<VV> LM Rear Antenna

[Dale Dewald]

At 12:00 9/01/2009 -0400, J R Read wrote:
 >>snip<<

>It has something to do with matching the length of the FM signal 
>wave.  Someone into radios can probably explain that.

Most FM broadcast antennas are set up as a 1/4 wave whip.  The length of 
the whip is easily calculated using the desired optimum reception frequency 
and the speed of light (which is also electromagnetic waves).

For the midpoint of the FM broadcast band, 98MHz, the optimum 1/4 wave 
length is 30.1 inches.

For the low end, 88Mhz, use a slightly longer whip: 33.5 inches
For the high end, 108MHz, use a slightly shorter whip: 27.3 inches.

Most ready made FM antennas I have measured are 31" long.  Adjust your 
telescoping antenna to about 31" or a bit longer if your favorite station 
is in the lower end of the band or a bit shorter if your favorite station 
is in the upper part of the band.

>You can get satisfactory FM reception by using an "AM" collapsible antenna
>and adjusting it to the FM length.  Now, if you are mounting in the rear,
>you might be putting a "rake" to it?  I'm not sure how that affects the
>length requirement.  Some "fine tuning" might be required.

Any rake (15 deg or less) will be insignificant considering the placement 
of the antenna near the rear corner of the car where the ground plane of 
the surrounding car body is asymmetric.  If you really want the best 
reception, place the antenna in the exact center of the roof.

Dale Dewald
Hancock, MI