Home › Forums › temp › AM Transmitting Antenna: Fat On Top, Thin On Bottom › Cage Monopole Ideas
Total posts : 45366
Hi Bruce MICRO1700:
I think the multi-pipe transmitter you are describing is a “cage-monopole” type antenna, and I have an experimental one right now operating at 107.1 FM with 4-elements.
According to some literature each element of a cage monopole added in parallel does not increase the wavelength of the antenna, but it serves to lower the impedance, which of course requires re-tuning.
I also notice that the FCC rules for AM and FM band do not limit the number of “elements” an antenna can employ. This becomes significant because in some bands the number of transmitting antenna “elements” are specified, as for example 15.235 Operation in the band 49.82-49.90mHz (3) The antenna shall be a single element, etc.
Based on those observations I take the position that adding parallel elements to an AM or FM transmit antenna does not exceed the rules.
By the way, one thing I notice with my 4-element cage-monopole is that the FM signal is very stable indoors, no interference is caused by my moving around within the field.