Slick,

The only thing about making the 48 inch mast non resonant is what's on the
other end of it? A ceramic insulator is not the same as a Radio Shack steel
house bracket. Since it is behind the first reflector, probably doesn't
matter in the real world.

Tam/WB2TT
"Dr. Slick" wrote in message
om...
"Tam/WB2TT" wrote in message
...
If the second reflector is actually doing anything, which I doubt, you
should be using a non metallic mast, and not bringing the coax down
between
the reflectors.

Tam/WB2TT


Actually, if you believe the yagi optimizer program, the second
reflector is indeed not doing TOO much, but it does affect the back
shape of the pattern, and improves the impedance bit, and also
increases the dBd of the outer edge of the pattern by tiny bit too.

And i did simulate having the mast at 48 inches from the DE, which
i was advised was the distance where the mast would have the least
affect. Changing the mast length in the YO program does affect the
pattern shape though, so i tried to make the mast a non-resonant
length.

At any rate, we could probably survive without the second
reflector, you are most likely correct, but maybe for the next design.

It first, i considered having the "T" section of PVC pipe move
outwards at a 90 degree angle, and then attach another section of 2"
PVC that would be sticking out, so that the coax would be totally
perpendicular to all the elements. But, this would be a bit unwieldy.



Slick

"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

"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

"Tam/WB2TT" wrote in message ...
Slick,

The only thing about making the 48 inch mast non resonant is what's on the
other end of it? A ceramic insulator is not the same as a Radio Shack steel
house bracket. Since it is behind the first reflector, probably doesn't
matter in the real world.


Tam,

The 48 inches refers to it's position relative to the DE, not it's
actually length, which in the ideal case would be at a non-resonant
length. The yagi optimizer program i have shows that the length does
indeed affect the propagation pattern.

I'm not certain how the user has the mast mounted, but it's
somewhere around 7.5 feet or so in length.

At any rate, he is getting out very well!

Err...i won't say at what wattage, someone might faint out there!

hahaha!


Slick

where is the center of radiation on this antenna ?
where will the strongest field strength be ?

i found this antenna page ,
can i use a coil for the use od decoupling the signal?

http://www.packetradio.com/images/6mdipole.gif

g subs inscribed:

i found this antenna page ,
can i use a coil for the use od decoupling the signal?

http://www.packetradio.com/images/6mdipole.gif

Yes. However you should investigate other matching methods - Google for the
"Ross Dipole". Also look at Gamma matches and Delta matches - both are
viable solutions for unbalanced feeder connection to a balanced antenna
(like a dipole).

--
BIAS COMMS

Everything gets easier with practice, except getting up in the morning!

(g subs) wrote in message . com...
where is the center of radiation on this antenna ?
where will the strongest field strength be ?


Looking at:

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

You will notice the lobe is greatest at the rear (180), and
this is because of an interesting use of the yagi optimizer program,
where we actually used two directors to act as reflectors.

So the strongest field strength will be where you point the driven
element (dipole), and the weakest ERP will be where the two reflectors are
facing.


Slick

(g subs) wrote in message . com...
i found this antenna page ,
can i use a coil for the use od decoupling the signal?

http://www.packetradio.com/images/6mdipole.gif


Well, if you look at the picture i posted, you will see
the 4 turns of 4" diameter loops that we placed in the RG-213,
just before the feedpoint.

Your site shows 10 turns of RG-58, which may or may not be
too many turns.

But barrel connectors have a bad reputation for higher losses
that if you used a mounted SO-239. For higher than 300 watts, you
should use a type "N" connector.


Slick