[TenTec] Re: Rudimentary SWR question...

[Dr. Gerald N. Johnson, electrical engineer]

DL6WU long yagis are designed to give a plain dipole driven element
center Z of about 50 ohms. Makes for less loss and easier feeds. Some
computer optimized (and commercial) medium length VHF yagis have that
dipole Z as low as 12 to 22 ohms. those often have a very close spaced
first director and tend to prove to be narrow band (which really hurts
when the driven element changes with rain or ice) and of poor
efficiency.

If you look at a dipole feed impedance as its parallel equivalent
instead of its series equivalent (e.g. g + jb instead of r + jx) you
will find the resistive component changes very slowly with frequency
while the parallel reactance changes fairly rapidly. I've built a few
compensated 80 meter dipoles with a parallel tuned circuit across the
feed point. The parallel tuned LC circuit parallel reactance changes in
the opposite slope of that from the antenna and can make an 80 meter
dipole have about the same SWR, under 1.3:1 without a tuner from 3.5 to
4.0 MHz. Outside that range the SWR rises rapidly.

73, Jerry, K0CQ

-- 
Entire content copyright Dr. Gerald N. Johnson, electrical engineer.
Reproduction by permission only.