<VV> LM Rear AntennaLM Rear Antenna

[Tony Underwood]

At 11:19 PM 9/3/2009, Gary Mierzwa wrote:

>
>
>JV Robertys wrote:
>
>"I can answer this one! It's the speed of light divided by the 
>frequency. So, 186,000 MPS x 5280 x 12 divided by 98 million hertz 
>gives you 120.6 inches. A quarter wave antenna would be ~30.1 inches. "
>
>
>
>Don't forget multiplying that answer by the Velocity Factor (K) of 
>electromagnetic enery in Free-Air (.985) verses Free-Space (1.00)
>
>
>
>Pickey, Pickey, Pickey   :>)
>
>
>
>Gary Mierzwa






A lot of aftermarket car antenna vendors (as well as factory supplied 
parts) were selling antennas that were 22 inches long... which was 
their "economical" answer to an over-all average although it was more 
like their being kinda cheap, shorter antenna means more of them can 
be made from the same quantity of material and they were stainless 
steel rod.    A 22 inch antenna ends up being about right for the top 
region of the FM band.

In the end, having an exact 1/4 wavelength antenna for receiving an 
analog FM radio signal isn't all that advantageous... you can give or 
take a bit without much difference at all.


It's the transmitting end where antenna length gets 
critical.     Most times, FM reception issues revolve around 
multipath distortion caused by reflections and not a genuine lack of 
signal strength.    The vast majority of FM transmitters crank out 
around 100KW erp (FCC limits) so strength is usually not an 
issue.    A few have grandfathered erp options where they transmit as 
much as 200KW or more, such as one station here in town that puts out 
191KW.   One or two have options according to their original FCC 
contracts to broadcast at an astonishing 500KW erp.    However, most 
of that is wasted in flat country since line of sight transmission 
doesn't really take much advantage of such power levels so the 
grandpa stations seldom ever run that much power simply because it's 
unnecessary and expensive.   The most I'm aware of is one or two that 
run ~300KW... and that's not continuous IIRC.

Those grandpa stations do get out.   And multipath still eats them up 
sometimes, quality-wise, in town.    But outta town, far down the 
road, you still hear them as far away as 250 miles or more if the 
transmitter is atop a mountain.    Even then, they need to be on a 
"clean" channel to avoid cochanneling with someone else equally as 
far away in the opposite direction, which is a problem with many AM 
stations at night when skip is rolling.

Almost no skip for FM or most other HF or VHF freqs but it does 
sometimes happen when sunspots are active and the atmosphere 
ionizes.   I once picked up a TV station broadcast from Corpus 
Christi TX (video was in and out but sometimes pretty fair with 
little noise, audio was mostly unintelligible) here in Roanoke VA 
with a fancy rooftop TV antenna on a rotor at the electronics shop I 
worked... it was an episode of The Three Stooges.   Since FM falls 
between VHF Lo and VHF Hi (or used to) it follows that FM skip would 
also be working... so a degree anyway.

...all of which has nothing to do with the length of a car's radio 
antenna for FM radio reception, so go with anything that's between 24 
and 36 inches and you probably won't go wrong.



tony..