[TowerTalk] dipole z vs height

[Jim Brown]

On 2/20/2013 7:18 AM, Jim Lux wrote:
>
> It will have an effect, but I don't know about a huge one.

I've done a lot of modeling, both with NEC and with SimSmith to study 
the effects of voth antenna height and transmission lines of different 
Zo and lengths. One thing that jumps out at you is that the (mis)match 
between the antenna and the line, combined with the impedance 
transformation of the line, will usually cause an APPARENT shift in the 
resonant frequency as observed on an SWR indicator in the shack. The 
shift can easily be 100 kHz on 80M.

To get a true picture of what the antenna REALLY looks like, I measure 
complex Z in the shack using a Vector Network Analyzer (a Vector Antenna 
Analyzer also works), find the electrical length of the line making a 
TDR measurement, export that data to SimSmith, and use SimSmith to 
"subtract out" the line by entering a line with negative length equal to 
what I've just measured.  I can than add that line back in to find the 
loss in the line, and I can try various matching schemes.

If I see that the Z of the antenna is closer to 50 ohms, I may choose to 
feed it with 50 ohm coax.

There's a VERY slick broadband matching scheme for antennas close to 75 
ohms that involves using a section of 50 ohm line that is some multiple 
of half-wavelengths followed by a quarter wave of 75 ohm line. This 
works VERY well on 80M for 50 ohm sections of a half wave, full wave, 
and 3/2 wavelength, such that an antenna cut for about 3675 kHz is 
better than a 2:1 match to a 50 ohm transmitter from 3500 kHz to 3875 
kHz.  It does NOT reduce the LOSS in the line due to mismatch between 
the line and the antenna, but it can eliminate the need for an antenna 
tuner. Around Northern California, Dave Leeson, W6NL, is often given 
credit for it but Dave says the technique goes back to at least the 
1940s.  I've included it in my tutorial on using SimSmith to design 
matching networks.

http://audiosystemsgroup.com/PacificonSmithChart.pdf

73, Jim K9YC