Thanks Simon, I WAS going to mention that the JC-1 is designed for home use, rather that Pro use. IF I was designing for the GD, as I did, 40 years ago, I would add a 1 - 2uH choke, just to be sure, but the JC-1 drives big electrostatic speakers just fine, in any case.
Define "braided cables." You mean like those woven computer ribbon cables that have hellaciously high capacitance when they're terminated as speaker cables?
Because I've used simple, four strand braided speaker cables for many years and have never had any problems using them.
se
The distribution cable I have seen was about 1-3/4" in diameter and had copper pipe as a center conductor.
Actually Scott after all the individual signals are combined at the headend, most operators use fiber to send the signals to within a half mile or so of the end user. At that point it can be turned back to RF. The standard trunk cable is about .75" in diameter and uses about a 12 gauge copper plated steel center conductor.
The interesting feature of the trunk cable is that it is built into a seamless aluminum tube 2500 feet long. The plant that makes it was built that long just to make the cable. The length is determined by the amount of loss that can be tolerated before an amplifier must boost the signal back to where it is designed to be.
Along the trunk cable are directional couplers that tap off some of the signal to deliver to each subscriber. These drops are typically RG6.
A well designed cable system tries to have no more than three amplifiers between the head end and the final user.
As to larger copper coax, that is often used for commercial broadcast users. It really was originally made from regular plumbing pipe! As 3" pipe is the largest common size, it was easy to use either 1/2" or 3/4" pipe for the center conductor. 3/4" was 75 ohms and 1/2" was 50 ohms. As the gap with 1/2" pipe allowed the internal voltage to be higher the military standardized on 50 ohms to allow greater power handling. Of course 75 ohm is easier to couple to a 300 ohm dipole so that is much more popular in non transmitting applications. For lower power applications smaller pipes were also used. The pipes were often filled with dry nitrogen or dried compressed air to keep the power handling up there.
I have done more than half a dozen cable headends and distribution systems. But only three broadcasting stations to date.
Commercial broadcast transmitters in my experience are 100% 50 ohm. The oldest AM sites could be 300 ohm above-ground open wire feeds, and few old timers don't have a story to tell about large birds straddling them. Momentarily.
The function of line dehydrators and nitrogen systems is mostly environmental, to prevent moisture buildup and a low impedance path in the cable.
The function of line dehydrators and nitrogen systems is mostly environmental, to prevent moisture buildup and a low impedance path in the cable.
John, it is a shame that whenever someone disagrees with you, you end up having to make it personal, suggesting that someone is somehow out to get you. That could not be further from the truth. Don't get so cranky. If we disagree, respond with a technical argument that the readers may be able to profit from. I enjoyed that beer at the RMAF, by the way.
Cheers,
Bob
I have seen a very few low power FM transmitters that were 75 ohm to be able to use standard FM receiver antennas as transmitter ones, but other than that I must agree virtually all commercial gear is 50 ohm.
In Canada I think we agree the major advantage of dry gas is reduced loss, but around the equator or other semi-tropical to tropical areas dry gas is a must. It also is nice that the pressure is a bit higher, that helps get the power handling up a bit at high altitudes.
Of course when the standard was being written, the transmitters were Morse code and the receivers were not very sensitive by today's standards. But I have not seen an installation that old, as I am curious as to what they would have used for the spacer insulators back then.
1. The internal inductance of a cylindrical conductor is 15 nanohenries per foot.
2. Dry nitrogen gas is used within coaxial lines to prevent flashover arcing within the cable. It is also used within hermetic military hybrids for the same reason. The design guideline for using nitrogen is a maximum voltage gradient of 70 volts per mil (thousanth of an inch).
3. Braiding conductors does not increase the capacitance of a speaker cable sufficiently to force oscillation of a power amp. The number of wires involved in the braid will be the most significant player in that regard.
As a general rule, for large quantites of braided wires, one can use 100 ohms as the characteristic impedance of a pair, and divide that by the number of pairs. By using an effective dielectric coefficient of 3, it is easy enough to calculate the resulting capacitance that the amplifier will see at it's break frequency, when the speaker has fully unloaded.
For example 12 pairs...(the example is cat5e, but most wire pairs exhibit roughly the same impedance..it's the insulation to conductor diameter that determines the numbers.)
100 ohm Z per pair gives 8.3 ohms. (100/12)
200 nH per foot per pair, gives 16.6 nH per foot. (200/12)
16.6 times C = 1034 times 3 (L in nh per foot, C in pf per foot.)
C = 1034 * 3 / 16.6 = 186 pf per foot.
A ten foot run...1860 pf.
Cheers, John
2. Dry nitrogen gas is used within coaxial lines to prevent flashover arcing within the cable. It is also used within hermetic military hybrids for the same reason. The design guideline for using nitrogen is a maximum voltage gradient of 70 volts per mil (thousanth of an inch).
3. Braiding conductors does not increase the capacitance of a speaker cable sufficiently to force oscillation of a power amp. The number of wires involved in the braid will be the most significant player in that regard.
As a general rule, for large quantites of braided wires, one can use 100 ohms as the characteristic impedance of a pair, and divide that by the number of pairs. By using an effective dielectric coefficient of 3, it is easy enough to calculate the resulting capacitance that the amplifier will see at it's break frequency, when the speaker has fully unloaded.
For example 12 pairs...(the example is cat5e, but most wire pairs exhibit roughly the same impedance..it's the insulation to conductor diameter that determines the numbers.)
100 ohm Z per pair gives 8.3 ohms. (100/12)
200 nH per foot per pair, gives 16.6 nH per foot. (200/12)
16.6 times C = 1034 times 3 (L in nh per foot, C in pf per foot.)
C = 1034 * 3 / 16.6 = 186 pf per foot.
A ten foot run...1860 pf.
Cheers, John
Interesting... ~0.002uF doesn't seem like it should cause trouble for any amp.
Most class-d amps have many, many times that much capacitance hung across the terminals (after the inductor, or course).
Most class-d amps have many, many times that much capacitance hung across the terminals (after the inductor, or course).
Guys, you're welcome to peruse the product specifications on the Commscope/Andrew and RFS/Cablewave sites to make your points. Nowhere have I seen power handling or loss figures differentiating between running air cable pressurized air, nitrogen or unpressurized, nor has it appeared in any engineering brief. Of the latter I have on my desk, the power and loss calculations are to three significant digits. Pressurization and gas type would be included if it mattered.
Re: moisture, Vancouver and Halifax have different stories to tell. Nearly every AM station on the continent buries its RF cable, in my case in a peat bog. I've seen AM sites with towers in lakes and ocean, CBC Vancouver an example of the latter.
Air dielectric cables in broadcast are used for their lower loss and higher power handling. Pressurization is not required to achieve those benefits. You are aware it's common practice to use air rigid and air coax unpressurized indoors? Its primary function is to prevent condensation, water leakage and contamination of the internal insulators. It would be madness to engineer a system in which any theoretical power handling advantage of, for example, nitrogen was a factor. The VSWR change from the first snow or icing would blow it up. I just spent a month dealing with repairing the feeders of a multi-user high power antenna system taken out by moisture leakage at a non-gas stop barrier from a cracked connector ring.
Re: moisture, Vancouver and Halifax have different stories to tell. Nearly every AM station on the continent buries its RF cable, in my case in a peat bog. I've seen AM sites with towers in lakes and ocean, CBC Vancouver an example of the latter.
Air dielectric cables in broadcast are used for their lower loss and higher power handling. Pressurization is not required to achieve those benefits. You are aware it's common practice to use air rigid and air coax unpressurized indoors? Its primary function is to prevent condensation, water leakage and contamination of the internal insulators. It would be madness to engineer a system in which any theoretical power handling advantage of, for example, nitrogen was a factor. The VSWR change from the first snow or icing would blow it up. I just spent a month dealing with repairing the feeders of a multi-user high power antenna system taken out by moisture leakage at a non-gas stop barrier from a cracked connector ring.
I'm confident that nobody in their right mind would forget to include the range of vswr in the use of coax cables, your assertion is entirely orthogonal to my statement..
As I stated, margin is a significant factor.
There are plenty of non-commercial applications that survive solely as a result of nitrogen filling of the cable..simply because the cables required to handle the voltages really do not exist. CERN is a good example, "Kickers" and "septums" being the loads, thyratrons being part of the source.. SF6 is another option, albeit not as cheap..
Perhaps had nitrogen been feeding the cable it would not have been taken out. Nitrogen is dirt cheap, and it would have been easy enough to monitor useage to determine seal integrity and repair the system at leisure. Apps I am involved with cannot tolerate down time..nitrogen pressurization allows us to repair such failures during scheduled maintenence, as the positive pressure would keep the moisture out.
Even WIKI explains the use of nitrogen within hard lines to reduce arcing..
Dry air is such a pain, I'm not really sure there is such a thing...we keep emptying the dry air line traps of water and oil..😱😱
Cheers, John
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Ah, the parable of the blind guys examining the elephant. We all use similar cables for different purposes.
That's all nice but Simon's original assertion was about the purpose of pressurization in broadcast. That stated purpose is incorrect. If you can provide a single example of pressurization used in the design calculations for broadcast, go to it. I'll give you hand, the CRTC and FCC websites contain full engineering briefs for every broadcast installation in North America. Another option is, again, to use manufacturer's data.
I don't know of a single broadcast installation that doesn't leak at some rate. This particular antenna system was running normally, to repeat this failure occurred as a non-gas stop barrier. Ironically, it was converted from air to foam decades ago in an attempt to solve prior failures.
Anything else you want to tell me about the nature of systems I've managed for 30 yearS?
I don't know of a single broadcast installation that doesn't leak at some rate. This particular antenna system was running normally, to repeat this failure occurred as a non-gas stop barrier. Ironically, it was converted from air to foam decades ago in an attempt to solve prior failures.
Anything else you want to tell me about the nature of systems I've managed for 30 yearS?
Which is probably why I've not stated anything about pressurized air or nitrogen being needed. We just keep ambient, and use the nitrogen to keep moisture out.
Had I asserted that, I would. Since I did not, your statement has nothing to do with me. Perhaps you should be addressing that with Simon.
Unless there are hermetic seals, everything leaks. I use barriers which are seals to prevent gas movement from gas to gas, and I use barriers which are seals to prevent gas to surrounding ambient. So it was not clear from your earlier statement which type it was.
Should I actually make a statement regarding the nature of broadcast systems, you'll be the first to know.
As I said, dry nitrogen is used to prevent arcing, and moisture is the typical cause. You're statements about your experience and expertise, while nice, are not mutually exclusive to these statements.
You said""Nowhere have I seen power handling or loss figures differentiating between running air cable pressurized air, nitrogen or unpressurized,""
I replied that dry nitrogen is quite useful, and is used in many applications where it provides higher dielectric withstanding. But generally, I see it used in apps which are pushing the envelope more than commercial broadcast would allow..
You do seem intent on argument where there really is none..at least it seems that way to me..
Cheers, John
Gee, I though you were the one who mentioned moisture in the dielectric increased the loss!
Are you now saying because it is not in the calculations it has no effect?
My apologies J, your interjecting comments in my discussion with Simon about broadcast cables caused me to mistake them for relevant.
My comments were relevant to why dry nitrogen is sometimes used, not to pressurization. I can see how pressurization would affect the dielectric withstanding, but have no information in that regard for altitudes where humans can survive.
These threads can get confusing at times. You did not owe me an apology, but thank you just the same...I am sorry it became confused..
Cheers, John
I'm always trying to stay away from cable debate, but I'm shock. I tried a Furutech power cord that I borrow from a friend, on my main system (Ayre K-3 and V-3 amp, Verity audio speakers) and I almost froze in place. From the first note, the difference was shocking. Even my wife heard it in a minute.
Everything is more, more dynamic, more life, more impact and details.
I'm sold.
I was using an home made DHLab cable + Hubbell and Wattgate connectors, not bad either, but this Furutech cable is marvelous.
Very recommended, I keep it 😉
Just my two cents.
Everything is more, more dynamic, more life, more impact and details.
I'm sold.
I was using an home made DHLab cable + Hubbell and Wattgate connectors, not bad either, but this Furutech cable is marvelous.
Very recommended, I keep it 😉
Just my two cents.
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