MW1CFN via rec.radio.amateur.moderated Admin
2018-04-03 07:35:25 UTC
Real-World Amateur Radio
Why would you...
Posted: 02 Apr 2018 08:48 AM PDT
... use 500W for WSPR?
Antenna inefficiency might be one answer, as a K6 station at much the same
distance was achieving the same S/N here with just 2W!
Phased verticals for 17m.
Posted: 02 Apr 2018 07:36 AM PDT
This week's work in progress is a return to several years ago, when I tried
out a pair of phased, elevated verticals for 20m. These worked well, but
saw little service as my tower was due to be installed where they stood.
With more room now, I've ordered a couple of new 7m fishing poles and 75Ohm
coax to make the necessary 1/4 wave stubs, this time for the 17m band.
The basic feeding format for two phased verticals is, of course, pretty
simple. It goes like this:
The devil, as always, is in the detail. The maximum gain of two phased
verticals, which is about 4.5dB over a single vertical, is achieved when
the spacing between each antenna is at 5/8 wavelength, remaining at much
the same gain to 3/4 wavelength. Gain falls significantly beyond 3/4 wave.
The pattern of phased verticals is markedly different from a single
element, in that you achieve a bi-directional beam, 'broadside' to the line
of the verticals, rather than along their line, roughly as shown in the
polar plot below (imagine one antenna at 0 degrees, the other at 180
Now, for 5/8 wave spacing, at 18.1MHz, the physical spacing needed between
elements is about 10.4m.
But each stub has to take the velocity factor (in this case, 0.8) of the 75
Ohm coax into account, meaning each 1/4 wave leg is shorter: only 3.3m or
so long. Twice that is 6.6m. This leaves us 10.4 minus 6.6 = 3.8m short
(1.9m each side of the 'tee') for the correct spacing, and then only if the
cable is dead-straight and doesn't have to reach an elevated feedpoint.
This is not the case for elevated verticals, and certainly not for vertical
dipoles, yet is almost always ignored in books and articles!
For coax that lies on the ground and has to climb, for this band, around
1.5m up a supporting fishing pole to the feedpoints either side, the total
coax length is significanly longer than the diagram above implies if, as is
likely to be the case, you are using a single, and not odd multiple of 1/4
wave for the stubs.
The shortfall is made up using the necessary length of 50 Ohm coax to reach
the antenna feedpoints.
If we take that 1.5m as a reasonable feedpoint height, we need 3.3m of 75
Ohm coax as calculated, and then 1.9m minimum of 50 Ohm coax without any
climbing to reach the bottom of each pole, and 1.5m on top of that to reach
the feedpoint. Total length comes out as 3.3 + 1.9 +1.5 = 6.7m per side.
In practice, you will probably have the feedlines slightly diagonal to the
vertical elements, so the feedpoint can be a little higher, or the feedline
length a bit shorter. In the worst case, a few notched sticks can be used
to elevate the entire feedline, although doing so might well cause matching
problems due to coupling effects.
Overall, better to cut your stubs, then set up the antenna system in the
field and see how much 50 Ohm coax is needed before cutting it off a drum!