There has been some online comment to the effect that commercially-produced Part 15 AM transmitters with internal loading coils typically cannot impedance-match the transmitter to antenna systems with radiating lengths longer than about 3 meters.
Clarifications/comments about this:
- Typical radiating conductors of Part 15 AM antenna systems consist of a ~3-m whip (or wire) attached directly to the tx "antenna" connector, and all of the conductors connected to the circuit bus common, chassis, or r-f ground terminal of the transmitter. (Note 1)
- The internal loading coil included in some commercial Part 15 AM transmitters usually has customer-selectable "taps" to vary its inductance. This may enable a better match of the transmitter to the impedance of the complete antenna system connected to it.
- Increasing the lengths of those radiating conductors decreases the inductance needed from the loading coil to achieve the best possible impedance match between the transmitter and the antenna system.
- A stock, internal loading coil typically cannot match the tx to its load impedance if the radiating length of the antenna system is much less than 3 meters, because the relatively high inductance needed to do that is not available from that loading coil.
- Using longer radiating lengths for the antenna system may enable finding a tap on a stock, internal loading coil where a reasonably good impedance match is possible. (Note 2)
1. Including a path from the transmitter system to an a-c power utility ground rod buried in the earth.
2. The "KENC case" is one example of this, where reportedly a transmitter with a 3-m whip attached and having an internal loading coil was installed and connected to the top of a radiating (yet "grounded") metal tower about 40-50 feet tall. Apparently that transmit system was able to be optimized for that configuration.
Hopefully this post will be understandable to everyone. Using (many) fewer words in it would not provide a sufficiently accurate description of this scenario.
Source Unknown
Foreign remark: "There has been some online comment to the effect that commercially-produced Part 15 AM transmitters with internal loading coils typically cannot impedance-match the transmitter to antenna systems with radiating lengths longer than about 3 meters."
Such an "online comment" would not have been made on a part 15 website because antennas in excess of 3-meters violate FCC rules for part 15 and would be regarded as supportive of pirate radio which violates website policy.
See the thread about "antennas" vs ground conductors on the ALPB forum.
Gnosis
Knowing context changes the discussion.
IIRC, there have been many, many discussions even here on Part15.org about "Part 15 AM" antenna systems with radiating lengths greater than 3 meters.
Gosh: "There have been many, many discussions even here on Part15.org about “Part 15 AM” antenna systems with radiating lengths greater than 3 meters."
Alright. You have the floor. Let's talk about 5-meters!
I've posted about this already, including how these systems function.
Probably you can research/review such in a search of this Forum.
Then why bring it up again?
You might take note that this is not the ALPB.
Don't take the bait Artisan.
Actually, my opening post in this thread contains some information I hadn't previously seen or posted on Part15.org.
I thought it would be of interest here, given the emphasis this site has given to FCC Part 15 issues.
I, being involved with the discussion over there about antenna length assure everyone that no illegal operation was discussed or I wouldn't have been involved in it.
Also from the Canadian standpoint an antenna/ground lead in excess of 3 meters is not illegal operation as we can also go by field strength without regard to power/antenna length. See the rule here.......see annex B2
https://www.ic.gc.ca/eic/site/smt-gst.nsf/eng/sf01320.html
You can see that we can also have 250uV/M@30meters and no power/antenna length restriction.
The discussion was about, since most commercially marketed part 15 AM transmitters have an on board tuning system suited for the 3 meter antenna making the antenna longer would not make your range better but worse, thus discouraging any adding to antenna length.
Mark -
Please check for a new private message here on Part15.org.
Such an “online comment” would not have been made on a part 15 website because antennas in excess of 3-meters violate FCC rules for part 15 and would be regarded as supportive of pirate radio which violates website policy.
Actually this is not correct. If you choose to comply with Part 15.209 rather than Part 15.219 your antenna can be a long as you want. You can have an elevated ground lead as long as you like. You can run unlimited current to your final stage. As long as you meet the emissions limits of 15.209.
TIB
Mark, and others interested...
Please see my post of this morning at
http://alpb.boards.net/post/15129
Information is Knowledge
Rich's morning post from the ALPB seems to indicate that the Procaster AM Transmitter has versatility that could be useful in some situations.
The information also gives rise to the question - can the other transmitters, the ones with user selectable taps on their internal loading coil, be modified to perform like a Procaster by the addition of an appropriate tuning capacitor?