I am posting this question and answer to TowerTalk
since they may be of interest to the readership.
See below. N4KG
On Mon, 08 Jan 2001 "Ted Leaf" <tleaf@hotmail.com> writes:
> Hi Tom,
>
> My inverted V, apex at 25', is trimmed to my center freq with a 1:1
> SWR--which is about what the books say--50 ohms.
>
> Was wondering why the V beam is 30 ohms.
>
> I don't recall what a V beam looks like. Please explain. And isn't
> the standard rule that reflectors are about 10% longer?
>
> 73, Aloha
> Ted Leaf, K6HI
> Kona, Hawaii
> <tleaf@hotmail.com>
>
A stand alone inverted vee will be close to 50 ohms.
The feed impedance of Yagi's is lower, depending
on tuning and spacing of the parasitic elements
whether the elements are straight or inverted V style.
(see W2PV Yagi Antenna Book or similar)
There is a chapter in the W2PV book showing
the effect (gain and F/B) versus parasitic element
tuning from +/- 2% through 7%. A good rule of
thumb is 5% longer for the reflector and 5%
shorter for the director. Beyond that, gain drops.
Closer tolerances produce higher gain but less
bandwidth and of course the dimensions are
much more critical (i.e. you better be right!)
de Tom N4KG
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