Hi Luis, Stefan,
just a short comment: I would second every bit of Stefan's advice, this
is really the way to go.
Antenna capacitance will not be much different between erecting the
same vertical on flat ground or on a tall building, maybe 5 pF/m vs 6
pF/m or so. This is because most of the E-field drop and reactive
resistance anyway happens near the wire surface, and capacitance
changes only logarithmically with wire diameter and height.
But the good thing is that effective height will be much larger,
because a significant part of the displacement current will not return
to the roof but go the "long way" down to ground zero. This is also why
a miniwhip on a pole receives larger signals. Details depend on the
exact geometry (eg. the relative thickness of the "mast"), but a good
guess is taking the geometric mean between the height of the house and
the effective height of the antenna itself if it was above flat ground.
In your case, this would be sqrt(80m*5.5m) = 21m, which is way above
average compared to most amateur antennas. Radiation resistance would
be 1.75 ohms, so in theory you'd need only 0.56 A to achieve 1 W ERP...
All the best,
Markus (DF6NM)
PS of course what you really want to do is sneak a thin sloping wire
from your roof up to the top of that other yellow building ;-)
-----Ursprüngliche Mitteilung-----
Von: DK7FC <[email protected]>
An: rsgb_lf_group <[email protected]>
Verschickt: Mi, 20 Mai 2015 9:57 pm
Betreff: Re: LF: EA5DOM vertical
Hu Luis,
Am 20.05.2015 18:34, schrieb VIGILANT Luis Fernández: Hi Stefan
Thank you for your detailedexplanations
I can recycle 16cm diameter PVCtube. As seen in the calculators and
assuming a copper wire withplastic cover to be 3mm diameter
can wind 300 turns in 90cmlenght which would be 2.33 mH.
That is not a good dimension. The length of the coil should be aboutthe
diameter of the coil or at least less than 2x the diameter.
A 16 cm PVC tube is a good choice but the 3mm diameter wire is to big.
But of course it is all your decision, i don't want to force you to
doit like i think, i can't do anyway ;-)
That can be a starting pointand the coil should be attached to the
bottom support
of the vertical antenna. See andold picture here showing the setup
https://dl.dropboxusercontent.com/u/7162072/Fotos%20varias/29-03-2011/DSC03912.JPG
Is the mast completely metallic? Or just the first piece? It can
behelpful to lift the coil a bit above the ground, say 50 cm onto
nonconducting stuff, to avoid losses. Just leave the tube 50 cm
longerthan necessary.
May be 5pF/m is the standardassumption for a vertical antenna against
horizontal ground plane. Buthere the counter poise will be different
and probably showing much lesscapacity.
Maybe the WX-station in the picture is grounded? It is a good EMC
testif it can handle 5 A of RF current ;-)
Also your horizontal loop can be a part of a counterpoise.
I can use a high voltagecapacitor in parallel with the coil to tune.
The variable capacitor I'm
already using with the loop, forexample. Would it make the trick to
use a smaller coil ??
It can help in first steps, when you still don't know the
actualresonance point, i.e. if you need more or less L. But in the end
youshould have no C in parallel. It won't handle the voltage anyway :-)
Ofcource this depends on your RF power.
If C is 55 pF and your losses are 50 Ohm and the power is 50 W, youwill
have 6 kV on the antenna.
73, Stefan
|
