It should also be mentioned that the front/back ratio is different at
various elevation angles. You won't get the free-space f/b ratio in
practice at most elevation angles. This is easily seen with modeling
software.

Good nulls also depend on a precise balance of strong fields, which all
have to be just right in order to add to zero. If any of the various
fields is just a little off, the null won't be what you expect -- it'll
be shallower and/or in a different direction. It takes only a small
difference to change the depth of a deep null by 20 dB or more. That
means you can't expect deep nulls to be exactly what a computer program
or other calculations predict, because there will inevitably be small
differences between the model and the real antenna. Or, for that matter,
from one antenna to the next one of the same type, even when built as
close to exactly the same as possible. You have the best chance of
achieving deep and repeatble nulls with the simplest antennas, like a
carefully built and balanced small loop. The larger the number of
elements, the larger the number of fields which all have to add together
to a value of zero, and the more opportunity there is for variation.

Roy Lewallen, W7EL

Bob Bob wrote:
Hi Art

In my experience (or maybe its just anecdotal) F/B ratios are very
frequency dependent for high dB numbers. You might for example get 28dB
F/B on 14.100 but it may drop to 15dB 100kHz away. The forward gain in
this case doesnt change much at all. How would you then advertise this
except by a graph?

Keep in mind also that F/B is expressed in a specific direction. 5-10
degrees off that and you will see the numbers fall off.

Max F/B rarely occurs at max forward gain...

Front to side is also good for attenuating signals you dont want.
(Assuming horiz polarisation)

If I was tackling this as a project I'd start defining some parameters
like 3dB beamwidth and the maximum unwanted lobe figures. Then plug it
into a modeling program (like 4nec2) I havent played much with modelling
but I assume that the optimising tools could do the trial and error
calcs for you. You are aiming for an antenna with moderate forward gain
but as low as possible radiation/reception in other directions..

In fact if you havent tried modelling it is well worth it just to see
the effect on the patterns.

You can also tune a (yagi/quad) reflector remotely if you want to peak
the F/B

Probably not what you were after sorry...

Cheers Bob VK2YQA (In Oz...)


wrote:

Regarding antenners, thats right,antennas.
Plenty of listings for various gains achieved with antennas
but I have not seen any similar listings for F/R and F/B.
Is there any around so I can see what can be or has been attained or
is blocking of rear signals of little consequence for todays hams. I
can see that being true
for when the band has just opened and propagation has not opened to
the rear or
with net operations so perhaps gain is every thing after all.
It is my opinion that if interest/efforts are increased first in
deflecting the rear action to the
front first and thereby INCREASING the front lobe width there will be
more opportunities
for increasing gains without the side lobes and narrow forward lobes
that accompany Yagi's.
as well as shorter booms. ( more deflectors/reflectors perhaps)
( no religeous, porno or genocide preaching please, fighting to see
which is best
could take place on boat anchor nets to see which has taken position
of the lowest level)
Regards
Art