FM Short Antenna Length and Field Strength
Posted on June 11, 2010
One method proposed to control the field strength and thus the range for part 15 FM is to adjust the antenna length. I did a mini study using my FM transmitter to find out how much control changing the antenna length provides.
My Ramsey FM-25A is enclosed in a metal box with the supplied telescoping antenna protruding through a hole in the top of the cabinet. To gauge the field strength I tapped into the AGC line of a EICO HFT-90 tuner (yep, vacuum tubes) and connected the 300 ohm differential antenna input to the output of a Motorola test set. The test set output was varied and plotted against the AGC voltage and, as expected, the relationship was not linear. A 1 foot long antenna was attached to the receiver and the other antenna terminal was grounded to the receiver chassis. This arrangement gives a relative measurement of the field strength by use of the graph to find the receiver terminal voltage corresponding to the AGC voltage reading. The receiver was positioned 3 meters from the transmitter.
The data were recorded for several lengths of the transmit antenna, corrected with the graph for the non-linearity, and normalized to be 100 at the maximum antenna length.
Length (inches) Rel. FS
17…………………..100
14…………………….85
12…………………….81
10…………………….77
_8…………………….62
_6…………………….15
_4…………………….08
This shows that for my particular transmitter the field strength drops rapidly as the antenna length is shortened below 8 inches. For lengths above 8 inches the relationship is approximately linear and deviates from this for shorter lengths. The curve for the AGC vs. antenna voltage has a small slope for voltages below those produced by the 8 inch length hence there is probably significant measurement error for these settings. Time permitting I may lengthen the receiver antenna to get a higher voltage on the AGC and repeat this.
Neil