tech stuff – Page 32 – Engineering Radio

Ubiquiti Nanobridge M5 IP radio

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 are a bit intensive.

There are many versions of these spread spectrum radios, some are licensed, and 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.

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 ASCII characters.

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 advertise 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.

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 manufacturer, 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 that 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.

AM Radio Improvement Plan

There has been lots of hand wringing and ink spilled regarding the sorry state of affairs in the senior service. AM is plagued with problems; interference, poor bandwidth, etc. To that end, the NAB has launched studies and initiatives and hired all sorts of pricey consultants to consult with. Here is my own AM improvement plan and it is rather simple:

Clean up the transmitter site.
Get rid of AM HD radio.
Variable IF bandwidth receivers.
Improve Programming.

How many of us have seen AM transmitter site dumps? Deferred maintenance, malfunctioning directional arrays, trees growing up on the ground system, flooded buildings and ATU’s, rusty towers, transmitters not a full power, ground system deteriorated or missing all together, just to list a few problems. Many AM transmitter sites are technical disasters. Think that these things have no bearing on the AM station’s signal? Think again.

differed maintenance, AM transmitter site — Differed maintenance, AM transmitter site (there is a tower in there somewhere)

Trees growing around the tower base can attenuate the signal by 30%. A comment from a well known engineering firm:

…Recently XXX field engineers had occasion to measure an AM station at XXXX kHz before and after removing vegetation in the vicinity. The station had a quarter-wave tower. The base area had grown up in brush and hardwood trees to a height of perhaps 30 feet (9m) and this extended from near the base across the entire ground system. After clearing (cutting, no ground system disturbance), the signal measured at some 16 locations on four radials went up a uniform amount of about 15% or 1.2 DB. That’s about a 30% increase in radiated power…

That is an inexpensive power boost and they didn’t even have to file with the FCC! A 1 – 2 dB power gain is pretty nice and can mean the difference between a listenable signal and static. How many times have I heard the lament that AM band is full of noise and not listenable. Certainly, there are major challenges in the urban listening environment. Putting forth a better signal will overcome some of this electrical noise.

There is a reason why engineering standards were developed for the physical plant; they work.

There is no cure for the noise that AM HD Radio puts out into the adjacent channels. This self interference benefits none, not even the station transmitting AM HD Radio. This dubious technology has proved itself a non-starter and should be discontinued. For smaller station owners, the cost of implementing AM HD Radio is prohibitive. Licensing of a proprietary modulation scheme, new transmitting equipment, specialized exciters plus any needed bandwidth improvements to AM antenna arrays can easily exceed $100,000.00. Unfortunately, it is often the small AM radio operators that are making a good showing, and serving their community of license and making money. These are the very stations that are hurt the most by adjacent channel AM HD Radio interference.

Receiver design over the last twenty to thirty years has been the greater issue with perceived low AM broadcast quality. AM receivers have an average bandwidth of just 3-4 KHz, which is slightly better than telephone quality. AM broadcasting has gotten a bad wrap because of this and there are many comments about how AM is “inferior quality” to FM. With a quality older receiver, AM can sound very good. Of course, the receiver manufactures all point adjacent channel interference as their rational for reducing IF bandwidth. Why not leave it in the hands of the user? The GE Superradio had this feature with a “wide” and “narrow” setting for AM reception. They worked remarkably well. A receiver could also be designed to automatically increase IF bandwidth at higher received signal strengths.

Finally, as the saying goes; Garbage in, Garbage out (GIGO). This holds true for many things including radio programming. Expecting that mediocre satellite syndicated news talk will garner great ratings and huge revenues is silly. For years and years, station owners have put minimal effort into AM radio and expected big returns. It is not working. AM stations that go against that trend; those with unique formats (Gasp! Music, on AM?), local content, and community oriented programming can and do succeed. They are fighting an up hill battle in both directions. With all of the business pressures from larger broadcast groups, interference issues and negative viewpoint on the viability of the AM band, one wonders how long they can last.

Restarting a Harris HT35 FM transmitter

This transmitter was retuned from 107.9 to 92.9 and put back into service. Retuning an HT35 transmitter is no small matter, there are 32 pages of retune instructions. This unit is now in service as the main transmitter for WEZF, Burlington, VT.

The transmitter power output is 22,000 watts into a four bay, three around panel antenna, which gives it an ERP of 46,000 Watts at a height of 824 meters (2,703 feet) above average terrain. The tower is at the summit of Mt. Mansfield, which is 1,340 meters (4,395 feet) above sea level.

This is the Mt. Mansfield FM transmitter room. There are two TV stations in this building as well.

Final frames are of the WVPS Nautel NV-40 transmitter.

The unitless coefficient of Zorch

Zorch is a term used to describe an over voltage or over current condition that usually leads to catastrophic failure, e.g. the power supply was zorched by lightning. There is also a quality to radio signals that defy and exceed theoretical definitions for service contours or power density. That is quality defined as:

Zorch (adj): The ability of an RF signal to be received in unlikely locations; outside of predicted service contour, in steel structures, underground facilities, tunnels, etc.

It brings to mind the saying, “antennas are not amplifiers and amplifiers are not antennas.”

During the earlier stages of FM broadcasting, there was a notion that costs could be reduced by increasing antenna gain and reducing transmitter size. While theoretically, ERP (Effective Radiated Power) is ERP, broadcasters soon learned that high gain antenna, low TPO (Transmitter Power Output) installations lacked building penetration and had other reception issues. Realizing that there is a trade off between antenna bays, transmitter power output especially in difficult reception areas, a great debate occurred and continues on what the optimal system is. The answer is, it depends on the receiving environment.

Where this technical detail can be really important is with lower powered FM stations; Class A and LPFMs to be exact. They are already battling against bigger stations that have tens or even hundreds of times more power. Certainly an LP-100 station has it’s work cut out for it. The choice of antenna is perhaps one of the most important technical decisions to be made. Choosing the right balance of antenna type, antenna gain, antenna height and transmitter power output can greatly influence reception reliability and thus coverage area.

A good study of this quality can be had by looking at various LPFM installations:

Station ERP (watts)	Antenna Type	Antenna Gain (power)	TPO (watts)*	Coefficient of Zorch
100	1 bay vertical	0.92	127	0.1
100	1 bay circular	0.46	253	0.4
100	2 bay vertical full	1.98	58	0.15
100	2 bay vertical half	1.40	83	0.2
100	2 bay circular full	0.99	118	0.5
100	2 bay circular half	0.70	166	0.7
100	3 bay circular full	1.52	77	0.46
100	3 bay circular half	1.01	115	0.52

*Includes 100 feet of 1/2 inch foam transmission line, Andrew LDF4-50A, loss of 0.661 dB at 100 MHz, or 0.859 power gain.

Stations should try to get the transmitting antenna as high up as permitted without reducing ERP. In other words, the FCC allows 100 watts ERP with 98 feet Height Above Average Terrain (HAAT) radiation center in their current LPFM rules. Being lower in height will reduce the coverage area. Going over 98 feet HAAT will cause the station’s power to be reduced, which will lower the coefficient of zorch accordingly. Therefore, getting as close to 98 feet HAAT, which is different than 98 feet above ground level in many places, will net the best performance.

If a singular polarization (horizontal or vertical) is desired, vertical polarization should be chosen, as most mobile reception is by a vertical whip antenna. For best reception performance, a circularly polarized antenna will work best, as receiver antenna orientation will not effect the signal reception. A circularly polarized antenna has better building penetration and multi-path characteristics. The FM broadcast circularly polarized antenna in not a true circularly polarized antenna, it is actually unpolarized.

The use of a multi-bay antenna has the effect of focusing the RF radiation outward, perpendicular to the element stack, thus limiting the radiation directly up or down from the antenna. This is more pronounced with one half wave spaced antennas, which may be an environmental consideration in heavily populated areas.

Thus, the best coefficient of zorch for an LPFM station would be a circularly polarized, 1/2 wave spaced, 2 bay antenna. This antenna would have some gain over a single bay antenna, take up less room on a tower than a full wave spaced antenna, offer good RF protection performance for the general public living and working under the antenna, reduce wasted upward radiation and offer good building penetration for the ERP. It would require a slightly larger transmitter and more electricity, but that trade off is well worth the effort.