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
> _______________________________________________
> TenTec mailing list
> TenTec@contesting.com
> http://lists.contesting.com/mailman/listinfo/tentec
>
_______________________________________________
TenTec mailing list
TenTec@contesting.com
http://lists.contesting.com/mailman/listinfo/tentec
_______________________________________________
TenTec mailing list
TenTec@contesting.com
http://lists.contesting.com/mailman/listinfo/tentec
|