[Antennas] SWR Isn't Enough - Working in a high SWR setting

[George, W5YR]

Briefly, Jim, there are two ways to get ladder line to your tuner without
bringing the line itself into the shack:

1. use paralleled coax; take two equal lengths of coax - any Zo will do -
and connect the shields together at each end; at the line end, don't
connect the shields to anything - let them "float;" connect the ladderline
to the two center conductors of the coax; at the tuner end, connect the
shields to the tuner chassis and the two center conductors to the tuner
balanced output. The coaxes do not have to be kept together so don't waste
any time fastening them together exactly parallel, etc. except for
convenience and appearance. This method allows the coaxes to be run into
the shack just as you would any coax.

2. terminate the ladder line in a 1:1 W2DU bead balun just outside the
shack wall - I hang the balun from the soffet of the roof - and run coax
from the balun to the tuner coax output.

Either method will work; both have some added loss in the coax but that is
the tradeoff for the advantages of NOT bringing ladderline directly into
the shack.

You may find that with certain antennas and certain bands that your tuner
may have difficulty handling the line impedance that it sees. In that
event, just add a few feet to the coax(es) on those bands.

I have three ladderline-fed antennas using (2) above and they work and tune
very well using an MFJ 989C T-section tuner. I strongly recommend the use
of the 1:1 balun over the 4:1 design since you are less likely to encounter
extremes of line input Z at the tuner. The current balun is clearly the one
to use since all you are trying to accomplish is to divert the current from
the side of the ladderline connected to the coax braid to the inner braid
only and to block it from the outer braid. In this application, the balun
is not intended to promote current balance in the feedline, although it may
have some influence on that. 

I prefer the bead balun for convenience and since I run QRP to 100 watts
max, it is more than adequate. For QRO, however, I would look at 1:1
current baluns using one or more toroids, with appropriate windings to
handle the power, since beads can get quite hot when exposed to high power.
All this is discussed in Walt Maxwell's classic article on baluns in QST
March 1983, page 36.

The coax sections, single or paralleled, may have quite high SWR on them,
but due to their short lengths the loss is negligible. The problem is that
if a voltage or current max occurs within the coax run, it will stress the
dielectric (voltage)and/or the center conductor (current). So I use RG-213
with solid dielectric that can handle very high voltage as compared to the
foam dielectric lines such as RG-8X. Again, this is a QRO problem.

I am putting up a new 80-meter tilted triangular full-wave loop antenna
that I will feed  with ladderline transitioning to parallel-coax balanced
lines (coming into into the shack along with several other single coaxes
going to other tuners) and running to a Johnson KW Matchbox balanced
output.

Ladderline has its champions and its detractors, and both camps offer good
arguments. Like most solutions in engineering, you end up using what does
the best job consistent with the drawbacks and costs, etc. Personally, I
like being able to get multi-band operation from wire antennas, and
ladderline has shown few disadvantages in my applications. Detuning in rain
is a minor problem that a touch on the tuner controls usually compensates.

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


Jim Shaw wrote:
> 
> Thanks for the tips on using ladder line, especially about keeping it away
> from objects by two to five times the line spacing.
>     Yes, I am leaning toward use of a tuner (because commercial ones are so
> readily available) although I haven't totally ruled out use of switchable
> matching stubs.  The indecision is due to considerations for getting the
> ladder line into the shack to a manual tuner versus using a remote auto
> tuner under the eve of the house.   My indecision must have come through in
> the last note.  Sorry for the confusion.