[TowerTalk] An Opposite View on the Log Periodic vs Selectivity Impact

[n4kg@juno.com]

LONG delayed N4KG response: 

The LOG manufacturer tries to discredit Yagi's as being
"non-directional" on non-resonant frequencies and claims
that they will experience MORE out of band interference.

To this I say:  B.S. !

The driven element of a Yagi antenna is a dipole.
Whether resonant or not resonant, a dipole has a
pattern that rejects signals off the ends.  F/B will
degrade out of band, but not F/Side.

Also, many (most?) Yagi's have some sort of matching
system which is TUNED and therefore provides some
SELECTIVITY out of band.

I suggest that the quoted Log Periodic manufacturer
is more interested in selling his antennas than providing
insight and truth.

de  Tom  N4KG

On Fri, 26 Nov 1999  "DavidC" <eDoc@netzero.net> writes:
> 
> My original question:
> 
> > >Since the log periodic is broadbanded does it amplify received
> > >signals between the Ham bands along with the Ham band signals and
> > >thus place a greater burden on the selectivity and filters of the
> receiver?
> > >If not why not, please?  I am just curious since it seems that it 
> would.
> 
> The following response came from a log periodic manufacturer.  (I 
> will not
> name him at the moment since he hasn't had the opportunity to give 
> me
> permission to share this piece from our private E-mail.)
> 
> > I don't think this would be the case at all.  In fact it could be 
> just the
> > opposite.  Your theory is reasonable but.......  Assume a triband 
> yagi on
> > your tower.  As a shortwave receive antenna , it would work quite 
> well
> > because of the large capture area.  Lots of aluminum up there.  
> Let's say
> > there is a strong sw broadcast station at 16 MHz.  Your tribander 
> wouldn't
> > care too much which direction the 16 MHz signal came from because 
> it is
> not
> > directional at 16 MHz.  It could conduct a pretty strong 16 MHz 
> signal
> > towards your radio.   Now consider your new log periodic, which 
> all your
> ham
> > buddies will envy......    It probably has a larger capture area 
> as far as
> > lots of aluminum is concerned, because it has more elements.  BUT, 
> it will
> > be directional as far as 16 MHz is concerned.  If the 16 MHz 
> signal is off
> > to the side or to the rear, there could be much less signal passed 
> towards
> > your radio.
> 
> I am beginning to see a case of neutralizing advantages and 
> disadvantages.
> 
> The yagi has been represented as less-resonant on non-Ham 
> frequencies
> and thus less likely than the logs to gather those signals ... in 
> the
> direction
> it is pointed ... sort of like a tuned circuit is frequency 
> selective.
> 
> But the yagi makes no intentional design effort to be directionally 
> biased
> against non-Ham frequencies off the side and back and as a "big hunk 
> of
> aluminum" may grab signals off the side and back that the log would 
> reject.
> 
> The log is intentionally directive across all design frequencies and 
> is thus
> more discriminating from the sides and back against all signals in 
> the
> design range.  However from the direction the log is pointed it will 
> amplify
> all signals, desirable or not, in its path and within its design 
> range.
> 
> Sounds as though either beam design may serve well in opposite
> circumstances.
> I guess we are back to the quality of the rig and any tuned circuits 
> between
> it and the beam we choose.  Yes?
> 
> Comments or clarifications?  Thanks! & 73, DavidC  K1YP
> 
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