El 14-11-14 19:12, Ralph Mowery escribió:
wrote in message
...
But others are correct. The antenna itself is an efficient radiator;
it's the matching network that lowers *antenna system* efficiency.


Any dipole type antenna will suck on 75M if mounted less than about
100 feet, or about .4 wavelengths. Below that you are warming clouds.


So now it's 100 feet? It used to be 60 feet. But I have proof that is
not the case. So do a lot of other hams I know.

Your "facts" are for an idealized installation. Reality is much
different, and will never get the ideal specifications you claim.

For a dipole over average ground:

Height Gain @ Elevation
lambda

0.1 3.89 90
0.15 5.55 90
0.2 5.95 90
0.25 5.81 62
0.3 5.80 48
0.35 6.00 40
0.4 6.38 35
0.45 6.86 31
0.5 7.41 28
0.55 7.76 25
0.6 7.87 23
0.65 7.76 21
0.7 7.54 20
0.75 7.30 18
0.8 7.16 17
0.85 7.15 16
0.9 7.26 15
0.95 7.47 15
1 7.71 14

At 75M .4 lambda is about 100 feet.

Generally for DX a takeoff angle of 30 degrees or less is the rule of
thumb for best general performance.

Of course the antenna still "works" at other heights, but if DX is what
you want to achieve, then best results, on the average over average
ground, the antenna will work best for that at a height of .5 lambda
or better.

Now is you happen to be in a salt water marsh surrounded by 100 foot
tall steel blimp hangers, your results may vary.

At a given height wouldn't all simple horizontal antennas (halfwave dipole,
g5rv) have the same take off angle and be equal in that respect ? Say you
had a halfwave horizontal and replaced it with the g5rv at the same height
there shold not be any differance it the takeoff angle. If there is any big
differance in the signal , it would probably be the loss in the feedline
going to the 102 foot long antenna.

You are right, if you use a G5RV that has no feedline radiation (both
ladder line and coaxial part), the shape of the H-plane elevation
pattern is the same, and practically the same for the E-field plane.


I have been tempted to put up a g5rv just to see what it would do. I have
an 80 meter dipole and also an OCF Carolina Windom at about the same heigth.
They are around 60 feet on the ends. There is usually not too much
differance in them except certain directions and distances. Then there can
be several S units at some directions. This is probably because they are at
right angles to each other.

The shape of the radiation pattern for an OCF (no feed line radiation)
at greater then twice the half wave frequency is different from that
of a originally center-fed half wave dipole. The current distribution
is different (you get phase reversals in the OCF antenna).

Just an example: When using an OCF at twice the half wave frequency,
it has no radiation in the H-plane (that is the plane perpendicular to
the wires, you get a 4-leaf pattern). A center-fed antenne has good
radiation in the H-plane at twice the half wave frequency. Of course
it has high input impedance, so one has to take care of the matching.

That means that if you have an OCF with its half wave frequency at
75m, it will not act as a good NVIS antenna at 40m.


--
Wim
PA3DJS
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