I've applied the Getsinger corrections for transmission line end effects
to leads in my coil calculator (http://ham-radio.com/k6sti/coil.htm) and
rods in my ground probe calculator (http://ham-radio.com/k6sti/gnd.htm).
These nifty corrections seem unknown to hams, perhaps because they've
spent the last 30 years behind an IEEE paywall. I thought it might be
fun to apply them to the transmission line in a hairpin match.
Optimized Yagi designs have a naturally low impedance. When forced to
exhibit 50 ohms with no matching network, forward gain can drop 0.5 to 1
dB. A hairpin match avoids this loss by efficiently transforming a low
impedance to 50 ohms.
I wrote a Windows program that calculates hairpin rod length and antenna
reactance for unity SWR given the other dimensions. In addition to end
corrections, it uses an HF expression for shorting strap inductance,
also paywalled. It accounts for the boom, which I always thought had no
effect. The program models feedline leads as orthogonal wires with no
mutual inductance or as a transmission line.
I have no data available to verify program results at the intended level
of accuracy. Perhaps hairpin match builders, armed with a NanoVNA, will
try it and provide feedback. You can find the program here:
http://ham-radio.com/k6sti/hairpin.zip
Your antivirus software may inhibit downloading the file. First pass the
URL to VirusTotal (https://www.virustotal.com). It will check the file
with dozens of antivirus programs and report the results. Once you're
satisfied the file is safe, temporarily disable your antivirus software,
download the file, and then reenable it.
Brian
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