[Elecraft] Baluns and 450 ohm line

[Don Wilhelm]

Folks,

If I recall the antenna in question was a dipole.  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.  When an antenna that is inherently has a 72 ohm feedpoint is pruned
to show 50 ohms on the meter, it will have reactance included - then the
length of the feedline becomes critical, the SWR is not zero even on a 50
ohm feedline, and other strange things rear their ugly heads.  To my mind,
it is better to either match the line to the antenna at the feedpoint, or
use an L network to match at the feedpoint, or just bite the bullet and feed
with open wire line.  Just because the transmitter wants to look at 50 ohms,
pruning the antenna to get that impedance is not the only important thing -
making the reactive component is more critical - and is the true indicator
of resonance of either tuned circuits or antennas.

Also, the parallel coax feedline is useful for maintaining balance, but the
matched line loss is the same as the matched loss for a single coax feedline
alone - which can be very high if that antenna is used for multiband work.
I often use parallel 93 ohm coax to bring a parallel feedline into the
shack, but I use only a short length and connect it directly to the 450 ohm
ladder line despite the impedance bump it creates in the line.

73,
Don W3FPR

----- Original Message -----
> ==========
> I'm still wondering why you would want to use a 100-ohm feedline to feed
> an antenna which you said your MFJ-269 indicated the feedpoint Z was 50
> ohms at resonance.
>
>