Hi Markus, Stefan,

I have renamed the discussion, just to match the real issue

Thanks again for all the advices. I'll use a thinner wire to get a shorter coil
as per Stefan advise. Anyway it will become a "Kebab Size" pipe almost 1m tall

>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.

Bottom mast is metallic. Top mast is fiberglass with the electrical conductor 
tape running inside
The idea is to install the coil up-standing parallel to the bottom mast. So I 
can place
a horizontal support rod from bottom mast to coil pipe top. I must fix the coil 
secure or wind
will reinstall it in any other place

>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...

This is quite promising. For sure I can not pass 1W ERP legal limit. Will I 
need to remove 1xBS170 in the U3 ? ;-)

>>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 ;-)

Markus ... You read my mind ! :-O
I do have friends living on that yellow tower at 100m horizontal distance and 
25m higer than my roof
But there is just an small step from Radio-amateur to Franklin-amateur
 
So, lets keep feet on radio and not to become the "spark aficionado" of the 
neighbourhood ;-)

73 de Luis
EA5DOM

PS: A fixed fishing line were a thin copper could be deployed and retracted 
along is in the projects pypeline
Call for papers on this issue !


-----Mensaje original-----
De: [email protected] 
[mailto:[email protected]] En nombre de Markus Vester
Enviado el: miércoles, 20 de mayo de 2015 22:47
Para: [email protected]
Asunto: Re: LF: EA5DOM vertical

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