[Elecraft] Baluns and 450 ohm line

[Ron D'Eau Claire]

Don, W3FPR wrote:

>   I just constructed a 40
> meter dipole a few weeks ago and cut it to resonance - that is I cut it
for
> no reactance shown on my MFJ 259 - and at that point, the impedance was
> 73+j0!!!  So I fed it with 75 ohm coax for the lowest loss on the
feedline -
> coiled up 15 feet of the coax on a 4 inch form and placed that balun at
the
> feedpoint of the antenna.  It works great, and yes, I have to suffer with
a
> 1.5 SWR at the transmitter or use the tuner on it to get to 50 ohms.  BTW,
> 72 ohms resistive is the normal impedance for a proper dipole at a decent
> height.
>
> What is the point you ask - well, I am wondering about the obsession with
50
> ohms.

Excellent observation, Don!

As I recall, turning over musty brain cells from long ago, 50-ohm coax was
frequently used for dipoles because it offered a better match when: 1) The
dipole is not very high in terms of wavelengths. The impedance at resonance
drops as the antenna gets close to the ground, and 2) An Inverted V tends to
show a lower-than-normal impedance, probably for the same reason because the
"ends" tend to be fairly low.

Back in the 50's when I hung up my first resonant antenna, 75-ohm "twinlead"
was commonly used for a dipole. But tons and tons of War Surplus 50-ohm coax
hit the market. Even those ops who had horizontal dipoles up high accepted
the slightly higher SWR to use the cheaper feedlines. After all, 90% of us
back then thought "SWR" was CW shorthand for "swear". We simply loaded up
our rigs for normal output using the pi-networks in our transmitters (darn
near as versatile as a KAT2 or a KAT1 except that they had to be adjusted
manually) and ignored whatever was going on with the feedline impedance.

Ron AC7AC
K2 # 1289