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

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Subject: [TowerTalk] Tuning a Force 12 Mag 620/340N
From: "Dick Williams" <>
Date: Fri, 8 Aug 2008 19:07:50 -0600
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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 

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 

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 

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???

            _____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; 
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 

            _____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


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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