[TowerTalk] Tuning a Force 12 Mag 620/340N

[Dick Williams]

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