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Author Topic: Tape Measure 2M antenna question  (Read 6205 times)
K5UNX
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« on: November 23, 2012, 02:07:28 PM »

I just finished a tape measure yogi from this recipe: http://www.ccars.org/Projects/TapeYagi/index.htm

I don't think it's working as well as it should. As I got it done, there happened to be a couple guys chatting on 146.52 locally. I went outside and checked the meter level on the stronger of the two signals on my FT-60R. I switched from my replacement whip to the yagi and it seemed to have a weaker signal even after turning around to find the strong direction.

I wonder if the coax cable I used is junk. I don't have any of this type of stuff sitting around the house so I was placing an amazon.com order for a few things and found this: http://www.amazon.com/gp/product/B001JT0CGI/ref=oh_details_o00_s00_i02

In stripping the outer insulator, I found that the outer conductor of the coax is something like tin foil with a few strands of very very thin copper wire in there. I mean really thin copper. I twisted it all together and soldered it like the instructions said.

I expected the receive signal to be stronger with the yagi that the whip on my HT. Not sure what to do next. I'll take it to our next club meeting for advice but I thought I would ask here in case someone has an idea. I am using a replacement whip that's about 16 inches long rather than the stock rubber ducky.

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KE3WD
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« Reply #1 on: November 23, 2012, 03:13:51 PM »

By all means suspect the feedline or the connections. 

That aluminum stuff meant for cable tv does not take to electrical solder. 


73
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WB6BYU
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« Reply #2 on: November 23, 2012, 06:41:58 PM »

A good check is to use put an ohmmeter across the coax connector:  if both
sides of the coax are connected properly you should see a short circuit (though
the beta match hairpin.)  If it didn't take solder well you can put a crimp lug
on it and solder that to the tape measure, or secure it with a sheet metal
screw through the tap and into the PVC pipe on the other side of it.

I've made a lot of tape measure yagis, and saw this design demonstrated by
Joe Leggio WB2HOL at Dayton.  (I generally use a different design that is
optimized for gain rather than F/B.)

Some construction suggestions:

1)  leave off the stub pieces on each end.  This allows you to use two T
connectors and one 4-way connector (saves weight and cost.)  Hold the
antenna just forward of the reflector.

2)  Use fiberglass strapping tape to hold the elements onto the PVC connectors
in place of hose clamps.  (Saves weight and cost.)  This might require replacing
every 5 years or so if it sees rough use.

3)  I like using the 5/8" tape measure elements rather than the 1" material.  By
reinforcing the center half or so of each element with a second piece it stiffens
it and helps it snap back into place faster.  You can get a 16' tape at Dollar Tree,
which is just right for one 3-element 2m yagi.
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WB6BYU
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« Reply #3 on: November 23, 2012, 07:37:57 PM »

Just to check:  you have the driven element is in the center, right?

The instructions say,
Quote

...Attach the longest (41 3/8 inchh) piece of tape rule to the cross connector closest to the center cross connector...


which may lead to confusion:  the long element (the reflector) should be at one end of
the boom (the end with the shortest boom section) rather than on the middle connector.

You probably have it correct, but that wording isn't very clear.
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KA4POL
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« Reply #4 on: November 23, 2012, 09:45:18 PM »

When trying to find the strongest receive direction, did you also change polarization of the yagi to vertical?
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KE3WD
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« Reply #5 on: November 24, 2012, 06:29:00 AM »

When trying to find the strongest receive direction, did you also change polarization of the yagi to vertical?

Good point.


73
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K5LXP
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« Reply #6 on: November 24, 2012, 08:01:30 AM »

I wonder if the coax cable I used is junk.

The only way the coax could be a factor is if it was shorted or open.  4 feet of coax no matter how crappy couldn't cause enough loss to see what you describe.

The hairpin match would be something to check.  Checking the antenna out with an analyzer could reveal some possible issues with that.


Mark K5LXP
Albuquerque, NM
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K5UNX
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« Reply #7 on: November 24, 2012, 10:51:15 AM »

When trying to find the strongest receive direction, did you also change polarization of the yagi to vertical?

I did not do that. I will try that.

I found the battery in my multi-meter was dead. Need to run to the store to get one, then I can check things out a little better.

Just to check:  you have the driven element is in the center, right?

The instructions say,
Quote

...Attach the longest (41 3/8 inchh) piece of tape rule to the cross connector closest to the center cross connector...


which may lead to confusion:  the long element (the reflector) should be at one end of
the boom (the end with the shortest boom section) rather than on the middle connector.

You probably have it correct, but that wording isn't very clear.

Yes I have it correct I think.  The driven element is in the middle. It's closer to the longest (reflector) outer element.


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WB6BYU
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« Reply #8 on: November 24, 2012, 10:58:27 AM »

Quote from: K5UNX
Quote from: KA4POL
When trying to find the strongest receive direction, did you also change polarization of the yagi to vertical?

I did not do that. I will try that.



The elements should be vertical for maximum pickup of a vertically polarized signal.
However, if you are trying it indoors you may have a lot of reflections from wiring, etc.,
and even moving the antenna a foot one way or the other can make a big difference
in signal strength:  you can't really compare two antennas unless they are in exactly
the same location.
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K5UNX
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« Reply #9 on: November 24, 2012, 11:12:55 AM »

Quote from: K5UNX
Quote from: KA4POL
When trying to find the strongest receive direction, did you also change polarization of the yagi to vertical?

I did not do that. I will try that.



The elements should be vertical for maximum pickup of a vertically polarized signal.
However, if you are trying it indoors you may have a lot of reflections from wiring, etc.,
and even moving the antenna a foot one way or the other can make a big difference
in signal strength:  you can't really compare two antennas unless they are in exactly
the same location.


I was in my driveway switching between both antennas more than once.

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K5UNX
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« Reply #10 on: November 24, 2012, 01:34:34 PM »

A good check is to use put an ohmmeter across the coax connector:  if both
sides of the coax are connected properly you should see a short circuit (though
the beta match hairpin.)  

I got a battery for my multi-meter and did what you suggested. It is shorted like you describe.

I looked the rest of it over and found a problem. There was a couple strands of wire shorting the center conductor and the shield together before they were attached to the tape measure pieces. I fixed that by cutting them out.

I'll re-test when I hear those guys again on simplex (146.520). They seem to be on most days.

I am going to get some better coax and replace the feed line at some point when I figure out where to get some good stuff.

EDIT: I just heard the guys on 146.520 again. I went outside and re-tested. Much better results. A lot stronger incoming signal than the whip this time. I think I fixed it by removing that stray wire! Gotta be more careful next time.
« Last Edit: November 24, 2012, 02:34:26 PM by K5UNX » Logged

K5UNX
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« Reply #11 on: November 24, 2012, 07:19:29 PM »

One more thing . . I noticed both of those guys had stronger signal while I held the yagi horizontal than they did while I had it upright.
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WB6BYU
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Posts: 13033




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« Reply #12 on: November 24, 2012, 08:52:56 PM »

Glad you got it working.  There isn't much that can go wrong with such an antenna other
than the coax connection.  (Sometimes the coax insulation melts when you solder it.)

I presume that when you held the antenna vertical that means that the boom was
horizontal and the elements were vertical.  (If not, that could be part of the problem.)

It might that the stations you were listening to were using horizontally polarized antennas.
It isn't the convention for FM work, but if they already have the antennas in place then
it may work for them.  You can also get reflections off of other objects that shift the
polarization. 

When you are hunting, generally you would rotate the boom to find the orientation that
gives maximum signal strength.  One tip is that, if you don't have about 20dB or so of
difference in signal strength when the antenna is cross polarized, you may be dealing
with reflections.  That's one way to try to determine which of two bearings is the right
one:  the one with the most consistent polarization is likely to be the more direct path,
since the polarization is often shifted on reflection.
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KA4POL
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Posts: 1921




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« Reply #13 on: November 24, 2012, 10:02:14 PM »

One more thing . . I noticed both of those guys had stronger signal while I held the yagi horizontal than they did while I had it upright.
If they are at their home locations it may be a good chance they are using horizontally polarized antennas. I'd just ask them. This way you can also find out about your TX signal. Of course each reflection rotates the polarization. Reflected signals usually carry kind of a low white noise in the background.
Anyway, it's good you found the problem.
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