[Antennas] Matching a Mobile Antenna

Sat, 09 Feb 2002 14:26:43 -0500

A shortened mobile antenna has a lower radiation resistance than a 1/4
WL vertical. So, for example, a 40 meter center loaded vertical, will
show somewhere around 8 to 12 ohms radiation resistance at resonance.
That's a 5 or 6 to 1 VSWR IF mated to a 50 ohm coax. The statement
regarding the tapped coil is 100% correct. It is an impedance
transforming device that will raise the impedance looking into the
tapped coil to 50 ohms RESISTIVE. Once a 50 ohm match is achieved AT THE
ANTENNA there is no need for a tuner. Also, the transmission line losses
are then minimal as stated in the post.

Now, many mobiles operate with a 96 inch whip and a tuner. It will work.
It is not optimum. The whip is a 1/4 WL on 10 meters with a moderately
low angle of radiation and a 1.4 to 1 vswr. As the frequency gets lower,
12, 15, 17 or 20 meters, the efficiency of a base loaded antenna
degrades faster than a center loaded antenna. Also, at lower frequencies
the base loaded antenna produces more of a 'cloud warmer' class of
antenna pattern.

Match the antenna to 50 ohms is best policy.

73, Deacon Dave, W1MCE
Manager, Textron Antenna Range, retired

[email protected] wrote:
> 
> Tom,
> 
> Normally you want a match from the radio to the transmission
> line AND a match from the transmission line to the antenna.
> With coax at 50 or 75 ohms a match at the radio is almost
> certain and the match at the antenna is normally done with a
> tapped coil at the base of the whip.
> 
> If you leave off the matching coil at the base of the whip,
> you will have higher losses in the transmission line and
> will need an antenna tuner of some type at the radio.
> However, those losses should not amount to much in a mobile
> installation with a coax run of probably less than 15 feet.
> The biggest losses in a mobile are in the poor efficiency of
> the antenna itself.
> 
> Regards,
> Frank Kamp
> K5DKZ
> 
> Tom Bates wrote:
> >
> > Greetings to the list...
> >  When matching a shortened vertical ( center loaded mobile ), is the antenna
> > efficiency ( read effective radiated power ) affected much by the location of
> > the matching network ?  I am wondering if a matching network at the base of the
> > antenna will have any advantage over an ATU right near the transceiver.  My
> > understanding of what W2DU had to say is that it doesn't matter--a match is a
> > match anywhere in the system ???
> >    Thank you, all.   73,  Tom  AA1NZ
> >
> > - - -
> >
> > Your moderator for this list is:
> > Larry Wilson KE1HZ [email protected]
> > _______________________________________________
> > Antennas mailing list
> > [email protected]
> > http://mailman.qth.net/mailman/listinfo/antennas
> - - -
> 
> Your moderator for this list is:
> Larry Wilson KE1HZ [email protected]
> _______________________________________________
> Antennas mailing list
> [email protected]
> http://mailman.qth.net/mailman/listinfo/antennas