222 MHz Yagi Rebuild

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Alan:
Hello All,

This is my first effort into VHF.

I bought this antenna, got it home and completely disassembled it. The aluminum was pitted and weathered so I wire brushed everything. It looks brand new.

I have just bought a 220 rig and want the Yagi to work right once I get it back together.

Here are the current measurements from the parts:

Boom Length....  48"
Spacing: REF/DE   9" O.C.
         DE/DIR  11" O.C.
        DIR/DIR  10" O.C.
        DIR/DIR  12" O.C.

Reflector.....   25 7/8"
Driven El.....   25
Director......   23 7/16
Director......   23 1/16
Director......   23

After taking measurements from the 5 elements, I'm coming out with dimensions that lead me to believe that the antenna is cut for 225.700 MHz, using one of the [internet] Yagi calculators. Do the numbers sound correct?  

If so, since repeaters are between 222-225 MHz, shouldn't I trim everything for 223.500 MHz, the center of the band?

I've been told by some that it won't make a whole lot of difference for this small a band.

What say you?

Many thanks for taking the time to respond!

Alan

Vincent Paul Sullivan Jr.:
The band allocation is small enough that you can quite successfully use the antenna as is.
You spoke of "trimming" the antenna. If you literally meant shortening elements DON'T...that will raise the frequency. You want to lower the frequency....that requires LONGER elements... to a degree at least...
so "tune" it with the matching device on the antenna as close as you can get it to 223.5 and you'll be good to go. (gamma match..tunable stub... whatever)

Good luck...I've always liked 220...I just don't have many folks to talk to on 220.

If you have a Cushcraft antenna you can download the antenna manual from the Cushcraft site.

Paul/W6VPS  

:
I will add only one things to VPS statements...

First and foremost in trimming elements, learned the hard way :o(  .

You cannot replace if you take off too much.  Trim slowly and with caution.  



Charles:
I'm coming out with dimensions that lead me to believe that the antenna is cut for 225.700 MHz, using one of the [internet] Yagi calculators. Do the numbers sound correct?

------------

Keep in mind that many "internet antenna calculators" do not take into account specific boom dimensions, element diameter - thickness of aluminum, mounting methods used by different antenna manufacturers, matching system variations and other similar parameters and/or design irregularities.

I am suspecting the antenna may already be "optimized" and designed to resonate specifically in the ham band portion of 220.

You need to ask yourself, why would this "ham radio antenna" be designed to operate above the ham radio band?? The fact is, it probobly isn't designed to operate anywhere else.

Triming the elements will only serve to increase the resonate frequency of the antenna. Good if you want to work on the 440 UHF band I guess.

Anyways, what is the SWR readings at the top and bottom of the 220 band? Did you try and plot the SWR? That will be a good indication of it's intended frequency of operation.

Remember the first rule about building antennas:

Always measure twice and only cut once.

Personally, I have been known to measure at least 5 or 6 times just to be sure. :)

73 de Charles - KC8VWM

Kevin J. Thompson:
I would leave it alone and fire it up after reassembly and check SWR.  It is probably already "tuned" for midband already.  If not there is probably a gamma match on the antenna itself so you can dial it in for lowest SWR.

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