[TowerTalk] Top loading HF-6V for 160

[Michael Tope]

----- Original Message ----- 
From: "Dan Zimmerman N3OX" <n3ox at n3ox.net>
To: <towertalk at contesting.com>
Sent: Friday, October 19, 2007 9:47 PM
Subject: Re: [TowerTalk] Top loading HF-6V for 160


> "Somewhere I have the modeling data which shows a
> modest improvement in radiation resistance when the top-hat wire is
> horizontal (as compared to sloping),  but it's not a make or break
> difference.
> "
> 
> It's make or break for a 34 foot vertical on 160 I think, based on
> another look at the model.
> 
> 34 foot vertical with 4 straight top loading wires 60 feet long,
> resonant 1.75 MHz, 5.6 ohms.
> 
> 34 foot vertical with 4 60 foot sloped down 30deg is resonant around
> 1.8MHz, 1.4 ohms, which is about the same as the radiation resistance
> of the base loaded version.
> 
> There's probably still a benefit over a very good ground system when
> you consider the low loss nature of the cap hat wires vs. a coil of a
> Q of a few hundred, which is going to have a loss resistance of  a few
> ohms (you need about +j920 to resonate)
> 
> Dan

Yes, I get similar results, Dan. The 60ft top hat wires sloping down at 
30 degrees (relative to horizontal) are only about 1dB better than the 
base loaded 34' vertical assuming that Q=200 and a ground resistance 
of 10 ohms.  The antenna with 60ft top hat wires perfectly horizontal is
about 5dB better than the base loaded antenna with no top-hat wires.
If you shorten the top hat wires to 25 feet with the same 30 degree 
slope and center load the antenna (20 foot level) with 26uH, you end
up only about 1.5 dB down from the case with 60ft horizontal top hat
wires. 

These results are in line with my suspicion that for sloping top-hat
wires, there is some optimum wire length that produces an optimum 
balance between top-loading, radiation cancellation, and inductor
loss. The next step would be to come up with an optimizer whereby
you put in the physical constraints (inductor Q, wire slope angle, 
radiator height, ground loss resistance) and then let the optimizer spit 
out an optimum position for the center loading inductor and a optimum 
length for the top hat wires. 

73, Mike W4EF.............................