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[Towertalk] Re. HF2V

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Subject: [Towertalk] Re. HF2V
From: (Tom Rauch)
Date: Sun, 7 Jul 2002 14:24:53 -0400
> I always thought that the radiation comes from the place where the
> current is the highest, ie at thepoint of feed for a dipole and a 1/4
> wl.  Hence the further away from the feed point (to limits of course)
> the inductive load is the better, then the next is to change the
> inductor to linear load.  That being all other variables constant.

Radiation is a directly tied to ampere-feet of the antenna, where the 
sections carry currents whose radiation fields add in phase.

As we make an antenna shorter for a given amount of applied power (or 
if we add out-of-phase radiation to form a directional pattern), 
currents must increase to compensate for the change until all power 
is radiated.

Making the antenna shorter increases I^2 R heating losses, and can 
also increase voltages.

Concentrating the current in one area is really not any different 
than making the antenna shorter. If resistive losses are kept 
reasonably low, nearly the same power radiates. The key is to keep 
losses low.

Distributed loading like a helical antenna is about like base or 
center loading, except the coil Q goes to heck and resistive losses 

Linear loading with a bent wire generally increases losses like using 
a poor coil would, and moves the effective point of loading to a new 
spot away from the point where the element is actually broken. 
Depending on how you do linear wire loading, it can be much worse 
than a good coil to about the same.

The best loading is with a large hat at the antenna's open end.     
> Now what is a good ground system?  I have read 4 radials is good but
> some say anything less than 16 is poor.  The idea that I like is 4
> elevated radials. 

We hear lots of things. Four radials are good, if  they are 1/4 to 
1/2 wl above normal earth.

One IEEE paper I have with actual measurements showed eight radials 
at 1/4 wl were only very slightly worse than a full system on 
the ground. I measured four radials at 6 or 8 feet on 80 meters about 
5-6 dB down from 60 radials on the ground using a 1/4 wl vertical. 
The change would be worse with a short vertical.

Interestingly enough, I was made aware of that paper by GAP! They 
were handing it out as proof the elevated feed they use helps! What 
the paper actually shows is eight radials at 1/4 wl height above 
ground are nearly equal to 120 1/4 wl radials on the ground, which is 
nothing like their antenna at all.

73, Tom W8JI 

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