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[Towertalk] Re: [Towertalk] Re. HF2V

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Subject: [Towertalk] Re: [Towertalk] Re. HF2V
From: (
Date: Mon, 8 Jul 2002 10:01:17 +0200
 > I always thought that the radiation comes from the place where the cu
> is the highest, ie at thepoint of feed for a dipole and a 1/4 wl. 

This is correct if the antenna current distribution is or remains 
almost linear, in other words if the antenna is full size or only 
slightly loaded.

> the further away from the feed point (to limits of course) the inducti
> load is the better, then the next is to change the inductor to linear 
> load.  That being all other variables constant.

When an antenna is very short one can think of it as a pseudo constant 
current radiator rather than anything else.

Inductive loading with real inductors (with ohmic losses) is infact in 
series with the antenna circuit and it's losses will reduce the current 
flow (ohm's law).
As much as you move the inductor away from the antenna base then more 
XL is required but since required XL increases esponentially as the 
coil is moved away from antenna base, also losses will increase.
With moderate antenna loading it exist an optimal place were to locate 
an inductor but when the antenna is very short moving the coil has no 
The coil losses will increase so much that the RF current will 
decrease, thus decreasing the current also below the coil.
In such a case the capacitive hat (capacitive loading) is the only 
possible improvement one can add, while moving the coil up and down 
won't change much the situation.
Overall system Q of an efficient short radiator can't be other than 
very high, but there's always something to pay...

> >If the ground system is poor, top loading can reduce loss by up to a
> >factor of three or four. If the ground system is good, efficiency
> >changes are often not measurable.
> Now what is a good ground system?  I have read 4 radials is good but s
> say anything less than 16 is poor.  The idea that I like is 4 elevated
> radials.  (If about 5ft will stop the kids running through the yard an
> hitting the guys.)

A good ground system is that one able to "collect" RF current keeping 
it in the circuit and not wasting, i.e. in the soil.
Under this perspective long radials with short antennas won't help like 
a bigger radial density that extends around the antenna proximity.

Elevated radials can work when when their coupling with soil (or 
structures below) is minimized, in this case anyway is incorrect to 
think about them as a ground plane because they are in effect a 
counterpoise, the other leg of the antenna, exactly as it happens with 
a vertical dipole. 

Mauri I4JMY

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