[TowerTalk] re; Ladder Line Fed Dipole Isssue

[Donald Chester]

Tony dxdx at optonline.net wrote:

> I manged to tweak the length of the 300 ohm ladder line to where the SWR 
> dropped to 1:1 in the middle of the 80 meter band. The SWR decreased on 
> 40 through 10 as well so I can now use my limited range auto-tuner on 
> all bands. The short 110 foot dipole doesn't seem to be an issue on 80.

Tweaking feedline length does absolutely nothing to affect the SWR on the line.  What it does is merely to allow the tuner to see a more favourable load impedance.  SWR in the transmission line is a function of the surge impedance of the line (in your case 300Ω) and the actual impedance of the dipole at the feed point.  Varying the feedline length does not change the SWR, but it  does affect the impedance the tuner sees at the transmitter end of  the line.

I never had a problem feeding a half-wave dipole  with a quarter-wave open-wire transmission line, which causes the tuner to see a high impedance load, on the order of 1000 ohms or more.  I configured my link-coupled balanced coupler for parallel tuning and it worked great.

Avoid lengths of feedline that terminate exactly mid-way between a voltage node and a current node; this occurs at odd eighth wavelengths (1/8λ, 3/8λ, 5/8λ, etc.).  These lengths present the tuner with a highly reactive load that is very difficult to efficiently match, and if air-dielectric variable capacitors are used in the tuner, they may flash over on modulation peaks even at low power.

Don k4kyv