Howdy,
Howdy,
OK how about this one: The coil can effectively 'lengthen' your short antenna (10' is very short for a MW antenna), by loading it in a small space and enabling it to resonate well, but the price for it is resistance in the wire, which reduces power. Soooo ....
If the power is reduced from, say, 100mw at the TX to say 1/2 that, 50mw, at the base of the antenna pipe, is there a reason why it couldn't be boosted back up to 100mw above the loading coil?
The only problem is that you can not have an external rf amplifier. (part 15 rules) Thats what would be required to get what would be equal to 100mw throughout the whole antenna. Your actual TPO would be greater than 100mw.
Its just simply the laws of physics. Full power AM broadcasters have the same issue as far as losses in the antenna system. A typical 50kw AM operating into a directional antenna may actually have a licensed TPO of 53kw or there abouts so what actually radiates is equal to 50kw.
In the spirit of part 15, our power INPUT to the final rf stage shall not exceed 100mw on the meduim wave band. So actually your TPO should be a bit less than 100mw.
If you changed the TPO from say 90mw to 50mw...no one would really be able to tell that much difference in your signal.
A quick note to clear up a possible misconception above, a part 15 operator in the AM band can have an external amplifier, that amplifier just can't have more that 100 mw DC input so it's of marginal value.
Several transmitter designs and at least one commercially available unit do place an external amplifier at the base of the antenna to overcome transmission line losses for a transmitter that is some distance (in feet) from the antenna but that final stage is still limited to 100 mw DC input.
I think a second challenge of boosting above the coil would be that the booster amplifier would still need to be matched to the section of the radiator above the coil which would require, yep, another coil...
Bob Carter:
"The only problem is that you can not have an external rf amplifier. (part 15 rules)"
Thanks ... I was thinking that as well, but I'm not sure the regs specifically say so. I couldn't find a reference, so it may require more digging when I get time.
"transmitter designs and at least one commercially available unit do place an external amplifier at the base of the antenna to overcome transmission line losses for a transmitter that is some distance (in feet) from the antenna but that final stage is still limited to 100 mw DC input."
But if the external amplifier includes a loading system, such as a tuned RF network, then what? Can't the previous tuning circuitry for loading the antenna be at 100 mw as final antenna input? Then you could potentially have a resonating system with little or no significant power loss at the physical antenna.
"I think a second challenge of boosting above the coil would be that the booster amplifier would still need to be matched to the section of the radiator above the coil which would require, yep, another coil..."
Ahh, yeah ... I forgot about that 😉 Could it be a small coil?
But if the external amplifier includes a loading system, such as a tuned RF network, then what?
Then you have a situation where you are essentially operating the same way as having the compete transmitter at the antenna base.
Can't the previous tuning circuitry for loading the antenna be at 100 mw as final antenna input?
Yes, but if there is radiation from the circuitry or transmission line it would most likely fall under the radiation restrictions of part 15.209.
Then you could potentially have a resonating system with little or no significant power loss at the physical antenna.
No, this would be no different than the situation where the complete transmitter is at the antenna base. Looking into the antenna feed point the loss will be the same for either situation.
Neil