[Antennas] Design questions for directional antennas

[Dr. William J. Schmidt, II]

Well, hold up... these is one more little issue with grounded vs. ungrounded 
elements for consideration.  Ungrounded elements had the capability of 
storing static charge (e.g. wind charging).  you may then have the 
equivalent of a small spark transmitter sitting on the antenna as the 
element discharges to the boom (under the right circumstances).  Same is 
true for verticals, and the reason I *always* ground my radiators (DC 
through chokes) and parasitic elements.

Sincerely,

Dr. William J. Schmidt, II  K9HZ
Trustee of the North American QRO - Central Division Club - K9ZC

Email: bill at wjschmidt.com
WebPage: www.wjschmidt.com

"If computers get too powerful, we can organize them into a committee; that 
will do them in."  -- Bradley's Bromide


----- Original Message ----- 
From: "David Robbins K1TTT" <k1ttt at arrl.net>
To: "'A10382'" <A10382 at snet.net>; "'Antenna reflector list'" 
<antennas at mailman.qth.net>
Sent: Sunday, July 31, 2005 2:40 PM
Subject: RE: [Antennas] Design questions for directional antennas


> 1) Some yagis have their non-radiating elements electrically connected
> to the boom (and therefore each other).  I copied a Cushcraft 900 yagi
> (with a metal boom), but made the boom on mine out of wood (using the
> same diameter and length radiators for all elements).  I did not connect
> the non-radiating elements - the directors and the reflector -- to each
> other.  While I did not notice any difference in performance between the
> two, I also do not have an antenna range to verify exact performance,
> alternatively relying on just operating.
>
> Q1: Is there any effect on performance by either connecting/grounding
> the non-radiating elements ??

no.  however, the lengths of the elements need to be adjusted to account for
a fat part in the middle when they go through the boom or are mounted on
plates attached to the boom.  Either of those arrangements makes the element
look bigger in diameter in the middle and require a slight shortening of the
element with respect to a free space design to achieve the same performance.


>
> 2) Commercially made broadcast TV receiving antennas usually have their
> elements in a "V" shape (sort of like a horizontal inverted V).
>
> Q2a: Other than possibly cosmetics (the antenna 'pointed' at the
> station) is there a design/performance reason for this ??
> Q2b"  Does this "V" shape have any effect on element length or bandwidth
> ?

yes it does affect the performance.  But also note that these elements are
normally part of a log periodic array not a Yagi (parasitic) array.  The
designs are very different and can't be interchanged.  The log periodic
array for tv is designed more for bandwidth than gain which is why it has
lots of close together fed elements.


>
> 3) A dual band yagi design like the 'Arrow' handheld has the elements
> for the 2 bands at right angles to each other.
>
> Q3:Does this type of element positioning eliminate any effect one band's
> elements has on the other band ?

not completely, but it gets rid of most of the effects.


>
> BTW: The one design that did not work well nagged me.  I ended up
> following a few suggestions, and the performance eventually (and
> seemingly) closely equaled the original design.  The change with the
> most impact was replacing stainless elements with aluminum (the original
> design was made with aluminum) - go figure...

steel is a relatively poor conductor of rf compared to aluminum.


David Robbins K1TTT
e-mail: mailto:k1ttt at arrl.net
web: http://www.k1ttt.net
AR-Cluster node: 145.69MHz or telnet://dxc.k1ttt.net

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