[TowerTalk] Broadband Match of 80/75M Dipole

[Jim Brown]

On 4/18/2019 7:04 PM, Grant Saviers wrote:
> IMO the simplest near full band 80m dipole/V antenna is made with the 
> 75 ohm matching section several folks have described.  However, it 
> needs to be modeled for the height above ground. See AC6LA AutoEZ 
> examples Part 5.

YES, and it works very well. Folks around Si Valley got the idea from 
W6NL, but Dave says the method is far older than him. The method is the 
first of many in this app note. http://k9yc.com/PacificonSmithChart.pdf

The executive summary is that you cut the antenna for around 3670 kHz, 
connect a half-wavelength of 50 ohm coax to the antenna, then a quarter 
wavelength of 75 ohm line. At the transmitter end of the 75 ohm section 
you'll see SWR below about 1.7:1 from 3.5 - 3.9 MHz, which is plenty for 
contesting and DXing.  Add enough 50 ohm coax to get to the transmitter 
output, or coil up any excess. Loss in those two lengths run with a high 
quality RG8/RG11 cables is less than 0.75 dB.  Loss and SWR are shown in 
slide #49.

My experience is that NEC will do a good job of predicting feedpoint Z 
if height and ground characteristics are known. To take the antenna to 
SimSmith, run an SWR plot with a lot of points, quit NEC, and find 
lastZ.txt.  Follow the instructions in AC6LA's ZPlots to find the file. 
Microsoft hides it. Rename it to antenna1.txt, and load that into 
SimSmith.   Or build the antenna, rig it at the height where you're 
going to use it, and measure it with an analyzer that can produce a 
Touchstone file of the complex impedance, measure the length of the line 
using TDR, and subtract out the feedline.

It's also possible to model the whole thing in NEC if you have a version 
that does real (lossy) transmission lines, and if you're willing to RTFM. :)

73, Jim K9YC