----- 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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