[Antennas] Think I've got this figured out, Re: AM Broadcast RX Antenna?

Sat Nov 14 01:16:33 EST 2015

No, don't bother with adding inductance. The car radio's input trimmer may 
be able to be adjusted for the added capacitance.
If not, a simple series capacitor at the input to the radio will do it; will 
restore the total antenna + cable capacitance to the
range the radio's antenna trimmer can handle. This is not so critical. I am 
guessing it would be circa 100 pF
or less. There will be no noticeable loss of signal with the added cap.
-Hue
-----Original Message----- 
From: Richard (Rick) Karlquist
Sent: Friday, November 13, 2015 9:43 PM
To: Hue Miller ; antennas at mailman.qth.net
Subject: Re: [Antennas] Think I've got this figured out, Re: AM Broadcast RX 
Antenna?
All modern car radios use a 30 inch whip, because that is
a quarter wave on 98 MHz, the center of the FM band.
Meanwhile, on AM, the radio is designed to resonate out
the total capacitance of the whip and the cable. Older
radios had an adjustment so you can change the length of
the whip and peak for maximum sensitivity. I doubt
that you will still find this adjustment.
If you want to convert to a CB whip, you need to put
enough inductance in series with it to cancel out the
additional capacitance the CB whip has. You don't want
to tune the whip to resonance, just reduce its effective
capacitance. This can only be done for one specific
frequency of course, but that is all you need.
Rick N6RK
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