Hi Jonathan - something is likely wrong, but you would need to make a
number of other tests to be sure, because arrival angles of signals
vary. It is conceivable that your DX station was arriving at an angle
where the antenna's F/B ratio is much less than the quoted peak. If it
is an assembly issue, then maybe you used the driven element dimensions
on the reflector
N6BT's Array of Light makes a very interesting point about yagis - (I'll
try to paraphrase) that the driven element's role is to inject RF into
the array, while the parasitic element's length determines the
directivity. Tune your radio too far above the optimum frequency, and a
reflector won't do its job very well. Tune too far below and the length
of the reflector will turn it into a director.
I was having a lot of trouble getting my F12 EF-240S to work well. At
Tom's suggestion, I think, I set up my MFJ-259B antenna analyzer with
coax to a short antenna some 300 feet from my yagi. Tuning the analyzer
with the antenna pointed away from this signal source, I could very
clearly see both phenomena described above. The antenna reversed at 7.1
MHz, making it clear that my reflector was much too short. Moving the
shorting bars quickly moved the "flipping point" comfortably below 7
MHz, and put the best F/B ratio in the middle of the CW segment where I
wanted it.
73, Pete N4ZR
The World Contest Station Database, at www.conteststations.com
The Reverse Beacon Network at http://reversebeacon.net, blog at
reversebeacon.blogspot.com,
spots at telnet.reversebeacon.net, port 7000 and
arcluster.reversebeacon.net, port 7000
On 9/19/2012 2:24 AM, SPWoo wrote:
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
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