<VV> LM Rear Antenna

[jvhroberts at aol.com]

 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. 


 


John Roberts

 


 

-----Original Message-----
From: J R Read_HML <hmlinc at sbcglobal.net>
To: virtualvairs at corvair.org; Dale Dewald <dkdewald at pasty.net>
Sent: Wed, Sep 2, 2009 1:31 am
Subject: Re: <VV> LM Rear Antenna










Dale,

Thanks for the cogent reasons WHY the FM antenna should be around 30 inches.

BTW.... how does the speed of light come into play?  What's the formula - if 
you happen to know.

As an aside...  Matt's favorite station is 94.7?  Is that what a 30 inch 
antenna would optimize?

Later, JR


----- Original Message ----- 
From: "Dale Dewald" <dkdewald at pasty.net>
To: <virtualvairs at corvair.org>
Sent: Tuesday, September 01, 2009 8:42 PM
Subject: Re: <VV> LM Rear Antenna


> 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
>
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