Subject: [Elecraft] Stealthy Antenna

[Charles Greene]

Mike,
  See below.

You wrote:
"All,
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). From the balun I plan to run twin feed
(20-30 feet) to a doublet or loop mounted under the eaves of my two storey
house (to avoid the anti-antenna covenants). I have some questions about my
proposed configuration that I'd appreciate some advice on:
1) Can anyone offer some experienced advice on how long or short the co-ax
can be?"

Just long enough to get from your rig to the outside.  I've got a couple 
antennas that way.  The losses depend upon the SWR on the coax.  If there 
is a miss-match on the balun, there will be a miss-match on the coax.

"2) Is it OK to mount the balun in a plastic box? "

Yes.  See:

http://www.njqrp.org/qhbextra/index.html

Click on QHB#8.  That's for the NJQRP kit version.

That's assuming you don't already have the NJQRP QHB.  It has some pics 
including one of the balun in a plastic box.

"3) Would you recommend a doublet or a loop?"

I never used a loop, but I understand it is a great antenna.  However, it 
may be harder to use it on several bands.  I have had good results using an 
off-center-fed antenna.  Feed at the 1/3-2/3 length.  In other words, for 
80, feed at the 45ft-90ft point, and for 40, feed at the 22.4-45ft 
point.  If using an OCF antenna, it is better to put the balun at the 
antenna and run coax all the way down.  The pic of the balun in the box in 
the referenced QHB was made up for just that purpose.  The current in the 
twin lead is not balanced because of the unequal lengths of the different 
sides of the antenna.  The current balun will strive to balance this 
current giving unintended results.  Doesn't really make much difference, 
but to be pure...

If you are using any antenna with a balun, do not put the balun at a 
current node.  The impedance will be too high.  You will have to cut the 
length of the feed line so that it plus 1/2 the length of a (doublet) 
antenna is not a 1/2 wave length or multiple thereof on any operating 
frequency.  I tested that particular balun at an impedance of 700 ohms (as 
high as my antenna analyzer would read), and it works fine but with 
slightly reduced efficiency.  I calculated that the voltage on the wires 
would exceed the insulation rating of the wires at about 1000 ohms and 100 
watts with a 2:1 safety factor, and dielectric losses will increase, so 
don't do it.  The impedance at a current node can easily equal several 
thousand ohms, and as Sevick said "an impossible condition for any balun."

Let us know how you make out.

"Mike
AC7MZ"
K2 #2599


73, Chas, W1CG
K2 #462