[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 2:20 PM
Subject: Re: [TowerTalk] Top loading HF-6V for 160


> On 10/19/07, Eric Hilding <b38 at hilding.com> wrote:
>> My understanding is that use of the 160m loading coil for the HF-6V 
>> limits
>> the power to 500w on 160m.
> I'd believe that. The voltage across the coil will tend to be pretty 
> fierce.
>
> 500W seems to give about 7600V across the coil. Full legal limit would be 
> 13kV.
>
>
>
>> Q: What if the 160m coil is left off, and 4 slolping top hat wires  were
>> attached to the top of the vertical?
>
> Commonly done and generally a decent way to top load, but in this case
> it doesn't seem to work out too well.  Maybe someone else can chip in
> but it seems tough to do sloping wires without cancelling a lot of
> radiation and driving the radiation resistance down.  Straight out is
> better.  It makes me wonder if even an inverted L with a sloping top
> wire is a very good idea.  I think it might not be.

Straight out gives the highest radiation resistance, but you still
get an overall radiation resistance benefit from sloping top-hat
wires on a short radiator. I've had good luck with 20 foot long top hat
wires sloping down at 45 degrees from the top of the 45' vertical that
I use when operating portable. I use 22uH of center loading in
combination with the  top-hat wires and have no problems running
1000 watts into the antenna.

I don't know if there are any simple formulas for determining the
optimum length for sloping top-hat wires. Presumably for a given
slope angle some particular length gives the best tradeoff between
top-loading and field cancellation. Based on some numbers that Dave
WX7G suggested, I used EZNEC and just kind of played around with
the loading coil  position and top-hat wires lengths until it seemed that
I was getting a pretty good balance (turned out that the number's
Dave gave me were very good). This was by no means an exhaustive
optimization, but for what it's worth the antenna seems to play very
well.

BTW, at home I use a 60' tall inverted L with a variable height mast
for the far end of the 60' long top-hat wire. When its fully erect, the
far end of the top-hat wire is at about 50' above ground, but the mast
is a bit flimsly so I drop it down to around 20' when it gets windy. I
always feel better when its up all the way, but I don't really notice a
difference in performance between the near horizontal case and the
sloping case. 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.

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