I posted the message on the Force 12 reflector, but I am sure there are some
Force 12 user here and/or guys that are a lot more versed in antenna theroy
than I !!
I have a Mag 640/340N that has been up for several years. When I put it up the
first time, I did not take the time to properly tune the 40 mtr part. I have
the antenna down right now and decided it was as good as time as any to tune it
correctly.
I decided the easiest way (but maybe not the correct way) is to take the
director and reflector off the antenna and then raise and tune them
individually.
The tower I am using is an 80 ft free standing one with a Force 12 Mag 717 on
it; and the closest other antenna is a 160 vertical about 70 ft away). I have
a pulley at the top and a rope with both ends at the bottom; thus it is very
easy to attach an element and hoist it up to various heights and check the
resonant frequency.,
I am using the Array Solutions AIM4170 Antenna Analyzer and I calibrated out
the coax and balun so I am getting the true reading of the element.
I "tried" to tune the 40 mtr director today and I am more confused than ever!!
With tip set to 4" as per the manual I came up with the following resonant
frequencies when rasing the element:
40 ft 7.52
52 ft 7.61
64 ft 7.64
If I lengthen the tips to 10 1/2 inches I come up with the following:
40 ft skipped
52 ft 7.51
64 ft 7.52
and finally I lengthen them 1 more inch to 11 1/2 inches and recorded the
following readings:
30 ft 7.34
40 ft 7.41
52 ft 7.48
64 ft 7.50
The element does shift upwards in freq as it is raised and I see about 100 kcs
shift for a 6" change in the tips (which confirms what the manual says).
However I am seeing much larger changes above 30 ft. than I though I
would/should.
What has me confused is what the manual says on tuning; especially steps e, f,
and g below. When and at what length am I satisfied???? According the
manual, I should tune the director to a freq of 7.5 MHz; but in a way it tends
to make you think it should be tuned at 30 ft (step e below), but then step f
says to raise it higher and again check. It seems I am seeing a lot more shift
in freq than I should???
Director:
_____a) Remove the short jumper from across the reflector
feedpoint screws.
_____b) Attach a 1:1 balun (i.e. Force 12 B-1) to the
feedpoint and attach a coax feedline to the balun.
_____c) Raise the reflector element to a height of at
least 15'. Elements will raise in frequency as they are raised higher, away
from ground. 15' should be about the limit of change for these elements;
however,
30' is better. Tests indicate a shift upwards of 15 kHz from 15' to 30'.
_____d) Using an antenna meter, such as an MFJ, AUTEK, or
AEA, sweep the reflector until a dip is found in the VSWR reading. The dip will
be higher than 1:1, which is normal. The frequency of the lowest dip in VSWR is
the frequency of the element.
_____e) Adjust the element tips to read a dip at 7.500
MHz at a height of 15'-30'.
_____f) If possible, raise the element higher to check
for any further change in resonant
frequency.
_____g) When satisfied that the element is on frequency,
remove the balun and replace the shorting jumper.
SO, the big question is do I leave the tips at 11 1/2 inches which puts it on
freq at 64 ft (when up it will be at 120 Ft), or do I leave it at 4 inches,
which, though I didn't measure at 30 ft, it should be close to 7.5 MHz?
Thanks for any help
Dick
PS I will again check the thing when it is on the antenna, but 40 ft will be
the most pracitcal place
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