600 ohm ladder line is classic, but in the case where you have a low
impedance load like your short dipole or a 1/2 wave resonant dipole and
a quarter wave of feed line it transforms those low impedance loads to
high impedances that are sometimes out of the range of the simple tuner
on 160 a band where getting the match with the L network of the 238B
requires more inductance than you have available. And subjects the
capacitors and connections to high voltage. A link coupled parallel
tuner like we used to make for ourselves (poorly approximated by the
Johnson Matchboxes) is an appropriate tuner for this 160 meter situation
if the capacitors have enough air gap to handle the voltage.
A 1:4 air core balun would be more appropriate than the current balun
which is 1:1. A ferrite cored 1:4 voltage balun is abused in your
situation and can lead to excess harmonics and core heating from
saturating a core that isn't wound for the high impedance. E.g. one
wound for 200 ohms on the high side won't probably work right with an
18K load. The high voltage will lead to core saturation and wave
distortion along with much core heating. Narrowing the feedline to lower
the characteristic impedance is simpler in my mind than any of the
alternatives.
73, Jerry, K0CQ
On 4/8/2011 1:36 PM, Art Trampler wrote:
> Thanks Jerry.
> Interesting that there is perhaps some mythology then about the benefit
> of "true ladder line." this is about 4.5" or 5" spacing, with machined
> spacers made my a ham-owned machine shop.
> I'll not rush into changing the feed line, as it took a little while to
> make it in the first place!
> Art
>
> --- On *Fri, 4/8/11, Dr. Gerald N. Johnson /<geraldj@weather.net>/* wrote:
>
>
> From: Dr. Gerald N. Johnson <geraldj@weather.net>
> Subject: Re: [TenTec] SLIGHTLY OT
> To: tentec@contesting.com
> Date: Friday, April 8, 2011, 1:29 PM
>
> A quarter wave center fed wire has low radiation resistance and is very
> capacitive. Bad phase angle. You run that through a 600 ohm line
> about a
> quarter wave long, converts low R to high R and C to L. So your balun
> sees high impedance that may be causing core saturation (for sure in a
> voltage balun) and you see high Z at the tuner.
>
> More wire at the antenna will shift the results, a quarter wave more
> feed line would shift the results, but the easiest change is to lower
> the characteristic impedance of the feeder by going to fatter wires or
> closer spacing. I take you are using 6" spacers. For 300 ohms you need
> only 1/2" spacers. A quarter wave of 600 ohm transforms a 20 ohm
> load to
> 18,000 ohms. A quarter wave of 300 ohm transforms that 20 ohm load to
> 4500 ohms which is somewhat easier to match. That 110 feet of feed line
> is close to a half wave or multiple of a half wave on higher bands, so
> the characteristic impedance has much less effect on the impedance seen
> at the tuner.
>
> 73, Jerry, K0CQ
>
> On 4/8/2011 12:13 PM, Art Trampler wrote:
> > It's on-topic, as I am using a 238B...
> >
> >
> > New antenna, 140' sloping dipole...apex about 50 to 55 feet, ends
> about 30 feet, so not quite an inverted vee,
> >
> > Feedline: 600 ohm (or thereabouts) 14 gauge stranded, insulated
> wire using 73CNC spreaders, Guesstimated length: 110 feet. Feedline
> and antenna are continuous pieces.
> >
> > Tuner: 238B, followed by 10 feet of LMR600 to a 1:1 current balun
> from Balun Designs (to get out of the shack).
> >
> > On 160 meters, I wound up in Hi-Z "5" position to get a 1.8:1
> match. Am I looking at Hi-Z because the antenna length is about 1/4
> wave? If so, would it help so add 20 feet to each end, even if it
> went at nearly a right angle to the antenna? Or is this really
> unimportant/not an issue as long as I get a match?
> >
> > Also, what recommendations do other owners have for additional
> capacitance if necessary? I have not replaced the internal
> capacitors with the Russian doorknobs but might at some point.
> >
> > Interested in your thoughts. If the Pegasus S-Meter is at all
> accurate, I was hearing Russians a full S-unit better on 20 meters
> last night with this antenna than with my AV640 vertical. Japan is
> off the end, so the vertical is better in that direction.
> >
> > 73,
> > Art
>
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