Hi All,
Thank you all for the replies.
I went to my antenna bible 'Array of Light' by N6BT and there are two chapters
(35 and 36) that made references on antenna tuning of shortened yagis such as
the Delta 240. There are a few interesting points noted by Tom.
- The point of lowest VSWR doesn't determine the frequency of best F/B. The
frequency of best F/B depends on the actual frequency of operation in relation
to the frequency of the reflector. In particular, the frequency of best F/B
will occur very close and just above the reflector's resonant frequency.
- If I want to operate say on 7.200 then the reflector frequency for a 37'
element should be set to 7.120. For 45' element it's 1.090. With this setting
the reflector will become a director, causing the pattern to reverse whenever I
operate below 7.120. The shorter the element, the closer the reflector must be
tuned to the driver than in a full sized yagi.
Since my antenna is resonant on 7.030 per the factory setting, the reflector is
most likely resonant on 6.970. Although the antenna won't have much F/B in the
phone bands it still has gain. If I decide to re-tune for best F/B in the
phone band then I will lose F/B when operating below the driver frequency at
7.200 or even worse experience reversed pattern whenever I operate below the
reflector's frequency at 7.120.
Another interesting point noted by Tom is that shortened yagis will have a
superior F/B than full sized antennas, and it normally exceeds 20db. This
might be true but I don't think the great F/B extends to a wide frequency due
to the narrow-banded nature of shortened yagis. Tom didn't elaborate on this
point. I'd be interested to run some tests with my D240 and determine what and
where the maximum F/B occurs. 20db of F/B sounds fabulous if it's true. The
book has given very detailed instructions on how to tune for maximum F/B. All
I need a very low level signal from a non-resonant antenna that is close by.
It seems like a Steppir is a much more elegant antenna on the lower bands. On
the higher bands we don't see these F/B quirks on most tri-banders. I feel
like my Skyhawk is comparable to a Steppir in terms of performance on the
higher bands..
73,
Jonathan Woo W6GX
Hi All, I have a D240 and I haven't
used it much since I put it up this past spring. Tonight I decided to
run some performance tests. I found a DX station and I turned the attenuator
on such that that my S-meter was reading S9 when the beam is pointed at him.
When I turned the beam 90 degrees away I saw an S7 which is 12 db less than the
maximum signal strength. When the beam was turned 180 degrees away I saw an S9
which is the same signal strength
as when the beam was pointed directly at him. I'm a little confused about
this. This is
supposed to be a yagi and not a rotary dipole! The listed spec. for F/B ratio
is 20db at its peak. I do know that F/B is not constant across the band for a
yagi. What kind of front to
back and side rejection ratios should I expect to see on this particular
antenna? I haven't asked Mark at F12 about this. I don't want to waste his
time until I have more info. Perhaps I'm missing something on the test or that
I made an error in my antenna assembly. Your help on this is greatly
appreciated. 73.
Jonathan Woo W6GX
_______________________________________________
_______________________________________________
TowerTalk mailing list
TowerTalk@contesting.com
http://lists.contesting.com/mailman/listinfo/towertalk
|