New FCC RF Safety rules come into effect on May 3 - what you need to know

The insertion loss of coax is usually such a small part of the equation (especially at HF) that it can be easily ignored, or calculated as a worst case calculation.

 

Hardly.

Above 20M, with a hundred+ feet of coax, your power estimate can easily be off by 10-30%+. Especially if the coax is a few years old.

It makes a difference between being in, or out, of compliance. YOU may choose to ignore it but the FCC REQUIRES that you KNOW how much power is going into the antenna--a priori. Not an unjustified brushoff.

The FCC didn't ask you to 'calculate a worst case', or do a SWAG analysis. They asked you to calculate the power that, effectively, goes into the antenna at your bands of use. That means you need to know 1) the power output of the TX system; 2) the insertion loss of the cable system, amongst other observables.

73
Chip W1YW

 

I'll just let them know my cable has 0 dB of loss. If it passes at that, it will pass with any other amount of loss that any "real" coax has. I think you are taking the regulation too literal, or too personal, or something.

 

Here we go with more 'personal' stuff.

Can you try to re-align yourself please? This 'personal' projection when someone disagrees with someone else is really inappropriate for these discussions.

If a lawyer saw your swag, he or she would then question evidence for every other single factor. For example, how do you KNOW that your TX power never exceeded X watts; how do you KNOW that no one was capable (or did) lie within the RF exposure distance? ETC.

The burden becomes YOURS to demonstrate a priori, not a posteriori. Why? Because that's what the REGULATION FOR COMPLIANCE says.

What's 'reasonable' and what's 'legal' are often two very different things.

73
Chip W1YW

 

What do you suggest? actual measurements at the antenna?
(that may come off as sarcastic, I really do want to know what we should be doing to be compliant)

 

You seem to not understand that any amount of coax loss can only lessen the total exposure, it can never increase that amount. If you assume that all of your TX power will arrive at the antenna, and you are still in compliance with that figure, no amount of cable loss will make you non-compliant. That is just physics.

 

See my earlier posts.

I think it is likely that we will see a 'how to' from the ARRL showing how to measure cable assembly insertion loss at MF/HF; we may also see vendors selling 'packages' of coax with a small chart showing the measured insertion loss for that item.

You have to show compliance up front. That's what's new as of May 3. For extant antenna systems you have 3 years, I believe. New or changed ones are right now.

73
Chip W1YW

 

He'll want you to use a lab grade wattmeter and a VNA to measure cable loss. Or maybe get some of that fancy stuff that actually measures RF density.

If the FCC really cared about minutia, they would allow us to use "rules of thumb" saying that SSB is 20% without speech processing. Maybe it's really 18% or 25%?

 

Again, you have made this personal.

I am pointing out the compliance regulation, as a legal requirement, not as a reasonable supposition.

Acknowledge you get that please.

73
Chip W1YW

 

Sure. And if I calculate my cable loss at 0 dB, then change it to any other cable, would I be out of compliance?

 

Lab grade wattmeter. We talking a Bird 43 or power meter? Recent cals?
What about the operators that don't have lab gear?
I either have or can get my hands on it but others won't have these resources.

 

On some website (can't remember what it was now) I saw many years ago, they had pictures and video of a mobile installation which was claimed to be a 70kW installation.

They've probably already burned out most of their brain cells. On the previously mentioned website there was also another video of a few guys looking over a high powered mobile installation and one of them jokingly said, "We're all gonna get cancer."

Which may not be too far from the truth. In the book Tower of Secrets written by a Russian KGB defector named Victor Sheymov, there is a description of a room used by the KGB headquarters in Moscow. I believe the room contained crypto equipment or something highly sensitive like that. To prevent eavesdropping on electromagnetic emanations, they installed powerful wideband transmitters inside the room to cover the electronic signals generated by the equipment. The room was also lined with metal walls, and when the door to the room was opened, it would disrupt TV reception about a mile away. The issue was that anyone who worked inside the room tended to get sick and develop diseases like cancer, and they went through employees very quickly, like having to put someone new in the room every couple of years or so because the others were getting sick and dying. The little regard the KGB and Russian government had for its own people was one of the motivating factors leading to Sheymov's defection.

 

This is the whole point. Most hams won't need any equipment to make the compliance calculations. Maybe a tape measure, probably not.

Only if your calculations showed you really close on compliance would some sort of actual measurement be needed.

 

Wrong.

Almost all digital wattmeters in the last 10 years have sufficient accuracy and precision to within a few watts at QRO. They are cheap and available on the used market. Not 'lab grade'.

And we already have cheap VNA's that do S12. The simple ones are available on Amazon for what, $50?



If you have a good one, you can normalize to a calibrated short piece of coax on a THRU setting, and then get the insertion loss of your 'under test' antenna coax directly. If not, you can measure the cal, then measure the coax under test, and determine the numerical difference.

Instead of falsely speaking FOR me, why don't YOU do everyone a favor and make a VIDEO showing how to measure insertion loss with a cheap hand held VNA?

THAT would be helpful.

73
Chip W1YW

 

Why would I do that? I am the one claiming it doesn't even need to be done (which it doesn't).

If I comply with 0 dB of calculated cable loss, I certainly will comply with .5dB of cable loss. The new rule only says you need to make a compliance evaluation, not actually measure anything.

For example, I have a 20M dipole at 35 feet. I run a 100 watt transmitter on SSB and CW. Am I compliant? Yes. And you can run all the numbers you want, on any type or brand of coax cable running to that antenna and the answer will always come out "compliant".