[Elecraft] Balun

[Ron D'Eau Claire]

Since you aren't trying to make the antenna self-resonant, you will need to
have an antenna tuner, as others observed. The tuner will convert whatever
impedance the antenna presents at the end of the feedline to 50 ohms as
required by the K2. That impedance will vary depending upon the band, and a
KAT2 (for the QRP K2) or KAT100 for the K2/100 will do FB. I do NOT
recommend the MFJ "T-Match" tuners for this duty, and I especially do not
recommend using their built-in baluns.  

About the length, unless you are going to work 160 meters, any length over
66 feet will be FB. The difference in performance between a half-wave center
fed antenna and a 1/4 wave center fed antenna is virtually nil. Once you are
over 1/4 wave long, the most important thing is height, especially for
working DX on the lower bands. 

I wouldn't invest one bit of energy getting an doublet like you are
contemplating any longer than 90 or 100 feet for 80-10 meter operation.
There's nothing to be gained by doing so. 

The ideal antenna tuner for you is a balanced tuner. None of the Elecraft
tuners are inherently balanced, although they are starting to show up on the
market again as more and more hams go to the type of doublet you are putting
up in order to easily work all the HF bands. A balun will probably work
okay, but they become unpredictable when the SWR on the line is high, as it
will be in your antenna. Sometimes people actually get out better simply
hooking one side of the open wire feeder to the K2 case and the other to the
center pin on the output of the ATU. That produces a large degree of
imbalance in the feeders, so they'll radiate. That can actually be a help if
your feeders are out in the clear and pretty much vertical. 

Nothing is as much "black magic" in his Hobby as antennas! 

Ron AC7AC

 

-----Original Message-----
From: elecraft-bounces at mailman.qth.net
[mailto:elecraft-bounces at mailman.qth.net] On Behalf Of Stuart Rohre
Sent: Thursday, June 09, 2005 3:39 PM
To: Vic Rosenthal; Richard Soennichsen
Cc: elecraft at mailman.qth.net
Subject: Re: [Elecraft] Balun


I guess one of the issues, are you ordering a K2 with its auto tuner?

In that case, my suggestions apply, to adding the Elecraft balun, and using
the internal K2 tuner for your antenna.  It is also true that a balanced
line tuner will work after the K2, and before your ladder line.

Since surplus Johnson Match Boxes do not match on all bands and are harder
to find in surplus; you might check out the new line of MFJ balanced tuners.
They have a double Tee network, and use large, well suited components.  They
go between the K2 and the ladder line, for manual tuning like a Match box
did.

I was impressed with them at Ham Com where they were on display. In addition
to a double network to balance the tuner, they float the tuner input with a
RF choke isolation, to insure you do not couple RF back to the rig on
outside of coax.

-Stuart
K5KVH



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