[TowerTalk] Help with Force 12 C3S 10-Meter Adjustments

[John Bayne N4EEB]

Hello friends,

This is my first post to the list.

My questions are based on a Force 12 C3S constructed to the 80 MPH diagram in this manual:  http://bit.ly/F12-C3S

I purchased a new C3S from he Colorado version of the Force-12 company shortly before they seemed to disappear.  Unfortunately, my kit was missing several element tubes, and a lot of the "pre-drilling" mentioned in the manual was not done.  That being said, after I ordered the replacement aluminum from DXE and fabricated the missing pieces, the antenna went together fairly easy after drilling the missing holes, etc.

Having done as much research on potential problems as I could do, I noticed hams commenting that the 10m 2:1 curve was VERY high.  So, in an attempt to compensate/mitigate this in the original build, I extended each tip on the 10m driver from the specified 36.5" to 37.25 inches.

The antenna was installed in the clear at 33 ft. - It is fed with 97 feet of LMR-400-UF terminating to a 5KW JK 1:1 balun.  2" long, 1/2" wide copper strap connects the balun to the 20m driver.

A blanket statement:  SWR curves on 20M and 15M (and performance) is perfect.  I am very happy.

As feared, the 10m SWR curve is way off for the type of operating I do.  I mainly operate CW, and phone from 28.3 to about 28.7.

The measured SWR curve on 10m is as follows:   (Measured on a RigExpert AA-30 and also the SWR display on a Palstar HF Auto tuner in bypass - both in fair agreement)

2:1 Lowside Freq:  Approx 29.220 MHz.
2:1 Highside Freq:  Approx 29.820 Mhz.

Frequency of lowest SWR:  Approx: 29.417 MHz.

Rotating the antenna 360 degrees does not change any of the SWR readings, so nothing around the antenna is skewing things.

I have bad O.C.D. with projects like this.  I know I'm probably overthinking the solution to this, and that's why I'm posting here.  I realize that simply lengthening the tips on the 10m driver will adjust the SWR curve lower.  I also know that experimenting prior to final riveting in place is the best option.  I do not climb.  I have a friend coming over on 11/21 to lower the antenna to allow drilling out rivets and making changes.  He does not have a lot of time.  I would like to pursue the best plan available first. That being said, here are my specific thoughts and questions:

It was suggested by someone familiar with this design to try simply lengthening the 10m driver tips each by 1.5 inches and seeing if that works.  (Leaving 10m reflector alone.)  What do you think about that?

My OCD brain has come up with a different first step:

Looking at the antenna diagram as printed, the length of the 10m reflector is 7.035% longer than the driver.  The distance between the elements is fixed and cannot be changed.

The manual states that a .750 inch adjustment in each 10m driver tip will result in an approximate change of 250 KHz.  

The distance of the SWR curve now vs where I need it to be, is approx. 1220 KHz.

Doing the math, the driver would seem to require a total lengthening of 4.88 1/4" increments to bring the lower 2:1 point down to approx 28.000 MHz.

If this way of looking at it is correct, the new total length of the 10m driver would increase to 207.26 inches, which would be accomplished by lengthening each tip to  approx 40 5/8" vs the current 37 1/4" tip lengths.

Am I thinking about this correctly?

Now, OCD on to the reflector!   How can one make such a large change in the driver and not the reflector?

If I change the total length of the 10m driver to 207.26 inches, the 7.035% difference in the stock reflector measurements goes out the window.

However, if I increase the total length of the 10m reflector to 221.84" by increasing each reflector tip by 4.42 inches (each tip now to be 47.75 inches instead of 43.5"), the totality of the adjustments are somewhat linear and symmetrical.  I think.

So, there you have my options.

1.  Lengthen the driver tips by 1.5 inches each, leave the reflector alone, and see what happens?

2.  Adjust the driver tips to best SWR curve and leave the reflector alone?

or

3.  Start with my math above and plan on starting with the full driver and reflector adjustments?

Any help would be appreciated.  I'm looking forward to learning a lot from your responses.

73, John N4EEB
n4eeb at arrl.net

(If you contact me off list, your email will bounce, but I will still get it and add you to my white list. - Thank you.)