Engineering Radio – Page 141 – One Hundred percent Human Generated Content for your Reading Enjoyment

Lightning Season

The rumble of thunder this morning let me know that lightning season is upon us here in the Northeast and likely the rest of the country as well. I used to enjoy the odd summer thunderstorm, especially the late afternoon pop-ups that cool off a hot summer’s day. Now whenever I see lightning or hear thunder I wonder if the phone is going to ring. Chances are good that it will not, as I invested many hours of my time and my previous employer’s money into lightning protection at the transmitter sites.

I go on the assumption that all tall steel towers will get struck, oftentimes repeatedly, during any particular electrical storm. Back in the day, I took a course by Polyphaser called “The Grounds for Lightning and EMP Protection.” It was a great primer on how to ground and bond equipment at a transmitter site to eliminate current flow, which is the cause of all EMP and lightning-induced failures. When I was in military communications, no expense was spared as they took uptime very seriously. Any downtime was a personal affront to the commanding officer of the unit in question.

Lightning is DC however, it behaves more like 10 KHz – 2 MHz AC. Therefore, lightning and EMP grounding systems need to be designed and installed to accommodate DC through 10 KHz AC voltages. This is easily done by choosing the correct conductors, ground bus bars, and bonding systems.

The other path lighting takes into a transmitter is through the AC mains. Utility company high voltage primary feeds act like large antennas for lightning-induced EMP. Fortunately, much of that is filtered out by the step-down transformers just before the building service entrance. It is still possible, however, for some impulse voltage to make it through the transformers and into the service entrance panel. On older tube type transmitters, this could often damage the plate voltage power supply because of the voltage multiplication factor of the plate transformer. Oftentimes, the transformer secondary would have “holes” punched through the insulation to ground. This is an expensive and time-consuming repair.

I would conservatively estimate that for every $10.00 spent on lightning protection, $1,000.00 dollars worth of damage and downtime is saved. Overall, a pretty good return on investment.

The basics for lightning ground bonding are thus:

Use the lowest inductance wire possible. The industry standard is #2 solid copper, however, if bonded properly, there will be very little current flow inside the transmitter building, so if #2 is not available, then any solid wire up to #8 will work. Tower ground bonding should be as heavy as possible.
Ground all guy anchor points, and bond all guy wires together and to the same ground rod or ground rod system.
Keep the bonding conductors as straight as possible, bends should be long sweeping turns to minimize series inductance.
All metal equipment should be bonded, no rack, telco demark, electrical panel, dummy load, bulkhead entrance grounding bus, combiner, door frame, etc should be left unbonded.
All coax outer shields should be grounded where it comes into the building.
All coax cables should go through a toroid before being connected to the transmitter.
All outdoor bonding connections should be exothermically (CAD welded) bonded to ground rods.
All grounding must go to a common ground point, AKA star grounding point. No individual ground points should be allowed in the building.
Multiple ground rods were installed around the outside perimeter of the building as deeply as possible. Some mountaintop transmitter sites may require special grounding material (Bentonite) and or to have a ground well drilled. Ground conductors should have as much surface area contact with Earth as possible.

The whole idea is to present a low-resistance ground path and keep all of the equipment at the same potential to minimize the current flow between equipment.

For the electrical building service entrance, a series surge protector installed before the service panel is the best method. Several are made and they need to be sized for the building service. A fallback is a parallel surge protector that will provide some protection. On the AC mains connections, any series inductance that can be added to increase resistance to the lightning pulse is good. All AC mains connections to the transmitter should go through a toroid before they are connected to the transmitter. This is a good idea for remote control and mod monitor wiring as well.

Quad Star Microphone Cable

Or Star Quad Microphone Cable, depending on who is making it.

Star Quad Microphone Cable diagram — Star Quad Microphone Cable Diagram

This has been around for quite a while, but many studio/broadcast engineers don’t understand it or don’t use it for some reason. Microphones and mic pickups produce relatively low signals when compared to line-level audio. Most microphone preamps have a gain of +50 dB, which means any noise gets amplified and even small things can become major problems quickly.

Under general conditions, most balanced shield twisted pair (STP) audio cable such as the standard Belden 8450 is adequate for stationary microphone cable for short runs. When the cable is not permanently fixed in place, as in hand-held microphones, microphones mounted on booms, or other nonfixed microphone applications, then a flexible cable must be used. Star Quad cable has better noise specifications than standard flexible microphone cable.

The advantage of Star Quad cable for low-impedance microphones (150 ohms) is that the parallel twisted pairs significantly reduce inductive reactance. In AC circuits, inductive reactance acts as a low pass filter, gradually rolling off as the frequency is increased. This effect is cumulative, the longer the cable run, the more inductive reactance is added to the circuit. The result is microphone audio can have smeared or ill defined high frequency audio.

Using two parallel twisted pairs is similar to parallel resistors when it dealing with inductive reactance, it halves the value.

In addition to reducing inductive reactance, the tighter twist found in Star Quad cables reduces the CMRR by about 20 dB. The Star-Quad configuration keeps the conductors in the same relative position to each other as the cable is flexed and moved around. All of this makes it superior to standard STP microphone cable.

Several companies manufacture Quad Star cables:

Belden: 1192A
Canare: L-4E6S
Gepco: MP1201
Mogami W2534
Cardas 4X24

The price of Star Quad cables runs about 40-60 cents per foot (more for the Belden, much more for Cardas) if purchased in bulk. That is about the same range for two conductor mic cables.

As good as this cable is, I don’t think they had this in mind when they made it:

I wonder what the centripetal force on that cable is when the microphone is in full motion. Also, I’d bet that SM58 was none the worse for were after it’s crowd surfing moment.

Newest Radio Thing

Just in time for the NAB, I’ve been working on this design since my college days. To give you some idea of how long that is, I am nearly 47 years old. I believe it has finally been perfected, now I just need to find somebody to make it. I guess I could send it off to China and get circuit boards made, but they would steal the design.

From the website xkcd.com, which has, perhaps the best website ever published in the history of the internet, here.

Oh yeah, and April Fools!

Radiation levels: Compare and Contrast

Not related to radio engineering, however, I’ve been doing daily radiation measurements at my house (upstate NY) since the Fukushima disaster. A few bits of housekeeping information first: This is a CD V-700 radiation meter, which is a model 6 manufactured by Anton. It was last calibrated in 1986. When I place the Geiger tube over the operational check source, it goes up to about 2 mr/hr as described in the owner’s manual. It may not be completely accurate, but it is accurate enough for this experiment.

This video was taken on March 17, 2011. It sets a good reference for normal background radiation levels:

This video was taken on March 27, 2011. It shows a significant increase in background radiation. Further, much of this appears to be gamma radiation, as the gamma shield is closed during this video:

Both of these videos were taken on the most sensitive (x1) setting. It shows that the radiation level is about 8 to 10 times above normal. It is a cause for concern, but not alarm. Not yet. If it continues at this level for several days or weeks, then the overall radiation exposure will begin to accumulate. Right now, it is about the same as taking two NY to Los Angles flights per day, according to this chart (0.35 mr/hr = 3.5 uSv per hour x 24 hours = 84 uSv per day):

As of March 28, 2011, the wind has shifted more to the southwest and the levels have dropped somewhat. From our beloved press corps, there have been a few reports here or there on this, most with the standard “this is nothing to worry about” disclaimer. I have also noticed a series of stories and reports that radiation is not all that bad, don’t worry about it, living next to a nuclear plant is fun(!), and we don’t know as much about radiation as we thought we did. I don’t know about all that, I’d rather base my opinion on the scientific body of evidence gathered over the last one hundred years or so. The conclusion of that information is that radiation is bad for human physiology and exposure should be limited.

There is also a crowdsourcing website called “Radiation Network,” which is showing all the levels across the US are normal. This makes me wonder about their instruments and or candor, you can draw your own conclusions.