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Author Topic: Factory tuned 440 Yagi. Can I trim for 445?  (Read 1305 times)
N6ADA
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Posts: 6




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« on: January 01, 2013, 05:05:23 PM »

Hello all,
The Diamond A430S10 (10 element UHF Yagi)  is factory tuned for 430-440. I understand that if used out of that "window", the SWR will be high. I would like to be using it around 445 MHZ.

My question is, can I simply trim the elements down to size (same size as yagis tuned for 440-450) and not worry about the matching unit on the driven element to achieve a low SWR?

Thanks,
Bob
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K7KBN
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Posts: 2805




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« Reply #1 on: January 01, 2013, 05:08:26 PM »

Possibly, or the two might work together.  Have you tried asking the manufacturer directly?
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73
Pat K7KBN
CWO4 USNR Ret.
N6ADA
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Posts: 6




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« Reply #2 on: January 01, 2013, 05:23:54 PM »

Thanks Pat I will shoot them an email. In the meantime, I was just wondering if other users have done this before.
BTW, I do not have any Yagis for the 440 band at this time. I'm shopping and the Diamond is what I can afford.
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W4OP
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Posts: 431


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« Reply #3 on: January 01, 2013, 06:31:37 PM »

VSWR should be  the third of  your concerns with gain and F/B being the two most important.

You might be able to achieve the VSWR and yet lose optimum gain and F.B.

I too would ask the manufacturer.

Dale W4OP
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KM3F
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Posts: 506




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« Reply #4 on: January 01, 2013, 06:37:31 PM »

I have two Cushcrafts for 70cm.
One for SSB and one for the FM portion.
The element lengths are about 3/16" difference between the two.
I have tried to pull the FM unit down to the lower half but the SWR still won't go near flat but in the usefull range of 2 to 1.
At these frequencies the adjustment of the Gamma match is very critical and sensitive to the antenna's surroundings so you need to make the adjustments loose enough to be able to 'bump' them with a long wooden dowel to keep out of the antenna's immediate field.
The coax needs to be mounted as it will be when up and the height to test the match needs to be well up as much as 20 feet to have a chance to end up with a decent match when in place.
Good luck..
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WB6BYU
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Posts: 13288




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« Reply #5 on: January 01, 2013, 06:51:01 PM »

If you're on a budget, try building one of these:

http://www.wa5vjb.com/yagi-pdf/cheapyagi.pdf

For the 432 MHz designs you can shorten the elements 1/2" to move them to the
440 - 450 MHz range and use the existing element spacings.

The 11-element version has a 60" boom, excellent Front-to-Back ratio and good
gain, and they are very easy to build.
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KQ6Q
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Posts: 979




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« Reply #6 on: January 02, 2013, 02:10:18 AM »

N6ADA - you'll also need to mount if for vertical polarization at 445 mhz.
You might be better off with an ELK 2m/440 beam - more broadband,
since it's a log periodic design.
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N6ADA
Member

Posts: 6




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« Reply #7 on: January 02, 2013, 05:10:16 AM »

Good points. I do plan on running two in phase vertically with a cross boom.

I also see your points about the gamma. It's hard to see the picture but it appears that the Diamond's matching system is concealed. That's my main reason for posting the question. Otherwise I would have no problem making adjustments. Never really thought about element spacing being that crucial for a 5 MHZ adjustment. Something to investigate.

Building one is also an option. The design that WB6BYU describes seems simple enough.

Thanks everyone for your inputs.
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W4OP
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« Reply #8 on: January 02, 2013, 06:47:10 AM »

I don't think you'll see any of the reputable antenna manufacturers using gamma matches for VHF let alone UHF. The uneven element illumination results in squint and there is considerable common mode on the coax. Those problems were the inspiration behind the Quagi design.

Look at M Squared, Innov, Directive Systems  etc to see how to properly drive a UHF antenna.

Dale W4OP
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