Tuning an antenna

perrybucsdad · #1 (**permalink**) 03-06-2013, 3:01 PM

I have a dipole antenna that is mounted in the attic and I recently purchased the YouKits antenna analyzer. I found that the 10M Dipole that I have is resonant around 25.5 MHz (well low of where I would like it which is 28.5).

I understand the basic process is to raise the resonant frequency I need to remove length from the two sections of the dipole and make it shorter. The question I have is this. Since I am looking to adjust the resonant frequency, can I remove the antenna from the attic and do this in my shop where I have better lighting and easier access to test this on the analyzer, or must I leave everything hooked up as is and do it in situ? Since the resonant frequency will have the lowest SWR anyhow, I don't think it should matter that I have removed it from the normal operating environment, but I wanted to check with others here for their opinions.

Thanks,

W2NJS · #2 (**permalink**) 03-06-2013, 5:22 PM

It sounds to me as if you're seeking perfection in your installation and that's not a practical idea, so...
Cut the antenna for the MIDDLE of the band you want to operate on. 10M is a pretty wide band, but you may wish to just work in the SSB portion. Don't make yourself crazy attempting to get the SWR down to 1:1 because the minute you move off the resonant frequency the SWR will go up, obviously. You probably would be best off using an antenna tuner for the final setup. Last of all, what you figure on paper as the desired resonant length will always be different from what you get when you put the wire in the air.

n8zcc · #3 (**permalink**) 03-06-2013, 5:30 PM

Since you have a plot and know where the antenna has the lowest SWR just work the math backwards to find the length you need.

1. First write down the total length of your new antenna after using the old formula. (468/freqMhz=total length in feet) Hopefully you kept notes!

2. Find the frequency (F) of the lowest swr of your first attempt dipole.
(Check several frequency points between top and bottom end of the band.)
Record the lowest swr frequency!

3. Multiply length from step 1, (L) times frequency (F) in step 2 of lowest swr to get new constant.

Step 1 L x Step 2 F = new constant

4. Now use the new "constant" obtained from step 3 above in the formula rather than the 468 to get new length in feet.

Here is the source for the above: Building and Tuning A Dipole The Easier Way - A Faster Way To Tune A Dipole Antenna Project

LtDoc · #4 (**permalink**) 03-06-2013, 5:41 PM

About moving the antenna to tune it in your shop. If you check that antenna when you get it to your shop you will see that the resonant point has moved, it won't be the same as in your attic. So, while you can certainly make the changes and replace it in the attic, count on it not being what you had in the shop. Any and every thing around that antenna will affect it's resonance to some degree. The best thing to do is tune it in place. That's almost never the 'easy' way, but it will certainly be 'easier' than making several trips up/down, attic/shop, etc.
There are several ways of tuning an antenna, the one outlined above does work. Using that analyzer and making small changes till it's where you want it also works. Pick the method easiest for you, tha'ts what the rest of us do...
- 'Doc

perrybucsdad · #5 (**permalink**) 03-06-2013, 6:17 PM

Okay, so leave it in place. Can I disconnect the coax that runs down to the basement while in the attic and just attach a short cable so I can attach my analyzer in the attic, or do I need to leave that all in place and keep running down to the basement after I make my adjustments?

Also, someone said not to cut the wires, but just fold them back. Anyone have any experience with that?

cabletech · #6 (**permalink**) 03-06-2013, 8:03 PM

First off a couple of questions.

1) did you build this antenna yourself? 2) what do you have the ends connected two? (dog bone insulaters or just to a nail in the rafters)?

The over all lenght of the coax will play a part in your tuning.

As for the overall lenght, what part of the band do you plane to use the most? If in the phone band,
(28.300-29.300) then divide in half for the center frequency, equals +/- 28.800.

Now divide 468 by 28.800=+/- 16.25ft. for the overall antenna, +/- 8ft on a side.

Also, you never want to built a wire antenna where the ends are right at the overall lenght, you want to put the insulaters in board about 6-8 inches so you can trim the antenna to the freq you wish and you do this by cutting small sections from both ends.

If you have a long ways between the antenna and the radio end, then get a friend to help with your cell phone, FRS, or your ham handheld (if they are also a ham).

perrybucsdad · #7 (**permalink**) 03-07-2013, 12:05 PM

Quote:

Originally Posted by n8zcc

Since you have a plot and know where the antenna has the lowest SWR just work the math backwards to find the length you need.

1. First write down the total length of your new antenna after using the old formula. (468/freqMhz=total length in feet) Hopefully you kept notes!

2. Find the frequency (F) of the lowest swr of your first attempt dipole.
(Check several frequency points between top and bottom end of the band.)
Record the lowest swr frequency!

3. Multiply length from step 1, (L) times frequency (F) in step 2 of lowest swr to get new constant.

Step 1 L x Step 2 F = new constant

4. Now use the new "constant" obtained from step 3 above in the formula rather than the 468 to get new length in feet.

Here is the source for the above: Building and Tuning A Dipole The Easier Way - A Faster Way To Tune A Dipole Antenna Project

Okay, so to make sure I understand this properly, let's assume that my current antenna is 17' long (not sure if I include the total length including the balun, or just the length of the two wires), and if the lowest SWR from my chart is 25.5Mhz, I would multiply 17 x 25.5 to get 433.5 as my new constant. If I want to set my center frequency at 28.4 (which would be the mid point for the 10M tech class license), I would then take 433.5 / 28.4 = 15.26 feet.

That's exactly what I was looking for. Obviously, I would want to sneak up on this final length to just be safe.

What about the comment that someone gave me last night (not on this list) that said not to cut the wires, but to just fold them back on themselves. He said this would shorten the length and achieve the same result but without removing actual wire. Anyone have any experience with that?

Thanks again,

dnoyeb · #8 (**permalink**) 03-10-2013, 3:14 PM

You certainly have all the equipment necessary to answer your questions. Though I sympathize with you running up and down the stairs. I have done this a lot with my computers. its good exercise but the household animals become quite confused

perrybucsdad · #9 (**permalink**) 03-11-2013, 9:34 AM

N8ZCC... I followed your steps and formula's to readjust the antenna, and it worked like a champ. I did have to do it a second time to fine tune it after the first adjustment, but that was not a bit deal.

Also, as opposed to cutting the ends of the antenna wires, I folded them back and wrapped them around the antenna like one of the old timers told me at a local ham meeting and that was spot on too.

LtDoc · #10 (**permalink**) 03-11-2013, 10:21 AM

Sounds like you got it, good.
That 'folding the wire back' thingy is about as normal as it gets, certainly easier than adding wire if you go too far.
- 'Doc

HF-Mobile-Antenna · #11 (**permalink**) 03-23-2013, 9:03 AM

You can add a 4:1 balun to broaden the usable frequency of any antenna.

LtDoc · #12 (**permalink**) 03-23-2013, 5:14 PM

Sorry, a 4:1 balun isn't going to do anything constructive for a typical dipole used on other bands. What would be the resulting input impedance, any idea? I'm certainly willing to bet that it won't be 'four times' on any band. In fact, if you do use a tuner and no balun at all you stand a better chance of getting an impedance match than with a balun. Baluns are not the 'cure all' some people think they are.
- 'Doc