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Author Topic: Can anyone ID this 70cm yagi?  (Read 5915 times)
K3GM
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Posts: 1817




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« on: June 17, 2012, 01:08:40 PM »

I got it for ten bucks.  It's a 21 element yagi with a square boom, appears nicely made.  Here's a picture of the entire antenna:
http://i9.photobucket.com/albums/a96/TwoSevenRight/2012-06-17_15-39-48_327.jpg

Here's a picture of the business end.  There appears to be a piece missing that connects the feedline connector to the driven element.  But I believe I can work around that with no problem.
http://i9.photobucket.com/albums/a96/TwoSevenRight/2012-06-17_15-31-02_640.jpg
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KM3F
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« Reply #1 on: June 17, 2012, 01:29:42 PM »

Don't know who's it is but the driven element is a folded dipole and will not match 50 ohm coax directly.
The fitting appears to be part of a conversion balun you will need.
You might consider converting the driven element to a Gamma Match, then your good for 50 ohm coax.
1/4" Alum rod and some vinyl tubing from  the hardware store plus a connector mount and your in biz.
Matching can be done by trial or a 259 analyzer.
Good luck.
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K3GM
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« Reply #2 on: June 17, 2012, 01:32:07 PM »

Yes, that was the plan.  Thanks for the affirmation.
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WB2WIK
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« Reply #3 on: June 17, 2012, 07:53:59 PM »

As a long time VHF-UHFer I know that antenna just fine, and owned eight of them.

It's an F9FT "Antennes Tona" yagi, made in France.
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K3GM
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« Reply #4 on: June 17, 2012, 07:57:23 PM »

Thanks, Steve!  In looking deeper into into feeding It, would it be simpler to fabricate a half wave hairpin match, putting the connections in a small plastic box?
« Last Edit: June 17, 2012, 08:13:39 PM by K3GM » Logged
KA4POL
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« Reply #5 on: June 18, 2012, 10:51:06 AM »

I can confirm it is a 21el Tonna F9FT. The antenna does not require any match. It is hooked up directly using an N connector, see http://www.f9ft.com/indexe.html.
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K3GM
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« Reply #6 on: June 18, 2012, 11:14:37 AM »

Thanks, Dieter, but if you look at the second photo I attached, you'll see a circular piece on the driven element.  It looks like something attaches there, but that part was missing.  I suspect that a 4:1 transformer with an N connector attached there.  Unless I'm mistaken, a folded dipole exhibits a feedpoint impedance of around 300 ohms, which would preclude this being fed directly with 50 ohm coax.  There are also two small holes on the side of the boom, just forward of the driven element that I suspect a matching network was attached to.
« Last Edit: June 18, 2012, 11:16:24 AM by K3GM » Logged
KB1TXK
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« Reply #7 on: June 18, 2012, 11:40:41 AM »

Some links that may or may not help:

http://f5ad.free.fr/Docs_ANT/Tonna%209el_144-20809_1.pdf

http://www.docstoc.com/docs/2772212/21-elements-432MHz-Antenna-Tonna-F9FT
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WB6BYU
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« Reply #8 on: June 18, 2012, 11:46:38 AM »

Quote from: K3GM

  Unless I'm mistaken, a folded dipole exhibits a feedpoint impedance of around 300 ohms...



Not necessarily.  A folded dipole steps up the base impedance by a factor of 4 (assuming
the two wires are the same diameter).  But it isn't uncommon to see high performance
yagis designed with an impedance of 12.5 ohms, so that the folded dipole steps this
up to 50 ohms.
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G8YMW
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« Reply #9 on: June 18, 2012, 03:04:30 PM »

Its an old 21 ele Tonna (1980's) the coax connects with spade connectors, the later ones connect with the "N" connector. I know I used to have one when I lived at Chesterfield.

Hope this helps
de Tony
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73 details Tony
Sent by WW2 Royal Navy signal lamp
WB2WIK
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« Reply #10 on: June 18, 2012, 03:48:55 PM »

Thanks, Dieter, but if you look at the second photo I attached, you'll see a circular piece on the driven element.  It looks like something attaches there, but that part was missing.

What goes there is nothing more than a "sleeve" made of hollow aluminum tubing.  The antenna is impedance matched fine without it, but it is intended to aid in decoupling.  The two small holes are for a "clip" that holds the sleeve in position.
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G8YMW
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« Reply #11 on: June 19, 2012, 02:59:12 AM »

WB2WIK wrote
What goes there is nothing more than a "sleeve" made of hollow aluminum tubing.  The antenna is impedance matched fine without it, but it is intended to aid in decoupling.  The two small holes are for a "clip" that holds the sleeve in position.

No its not, that is only on later 21 ele's
This one (As I said) only the coax connects directly on with spade connectors, the only bit missing is the boot that covers the connection
I bought THIS model in 1982 (I think) and certainly took it and a 16 ele (Might have been 17) for 2 metres to GD land in 1983.
The old 21 ele is no longer with us but my XYL bought another about 2 years ago (With the N connector and sleeve)
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73 details Tony
Sent by WW2 Royal Navy signal lamp
K3GM
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« Reply #12 on: June 19, 2012, 04:20:31 AM »

Yes, Tony, there appears to be two different version of connecting the feedline to the driven element.  I will try direct feeding it first and see what I get.  If it works, I'll use tab receptacle lugs on short pigtails to connect to the tabs.
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KA4POL
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« Reply #13 on: June 19, 2012, 05:20:15 AM »

I checked my old 2m Tonna and found the lugs as well. Looks like car electric and is astonishing for 70cm.
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K3GM
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« Reply #14 on: July 06, 2012, 01:59:42 PM »

F9FT epilogue....
Today I had an opportunity today to assemble the antenna and try it out atop a 15' temporary mast.  At 432.100,the Bird shows about 18 watts going up and a shade over 1 watt reflected; not too bad. As noted, the antenna was directly fed with 50 ohm coax with short pigtails.
« Last Edit: July 06, 2012, 02:12:08 PM by K3GM » Logged
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