radio8z

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