Tom Horne wrote:

As to why not put the coupler at the feed point I would point out that
the statement "The box *is* a
bit heavy," would pass as a masters dissertation in understatement. An entire center support would have to be created or I would have to rig a catenary at the present support points and lower the antenna closer to the ground to permit the catenary to take the weight of the antenna coupler. I would point out that this present Folded Terminated Dipole is not Tilted but instead is rigged as a flat top. Height is might; so the old radio axiom goes; so I would rather not have to lower the antenna just to avoid using ladder line. Additionally in order to compensate for the bending of the trees that support the antenna now I would have to rig a second counterweight and puller system to keep the antenna itself from being torn apart since the first one would be in use to keep a constant tension on the catenary. That is a lot of work in order to avoid the use of some ladder line that probably has a lower loss than the additional coax that would replace it.



OK.. makes sense.. So you'd have antenna in the air, open wire line to
tuner which is sitting on the ground or somewhere convenient, then coax
to the rig.

As for loss.. open wire line is lower loss when matched than coax, but
in the highly mismatched case, the loss might be greater than the loss
of the coax, if you move the tuner.

This is kind of unintuitive, so we'll run some examples




What's the feedpoint Z of a 90 foot dipole in the various bands?
using W9CF's online loss calculator with 33 ft of 450 ohm window line
For 3.8 MHZ (24-541j), loss is 1.3dB (tuner sees 13.3-15.5j)
For 7.2 MHz (29+768j) loss 0.79 dB (tuner sees 11.6-247j)
For 14.2 (124-681j) loss 0.51 dB (tuner sees 135-663j)

For comparison 33 ft of RG-8X has .17 dB of loss (matched) at 3.8MHz,
..25 at 7.2, and 0.35 at 14.2

Now, the question might be, what's the loss in the tuner matching
29+768j to 50 ohms (tuner at feedpoint) vs loss in tuner matching
11.6-247j to 50 ohms (tuner feeding 33 ft of 450 ohm window line to antenna)

W9CF's tuner applet says 0.3 dB for the first case and 1.7 for the second..


SO..
Tuner at feedpoint at ladder/coax transition
Loss in 450 ohm line 0 0.79
Loss in Tuner 0.3 1.7
Loss in 50 ohm line 0.25 0
Total: 0.55 2.49
That makes the tuner at the feedpoint almost 2 dB better..

Similar numbers for 3.8MHz
24 -541j 2dB tuner loss to 50 ohms + 0.17dB feedline = 2.17
13.3-15.5j 0.9dB tuner loss + 1.3 feedline loss = 2.2 dB

pretty much a wash

---- take home message 90 feet is probably a "bad" length to use for
this application
---- take home message2 open wire line is only low loss when well
matched.
---- take home message 3 it's worth it to put the matching network at
the feedpoint, when there is a large mismatch.





Running a quick NEC4 model of a 90 foot dipole 10 meters off the ground

FREQ. - - UNNORMALIZED IMPEDANCE - -

RESISTANCE REACTANCE MAGNITUDE PHASE
MHZ OHMS OHMS OHMS DEGREES

3.500 1.93208E+01 -6.73980E+02 6.74257E+02 -88.36
3.600 2.08586E+01 -6.28432E+02 6.28778E+02 -88.10
3.700 2.25078E+01 -5.84283E+02 5.84717E+02 -87.79
3.800 2.42755E+01 -5.41393E+02 5.41937E+02 -87.43
3.900 2.61693E+01 -4.99631E+02 5.00315E+02 -87.00
4.000 2.81973E+01 -4.58878E+02 4.59744E+02 -86.48


FREQ. - - UNNORMALIZED IMPEDANCE - -
RESISTANCE REACTANCE MAGNITUDE PHASE
MHZ OHMS OHMS OHMS DEGREES
7.000 2.38419E+02 6.75860E+02 7.16680E+02 70.57
7.100 2.56679E+02 7.21609E+02 7.65901E+02 70.42
7.200 2.76540E+02 7.68679E+02 8.16910E+02 70.21
7.300 2.98181E+02 8.17175E+02 8.69877E+02 69.95

FREQ. - - UNNORMALIZED IMPEDANCE - -
RESISTANCE REACTANCE MAGNITUDE PHASE
MHZ OHMS OHMS OHMS DEGREES

14.000 1.39739E+02 -7.64444E+02 7.77111E+02 -79.64
14.100 1.31779E+02 -7.21947E+02 7.33876E+02 -79.66
14.200 1.24816E+02 -6.80698E+02 6.92047E+02 -79.61
14.300 1.18762E+02 -6.40602E+02 6.51518E+02 -79.50