In article ,
Ham op wrote:
I did a SWR plot before and after changing it and here's the
sticking point: Before, I had a 2:1 range from 6.900-7.280 and
now I've got 7.000-7.300 but the minimum SWR went from
1.10:1 to 1.55:1. My Elecraft KAT2 autotuner handles that very
easily but I'm wondering what would happen if I added 12 feet
to my 50' of 450 ohm ladder line? Would the minimum SWR
get better or just move? Or both? Does it really matter? (I don't
think it matters based on what I've read here).

SWR has nothing to do with the length of line [assuming low loss line].

True in one respect, but not necessarily a complete answer in this
case (I think).

SWR is determined solely by the antenna to transmission line impedance.
[e.g. an antenna input impedance of 70 ohms and a 50 ohm cable has a SWR
of 1.4:1 ALL the time].

True, but that's not the whole story here.

If I understand the OP's setup properly, he's trimmed his antenna wire
down to be a half-wave 40-meter dipole. He's then feeding it with a
combination of ladder line, a coaxial balun, and coax to the shack.

If that's a true picture of the situation, then I would expect the SWR
at the shack end of the coax to vary quite a lot as the length of the
ladder line is changed.

Reasoning: the antenna's feedpoint impedance wouldn't be too far from
the 50-to-70 ohm range, give its resonant length and its height above
ground. This impedance will be transformed by the 450-ohm ladder
line, and the impedance at the ladder-line/coax joining point will
depend on the degree of transformation.

If the ladder line is cut to be an integral number of electrical
half-wavelengths, it would mirror the antenna feedpoint impedance to
the coax. There's be a good match, the SWR on the coax would be
somewhere between 1:1 and 1.5:1, and the rig would quite possibly be
happy without even needing its internal ATU. This is Good.

On the other hand, if the ladder line is an odd number of
quarter-wavelengths, it would transform the antenna's feedpoint
impedance up to a much higher value (in excess of 1000 ohms). This
would result in a severe mismatch at the ladder-line/coax joining
point, and a high SWR on the coax. I'd expect both high losses, and a
significant amount of RF appearing on the coax braid (the coiled-coax
choke balun wouldn't have enough choking reactance). The rig's ATU
might not be able to match this load at all. This would be Bad.

In these two extreme cases, the SWR on the ladder line would be high,
and wouldn't change appreciably as the ladder-line length was changed.
Losses in the ladder line should be minimal.

The SWR on the coax section, and the impedance presented to the rig,
would vary anywhere from "utterly benign" to "nasty and unacceptable"
depending on the length of the ladder line.

The same basic reasoning would seem to apply, I think, for any antenna
feedpoint impedance other than one which matches that of the ladder
line. Although changing the ladder-line length won't affect the SWR
on the ladder line, it can affect the SWR on the coax section
(possibly by a lot!) due to the varying transformation of the
antenna's feedpoint impedance.

This is, in a way, the converse of Cecil's "no tuner" matching
arrangment, which uses a tuned ladder-line feeder to transform a
dipole's feedpoint impedance down to 50 ohms on multiple bands by
adjusting the feeder length.

--
Dave Platt AE6EO
Hosting the Jade Warrior home page: http://www.radagast.org/jade-warrior
I do _not_ wish to receive unsolicited commercial email, and I will
boycott any company which has the gall to send me such ads!

----- Original Message -----
From: "Dave Platt"
Newsgroups: rec.radio.amateur.antenna
Sent: Tuesday, July 26, 2005 8:08 PM
Subject: Raising a 40m dipole.....It's up! Now what?

snip

If the ladder line is cut to be an integral number of electrical
half-wavelengths, it would mirror the antenna feedpoint impedance to
the coax. There's be a good match, the SWR on the coax would be
somewhere between 1:1 and 1.5:1, and the rig would quite possibly be
happy without even needing its internal ATU. This is Good.

On the other hand, if the ladder line is an odd number of
quarter-wavelengths, it would transform the antenna's feedpoint
impedance up to a much higher value (in excess of 1000 ohms). This
would result in a severe mismatch at the ladder-line/coax joining
point, and a high SWR on the coax. I'd expect both high losses, and a
significant amount of RF appearing on the coax braid (the coiled-coax
choke balun wouldn't have enough choking reactance). The rig's ATU
might not be able to match this load at all. This would be Bad.

snip

Would that explain why my tuner can only get a 2.6:1 match on 12m
with a 50 foot section of ladder line? Here's my (probably wrong) math:

((246/24.940)(0.92)5)= 45'4.5"

That's 246 / Freq x Velocity Factor x 5 (odd quarter wavelength)

On a related issue, my lowest SWR went from a broad 1.6:1 to a
narrow 1.1:1 after it stopped raining. Was that the wet asphault shingles
effecting the ladder line or just the wet ladder line? If it's the shingles,
I could elevate the line but either way, my tuner handles it. Should I care?

I've now plotted the SWR and (dry), the curve is nearly the same
as it was before I raised the antenna - it's just centered about 45 khz
higher and a bit narrower. This is with the tuner in bypass mode.

Thanks for all the help, guys!

Ken
--
Just my 2¢... 73 es gd dx de Ken KGØWX
Grid EM17ip, Flying Pigs #-1055
Proud builder & owner of Elecraft K2 #4913

Ken Bessler wrote:
((246/24.940)(0.92)5)= 45'4.5"

That's 246 / Freq x Velocity Factor x 5 (odd quarter wavelength)

Calculated values rarely ever take all the variables into account.
Virtually all calculated values have to be adjusted, i.e. cut
and tried, to achieve the desired results. What SWR do you get
when you increase the length by one foot? What SWR do you get
when you decrease the length by one foot?

On a related issue, my lowest SWR went from a broad 1.6:1 to a
narrow 1.1:1 after it stopped raining. Was that the wet asphault shingles
effecting the ladder line or just the wet ladder line? If it's the shingles,
I could elevate the line but either way, my tuner handles it. Should I care?

Do you care? :-) It is a good idea to keep ladder-line a few
inches away from anything that might adversely affect it. That
includes wet asphalt shingles.
--
73, Cecil http://www.qsl.net/w5dxp

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