for the cameras, computer, IP-RS232 converter and anything else that may need to be added in the future. One thing we may try is an Audio of IP (AoIP) bridge like a Barix or Tieline for program audio and room audio.
I found this on the floor at an old transmitter site:
Polyphaser IS-PT50HN-B DC block surge suppressor
Since it appears to be discarded, I ignored the dire warnings and opened it up to look inside:
Polyphaser IS-PT50HN-B DC block surge suppressor
This is is a DC blocked lightning surge suppressor designed for 890-980 MHz, 750 watts maximum. The two parallel wires represent a capacitor, coupling the radio to the antenna, the inductor acts as an RF block to the gas discharge tubes. The design is such that the inductor acts to block the normal in use radio frequencies but will allow the 10-30 KHz lightning pulse to pass to the gas discharge tubes thence to ground. The inductor and gas discharge tubes are on the antenna side of the unit. I measured these units with a DVM and they all appear to be good.
My only comment on this unit is that there is no effort to maintain the transmission line impedance. At the upper end of the UHF spectrum, this can lead to return loss and wasted power. For a receive application, it may not be so bad, but for a transmitter, I would rather use something else.
For lower VHF frequencies, something like this can be DIY fabricated with minimal expense and effort. The case must be bonded to the station ground.
This is a universal truism that can also be expressed as “Murphy’s Law.” I don’t rightly know how Murphy received credit for this, however, I chalk it up to either the luck of the Irish or the gift of self promotion. Either way, that principle was demonstrated again with a 950 MHz STL link between Mt. Beacon and Peekskill, NY for WHUD.
I had noticed, while doing some transmitter maintenance, the receive signal strength of the STL had dropped from 300 µV to 30 µV. That is an alarming development. Therefore, we scheduled a tower crew for the next day, not wanting to go off the air over the coming holiday, which would be a sure bet otherwise. Upon arrival, the tower crew noticed a strange thing in the STL transmission line at the base of the tower, which looked like some type of a splice. Truth be told, I have been associated with this station since 1999, and had never noticed the splice before. This STL system was installed in 1998, when the station’s studio moved from Peekskill to Beacon. I can say, of all the things that have gone wrong over the years, this STL system was always very reliable. Regardless of that, I quick check with a spectrum analyzer showed a 3 dB return loss at 137 feet (41.75 m), exactly the distance from the transmitter room to the base of the tower.
3 dB return loss, distance to fault 137 feet
A 3 dB return loss coincides exactly with the drop in received signal strength at the other end of the path. Thus, the tower crew took apart the splice and water poured out of it. I would estimate at least 4-6 ounces of water (180 ml), perhaps more.
7/8 coax cable splice connector, opened up
We then began to take in the details:
- The 7/8 coax coming out of the building was Cablewave FLC78-50J
- The 7/8 coax going up the tower was Andrew LDF4-50A
- The splice connector was Andrew L45Z
- The center conductor threaded connector did not fit properly into the Cablewave cable, it was too loose.
- The cable was chaffing on a tower leg, about 50 feet above the splice because it was not properly secured to the tower
- The 7/8 splice connector was missing an O ring on the backnut of the Cablewave cable
Thus, water ingress causes the high return loss. Problems with this system began immediately after Hurricane Irene, the end of last August. We were able to make a temporary fix using two type N connectors of the proper manufacture for each type of cable. The radio station returned to air just before noon, about 45 minutes after turn off. After the repair, the return loss dropped to about 20 dB, which is good.
The permanent fix is for the entire run of cable from the transmitter room to the STL antenna to be replaced. That type of line splice should have never been used on a 950 MHz STL, and it was certainly wrong to mix cable types with an Andrew connector. Those little details will always manifest themselves eventually.