Possible doublet configuration

[Glen Torr-2]

Hello All,

Following this thread with much interest. I need a stealth antena and
a doublet may do the job.
Could I feed my K3/100 with ATU into a BL2 right at the antenna
connector, the output of the BL2
to home made ladder line, (2 to 4 inch spacing) to a doublet approx
1/2 wavelength on the lowest
freq?

Is it OK to bring ladder line into the shack?

The 1:1 or 4:1 of the BL2 might provide matching options in difficult cases.

Thanks for any thoughts,

Glen VK1FB
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[Don Wilhelm-4]

Glen,

Yes, you can certainly do that, and it should be quite efficient.
You can bring open wire feeders or ladder line into the shack - the old
timers did that all the time.  The greatest problem is making the
conductive path through the wall/window or other part of the structure.  
Replacing a piece of glass with plastic is often done, or with double
hung windows, put a plastic panel beneath the lower sash and close the
window on it.  Don't forget to leave a drip loop at the point just
outside, otherwise rain will drip into the dwelling.

Like any other, the feedline length will act as an impedance transformer
(a tuner does the same thing with lumped components) - so the matching
range required may be quite large for some feedline lengths.  The
switchable balun may help with that, but try to stay away from feedline
lengths that produce a very low  or very high impedances in the shack
because the balun can become ineffective at impedances far removed from
its nominal 50 ohm or 200 ohm design points.

I will not give any specific antenna lengths and feedline lengths, but
you can experiment with either the real antenna or do some work with the
TLW software (download from ARRL website) to tell you what lengths will
work for you.

73,
Don W3FPR

Glen Torr wrote:

> Hello All,
>
> Following this thread with much interest. I need a stealth antena and
> a doublet may do the job.
> Could I feed my K3/100 with ATU into a BL2 right at the antenna
> connector, the output of the BL2
> to home made ladder line, (2 to 4 inch spacing) to a doublet approx
> 1/2 wavelength on the lowest
> freq?
>
> Is it OK to bring ladder line into the shack?
>
> The 1:1 or 4:1 of the BL2 might provide matching options in difficult cases.
>
> Thanks for any thoughts,
>
> Glen VK1FB
>  
>

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[Craig Smith]

<> Could I feed my K3/100 with ATU into a BL2 right at the antenna
<> connector, the output of the BL2
<> to home made ladder line, (2 to 4 inch spacing) to a doublet approx
<> 1/2 wavelength on the lowest
<> freq?  Is it OK to bring ladder line into the shack?

This might work, but I think you will be better served with a few
modifications.  First of all, stay away from 1/2 WL on the lowest freq band,
as higher bands will be harmonically related and can give you difficult to
match impedances.  I use 105 ft for my 80 thru 10  contest antenna.
Secondly, stick with the published ideal feedline lengths and impedances,
for example 30 ft of 300 ohm line for the 105 ft doublet.  Ladder line can
be brought inside, but coax is MUCH more user-friendly and can avoid lots of
problems.  Keep it as short as possible.  Here, I use 6 ft of RG-213 to go
from the K3 to the window termination with the ladder line.  I've tried the
BL2, and it worked OK most of the time but sometimes got quite warm - not a
good sign.  Instead I made a choke balun consisting of 4 turns of LMR-240
wound on two of the Fair-Rite type 31 big clamp-on ferrites (Mouser PN
623-0431177081).  This unit works GREAT based on both impedance measurements
and my actual experience.  It sits right in the window opening and
interfaces the ladder line with the RG-213.   The K3 internal tuner works
just fine with this setup on 80, 40, 20, 15 and 10.  Haven't really tried
the WARC bands.

         73        Craig   AC0DS

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[qrp_1]

Following this thread with much interest. I need a stealth antena and
a doublet may do the job.
Could I feed my K3/100 with ATU into a BL2 right at the antenna
connector, the output of the BL2
to home made ladder line, (2 to 4 inch spacing) to a doublet approx
1/2 wavelength on the lowest
freq? [[snip]]

Sure you can do it that way, but the ATU may or may not tune it on all
bands - depends on that matching range of the ATU and the impedance of
the antenna system at each frequency.  Impedance does change with
frequency, and as long as it stays within the matching range of the ATU,
it should work just fine.  But I think you would have much greater
success with a non resonant length for the doublet - say an 88' doublet
for 10 thru 80 meter coverage, or a 44' doublet for 10 thru 40 meter
coverage.  These lengths aren't carved in stone either, the theory is to
stay away from 1/2 wavelength per side of the doublet to avoid extremely
hig impedance.  You might even want to look into an off center fed
doublet fed with balanced feedline with the balun at the end of the
balanced feedline and a short run of coax coming into the shack.  The
idea behind the off center feed is that the impedance stays fairly close
to the same (around 200 to 300 ohms)  over a wide frequency range and
allows a 4:1 balun to convert it down close to 50 to 75 ohms (or
thereabouts).  Do a google search for the FD-3, or FD-4 antenna to get a
better idea of what I mean.
 
Is it OK to bring ladder line into the shack? [[snip]]

Sure !  Just keep it clear of long runs that are parallel to large metal
objects and it will work just fine.

73 - Rick McKee, KC8AON
Southern Ohio - EM88sn
www.angelfire.com/electronic2/qrp
With God all things are possible ~ ><(('> ~
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