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Author Topic: 2 meter yagi gamma match question  (Read 2256 times)
K8POS
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« on: April 01, 2009, 06:09:22 AM »

I built a 2 meter yagi based on these plans:

http://users.belgacom.net/hamradio/schemas/yagi_vhf_antenna.htm

I cut the antenna for 144.5 (want to use it for SSB).
When I went to tune it up with a friends analyzer I could not get the load above 42 ohms.  The SWR was 1.2 to 1 on 144.8 (Close enough) But could never get it to 1-1 anywhere on the band. Highest was 2.1-1 at the high end of the band. (as expected).
I think the gamma match arm is too short. According to the plans it should be under 4".  Most other 2 meter yagi's I have seen the gamma arm is more on the order of 9-10".  From what the antenna book, and some of you on here have taught me, I am thinking there is not enough arm there to make a good coupling to the driven element.  I even made sure to clean the driven element, gamma arm and the strap with scotch brite to polish them up before assembly.
This antenna should out perform the 4 element quad it replaced. (same height and feed line).  While the beam width is more narrow, the gain which should be better is worse.
Am I on the right track?
Going to take it down when it warms up again.

Bob
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N3OX
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« Reply #1 on: April 01, 2009, 06:50:53 AM »

"The SWR was 1.2 to 1 on 144.8 (Close enough) But could never get it to 1-1 anywhere on the band"

Dude, you're done, especially if that's a measurement up near the antenna not at the shack end of the coax.  But either way, it's probably fine.

For all I know, the antenna is designed to have a 42 ohm input impedance which is close enough, period.

Getting 42 ohms instead of 50 could happen if you're off by a tiny, tiny bit in all kinds of ways, and in my opinion, it's almost certainly not worth the trouble to bring it down.

You can go ahead and measure some interesting things instead of SWR too...

Dowload Polar Plot from G4HFQ's site:

http://www.g4hfq.co.uk/

Get your radio plugged into the sound card, turn off the AGC on your radio and and get someone close to key down for a minute while you rotate the antenna and collect a pattern.

You could build a simple dipole and put it on the other end of your roof facing toward a loud signal source and actually compare the gain of the yagi vs. the gain of the dipole (you can do that in Polar Plot too, just switch back and forth and you'll get a step change calibrated in dB )

Or heck, compare it directly against your quad!

This will tell you a lot more than anymore SWR reading will ;-)

73
Dan
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73,
Dan
http://www.n3ox.net

Monkey/silicon cyborg, beeping at rocks since 1995.
N3OX
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« Reply #2 on: April 01, 2009, 06:56:52 AM »

"Most other 2 meter yagi's I have seen the gamma arm is more on the order of 9-10". "

Yagis with very low intrinsic feed point impedances have to be tapped out further than those with a bit higher.

If you swapped in a split (normal dipole type) driven element and measured that with your analyzer, maybe you'd see 30 ohms where maybe a yagi that needed a longer gamma arm would give 15.

If you've got 42 ohms resistive with the analyzer right up at the antenna, you are VERY near the exact tap point you want.





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73,
Dan
http://www.n3ox.net

Monkey/silicon cyborg, beeping at rocks since 1995.
K8POS
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« Reply #3 on: April 01, 2009, 07:27:22 AM »

Ok, sounds good.  Only other thing I can add, I did NOT have a metal mast in it when I tuned it up.  I read in one of the many web pages I looked at where that could throw things off a bit.  Will have to test it again with the antenna in the operating position and just give it more time.  Band has not been the best lately.

Bob
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N3OX
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« Reply #4 on: April 01, 2009, 07:42:08 AM »

"I did NOT have a metal mast in it when I tuned it up."

If it's going to be horizontal for SSB work, it shouldn't matter a bit.


73
Dan
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73,
Dan
http://www.n3ox.net

Monkey/silicon cyborg, beeping at rocks since 1995.
WA3SKN
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« Reply #5 on: April 01, 2009, 08:08:28 AM »

Have you ever heard of "tolerance"?
1.2:1 is really close enough.
But why you could not get an exact match could relate to the tap location on the element, the capacitance and the inductance of the gamma presented, and lots of other factors surrounding the antenna.
That's why you have adjustments.  The build shows the gamma soldered in place after making adjustments.
Was there ANY metal within about 10 wavelengths of the antenna when you tested, and how high above ground was the antenna located? This could cause a lower impedance.

-Mike.
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K4JSR
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Posts: 513




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« Reply #6 on: April 01, 2009, 10:08:48 AM »

I am surprised that my friend Charles, KC8VWM, hasn't
popped up on this topic.  Charles frequently haunts
the Elmer's Forum.  If he doesn't pop up soon, you may want to email him at the email listed at QRZ.com.

73 and good luck!
Cal  K4JSR

Ps.  I also pretty much believe that all that is left for you to do is put that sucker up and talk!
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