KAT2 without balun - which doublet length is better?

[ub5_073_oleg]

Hi,

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')

Quick modelling of 2x20m + 13.5m feeder antenna shows that
KAT2 should be able to do the job, as its 20uH and 2400pF
are enough on all bands.

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?

73  Oleg


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[Dave Van Wallaghen]

Hi Oleg,

Well, I wish I could take credit for implementing this configuration, but it
was at the urging of Ron, AC7AC and Don, W3FPR who stated that if the
doublet was fairly well balanced, that connecting it directly to the KAT2
should work well and eliminate any losses due to the balun. There was a
pretty good discussion of this about a year ago I think.

The doublet I used was 66' across with about a 33' feeder. I was able to get
a good match 80m - 10m. The only bands I had some trouble tuning were 17m
and 12m. Again, at the suggestion of Ron, I added a 6 foot stub feeder and
got it to tune up pretty well on those bands.

I wish I could tell you that I modeled this setup and designed it to work
properly, but alas I did not. I simply built it up with convenient lengths
and stuck it up in a tree and had a ball with it all summer long. As I learn
more about antennas, I will surely come back and revisit it and try to
figure out why it seemed to work very well for me.

One thing I did notice toward the end of summer and beginning of fall, is
that I started to have a harder time getting a match on 20m. My guess is
that as time and weather took some toll on the antenna that it became
somewhat unbalanced. It eventually succumbed to a branch knocking down one
leg after a nasty storm here and I took it down for the season. Looking
forward to putting it back up when the weather gets nice here. In fact, I'm
looking to use my K3 out there this summer as well. I just bought a solar
controller from Don Brown, KD5NDB, and may try to run the K3 (and K2) QRP
from a solar power configuration. Sounds like fun anyway.

If I can answer (unscientifically) any more questions, let me know.

Take care & 73,
Dave W8FGU

> -----Original Message-----
> From: [hidden email] [mailto:elecraft-
> [hidden email]] On Behalf Of ub5_073_oleg
> Sent: Monday, March 17, 2008 5:22 PM
> To: [hidden email]
> Subject: [Elecraft] KAT2 without balun - which doublet length is better?
>
> Hi,
>
> 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')
>
> Quick modelling of 2x20m + 13.5m feeder antenna shows that
> KAT2 should be able to do the job, as its 20uH and 2400pF
> are enough on all bands.
>
> 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?
>
> 73  Oleg
>
>
> _______________________________________________
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> You must be a subscriber to post to the list.
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>
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> Elecraft web page: http://www.elecraft.com

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[David Woolley (E.L)]

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.
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