From time to time there have been discussions about the effect of using coaxial cable between the r-f output connector of a Part 15 AM transmitter and the feedpoint of an elevated antenna system.
From time to time there have been discussions about the effect of using coaxial cable between the r-f output connector of a Part 15 AM transmitter and the feedpoint of an elevated antenna system.
To investigate this, NEC-4.2 software was used to analyze a system using ~50 feet of coaxial cable with an elevated 3-meter whip, and compare its performance to a system mounted at the surface of the earth, with other conditions the same.
In both cases the applied power was 35 mW on 1650 kHz, and the r-f ground reference was a 3-meter vertical conductor buried in earth of conductivity = 5 mS/m, d.c. 13 (average soil characteristics).
The system using ~50 feet of coax generates a groundwave field intensity at 1 km that is almost 6 times that of the system mounted at earth level -- for the same applied power and ground conditions. This is due to the longer path for r-f current to flow along the conductors of the elevated system, which produces more radiation from this configuration
This performance is very similar to mounting a Part 15 AM transmitter on top of a 33 foot metal flagpole connecting the transmitter r-f ground terminal to the same buried r-f ground as in the NEC study.
The FCC has issued a few NOUOs for the "flagpole" type installations, and it remains to be seen what action they will take with an elevated antenna system using coaxial cable between the transmitter and the 3-meter whip.
NEC study results:
rich
a few here have filed a FOIA request for the missing THII/iAM/ATU documents as well as the rangemaster.
if you care to contact me i will forward you cut and pasts of the info to put in the boxes for each request and you can submit as well.
the more who submit a request the more they will realize this subject can not be swept under the rug and the better chance we have of getting the missing info.
forumsnotify at gmail dot com
Thanks Rich.
That is exactly what I wanted to look at.
"the more who submit a request the more they will realize this subject can not be swept under the rug and the better chance we have of getting the missing info."
Did my FOIA yesterday. The more the better.
8)
RFB
"it remains to be seen what action they will take with an elevated antenna system using coaxial cable between the transmitter and the 3-meter whip."
Before more suggestive claims to make reality out of thin air, it is not. It is a reply to this particular topic.
Rich, given the simulation data, applying that into the real world, does that not present a rather interesting conundrum within the 15.219 rule?
RFB
... given the simulation data, applying that into the real world, does that not present a rather interesting conundrum within the 15.219 rule?
A correctly constructed NEC model shows real world results, actually. The FCC accepts NEC analyses of arrays proposed for licensed, directional AM broadcast stations, and such pattern performance is measured and confirmed to meet FCC-authorized field intensity values after the installation of the array, and before the station can use it, commercially.
As to how an FCC on-site inspector (or the OET, or the EB) would view the performance with respect to §15.219(b) of a "Part 15 AM" system using a coaxial cable between the transmitter and the feedpoint of an elevated, Z-matched 3-meter whip -- that remains to be seen.
Indeed. Thanks for that data and prospective. It's how the FCC et
al would view the situation.
If I recall, a recent FOIA request for the 22 page field agent
manual, there is a paragraph stating the field agent must
determine if a system is operating under one sub part or the
other..specifically it states 15.209 or 15.219, and each one's
own sub parts below that.
The reason why I put "Exemption" in my previous post is that I do
not know of any exemptions made to a device that goes outside of
the very specific rule to which said device was certified for
when that rule clearly doesn't allow certain things.
RFB
Rich, at first I became quite interested and thought your simulation would be usefully informative, but it is rendered incomplete by failure to mention a loading coil, also known as an ATU (antenna tuning unit).
Therefore, might I ask you re-submit the simulation including the loading coil, which would serve to give the study its fullest meaning.
I'd open that sucker up and SEE what's goin' on! A couple dabs of silicone sealer would fix it back up as good as new.. 😉
BTW..
Thank You Rich! I always appreciate your input!
rich posted
"In both cases the applied power was 35 mW on 1650 kHz, and the r-f ground reference was a 3-meter vertical conductor buried in earth of conductivity = 5 mS/m, d.c. 13 (average soil characteristics)."
From previous communication I believe you to be a very literal man, and to say "a 3-meter vertical conductor buried in earth" raises this concern:
In actual practice a 3-meter vertical would be insulated from earth, albeit at earth level. Very significant.
What are we saying?
carl posted: your simulation would be usefully informative, but it is rendered incomplete by failure to mention a loading coil, also known as an ATU (antenna tuning unit).
Both systems need a loading coil, normally located physically adjacent to the bottom of the 3-meter whip.
If their r-f losses both were about 10 ohms, then the elevated system still would have about 6X the field improvement at 1 km over the earth-mounted system -- the same as I posted.
That is why I chose not to include this loss in the models, which I thought were complex enough for many viewers, already.
carl wrote: From previous communication I believe you to be a very literal man, and to say "a 3-meter vertical conductor buried in earth" raises this concern: In actual practice a 3-meter vertical would be insulated from earth, albeit at earth level.
But not a 3-meter vertical conductor that is buried in earth.
A 3-meter conductor can exist in any physical location -- in free space, just above the surface of the earth, immersed in Earth's seas, rivers or lakes, or buried below the surface of the earth.
The 3-meter antenna buried in earth is a short circuit with the negative ground side of the system, isn't it?
I wish I was as smart as I am before these confusions.
I think the confusion here is between the 3 meter element above the surface, and the one slammed into the earth surface..ie ground rod..which those are right around 3 meters or so.
RFB
carl wrote: The 3-meter antenna buried in earth is a short circuit with the negative ground side of the system, isn't it?
A 3-meter vertical conductor buried in the earth is not an antenna.
It is part of the r-f ground reference used by conductors functioning as "the antenna," present in the physical space above the surface of the earth.
This is getting a bit complicated. I suggest that any who have questions, and want a more detailed understanding of the performance of monopole antenna systems might buy/borrow, and read some of the antenna engineering textbooks written by Terman, Kraus, Johnson&Jasik, Balanis, etc.
Added: carl - you might also want to study the graphic I posted earlier that includes a 3-m buried conductor with above-ground radiating conductors: