Dave Shrader wrote in message news:CA44c.6318$C51.52257@attbi_s52...
Interesting photo but NO PATTERN


Opps! Here is the H-plane plot:

http://www.drslick.org/Temp1/yagiplot.jpg


It was an interesting usage of the program, in that we used a
"dummy" reflector that was very far away from the DE, and then used
two directors as reflectors (notice on the plot that the lobe is
actually pointed in the 180 degree direction).


Taking a look at the photo again:

http://www.drslick.org/Temp1/yagi929.jpg

You will notice the 4 turns of 4" diameter coils in the RG-213
coax, just before the feedpoint (SO-239), using a plastic tie-wrap.
You can use a balun instead, but i have never done this before,
perhaps one of you out there has something i could try in the future.
It would be a 1:1 in this case.
You'll also notice that i didn't just leave the coax hanging off
the feed point, as this might ruin the pattern and impedance. I tried
to keep the coax at 90 degrees to the elements by tie-wrapping it to
the PVC.

It would be nice to borrow an anechoic chamber with the proper
field-strength meter, and see how closely the actual unit matches the
YO program in the H-plane.

Great fun, and it should be able to handle up to 500 watts or so,
but i don't recommend using anything more than 1 mW or so... hehehe!

Questions and comments appreciated.


Slick

If you put the array on a rotator and put a receive antenna with some
sensitive output indicator beyond the near field, you can plot relative
field, and thus accurate F/B by adjusting the power to the antenna for same
received F/S at, say, 10º increments. Nothing else should change, so the
relative field strength in the various directions should relate to each
other. Ground reflections and absorption, and all the other artifacts will
be constant unless you change something else.

--
Crazy George
Remove N O and S P A M imbedded in return address
"Dr. Slick" wrote in message
om...
Dave Shrader wrote in message
news:CA44c.6318$C51.52257@attbi_s52...
Interesting photo but NO PATTERN


Opps! Here is the H-plane plot:

http://www.drslick.org/Temp1/yagiplot.jpg


It was an interesting usage of the program, in that we used a
"dummy" reflector that was very far away from the DE, and then used
two directors as reflectors (notice on the plot that the lobe is
actually pointed in the 180 degree direction).


Taking a look at the photo again:

http://www.drslick.org/Temp1/yagi929.jpg

You will notice the 4 turns of 4" diameter coils in the RG-213
coax, just before the feedpoint (SO-239), using a plastic tie-wrap.
You can use a balun instead, but i have never done this before,
perhaps one of you out there has something i could try in the future.
It would be a 1:1 in this case.
You'll also notice that i didn't just leave the coax hanging off
the feed point, as this might ruin the pattern and impedance. I tried
to keep the coax at 90 degrees to the elements by tie-wrapping it to
the PVC.

It would be nice to borrow an anechoic chamber with the proper
field-strength meter, and see how closely the actual unit matches the
YO program in the H-plane.

Great fun, and it should be able to handle up to 500 watts or so,
but i don't recommend using anything more than 1 mW or so... hehehe!

Questions and comments appreciated.


Slick

If you put the array on a rotator and put a receive antenna with some
sensitive output indicator beyond the near field, you can plot relative
field, and thus accurate F/B by adjusting the power to the antenna for same
received F/S at, say, 10º increments. Nothing else should change, so the
relative field strength in the various directions should relate to each
other. Ground reflections and absorption, and all the other artifacts will
be constant unless you change something else.


ok, that sounds like a good idea, with the rotator, keeping everything else
the same. This site:

http://radioproshop.com/antennaconcepts.htm

Says that the near-field is defined by Rmin=(2*D**2)/wavelength, where
D is the largest dimension of the antenna. So if we roughly say one wavelength
is 10 feet, and the largest dimension is the dipole, then we get Rmin=5 feet,
or about a 1/2 wavelength. This sounds a bit too close to me, perhaps the
far-field is better defined as, like, ten wavelengths or so?


Also, i don't really have a transmitter that I can vary the power that
much...maybe that's something i should make or get. Something variable at
least by 20dB or so.


Slick

OK, one alternative is to use a known accurate (1dB) step attenuator in the
receive line to get the relative numbers. Always go for the same strength
indication on the receiver. Start at the azimuth with lowest indication,
and add attenuation as it rotates.

--
Crazy George
Remove N O and S P A M imbedded in return address

"Crazy George" wrote in message ...

OK, one alternative is to use a known accurate (1dB) step attenuator in the
receive line to get the relative numbers. Always go for the same strength
indication on the receiver. Start at the azimuth with lowest indication,
and add attenuation as it rotates.


Yeah, that's a good idea. I made an amplified RF sniffer from
plans in the ARRL handbook that uses an op-amp, but i never calibrated
the meter.

i could do this if i had bought one of those step attenuators at
that ham fest i was at. Even if i had bought the somewhat inaccurate
one that rotated, i could have still measured how off it was on a
power meter.

Yeah, i can set the sensitivity of the sniffer with a pot that
controls the feedback resistor, and i can set for full scale
deflection at the rear lobe, which should be the lowest field
strength, and then increase the attenuation to maintain the same
needle position.

I'd have to get it back from the guy and take it off the rooftop
again! This experiment will have to wait!

He's getting out just fine...



Slick

"Radio913" wrote in message
...
If you put the array on a rotator and put a receive antenna with some
sensitive output indicator beyond the near field, you can plot relative
field,

SNIP


Also, i don't really have a transmitter that I can vary the power that
much...maybe that's something i should make or get. Something variable at
least by 20dB or so.


Slick


You don't need to pump power to measure antenna patterns. A simple signal
generator with a milliwatt or so will work fine. If you want to be really
serious about designing antennas, then you need to start acquiring serious
tools. See eBay; start bidding on a sig gen, some pads (attenuators), a
rotor, a field strength meter.

Ed

"Ed Price" wrote in message news:6wH4c.5720$uh.5339@fed1read02...


You don't need to pump power to measure antenna patterns. A simple signal
generator with a milliwatt or so will work fine. If you want to be really
serious about designing antennas, then you need to start acquiring serious
tools. See eBay; start bidding on a sig gen, some pads (attenuators), a
rotor, a field strength meter.

Ed


I've got an MFJ antenna analyzer, which is somewhere around 1 mW
or so.

If i calibrate my RF sniffer correctly, i think i can get a pretty
accurate plot, accurate enough for my purposes.


Slick