[Elecraft] Stealthy Antenna

Mon Jan 20 12:57:01 2003

Seldom do Vic and I differ on anything of consequence, but I must
mention that the length of coax from the balun to the rig does enter
into the equation, in a couple of ways.

First, Vic is 100% correct: it *must* be as long as the distance from
the balun to the tuner!  <:}

Second, on some bands the SWR on the coax can be quite high and
thereby some loss will be present.The amount depends upon the length
of the coax and the frequency for a given SWR. There are tables that
you can use to find this. Or the TLDetails program from AC6LA will
calculate it and a jillion other interesting things for you. But
practically speaking, a length of "a few feet" will be necessary and
we must accept whatever loss we get - just don't try to get by with
RG-174 if you can use RG-58 or 8X instead.   <:}

At QRP, the voltages and currents involved are low so even though the
peak voltage on the coax (and the peak current) are multiplied by the
sqrt SWR on the line, even RG-58 is unlikely to suffer any damage from
high coax SWR. I occasionally run 100 watts <shame!> so I use RG-213
from my outside baluns to the tuners - anal overkill in the name of
confidence.

The coax length has a direct bearing on the impedance presented to the
tuner so you may find on some bands that the tuner has difficulty.
That being the case, just change the coax length a couple of feet
(shorter if possible) and try again. If this affects only one band,
just make up a coax jumper that you stick on the line for that band.

Finally, I strongly suggest that you use a 1:1 current balun instead
of the 4:1 balun. The antenna is non-resonant on most bands so the
input Z at the ladderline can be anything from a few ohms resistive
and either a little or a lot of either capacitive or inductive
reactance to a lot of ohms resistive with the reactances. So, there
may be times when the 4:1 might seem appropriate, but the hooker about
baluns is that they do NOT WORK LIKE TRANSFORMERS.

If a 4:1 balun is designed to be fed with 50 ohm coax, then any load
other than 200 ohms resistive will NOT appear at the input as that
load impedance divided by 4. The impedance will be determined by the
properties of the balun and can be all over the lot "depending."  So,
you might think that a very high line input Z would benefit from a 4:1
reduction, but such a reduction may or may not occur - usually not.

On the other hand, if the line input Z is quite low - a few ohms is
not unreasonable on some bands for some antenna and line lengths, then
reducing the Z even further - even if not by "4" - can make the system
harder to tune and load. So, on average, you are better off with a
simple 1:1 current balun.

I strongly recommend against using a voltage balun for this
application although many do and have with seeming success over the
years  - but there are a lot of potential gotchas that the current
balun avoids. The famous W2AU balun is a voltage balun that was the
most common before W2DU gave us his current bead balun and Jerry
Sevick did a lot of work on current baluns using ferrite rods and
toroids. But, current practice is to use a current balun with
transmission lines since the result is more likely to be balanced line
currents with minimal radiation and noise pickup from the line.

So, let the balun do what it is designed to do: make the transition
from a balanced line to an unbalanced line and thereby limit or
prevent current flow on the coax outer braid. Let the antenna tuner do
its job: impedance matching.

Vic,  pardon the interruption but I felt that I needed to tack these
points on to your posting. That makes me a fair target for you on my
next posting!   <:}

73/72, George
Amateur Radio W5YR -  the Yellow Rose of Texas
In the 57th year and it just keeps getting better!
Fairview, TX 30 mi NE of Dallas in Collin county EM13qe
K2 #489  IC-765 #2349 IC-756 PRO  #2121 IC-756 PRO2 #3235

----- Original Message -----
From: "Vic Rosenthal" <[email protected]>
To: "Mike Walkington" <[email protected]>
Cc: "Elecraft Reflector" <[email protected]>
Sent: Sunday, January 19, 2003 10:55 PM
Subject: Re: [Elecraft] Stealthy Antenna

> Mike Walkington wrote:
> >
> > I'm planning to use a short piece of co-axial cable (6-7 feet) to
get from
> > my K2 (with KAT2) through the wall of the house to an externally
mounted 4:1
> > current balun (the W1CG model).
> ...
> > 1) Can anyone offer some experienced advice on how long or short
the co-ax
> > can be?
>
> Long enough to reach from the rig to the balun neatly.  Reasonable
lengths won't
> have enough loss to matter.
>
> > 2) Is it OK to mount the balun in a plastic box?
>
> I've done this with 'Tupperware' type plastic containers from the
supermarket.
> Ultimately the UV in Central CA destroys them, but it takes a long
time.
>
> > 3) Would you recommend a doublet or a loop?
>
> I'm just guessing, but perhaps a loop would couple more effectively
(read: not
> good) into house wiring, like a big transformer.  Probably someone
has practical
> experience for you.
>
> 73
> Vic K2VCO