[Antennas] Design questions for directional antennas

[David Robbins K1TTT]

That is true, however I considered it beyond the scope of the original
question.


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
 

> -----Original Message-----
> From: antennas-bounces at mailman.qth.net [mailto:antennas-
> bounces at mailman.qth.net] On Behalf Of Dr. William J. Schmidt, II
> Sent: Sunday, July 31, 2005 20:18
> To: David Robbins K1TTT; 'A10382'; 'Antenna reflector list'
> Subject: Re: [Antennas] Design questions for directional antennas
> 
> 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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