Dave wrote:
N4aeq wrote:

Im trying to match 50 ohm coax (lmr400) with a log peroidic which
should have a 75 ohm feedpoint
the SWR is flat across the 6 m band but high. Im thinking i need a
better match but not sure
where to start.


You have not stated which LP you are using. So, my comments are general in nature.

The feedpoint impedance of a LP does vary as a function of frequency and the log
characteristics of the elements, their spacing, and the boom geometry. The
feedpoint impedance will cycle between a low to high value and generally remain
below 1.7:1.

Example: my HF LP [Tennadyne T8] can have any feedpoint impedance between 29.4
and 85 ohms as I tune from 13.5 MHz to 30 MHz and it will have numerous
excursions within the frequency range. That is the Physics of LP design.

Since the feedpoint impedance is a variable it is not easily matched to a
standard coax line.

Now, a VSWR of 1.7:1 relates to approximately 93% of the rigs power being
coupled to the antenna.

To your comment. What is the 6 meter VSWR?

If it is less than 2:1 [90% coupling to the antenna] then forget it!

Im using a double boom LPA, 50 coax connected to the front with shield
to one boom and
cc to the other. The back of the boom is shorted with a bar, it is a
monoband 50-54mhz
5 element. Swr nowhere near 2:1 (i wish) and i have checked the line
so its ok so maybe
i should have ask how do you adjust the impeadance on a double boom lpa?