[TowerTalk] Re:

[art unwin]

>>>
>>>I have long wondered why the reflector on a yagi is longer than the driven
>>>element and why the directors are shorter.  Also why do multiple directors
>>>increase the gain but multiple reflectors do not?
>>
>>
>>This is incorrect when speaking of multi (high density) elements
>>or optimum elements per array wavelength.
>>
>> When you have multiple directors then multiple reflectors
>>   DO  assist the gain as well as INCREASE the front to rear/back
>>..... especially when TOTAL BAND SPREAD is considered.
>>
>>Reflectors in such cases are NOT always longer than the driven element
>>( must not exceed 3 in number ,total 13 elements max per wavelength of
>array) 
>>......because the self impedance of an element is of more  importance than 
>> physical length 
>
>
>

I thought I would add some specific pertinent data in response to your
question
(words are always cheap)
A beam with  13 directors, 3 being reflectors on approx 1 WL boom
gives a PEAK F/B of over 60 db along with  a F/R of around 50.
Don,t ever recall running into those sort of figures on other 
designs with a single reflector (maybe someone will enlighten me)

With respect to your second question, none of the above rear elements
(or directors) are physically longer than the driven element and 
gain is still around 16 dbi or a hair less across the band as I 
remember. The term 'reflector' is really a misnomer 


A point regarding reflectors is that with high density 
elements required for consistent across band gain there 
is NO optimum placing for a single reflector.
 Even on designs of three or four element beam for any band
the reflector will be in a different place if it is designed for the 
lower part as it would for the higher part 
 Same goes for the directors but then we live with compromise.
True, the smaller beams do have a high F/B for a single point 
(tho not all that high) but this rejection rapidly falls off
on either side, which is not the case with multi reflectors designs.
 
When considering 'less optimum' designs such as those generally in use
today one can feel comfortable with the idea or notion of a 
single reflector and a minimum number of directors.
 But one should not be mislead by those who incorrectly extrapolate 
or project such 'factors' as the mantra for 'all' designs without 
verification or data.

The saying still goes, trust but verify(the facts Mam,just the facts) 
and it is not really that hard to do !

Regards
Art Unwin KB9MZ






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