Hi there HP,

Would you happen to know what that translates into going to watts at the transmitter?

For 100uV/m at 10 meters and maybe at 3 meters, into watts at the transmitter. I dont work with radio very much and i am getting numbers that are just too unreasonable.
So the question is, given a perfect measurement at 10 meters of 100uV/m, what is the transmitter power.

As @Aleph(0) points out -- there are many 'complicating factors', I suggest THIS RESOURCE as a starting point but with the caveat that, inasmuch as environs alone can (and, as a practical matter, do) effect radiation patterns (and, hence, 'gain symmetry') you are advised to take the claim of 'maximum possible field-strength' with the proverbial 'grain of salt'! Hope you find this helpful!

Very best regards
HP

 

Closest thing I can think of to isotropic radiator is star which is just big a$$ pan-spectrum noise source

--EMPHASIS ADDED--

...And I would say that's a tad ambiguous inasmuch as it's as applicable to stars in the sky as to those in Hollywood!

TTFN
HP

 

Or, going back to basics, what about a theoretically perfect point source spherical radiator? As I recall, *all* antennae are modifications and variations of this starting point.

ak

 

Hi there HP,

Would you happen to know what that translates into going to watts at the transmitter?

For 100uV/m at 10 meters and maybe at 3 meters, into watts at the transmitter. I dont work with radio very much and i am getting numbers that are just too unreasonable.
So the question is, given a perfect measurement at 10 meters of 100uV/m, what is the transmitter power.

http://www.antenna-theory.com/basics/friis.php
http://www.random-science-tools.com/electronics/friis.htm

 

Hi,

Is there anyone here that can actually calculate with any antenna?

Pick an antenna that has gain of 1, assume an omni pattern, assume no other interference, etc. DO whatever it takes to make a single calculation.
Thanks

In the mean time i'll read the links.

BTW, there are calculations that convert from watts to dBuV/m.
No antenna required.

An application would be to see if a transmitter theoretically matches the FCC spec which is 100uV/m. The wattage of the transmitter would be known. With the straightfoward calculations i get 20nWatts which sounds way too low. I would expect at least 1mw at least. On one of the other frequencies they specify 100mw for example not in units of uV/m.

LATER:
Oh ok, i found some good info in HP's link. Thanks HP.
Yes that's what i needed, the maximum possible uV/m.
I'll try that next...

OK, same result: 20.32 nano watts produces 100uV/m at a distance of 10 meters. Sounds too low. Who uses a transmitter that's only 20nW ???

 

Hello again,

Ok it is clear what they are doing now...

They take the transmitter power and project that onto the surface of a sphere of radius r=distance from transmitter (10 meters for example). That creates the power distribution. Then they use a form of Ohm's Law V=sqrt(P*R) to calculate the voltage per meter and use 377 as the resistance of free space, that gives a quantity V1 in volts per meter. Then they take the gain G of the half wave dipole (a common antenna) and multiply V1 times that gain and arrive at the 'final' field strength in volts per meter:
V/m=V1*G
The gain comes from the distribution of the real antenna relative to the distribution of the theoretical isotropic antenna. They state G=1.28 but i'll test that later.

That's really all i wanted, but it still comes out sounding too low, only 20nw to produce the 100uV/m at 10 meters out.
I have a feeling that most real setups are going to have a lot of losses before the signal gets out there anyway. I'll have to read more on this.

 

Pick an antenna that has gain of 1, assume an omni pattern, assume no other interference, etc. DO whatever it takes to make a single calculation.
Thanks

Please be advised that the isotropic (i.e. spherical pattern) radiator is the only 'pure' antenna having a gain of 1 (i.e. 'unity gain') -- Such is a hypothetical 'abstraction' without a 'real world' counterpart.

The link I provided above supplies the (rather pedestrian) math descriptive of the isotropic radiator accompanied by coefficients required to emulate 'dimensioned' monopolar radiators...

Please understand that, as a practical matter, compliance can be assured only by field assessment via measurement -- while formulary may grant insight, such is an unreliable predictor as applied to 'real world' dynamics (owing to unreckoned contingencies) - and utterly inadmissible in the courtroom!

Best regards
HP

 

I have a feeling that most real setups are going to have a lot of losses before the signal gets out there anyway.

And, not uncommonly, 'concentration/focusing' in certain directions producing greater field strengths than calculated at points upon the periphery -- Again, if this is a matter of abstract study with you then all's well and good! - On the other hand, if you're looking for a 'way around' field assessment you're riding for a fall

Best regards
HP

 

Or, going back to basics, what about a theoretically perfect point source spherical radiator? As I recall, *all* antennae are modifications and variations of this starting point.

ak

That'd be the isotropic radiator -- If but practical systems were so pristine!

Best regards
HP

 

Please be advised that the isotropic (i.e. spherical pattern) radiator is the only 'pure' antenna having a gain of 1 (i.e. 'unity gain') -- Such is a hypothetical 'abstraction' without a 'real world' counterpart.

The link I provided above supplies the (rather pedestrian) math descriptive of the isotropic radiator accompanied by coefficients required to emulate 'dimensioned' monopolar radiators...

Please understand that, as a practical matter, compliance can be assured only by field assessment via measurement -- while formulary may grant insight, such is an unreliable predictor as applied to 'real world' dynamics (owing to unreckoned contingencies) - and utterly inadmissible in the courtroom!

Best regards
HP

Hi HP,

Yes thanks for the link that helped the most. Yes the pedestrian math, it was so pedestrian i ran right through it (har har har)

Yes a measurement would be best if this were to go practical, but the FCC does allow "good engineering practices" too believe it or not. I would measure anyway though because i am quite sure the field strength from the wattage calculation would be far too high and misleading because it did not take into account mismatches in the system. So i'd be cheating myself if i went strictly by the calculation. I just wanted it for insight into what the FCC is actually asking there in part 15. Apparently they want a very small field strength for some frequencies. That's the main issue, what frequency to use. Certainly 10MHz isnt good unless you can put up with low signal level.

See what this was really all about was an exploration into what the FCC was asking for when operating at various frequencies, and when they state 100mw for some frequencies that's easy to cope with because we just make the transmitter able to put out 100mw or less. But when they specify field strength at a certain distance, we have to be able to translate the power of the transmitter to the field strength to get some idea what the difference if any would be for those frequencies.
What i learned was that using some frequencies would be better than others because they allow a greater field strength. For my two test points, the ideal power comes out to either 100mw or 20nw, QUITE a difference, so obviously, no matter what setup you use or antenna you use, the 100mw is going to outshine the 20nw transmitter unless there is some very special circumstance.
So really this was a comparison between what is allowed on two different frequencies, which is more or less independent of the antenna. If we go ideal with one then we go ideal with the other, if we included gain with one then we include gain with the other.
The results are comparative and very informative.

There is a drawback to the 100mw allowed level however and that is the max frequency is about 1.7MHz, while the 100uV/m frequency could be 10MHz for example.
I also doubt i would use a 1/2 wave dipole either because i would need it to be smaller and thus less radiated power too.

So you see what i was after now and why i asked the questions i did?

Another issue is aimed more at the original point of this thread too, and that is about the harmonics. If we have a square wave at 1MHz and we let the third harmonic out (3MHz) it could be 1/3 of the fundamental, and that is OUTSIDE of the band allowed for 100mw. So if we used 100mw we'd have to ensure that the third harmonic was cut by A LOT in order to ensure the power for that frequency is 20nw or so, ideally. That means some serious filtering.
Of course we also have the 5th and higher, which will be out of band too, so the filter has to cut those too.

 

I would measure anyway though because i am quite sure the field strength from the wattage calculation would be far too high and misleading because it did not take into account mismatches in the system.

the ideal power comes out to either 100mw or 20nw, QUITE a difference, so obviously, no matter what setup you use or antenna you use, the 100mw is going to outshine the 20nw transmitter unless there is some very special circumstance.

Indeed! With regard to radiating structures applied to typical 'type 15 systems' - You'll find the 'legal limit' drive 'errs' rather nearer 7 dBm than -47 dBm! -- While I'm undecided as to whether 'life imitates art' -- I'm certain it imitates the IRS!

Apparently they want a very small field strength for some frequencies.

FWIW Years ago ≤ 20 dBm (i.e. 100mW) 'antenna drive' was the generally applied standard for 'part 15' devices -- then 'peeps' became wise in the ways of antenna design - Thus the 'new' rules' adoption of (essentially) 'range' --as opposed to drive power (or even ERP)-- as the standard... Killjoy bureaucrats!

So you see what i was after now and why i asked the questions i did?

Indeed I do! Moreover I commend both your initiative and desire for intellectual exploration of the matter!

Another issue is aimed more at the original point of this thread too, and that is about the harmonics. If we have a square wave at 1MHz and we let the third harmonic out (3MHz) it could be 1/3 of the fundamental, and that is OUTSIDE of the band allowed for 100mw. So if we used 100mw we'd have to ensure that the third harmonic was cut by A LOT in order to ensure the power for that frequency is 20nw or so, ideally. That means some serious filtering. Of course we also have the 5th and higher, which will be out of band too, so the filter has to cut those too.

Of course non-sinusoidal waveforms require filtering -- that said, development of a compliant signal is requisite of far less 'stringency' than suggested by an ideal model!

Very best regards
HP

 

Hi.
Could a plain 10 MHz monolithic crystal oscillator drive a LT1010 amplifier for short range (under 50m) AM transmitter ?
What undesirable effects a non-smoothed to sinewave RF carrier produces, and implications on AM ?

Yes, needs to be around 10MHz; as compact as possible.

Once or twice I see beginner book projects to amplitude modulate the Vcc of a TTL oscillator can.

However - sticking an antenna on a square wave source is a really bad idea, it has harmonics from here to forever!

You could probably clean it up a bit by coupling it into an LC tank - but you should keep the ERP way down low unless you want an expensive letter from the feds.

 

Once or twice I see beginner book projects to amplitude modulate the Vcc of a TTL oscillator can.

However - sticking an antenna on a square wave source is a really bad idea, it has harmonics from here to forever!

You could probably clean it up a bit by coupling it into an LC tank - but you should keep the ERP way down low unless you want an expensive letter from the feds.

---Emphasis added---
Buy that man (@ian field ) a Miller Newcastle's!!! Which is to say Agreed!

Very best regards
HP

 

Buy that man (@ian field ) a Miller Newcastle's!!!

Why not a Landlord Pale Ale?

 

Why not a Landlord Pale Ale?

Abbot ale - the beer with exhaust crackle additive.....................

 

Hi again,

Just a quick note to thank HP and nsa for the good links that helped me get a handle on this. From this it is certainly clear that the FCC rules have changed quite a bit.
When i was much younger it was 100mw across the board as far as i remember. I just wonder if they had a good reason for changing it.
So now i just have to get up the money to bail out all the people who i told could run their transmitter of any frequency at 100mw

 

Hi again,

Just a quick note to thank HP and nsa for the good links that helped me get a handle on this. From this it is certainly clear that the FCC rules have changed quite a bit.
When i was much younger it was 100mw across the board as far as i remember. I just wonder if they had a good reason for changing it.
So now i just have to get up the money to bail out all the people who i told could run their transmitter of any frequency at 100mw

Its a lot easier to build a transmitter than study how to avoid ending up in the brown sticky stuff...................................

 

When i was much younger it was 100mw across the board as far as i remember. I just wonder if they had a good reason for changing it.

As per post #31 - it seems folks discovered that, provided a properly designed antenna, 100mW was actually useful (perish the thought!) - hence the commission's adoption of 'range' -- as opposed to power -- as the regulated entity

I just wonder if they had a good reason

That's asking way too much of bureaucracy! -- Rationalization being said system's sole stock, trade and, come to that, purview!

Very best regards
HP

 

the brown sticky stuff

Now you're confusing Landlord's with Stout

TTFN
HP

 

Now you're confusing Landlord's with Stout

TTFN
HP

I'll just leave you to your weird fetish................................