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Author Topic: GMRS Antenna Build help  (Read 11046 times)
MDNITERDER
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Posts: 146




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« on: February 13, 2012, 12:00:48 PM »



 Does anyone have a good instructional build up on a GMRS or a 462/467mhz yagi or 5/8 wave antenna.

 I know one could buy these antennas but I kinda like building them, I have access to 3/8 aluminum tube for a nice yagi and stainless rod for 5/8 wave

 Any info I would love it.

Thank you in advance
John Smiley
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WB6BYU
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Posts: 13574




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« Reply #1 on: February 13, 2012, 12:29:30 PM »

I needed something on short notice to track down some interference to the Sheriff Dept.
around 465 MHz so I threw together both a 4-element quad and a 5-element yagi, the
latter using #12 copper wire.  Both worked well.  The quad used the W4RNL formulas.
I'll have to dig up the dimensions - the yagi will have to be adjusted somewhat for 3/8"
diameter elements.

What sort of lengths / gains do you need?  You can use the calculator for DL6WU yagis: 

http://www.k7mem.150m.com/Electronic_Notebook/antennas/yagi_vhf.html

for longer beams (boom lengths over 5 feet or so.)  Just about any 440 beam design can be
scaled up to 465 MHz by multiplying the element lengths and spacings by 440/465 or
432/465 (depending on the original design frequency.)  There are lots of other 70cm beam
designs out there, too, but the one above has the advantage that you can design it
for a specific frequency.

Personally I find the WA5VJB "Cheap Yagi" construction method one of the easiest to use:

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

This is how I built my 465 MHz beam, but using PVC pipe for the boom.  A bit more difficult
to bend 3/8" tubing around, however.
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MDNITERDER
Member

Posts: 146




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« Reply #2 on: February 13, 2012, 12:55:35 PM »

 How do you connect the feed line on the http://www.k7mem.150m.com/Electronic_Notebook/antennas/yagi_vhf.html is it direct connect or does it need a form of matching device, thank you for your quick reply.

John
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WB6BYU
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Posts: 13574




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« Reply #3 on: February 13, 2012, 01:20:32 PM »

You'll have to read through the site, but I'm pretty sure it is a direct 50 ohm match.

You'll probably want a 1 : 1 balun of some sort, or a 4 : 1 balun and use a folded dipole element.
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MDNITERDER
Member

Posts: 146




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« Reply #4 on: February 13, 2012, 02:16:29 PM »

I'm still new to all this so it is a bit confusing, was hoping to find a direct feed or someone who already made one, I am at a loss when it comes to balun, it mentions that it is a 300ohm loop driven element.

[edit] Although making a balun is something i should learn to do. [/edit]


John
« Last Edit: February 13, 2012, 02:41:36 PM by MDNITERDER » Logged
WB6BYU
Member

Posts: 13574




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« Reply #5 on: February 13, 2012, 03:02:00 PM »

Here is the page about feeding the antenna, which you probably found:

http://www.k7mem.150m.com/Electronic_Notebook/antennas/yagi_vhf_feed.html


Although he says the folded dipole has a 300 ohm impedance, that specific antenna is
designed for 50 ohm feed IIRC, which means a folded dipole would be 200 ohms instead
of 50 ohms.

That makes life simple - see the drawing of the 4 : 1 balun using 75 ohm coax?  You'd
do the same thing using 50 ohm coax.

Or you can just feed the driven element directly, and you should get a low SWR.  But
for a clean pattern and predictable performance you really want to add a balun - if you are
going through the effort to build the antenna in the first place, you might as well finish
the job right.  There are lots of kinds of baluns, and it can be as simple as slipping a
piece of braid over the end of the coax.

But that is one of the reasons I like the WA5VJB "Cheap Yagi" approach - it is simple to
build with wire and you can just solder the coax directly to the element (presuming you
use copper wire for the element, of course.  Soldering doesn't work well with aluminum.)
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KK9CQ
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Posts: 6




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« Reply #6 on: February 13, 2012, 04:53:36 PM »

I built and used a Quagi for 465 Mhz. Easy to make and worked fine.

Be safe.
Bob
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MDNITERDER
Member

Posts: 146




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« Reply #7 on: February 14, 2012, 08:58:55 AM »

I wish i was better at this antenna building, I mean i have built a 5/8 wave once a long time ago from an ARRL book i borrowed and if I recall it outperformed a store bought version from antennex. That may have been due to mine was direct coupled to the car vs the other was a mag mount. Boy do some of the formulas in the ARRL book get tough or in my eyes they do.
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MDNITERDER
Member

Posts: 146




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« Reply #8 on: February 14, 2012, 10:49:32 AM »

OK, so i will build this up tonight, for testing I will build it with PVC as the boom, if all works I will upgrade to fiberglass. if i like it i may make it with the through boom design with metallic boom and weld the elements.
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WB6BYU
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Posts: 13574




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« Reply #9 on: February 14, 2012, 12:23:00 PM »

You'll want to adjust the element dimensions somewhat when you go to the metal boom - the
website has links to the equations to use.

When I can get my other computer to cooperate I'll post the dimensions I used for the
WA5VJB-style "Cheap Yagi" beam that I built out of copper wire - might make a good portable
design.
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MDNITERDER
Member

Posts: 146




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« Reply #10 on: February 14, 2012, 12:30:35 PM »

 I ordered push nuts to hold the 3/8" Aluminum tube, I used 3/8 cause I have access to it by the 100's of feet. its 1/16" wall I am setting the jig up to bend it a little bit.

 Sadly all I have is RG8x for coax at this time. I know everyone says UHF to use a higher grade.
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MDNITERDER
Member

Posts: 146




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« Reply #11 on: February 14, 2012, 02:47:23 PM »

 Maybe someone could answer, on the loop of a folded dipole can one cheat and use two straight pieces or i should say 3 with some form of strap coupler at the ends
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WB6BYU
Member

Posts: 13574




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« Reply #12 on: February 14, 2012, 07:39:00 PM »

Yes you can.


Meanwhile, here is a link to GM3SEK's Long Yagi Workshop page:

http://www.ifwtech.co.uk/g3sek/diy-yagi/index.htm

If you scroll down to the Practical Construction section you'll find an article on folded dipole
driven elements.
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MDNITERDER
Member

Posts: 146




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« Reply #13 on: February 20, 2012, 09:50:26 AM »

The first build of this yagi is almost complete, I am using 3/8 aluminum 6061 T6 with a .065 wall The boom is 1" Sch 40 pvc, I will change it out to 1-1/4" square fiberglass tube once everything checks out.

 I will post photos once it is done, My question is at 467mhz at the feed end of the entenna should I use a Type N connector or should a simple PL-259 and SO-239 be ok?
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WB6BYU
Member

Posts: 13574




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« Reply #14 on: February 20, 2012, 10:38:38 AM »

There might be a slight improvement using an N connector.  Whether or not it actually makes
a practical difference depends on your use.
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