Many articles have been written on the topic and it is still a black art to some. Making a Medium Frequency (MF) antenna that has enough bandwidth to pass 10 KHz audio can be challenging, to say the least. The VSWR out to +/- 15 KHz carrier needs to be kept at a minimum and the power needs to be evenly distributed between the two sidebands. This can become problematic with complex Directional Arrays or towers that are tall or short for their operating frequency.
When we were working on the WFAS-AM tower in White Plains, NY, it became apparent to me that something was not right. The tower is skirted and now holds the antenna for W232AL, a 250 watt translator broadcasting the WPLJ HD-2 channel. After installing the FM antenna, some tuning of the AM antenna was required and this is the graph of the resistance and reactance curves:
This looked very similar to the resistance and reactance curves before the FM antenna work was done. Red line is resistance, the blue line is reactance. I think it had been like this for a long time. While it is not terrible, it is not that good either. As alluded to in a previous post, some re-working of the ATU was needed. After some trial and error, this is the circuit that we ended up with:
Not quite what I expected, however, it was designed with the parts on hand, excepting the vacuum variable output capacitor, which was donated by me. That part was key in making the proper adjustments.
After my redesign and tune up of the ATU, this the resistance and reactance curves at the input terminal of the ATU:
The graphs have a slightly different format, but you get the idea. The red line is resistance, the blue line is reactance and the green line is overall impedance. The resistance is symmetrical about the carrier as is the reactance. Truth be told, I think there is a little more that can be had here, but for now, there is no reason to go any further. I made the initial measurements at the input of the ATU and confirmed them again at the output terminals of the transmitter. When we turned the transmitter back on, I noticed that the modulation index had dropped by about 15 percent. I think the reflected power was getting back into the RF sample and fooling the mod monitor. I also noticed that the high end in particular sounded much nicer.
The ATU building is a little cramped and it is hard to get a good picture. The vacuum variable capacitors were salvaged from a scrapped AM transmitter years ago. The tower is 202 degrees tall, which is also a factor. It will be interesting to see what seasonal changes there are with snow cover, mud, etc.
Overall, this was a fun project.