[Elecraft] Balun with a G5RV?

[George, W5YR]

It turns out, Ron, that the coax shields are tied together at each end of
the coax/balanced-line run with the coax tied to the tuner ground point at
that end and floating at the "other" end. The two coaxes can take any path 
from the tuner to the "other" end - they do not require any particular
attention to spacing, etc. Neither do they need to be kept away from other
conductors, any more than a regular coax run. Main requirement is that they
be about the same length - to aid in maintaining balance in the system.

The coax balanced lines have the same loss as single coax since the same
current is flowing on the center conductor where the principal HF loss
occurs as I^2R loss. The return current is on the other center conductor
instead of the inside of the braid. At HF, dielectric loss is negligible.

This technique for bringing balanced lines into the shack is widely used
and is very effective. Of course, the coax is subjected to the same peak
currents and voltages as the regular open-wire or ladderline since the SWR
can be quite high in the coax section. For that reason, it pays to keep the
coax run short.

The Z mismatch going from 450-ohm ladderline, for example, to 100-ohm
coax-pair line (two 50-ohm coaxes) is a non-issue since the entire line
operates with standing waves.

Interesting stuff . . .this has been written up in QST and the Handbook and
Antenna Books for many years. I am installing a new 80-meter full-wave
tilted triangular loop that will be fed with ladderline and then transition
to parallel coaxes into the shack for about 10-12 feet to a Johnson KW
Matchbox tuner. Should handle my 5 watts just fine!   <:}

73/72/oo, George W5YR - the Yellow Rose of Texas         
Fairview, TX 30 mi NE of Dallas in Collin county EM13qe   
Amateur Radio W5YR, in the 56th year and it just keeps getting better!
QRP-L 1373 NETXQRP 6 SOC 262 COG 8 FPQRP 404 TEN-X 11771 I-LINK 11735
Icom IC-756PRO #02121  Kachina 505 DSP  #91900556  Icom IC-765 #02437


Ron D' Eau Claire wrote:
> 
> > The two coax lines in parallel are being used as a balanced feed to the
> > antenna.  Only the center conductor is being used for the feedline.  It
> > acts like any other balanced feedline with the exception that you don't
> > have as many "gotchas" as with the "real" thing.
> > 73 de John - KC4KGU
> > K2 #2490
> 
> Hi, John.
> 
> Maybe I misunderstood. I thought that your schematic showed the shields
> being grounded. If they are you have losses caused by leakage and dielectric
> losses between the center conductor and the shield. These losses increase as
> the SWR or the operating frequency increase.
> 
> If the shields are not connected to anything at any point, you are right.
> You won't have losses in the dielectric between the center conductor and the
> shield. But you will have losses through the dielectric to anything the
> cables come in contact with.  You need to be sure that each coaxial line is
> carefully spaced and well insulated from everything around it including the
> other coaxial line.
> 
> In that case I  would think that it'd be easier (and cheaper) to use a good
> air-insulated open wire line.