Topband: fishbone and comb antennas

[Tom Rauch]

Hi Sam and all,

When you find an obscure array that is not often used, you can be 
sure it is unpopular for a more than one reason.

> i recall some detailed articles i think in an old RSGB handbook
> but couldn't find a reference in a more recent edition
> the arrl antenna handbook has about one page on the fishbone
> my internet searches yield only dictionary entries
> the comb is just half a fishbone

The sole benefit is the elimination of complex phasing lines. 
Nothing more. Everything else is pretty much a negative.
 
1.) The Fishbone has high F/B only when elements are spaced 
1/4wl, unless you modify phasing by altering element lengths 
and/or coupling at each element (making it a narrow band 
antenna). We sometimes call that modification a Uda-Yagi or Yagi. 
  

2.) It has to be very long, and the transmission line must have a 
velocity of propagation of freespace. No velocity-slowing dielectrics 
allowed in the transmission lines!!   

3.) There can not be uncontrolled standing waves along the length 
of the feed system if the antenna is to have any useful F/B ratio. 

Even if you manage all that, since distribution is uniform (instead of 
binomial) the pattern is not clean. It is also not very directive for its 
physical length.

If we somehow manage to modify the phase-delay by making it 
faster-than-light in the transmission line, providing increased 
directivity (Hansen-Woodyard phasing), you won't have a snowball's 
chance of having current distribution correct.

Log Periodics and other Cross-fire arrays are actually improved and 
modified versions of the Fishbone, where alternate elements are 
transposed. This causes destructive radiation from the elements, 
and greatly increases directivity and F/B ratio for a given length 
array.

Since logs and cross-fires are better in almost every respect, they 
largely replaced the Fishbone except in applications where space 
can be wasted and where transposing element phase might be a 
chore. 

Some UHF antennas use modified Fishbone elements because 
elements can be attached directly to a supporting boom that can 
be part of a transmission line, or can use a surface wave. They 
also generally back the Fishbone up with a reflector system that 
helps correct the poor inherent F/B ratio of Fishbone's.   

> version of the comb relies). the theory looks ok and should provide
> similar pickup to a 1000 metre beverage, but can't find any reference
> to them as yet and i can't believe that a top antenna would have been
> overlooked by the topband gang for so long. someone out there will
> have played with these for sure.

A 1000 meter Beverage is NOT a good 160 antenna for many many 
reasons. With that in mind, it is safe to say a Comb or Fishbone 
antenna would be just as good (or bad).

These antennas were well known in the very early days, and grew 
unpopular quickly. They did so for good reason.

73, Tom
(W8JI@akorn.net) 


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