ub5_073_oleg wrote:
I'm going to use my K2 with KAT2 internal ATU to operate a
doublet from 80m to 10m including WARC bands without a
balun. Among others, Dave's W8FGU home page inspired me to
try this.
I hope this is not a problem, especially when battery-fed
during portable operation, as there is [almost] no
HF-connection to the ground (the whole system is 'floating')
Even though the case is floating, it doesn't avoid RF on the chassis
problems. Although the output transformer will see 50 ohms, the two
legs of your doublet will have a much higher impedance which means that
the RF output will cause the chassis voltage to vary by much more than
would be implied by assuming half the power into 25 ohms. Instead of
under 10 volts, you are likely to get up to 100 or so volts. I've found
that to my cost with a short indoors doublet on 160m, even though it has
a simple balun.
The high chassis voltage variations will drive any appendages, like
battery cables, key cables, headphones, etc.
Unfortunately, for baluns to work well they really need to be on the
transmitter side of the ATU, where they see about 50 ohms differential
mode, and that isn't compatible with an internal ATU.
My current best guess as to what is needed is to tune out the reactive
component of the ground side leg of the doublet, and therefore get the
impedance down to 25 or less. Logically this is equivalent to end
feeding against a counterpoise, but, in my view, the distinction between
a counterpoise and the other half of a doublet is somewhat artificial.
I haven't tried this in anger, and one question is how to determine when
you have canceled the reactance. The instructions for commercial
"artificial earth" tuners tend to suggest tuning for maximum current,
but, it seems to me that that can only be done if the formal ATU is
tuned to cancel the reactance in the non-ground end. As that setting,
in turn depends on having zero reactance on the ground end, one, has an
interdepence problem.
My question is: can/should I make it easier for KAT2, e.g.
choosing other antenna or feeder lentgh or maybe
adding/removing a piece of feeder on some bands? Are there
some proven length to use?
The G5RV was designed to have a relatively good match on all the
pre-WARC bands.
--
David Woolley
Emails are not formal business letters, whatever businesses may want.
RFC1855 says there should be an address here, but, in a world of spam,
that is no longer good advice, as archive address hiding may not work.
_______________________________________________
Elecraft mailing list
Post to: Elecraft@mailman.qth.net
You must be a subscriber to post to the list.
Subscriber Info (Addr. Change, sub, unsub etc.):
http://mailman.qth.net/mailman/listinfo/elecraft
Help: http://mailman.qth.net/subscribers.htm
Elecraft web page: http://www.elecraft.com
