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[Towertalk] Log Periodics vs. Monobanders

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Subject: [Towertalk] Log Periodics vs. Monobanders
From: (
Date: Tue, 2 Jul 2002 22:20:11 -0600
Thanks for clarifying K7UP's arrangement Jim.

With his 5L10 at 98 ft, assuming FLAT terrain, there are
two main lobes supported by the ionosphere, namely from
3 to 8 degrees and from 13 to 18 degrees with a significant
NULL from 9 to 12 degrees.  Another 10M antenna mounted
anywhere from 35 to 50 ft would fill in this Null quite nicely.
(Angles listed are for -3 dB points relative to the peak angles).

With his 5L15 at 80 ft, again assuming FLAT terrain, there
are two main lobes supported by the ionosphere, from 4 to
12 degrees and 21 to 29 degrees with a significant NULL 
from 13 to 20 degrees.  Another 15M antenna mounted
from 35 to 45 ft would fill in this null quite nicely.

The above pattern nulls fall right in the middle of the
useful range of angles supported by the ionosphere
during daylight hours.  It's no wonder that LOWER
antennas often outperform these (high) antennas.

A simple solution to K7UP's high band nulls would be
the addition of a LOW Tribander, 5 Bander, or LP at
35 to 45 ft (as I have preached for decades).  

NULLs in the vertical pattern of horizontally polarized antennas
cannot be avoided at heights above 3/4 WL.  The BEST 
solution is to have High AND Low antennas which complement
each other, filling in the nulls.  The depth of these nulls
GREATLY exceeds any small differences in antenna gains
for reasonably sized antennas.  

Note that the difference between a 2L Yagi on a 0.15 WL 
boom (~6.5 dBi) and a 5L Yagi on a 0.75 WL boom 
(~10.5 dBi) is only 4 dB vs. the 6 to 20 dB (or more)
depth of pattern nulls for High Horizontally Polarized

One final comment about moderate sized LP antennas.
My rule of thumb is that their gain is roughly equivalent
to a 2L Yagi.  The Force 12 C3 series and the popularity
of 2L 'shorty forties' has shown just how well 2L Yagi's
perform.  There is a lot to be said for an antenna that
covers 5 bands on one coax that can be rotated by
a modest sided rotor and supported by a modest tower!

A stacked pair of such antennas with an upper / lower /
both switch is an even more powerful combination.

Tom  N4KG

On Tue, 02 Jul 2002 Jim Henderson <> writes:
> Mel Martin wrote:
> > I notice in Jim's post that he had 4 antennas within 20 ft
>     Negative.
>     First, they were not mine, they were K7UP's.
>     Second, they were on 2 towers, 100 feet apart. As I recall, the 
> 40 KLM
> and 10m 5el were about 18 ft apart on one tower, with the 10m above 
> the 40
> at approx 98' and 80';  and the 15 and 20m were on the second tower 
> at about
> 80' and 65' respectively.
>     I would have to ask him, as even though I helped him work on 
> these
> antennas, it has been a long time.
>     But modeling not withstanding, he always placed high in the SS 
> and CQWW,
> etc so his system played well.
>     Mine did too, although I was after other goals. As I stated, my 
> LP
> opened and closed the bands a good %75 of the time compared to the 
> LJs. Hard
> to argue against results, no matter what the model says. There are 
> more
> programs now, and many more (and improved) choices for interlaced MB 
> yagis.
> At the time and for what I wanted to do, it was the best choice for 
> me.
> Jim, KF7E
> --

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