Electrical code, should you decide to follow it, requires that all grounding electrodes be bonded by AWG6 or larger conductor (there are some exceptions, but that's the general rule).

I'd find it hard to believe you're totally isolated, unless all your gear is battery powered and you never hook up something with a wall wart or battery charger.

But yes, AWG 6 is the way to go.  Easy to connect, easy to buy, etc.

For the same amount of copper, the inductance of strap is not much lower than solid or tube conductors.  You're thinking skin effect where surface area matters, but that's about AC resistance.   For lightning, inductance effects dominate, so use what ever is cheaper.  If its an RF ground for a vertical antenna, resistance matters, use strap.

This applies at HF too, not just microwaves and UHF. 
It's all about heat absorption and getting cooked, literally.
(there's also a "shock" issue, but that comes up a lot less frequently with RF)

Here's some basic numbers:  Sunlight is about 800 Watts/square meter, or 80 mW/square centimeter.   If I took you out, dressed you all in a black ninja suit and staked you down in the noon sun, you'd be in a world of hurt after an hour or two.  Maybe you'd have heat stroke, maybe not, but nobody would argue that this is totally benign.
Likewise, if you hold a 25 watt lightbulb in your hand (about 250 square centimeters), you're getting exposed to about 100 mW/cm2, and that's going to be pretty uncomfortable if you keep it up for long, and it might result in damage.

So the problem with RF, particularly at MF,HF and VHF is this.. it gets absorbed throughout your body (RF skin depth is meters at those frequencies in a conductor like you).  And it gets preferentially absorbed in some tissues, so while your body as a whole may not get too hot, some pieces might, and you won't feel it.  You don't have temperature sensing nerves *inside* your body, just on the skin surface.  There's also an issue of "how much RF does a person absorb in a given field".. VHF is worst, because the body is comparable to a wavelength, so you make a real good absorber.  At lower frequencies, you absorb less.  That's why the exposure limits are higher at lower frequencies.  At higher (microwave) frequencies, the limits are higher because all the power is absorbed in the skin, and getting a "sunburn" is considered less bad than "cooking your internal organs". You're also more reflective at microwave frequencies (which is how motion detectors work, after all).

They've done a LOT of studies on what happens when you heat up various parts of your body (some on laboratory animals, some done on willing subjects, some on not so willing subjects), so there's a very well established science of "how much energy can we put into component X" of the body without causing adverse effects.  (Called Specific Absorption Rate in the literature).  They turn that into a EM field strength that will cause that energy density (using some carefully derived models of the body's RF properties, also based on measurements, some of which I've done myself).

They take those numbers, pick a nice safety margin (like a factor of 10) and draw some straight lines on the graph to make it easy to figure out, and that becomes the limit.   The reason for the safety margin is several fold:
1) you don't know if the person being exposed happens to be especially sensitive
2) you don't know if the environment the person is in happens to be bad. Full Sun falling on you if you were naked in below freezing might be wonderful, not so wonderful in 120F heat.
3) it's hard to accurately measure the field.

Don't forget that the limits are not "instantaneous" power.. they're averaged over a time period (e.g. 30 minutes).  5 minutes in the sun in that ninja suit followed by 25 minutes in the cool shade is probably not going to have the adverse affect that sitting continuously is.  Most hams are not running 100% duty cycle. Most people working near broadcast transmitters are not exposed to full field 100% of the time. (A friend tells me that on Navy ships with lots of externally mounted antennas radiating high power, they have signs in high field areas that say "Sailor, keep moving")


So when you say there are no cases of RF injury at HF, you are just wrong.  There are plenty of them, and the references are all in the back of the C95.1 spec, pages and pages of them.  Industrial accidents, diathermy treatments, etc. All at HF kinds of frequencies. This isn't some sort of speculative, oh, maybe the exposure shows up 20 years later thing.  We're talking about easily discernable acute injuries and their subsequent effects: burns, reddened skin, tissue necrosis, gangrene, the whole shooting match.

If you've ever felt that "RF tingle" on some band when you touched the mic or chassis: you've just exceeded the MPE for RF.  Odds are, you don't sit there hanging on for half an hour, though, any more than when you get zapped by touching the 110V line, you just hang on for the thrill of it: like those crazy people taking an "electric cure" at the turn of the last century.

RF burns are nasty. It turns out that unlike a DC or line frequency shock/spark, RF is high enough in frequency to not trigger the pain nerves in your skin.  The current tries to flow in the low resistance tissue (i.e. inside your skin) so they tend to be deep burns with small entrance wounds, and remarkably painless when they happen (because although the current density is high, burning the tissue, the total energy deposited is small, so it doesn't get noticeably warm). It's like having a very sharp hypodermic needle poked into you when getting a tetanus injection: you don't feel it going in, but the tissue damage or inflammation sure hurts later.

The IEEE/ANSI standard (from which OET Bulletin 65 limits are derived) has the research that establishes the safety limits. It's not opinion.  Let's be clear here, while cancer connection is sketchy, damage from thermal effects is fully substantiated and backed up by data.  There is *tons* of data on what power dissipation causes a burn, *tons* of data on absorption (particularly at non-microwave frequencies), so there's no doubt about the harm resulting from excessive RF exposure, any more than doubt about harm resulting from grabbing onto a 440VAC power line.  I'll bet there's more than one person reading this who has actually gotten an RF burn.

The safety limits are NOT talking about instant death.  They are talking about injury or measurable adverse effects.  You might consider accidentally smashing your finger with a hammer benign, but it's an injury none-the-less.

The limits are also set lower for the casual public, because they, presumably, are not deriving any benefit from the exposure, so their presumed risk tolerance is less.  This is true for virtually all "exposure to hazard" kinds of safety rules and is no different for EM fields.


One thing to keep in mind, when considering chronic vs one-time exposures, is the cumulative effect over time.  So far, there appears to be no such cumulative effect for EM fields (a few studies showing correlation with some cancer, but nothing conclusive, yet) as there is for ionizing radiation or things like smoking.  (that is, one cigarette won't cause any appreciable damage, smoking them for years definitely will)

Limits are all the more important when it comes to your neighbors and the general public. I don't much care if you want to go out and juggle running chainsaws in your backyard, but I certainly don't want you doing it with me standing next you, exposing me to the hazard.

Do the analysis, keep the records.  It's not that hard. 

doh..

ok, so you're definitely in the near field...

I guess you haven't read IEEE/ANSI C95.1-2005, which is the RF exposure standard.  It's got about 100 pages of analysis in there, and yes indeed, there are plenty of cases where people have suffered ill effects from exposure to RF. Notable cases are crewmen on an aircraft carrier where significant VHF power was coupled through them between an aircraft on the deck and the deck, leading to ankle and wrist pain.  There are also numerous documented cases of people exposed to high RF power in transmitter and radar situations with burns and injuries, not to mention thyroid damage and cataracts.  There's a reason for the "don't look into the open waveguide with your remaining good eye" signs are around.

There's a ironically humorous FCC enforcement action from about 10 years ago where a worker was up a tower with a FM transmit antenna running at reduced power. Station manager was getting calls during drive time about the signal being weak, so he jacked the power back up.  The tower jockey's protective clothing caught fire.

In general, the limits are set based on documented thermal effects.  The cancer/RF connection is more tenuous. There's been some studies in vitro, and some more recent studies with cellphone exposure, but it's not conclusive.

The maximum permissible exposure for "controlled exposure" (i.e. you're working with it) is typically set at 1/10th of the level where effects have been seen. The MPE for "uncontrolled" (i.e. general public) is typically 1/10th of that.

And because that antenna is huge, the power density is fairly low.  If it's 20 dB and 250kW, that's 25 MW E*I*RP, but it's still just 250 kW being radiated, just over a smaller angle than the whole sphere.   The Delano VOA station radiated 500kW in a 12/6/0.5 array.. that's 12 dipoles wide and stacked 6 high, so it covers a huge area. Say they were radiating on 40m.. those 12 dipoles mean the antenna was 240 meters wide and probably half that high.   Spread 500kW across 20,000 square meters and the power density gets pretty low (25W/m2 or 2.5mW/cm2)  which is way below the MPE..
(MPEs are frequency dependent, but 10mW/cm2 for controlled exposure is about as low as they get)

As a practical matter, the EPA measured the field at the fence by the Delano VOA station (because of a suspected cancer cluster in McFarland) and I think the peak was <10V/m E field, which is about 0.025 mW/cm2.     

because the field isn't all that high.  Stand in front of a 10kW average power magnetron fed into a horn, and you've got problems.

That's hardly a valid test, and you know it.

146 MHz = a bit more than 2 meter wavelength.  6 feet away is only two wavelengths..

All those online calculators use "far field approximation"   (that is, they convert to an Effective Isotropic Radiated Power, and assume a point source) and a couple wavelengths away isn't "far field"..  The division between far and near field is a fuzzy one, but in general, one likes to say "several wavelengths" to be sure. (and don't get confused by  the term "far field" used in antenna range measurements: the 2*D^2/lambda criteria.. that's not relevant here.. that's where a "plane wave approximation" is valid)..

The problem with the near field is that there are antennas with very large near field energies (compact HF loops are notorious) even if their gain is terrible in the far field.  You have to look at the actual magnetic and electric fields to determine whether you are within the limits.

Another notorious problem is being close to the bottom of a HF vertical on low bands.  Wavelength is 40 or 80 meters, and most of the time, the antenna is sitting your backyard, and most people's backyards aren't 80 meters or 160 meters long.

OET bulletin 65 gives you some "safe harbor" calculations for things, and if you happen to exactly match one of their canned solutions, then you've met the legal requirement.  They've actually gone out and measured/modeled the near and far fields and generated the tables accordingly.

Do anything else, and you're on the hook to do a real analysis (or find someone who can).

I think the best long term solution for <100 Watts and limited space (but not severely limited) is a remote automatic antenna tuner at the feed point of whatever length wires you can put up.  it doesn't have to be perfectly symmetrical (or even close). There are some "particularly" bad lengths (you don't want the overall length of the antenna to be exactly 1 wavelength), but other than that, you get all band operation, coax feedline, minimum losses in that feedline (since it's always matched).    You can also put multiple wires of multiple lengths up, which will help the pattern on higher bands, if nothing else.

As others have noted, the other choices either have losses in the antenna (and how ever much loss there is in the autotuner, it's probably not 6dB, like the typical Terminated Folded Dipole) or require a low loss feedline and a tuner at the shack.

I've fooled with a variety of "no tune" or "multiband" antennas of one kind or another, and they're fine if you've got more time than money, because you're going to be doing a fair amount of fiddling around to get them to work right.

power density isn't as troublesome in the magnetics though. You can always get bigger cores, etc.   The problem with the transistors is that the dice are just so small, so that's where the big thermal issue is.

(After all, there are millions of liquid cooled transformers around already)

This is incorrect.  In fact, black things radiate better than other colors, although there isn't much difference.  What you're talking about is a property called emissivity, and most painted surfaces run somewhere around 0.9, regardless of color. Shiny things, though, have low emissivity in general (0.03 for silver and gold)

You need to be very careful, too, about comparing properties  in the visible spectrum (e.g. solar collectors) and far IR (where heat sinks radiate).  Well designed solar collectors are actually designed to have high absorption in the visible, but low radation in the far IR (heat).  Black paint is actually non-optimum for a solar collector.

Interestingly, shiny metal actually radiates worse.  (this is why those seat belt latches and shiny metal railings get hot in the sun..)  It has to do with the ratio of absorption and emission (so called alpha over epsilon ratio).

here's a table of a, e, and a/e: http://www.solarmirror.com/fom/fom-serve/cache/43.html

All that said, heat sinks generally cool by conduction to the surrounding air, not by radiation.  Even if there's no fan, they rely on hot air rising (fanless heatsinks should have the fins oriented vertically).  For conduction, thinner paint is better.  Most aluminum isn't painted, it's "chemical conversion" treated: anodizing, alodining, etc.

Most Tesla Coils (esp big ones) are in the 100 kHz range.
And these aren't spark gap transmitters.  They are basically a solid state power oscillator (usually using big IGBTs) driving an RF transformer, and actually have fairly well controlled RF spectrum. (google for DRSSTC)

However, given that they are operating in the 3km band, the "antenna" is pretty small and inefficient.

I've done quite a lot of analysis and tests on this kind of thing and the dominant RFI from them is actually not from the fundamental, but is the transients from the spark.  An individual spark is discharging  the capacitance of the top terminal (some 10s of pF, at a few hundred kV) over a loop that's several meters in cross section.  That is a pretty decent di/dt and generates a very nice waveform to kill victim equipment that's in the area.

Right now, liquid cooling is pretty complex and very individual design sensitive: that is, every design needs a different cooling design, either from a thermal standpoint or a packaging standpoint. That translates to expensive.  What's needed is something that is mass produced to meet a consumer need, which we as amateurs can then repurpose to our own needs.

There is some hope because the increasing power density the CPU and GPU world means that just about every laptop made these days has a heatpipe in it, and there are quasi-off-the-shelf liquid cooling schemes for PCs.  The cost of chillers has has also come down: either using Peltier devices (very inefficient) or just because of the inexpensive off-shore-manufacturing of simple mechanical devices (kegerators, bar fridges, etc.). You see chillers for aquariums at remarkably low prices compared to what we used to pay for a standard lab chiller.

Sooner or later some inventive ham is going to start looking at how do you bolt a set of FETs onto the "coldplate" of a CPU chiller, and still get the RF part of the design to work ok.  And gradually, the articles will change from "how to build a chimney and blower for your power grid tube"  and "how to make your own socket for that broadcast pull" to "how to turn an icemaker into a chiller" and "how to safely bend the heat pipe on a laptop CPU chiller".

What engineering basis do you have for recommending more rods?  As you point out "installing them too close... is poor economy", and I think the same would apply to number as well as spacing.

Not that more rods isn't better (it is, in general), but where is the cost/benefit tradeoff? 

Let's look at the physics.  There's three reasons to connect to the soil:
1) RF ground for an antenna where the soil is part of the radiating system
2) Electrical safety, in case a power line or other live conductor comes in contact with the antenna
3) Lightning energy dissipation

Ground rods in the middle of the bonding run do nothing for #1, because the AC resistance of the wire connecting them is sufficiently high at amateur HF frequencies that it's like hooking a big resistor between the "ground" and the antenna.

Ground rods in the middle of the bonding run do nothing for #2, because the DC/60Hz resistance of AWG6 (<0.5 ohms in 100 ft) is so low that the voltage drop along the wire is small in the event of a fault, so that all the equipment stays close to "bare feet" potential. (one of the reasons for NEC grounding)

Ground rods in the middle of a bonding run don't do much for #3, because the inductance of the 20 feet of wire to that middle rod is about 6-7 microhenries.  At the nominal 1 MHz for lightning, that's an impedance of 18 ohms, and at, say, 1 kA, that's a voltage drop of 18 kV.  Buzzing along to the next rod, the current is less, but the inductance is more... The net result is that the added rods don't add much to the effectiveness of a lightning ground.    If you're worried about reducing the impedance of your lightning ground, drive more rods near the antenna, or even better, bury some radials, or, if the antenna has a concrete base, make sure it's well connected to the grounding system.

Take home message:

The bonding rules aren't for lightning or RF.  They're for line frequency faults and shock hazard.  From that standpoint, it turns out that the bonding really doesn't need to be connected to "earth ground" very well at all. (and in fact, the NEC code makers are gradually changing the wording from grounding to bonding)

Long wires (unless buried) make terrible additions to RF grounding system.

Lightning is all about keeping the strike energy somewhere other than delicate stuff,  and making sure that all equipment goes up and down together.  The currents are so high and the rise time so fast that trying to keep the voltage "low" is impossible.

Ground rods worked great for Ben Franklin, because they replaced *nothing*.  But there's a lot of better ways to do it now, and we know a LOT more about the underlying physics and applications.

the answer to the question is the classic "it depends".

If it's a SDR approach where you're basically feeding bandlimited raw samples to a PC for processing, then USB is probably easier.  If it's an approach where you have some "smarts" in the box between sampler and interface, or where you might want to have multiple PCs and multiple sample streams, Ethernet is going to be more appropriate.

If it's an SDR (a'la Flex) where you are leveraging inexpensive high volume gear (audio interfaces), then you want whatever the inexpensive audio gear uses, today, that's USB 2.  Who knows where it will be in the future?  Thunderbolt? USB3? something new?

People have been professionally building, using, and fixing tube amps for almost 100 years, and solid state amps for a bit less than half that.  Hams are running somewhat behind that.. I would say that the fraction of hams who have built and modified and fixed solid state amps is perhaps a few percent of those who have done the same with tube rigs.

In another 30 years, I suspect it will be different.