Measuring Pt 15 AM Field Strength

[dickfry]

Re: Lets cut to the chase...

> Please note that I don't care in the least about the
> configuration of the antennas that Part 15 AM users install
> and use, or how much field strength their systems produce.
> I have only attempted to inform Part 15 users of the
> provable technical performance of these configurations that,
> judging from what I read on these boards, they may not have
> recognized beforehand. They are free to use or reject this
> information as they see fit. But -- at least now they might
> know what they are really doing.
>
> //
>
Woof woof woof. Arf arf. I hear that barking sound again. Can't the neighbors train that mutt to shut up when there's nobody home?

-The Other Dick

 

[dickfry]

> The FCC charts even include instructions about how to use
> them for any power level.
>
> //
>

The FCC also warns in their RF field footnotes that the charts should not be used for near field measurements. I think they limit it to beyond 10 wavelengths, if I remember correctly. That's pretty much the coverage area of most Part 15 stations.

When you get too close to an antenna, you must switch to the more reliable free space measurements. Free space curves show much less loss than the distant field charts. I know you know this, or at least you would have to have known it at one time. So don't hand us this warm steamy load and pass it off as science.

Great, you have a computer program that runs FCC charts. And so do I. But I also know when NOT to use that program. That time would be anytime the fields being measured are so close to the antenna that the charts are inaccurate, and therefore irrelevant.

The Glory Days are over. Stop embarassing yourself by putting out misleading "facts." Yeah, some people will buy anything from a guy in a nice suit, but a lot of us know better.

-The Other Dick

 

[rfry]

Re: Insanity is... (Re: Lets cut to the chase...)

> and antenna height *does* significantly
> affect Part 15 AM range, unlike full-power
> AM stations with full-size towers.
______________

If you want to prove to yourself whether or not this belief is true:

1. Using the signal strength meter on a ham receiver, measure the field strength received from your present, elevated Part 15 AM antenna+long ground wire system.
2. Disconnect the audio, power and ground lead/wire from the Part 15 tx, leaving only the 3 meter antenna and loading coil connected to the tx.
3. Just below the tx, temporarily install a battery pack of the correct voltage for the tx, and connect its power leads with the right polarity to the tx. Be sure that the entire battery pack is insulated from contact with anything but the tx, and that the case or chassis of the tx is not in contact with any metallic path to ground.
4. Re-measure the field strength, using the same receiving conditions as in step 1.

If the field strength measured in step 4 goes down, you will know that the reason for the higher field strength from your original elevated antenna is not due to the height of the 3-meter section above ground, but to the added radiation from using the long ground conductor(s).

If you or anybody else would do this and report back, it would be a good reference point for everybody.

//

 

[JasonW]

Re: Insanity is... (Re: Lets cut to the chase...)

Why don't *YOU* do it? I -know- that elevating the antenna increases the range, from my own experience and the repeated experiences of many other Part 15 AM broadcasters over the years.

I don't really care -why- it works, only that it -does- and that it's legal.

If you want to be Marconi Jr. and conduct controlled experiments to find out whether the range increase is due to the longer line-of-sight from the higher antenna, a lowered angle of radiation, a radiating ground, or some combination of all three, more power to you. -- JasonW

> > and antenna height *does* significantly
> > affect Part 15 AM range, unlike full-power
> > AM stations with full-size towers.
> ______________
>
> If you want to prove to yourself whether or not this belief
> is true:
>
> 1. Using the signal strength meter on a ham receiver,
> measure the field strength received from your present,
> elevated Part 15 AM antenna+long ground wire system.
> 2. Disconnect the audio, power and ground lead/wire from the
> Part 15 tx, leaving only the 3 meter antenna and loading
> coil connected to the tx.
> 3. Just below the tx, temporarily install a battery pack of
> the correct voltage for the tx, and connect its power leads
> with the right polarity to the tx. Be sure that the entire
> battery pack is insulated from contact with anything but the
> tx, and that the case or chassis of the tx is not in contact
> with any metallic path to ground.
> 4. Re-measure the field strength, using the same receiving
> conditions as in step 1.
>
> If the field strength measured in step 4 goes down, you will
> know that the reason for the higher field strength from your
> original elevated antenna is not due to the height of the
> 3-meter section above ground, but to the added radiation
> from using the long ground conductor(s).
>
> If you or anybody else would do this and report back, it
> would be a good reference point for everybody.
>
> //
>

 

[rfry]

See responses below...

> The FCC also warns in their RF field footnotes that the
> charts should not be used for near field measurements. I
> think they limit it to beyond 10 wavelengths, if I remember
> correctly. That's pretty much the coverage area of most
> Part 15 stations.

The near field of an antenna is defined as all regions within [2*(Antenna Length)² / Wavelength] radius of the antenna radiation center. See almost any antenna engineering textbook to verify.

So the distances in my earlier post are in the far field, well beyond that distance for a 3-meter Part 15 AM antenna.

> When you get too close to an antenna, you must switch to the
> more reliable free space measurements. Free space curves
> show much less loss than the distant field charts. I know
> you know this, or at least you would have to have known it
> at one time. So don't hand us this warm steamy load and pass
> it off as science.
>
> Great, you have a computer program that runs FCC charts. And
> so do I. But I also know when NOT to use that program. That
> time would be anytime the fields being measured are so close
> to the antenna that the charts are inaccurate, and therefore
> irrelevant.

Incorrect. My numbers were based on the inverse distance field. The FCC charts show field strengths over the range from 0.1 kilometer to 5,000 kilometers, including the inverse field value. The inverse field is the unattenuated groundwave field produced over a path with zero ground loss. Probably this is what you are calling the "more reliable free space measurements."

> The Glory Days are over. Stop embarassing yourself by
> putting out misleading "facts." Yeah, some people will buy
> anything from a guy in a nice suit, but a lot of us know
> better.

My posts have not been proven to be technically incorrect, or non-factual -- therefore they cannot mislead anyone. They may give information that some readers may not want to accept for some reason, but not from lack of their accuracy. More likely because of their accuracy.

//

 

[dickfry]

>
> Incorrect. My numbers were based on the inverse distance
> field. The FCC charts show field strengths over the range
> from 0.1 kilometer to 5,000 kilometers, including the
> inverse field value. The inverse field is the unattenuated
> groundwave field produced over a path with zero ground loss.
> Probably this is what you are calling the "more reliable
> free space measurements."

Yeah, quantifying an unatennuated signal over a path with zero ground loss would probably qualify as a free space measurement.

Sorry if you're having trouble following this stuff.


-The Bigger Dick