[TowerTalk] Log Periodics

[Peter Forbes]

Brian, 

That is exactly the point I am making. K7GCO claims that "a wider horizontal
pattern means a wider vertical pattern" - I would question this.

In the case of a 2 element yagi (or even a multi-element yagi) you can
change the free space vertical pattern dramatically by tuning for max gain/
max F/B, reflector or director etc.,  but in the case of a log cell, the
situation is entirely different.

I take your point about about super-long arrays where the odd harmonic
effect of the longer elements can be reduced by terminating the longest
element(s) with a shorting stub.

Cheers

Peter  VK3QI



At 01:28 PM 7/12/00 +0000, you wrote:
>I would say that both of these posts are in the realm of speculation.
>
>There is no calculation for horizontal antennas I have done in
>EZNEC/AO that shows much modification of the vertical pattern due to
>antenna design with a few notable exceptions.  The exceptions are
>extremely long "boom length" antennas.  By long, I mean 5 wavelengths
>or more.  It may be possible to build such a LDPA so that at its
>highest frequency the boom is this long.  However, the design's I've
>seen for LPDA'S all have booms comparable to commercial yagi's.
>
>If you guys would provide an LPDA model for EZNEC, I'd be glad to run
>it and end the speculation.
>
>73 de Brian/K3KO  
>
>
>
>Peter Forbes wrote:
>> 
>> K7GCO writes:
>> 
>> > Cheers,    Steve    K7LXC >>  This is very true but there is another
>> >variable.  IF THE HORIZONTAL PATTERN IS WIDER, SO IS THE VERTICAL
PATTERN and
>> >this fills the higher angles with far more RF than the loss of gain and at
>> >certain times of the day do a bang up job on DX also.  This is comparible to
>> >a 2 element quad at say 1/2 wave or slightly higher do so well.  It's the
>> >lack of gain that makes it work closer to the ground.  High gain beams don't
>> >work well close to the ground.  k7gco
>> >
>> 
>> Is it NECESSARILY SO that a wide horizontal pattern means a wide vertical
>> pattern as well?
>> 
>> The vertical pattern for a horizontal log periodic array is a function of
>> several things:
>> 
>> (1) ground reflection effects
>> (2) ground impedance effects (when within one wavelength height above ground)
>> (3) the spacing of the elements within the log cell which are active at that
>> frequency (in other words the Design parameter of the array (tau) and the
>> relative element spacing constant (sigma)).
>> 
>> In all my literature searches on the log periodic (both amateur and
>> professional) there is little discussion of the effect on the vertical
>> pattern when near the ground, with most computer modelling being free space
>> situations.
>> 
>> Cheers
>> 
>> Peter   VK3QI  ( an 18-29 mhz log periodic user)


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