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Author Topic: 6 meter 5 element yagi  (Read 1753 times)
KE7MYL
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Posts: 13




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« on: July 11, 2008, 10:47:56 AM »

I have built a 6 meter 5 element yagi as presented in the ARRL antenna book, 21 edition, first printing, page 18-27 & 18-28.
The dimensions are from table 6 file 506-12W.YW center freq. 50.6 MHz.
This is my first homebrew yagi and I have encountered a problem. After repeated attempts, I can not get this antenna to tune. This antenna is fed with a ½ wave phasing line / coax balun and hairpin. I am a machinist and have constructed this antenna EXACTLY as presented. Here are a few notes on the construction:

Balun checked with antenna analyzer- 182 degrees @ 50.6 MHz

Antenna connected directly to coax- Rs=20   Xs=34

Antenna connected with balun and hairpin- Rs=15   Xs=44

Antenna connected with just hairpin- Rs=141   Xs=92

Antenna connected with just balun- Rs=7   Xs=4  

As per the book the parasitic elements are electrically connected to the boom and the driven element is insulated from the boom. Also the coax shield is not connected to the boom.
If you are familiar with this antenna, or can Elmer me I would appreciate it.

73 de KE7MYL
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WB6BYU
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Posts: 13174




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« Reply #1 on: July 11, 2008, 11:31:14 AM »

Were all the reactances positive (as you wrote them) or
were some of them negative?  It may make a difference.
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KE7MYL
Member

Posts: 13




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« Reply #2 on: July 11, 2008, 12:06:29 PM »

All positive.

Thanks...
KE7MYL
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WT0A
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Posts: 922




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« Reply #3 on: July 11, 2008, 01:01:24 PM »

Take a look at Chips (N6CA) site for designs using T match. They work well for me.
Glen WT0A
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WB6BYU
Member

Posts: 13174




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« Reply #4 on: July 11, 2008, 02:36:59 PM »

Are you measuring the impedances right at the antenna
connector, or at the end of a patch cable?

The impedance without the stub should have a negative
reactance in order for the stub to work.  If it is positive
right at the feedpoint then you may need to shorten the
driven element.
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KE7MYL
Member

Posts: 13




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« Reply #5 on: July 11, 2008, 04:27:38 PM »

I am measuring the impedances at the end of a 20 foot run of RG-8.

Thanks…
KE7MYL
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K5LXP
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Posts: 4475


WWW

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« Reply #6 on: July 11, 2008, 08:38:53 PM »

Is the antenna in the clear or near the ground or other objects when you're measuring it?  I went round and round with a 6M beam I had until I put it on a pole and got it 10 feet off the ground, where it suddenly started to work perfectly.

Mark K5LXP
Albuquerque, NM
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WB6BYU
Member

Posts: 13174




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« Reply #7 on: July 12, 2008, 09:02:40 AM »

The reasons why the impedances don't make sense is because you are
measuring them through a length of coax rather than right at the
feedpoint.

Go to VK1OD's transmission line calculator here:

http://www.vk1od.net/tl/tllc.php

Set the "Mismatch" field to "Zin".  Enter your measured values in the
window beside it in the form "30+j50".  Set the length and type of the
coax and the measurement frequency.

After you hit CALCULATE look for the line that says "Zload" in the output.
That should be the actual impedance at the feedpoint.  A quick check
with EZNEC says it should be about 20-j60 without the balun or the
matching stub.

Do make sure that the grounds of all the coax connectors are hooked
together.  (They were mounted on a common aluminum plate on the
original.)  It should be possible to ground this common connection to
the boom without adverse effect.


The design is, indeed, a bit puzzling.  I'm not sure why they use a beta
match to go from 20 to 200 ohms, then step it down to 50.  I think the
SWR bandwidth would be better (and adjustment easier) using a beta
match from 20 to 50 ohms and a 1 : 1 balun.
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KE7MYL
Member

Posts: 13




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« Reply #8 on: July 12, 2008, 10:11:10 AM »

WB6BYU:

Thanks for the input! I found VK1OD's web page this morning.
The matching of this antenna is rather convoluted and I have been advised to “just Gamma or Beta match it” and I may take that advice. I am testing this antenna pointing up with the reflector about 1.5 feet off the ground supported with a length of plastic pipe.

Thanks to all for the help, it is greatly appreciated.

73 de KE7MYL
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WB6BYU
Member

Posts: 13174




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« Reply #9 on: July 12, 2008, 10:47:22 AM »

You could try simply matching it with some cut-and-try.

Ideally you want a way to make the length of the beta matching stub
adjustable:  this can either be a pair of rods with an adjustable sliding
short, or a piece of wire bent into a long U shape with sliding contacts
where it attaches to the driven element.  (You might start by making a
sliding wire short across the existing stub.)  Then try changing the stub
length and see what it does to the SWR.  Also try changing the length
of the tip sections of the driven element by sliding the ends in and out.
If you look at the indicated SWR rather than the impedance, it won't
make a lot of difference that you are measuring it through the coax.
Basically keep tweaking the two adjustments for minimum SWR.  If you
want to try direct feed, remove the balun and do the same thing but
start with the driven element somewhat longer - this may require tip
lengths of about 20 to 21 inches on the driven element (for a total
element length of 132 to 134 inches).
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KE7MYL
Member

Posts: 13




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« Reply #10 on: July 12, 2008, 12:50:44 PM »

WB6BYU:

Amazing! This morning, using a short stub to feed the antenna and using a sliding length of U shaped aluminum as the hairpin, I am now bringing the antenna to resonance. Somehow, I am receiving your help directly instead of by your posting.
…KEEP SENDING!!!

73 de KE7MYL
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WB6BYU
Member

Posts: 13174




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« Reply #11 on: July 12, 2008, 01:10:06 PM »

No, it is a logical progression of what to try next.  You're running a bit
ahead of me because of the delay in posting and responding.  It probably
will take longer for you to respond this time because you will have
finished your lunch and be back playing with the antenna.  Next thing
you know you'll be working DX.

If I had a rig and an antenna for 6m I'd point it down your way and see
if I could hear you, but unfortunately I don't.

Good luck!

- Dale WB6BYU
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KE7MYL
Member

Posts: 13




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« Reply #12 on: July 12, 2008, 05:48:04 PM »

WB6BYU

Success!!  By making a long sliding hairpin and adjusting for the best SWR, I was able to arrive at the proper length. Then, just a matter to cut off and flatten the ends, drill holes and connect to the feed point and center neutral point to the boom. I also took your suggestion to connect the common coax shield bar to the boom; it seems to flatten the SWR just a bit. The antenna exhibits about 1.2:1 SWR over the design frequencies of interest, 50.1 to 51.1 MHz. It took a little length adjustment on BOTH the driven and first director.

Now, to get it up on the tower…. Hmmm…

Thanks to you Dale and all of the others that had input on this project.

73 de KE7MYL
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