[ICOM] Pro II Antenna Tuner question

[D C Macdonald]

Don't forget the Velocity Factor of the feedline!
If you are using "normal" coax, the VF is .66,
which means 1/4 wavelength on 80 meters
is around 40 feet!

Use of 1/2  ELECTRICAL  wavelength of feedline of  ANY
impedance gives you exactly the same impedance at both
ends.  This  MAY  give you an  EASY  mismatch to deal
with if your antenna is close to resonance.  It should be
no problem, for example, to TUNE a 75/80 meter dipole
antenna SYSTEM with reasonable ease in the matching if
you use 1/2 wave (electrical) of feedline.

Quarter wavelength feedlines act as impedance transformers.
For example (assuming use of 50 Ohm coax), if your antenna
impedance is 100 Ohms and you use 1/4 wave of feedline,
the impedance at the transmitter end will be 25 Ohms.
If your antenna (loaded mobile whip) is 4 Ohms, transmitter
will see over 600 Ohm as a load.


TX Z     FL Z
----  =  ----
FL Z     ANT Z


73  ---  Mac, K2GKK/5



----Original Message Follows----
From: "David J. Ring, Jr." <n1ea at arrl.net>
Reply-To: ICOM Reflector <icom at mailman.qth.net>
To: "ICOM Reflector" <icom at mailman.qth.net>
Subject: Re: [ICOM] Pro II Antenna Tuner question
Date: Sat, 10 Jul 2004 18:03:18 -0400

Also, avoid feed line lengths that are an odd number of quarter waves on the
operating frequency!

I've found that lengths of 50 feet seem to work fairly well on 40/80/160
meters - but you might have to trip the line for best SWR.

73

DR