SparkyGuy wrote:
I built this, actually, a while back. Toss in a director (what is one
more wire?); I found that to be a worthwhile mod ...

Regards,
JS

Suggestions re. dimensions and location of a director?

Thanks.


I believe I tried lengths of a 3% to 5% shorter length with the
director, than the drive element (i.e., the one in front of the
reflector), settling on a 4% shorter length ... anyway, that is common
in HF parasitic arrays ... this is equiv. to a 3-element quad on HF.

I just dug it out and measured it, this is what it appears to be,
alright. With a spacing between driven element and director of ~0.2
wavelength, center-to-center.

Regards,
JS

SparkyGuy wrote:

...
Suggestions re. dimensions and location of a director?

Thanks.


Had company today, been in and out all day ... sorry I didn't offer more.

A wavelength at 2.4Ghz will be approx. 12cm, half-wave ~6cm, 1/4 wave
~3cm. (with a very thin wire 1/2 wave monopole-omni-element I find
~12.25cm works for me, element dia. will affect this, as other factors ...)

Anyway, I looked though my old emails for something a friend sent me,
thought if you were interested in the 2.4g fsm, you might be interested
in this, if nothing else, just because the dude is so complete!:

http://www.users.bigpond.com/darren....enna_for_2.htm

You will notice, he uses a plate reflector ...

Regards,
JS

I believe I tried lengths of a 3% to 5% shorter length with the
director, than the drive element (i.e., the one in front of the
reflector), settling on a 4% shorter length ...

So about 26.8mm square?

With a spacing between driven element and director of ~0.2
wavelength, center-to-center.

So the director should be 25 mm forward (toward the uwave source) of the
driven element? (3e8 / 2.4e9) x 0.2 = ~25mm

Do these numbers look right?

Thanks.

On Sun, 07 Dec 2008 21:40:38 -0800, John Smith
wrote:

A wavelength at 2.4Ghz will be approx. 12cm, half-wave ~6cm, 1/4 wave
~3cm. (with a very thin wire 1/2 wave monopole-omni-element I find
~12.25cm works for me, element dia. will affect this, as other factors ...)

Anyway, I looked though my old emails for something a friend sent me,
thought if you were interested in the 2.4g fsm, you might be interested
in this, if nothing else, just because the dude is so complete!:

http://www.users.bigpond.com/darren....enna_for_2.htm

You will notice, he uses a plate reflector ...

Regards,
JS

I also noticed that he cut his elements to a precision of 0.1mm. That
would be correct if the boom were an insulator or the elements were
inserted in insulating sleeves. However, since the 10mm square boom
is metal and effectively "shorts" part of each element, the actual
element length includes part of the diameter of the boom, thus adding
approximately 5mm to each element length (not including the reflector
and driven element). A VSWR sweep test would have shown the problem.

--
Jeff Liebermann
150 Felker St #D http://www.LearnByDestroying.com
Santa Cruz CA 95060 http://802.11junk.com
Skype: JeffLiebermann AE6KS 831-336-2558

SparkyGuy wrote:
I believe I tried lengths of a 3% to 5% shorter length with the
director, than the drive element (i.e., the one in front of the
reflector), settling on a 4% shorter length ...

So about 26.8mm square?

With a spacing between driven element and director of ~0.2
wavelength, center-to-center.

So the director should be 25 mm forward (toward the uwave source) of the
driven element? (3e8 / 2.4e9) x 0.2 = ~25mm

Do these numbers look right?

Thanks.


My elements are made from #14 bare copper wire. It is difficult to
measure each side, individually, when constructing the antenna. Better
to cut the wire to 1 wavelength, then make your bends forming it into a
square. Of course, every attempt possible should be made to keep each
side 1/4 of this length, exactly.

In measuring the one before me, which I constructed, this total length,
about the quad, is ~12.5cm.

However, here is a calculator I referenced in my constructions:

It is a java application, you must have java on your computer. Let me
know if you can't access it and I will give you the figures it spits
out, 2.4ghz = 2400mhz. Just let me know what wifi channel you wish to
center on (frequency.)

Regards,
JS

John Smith wrote:

...
However, here is a calculator I referenced in my constructions:

It is a java application, you must have java on your computer. Let me
know if you can't access it and I will give you the figures it spits
out, 2.4ghz = 2400mhz. Just let me know what wifi channel you wish to
center on (frequency.)

Regards,
JS

Well, da! It would help if I cut loose with the URL. sheepish smile

http://www.n6mrx.com/Antenna/Cubical-Quad1.htm

Regards,
JS

Jeff Liebermann wrote:

...
I also noticed that he cut his elements to a precision of 0.1mm. That
would be correct if the boom were an insulator or the elements were
inserted in insulating sleeves. However, since the 10mm square boom
is metal and effectively "shorts" part of each element, the actual
element length includes part of the diameter of the boom, thus adding
approximately 5mm to each element length (not including the reflector
and driven element). A VSWR sweep test would have shown the problem.


I have always used #8 wire as a boom on ghz yagis' (constructed a 7
element once.) Since the boom is at a current antinode (voltage node),
I ignored the boom.

The boom you mention is rather LARGE, give the freqs in question.

One of my next projects is a GOOD VSWR meter for ghz ... unfortuantly,
on the list of priorities here, it is low on the list ... :-(

I have just tended to take the % signal readings on the chans as good
enough, and trimmed my antennas accordingly. I know, this does NOT take
into account the functioning/power-fluctuations/antenna(s)/etc. of the
AP over these ranges ...

But, excellent point, glad you mentioned it ... but you are right, my
elements end up being filed down a bit from the figures the calculator
spits out ...

Warm regards,
JS

On Mon, 08 Dec 2008 08:37:34 +1100, Clifford Heath
wrote:

Bill wrote:
looking at the photos it appears to be being used
with a DVM, depending on how you want to use it you may find an analogue
meter easier. It will show changes better than a DVM.

The author says he tried that and it didn't work as well:
"I tried connecting the antenna directly to a micro amp moving
coil meter, however there was very little meter deflection from
a Wireless LAN card."

Impedance too low, perhaps?

Clifford Heath.

Maybe, but more likely, not enough RF to do anything useful with an
un-amplified meter.

What's missing from everyone's posting is what they plan to do with
this field strength meter.

Let's play with the numbers. The average wi-fi access point belches
about +12dBm (32 mw) RF into an antenna with perhaps 2dBi gain. The
FSM has a gain of perhaps 0dB. At a distance of about 3 meters (10ft)
the path loss at 2.4GHz is about 50dB which will deliver about -30dBm
to the diode. On the most sensitive voltage range, the DVM has an
input impedance of at least 1Mohm and can be treated like an open
circuit load to the detector.

-30dBm into 50 ohms is 0.224 vrms. Transforming linearly from 50 ohm
reference to perhaps 150 ohms loop antenna impedance yields about
600mv rms. (Note: I know this is a bad guess but it's midnight and
I'm tired). Resonating the loop probably raises the impedance even
higher, thus resulting in even more rectified voltage.

The resultant DC voltage will be about the same at about 600mv DC.
However, for wi-fi use, the xmit duty cycle is quite low, sending just
beacons, when there is no traffic moving. The 1000pf cap will need to
be paralleled with a much larger cap in order to form a usable peak
detector.

So, with a high imput impedance DVM the antenna and detector is quite
efficient and can generate quite a bit of voltage. However, when the
DVM is replaced with a non-amplified meter, the relatively low
impedance of the meter shorts out the diode and signifigantly reduces
the Q of the resonant loop antenna, resulting in much lower detected
voltage.

I've built several transmitter hunt "sniffers" using almost exactly
the same circuit, except that I use a hot carrier Schottky diode for
the RF detector, and the cheapest Harbor Freight DVM (because it
always gets destroyed or lost during the hunt). For 2.4GHz, I use a
dish antenna, RF amplifier, step-up xformer (coax balun), diode
detector, and DVM.

I also made a really ugly hack that uses a satellite TV signal meter.
http://www.sadoun.com/Sat/Products/Accessories/Meters/SF95L-DSS-FTA-Satellite-Signal-meter.htm
These work from about 900 to 1800MHz with a minimum detectable meter
sensitivity of about -70dBm. They are comatose at 860 (cellular) and
2.4GHz but are great for 900 and 1900MHz (PCS) sniffing. The input
port is connected to a 900MHz or 1.2GHz antenna of any sorts. The
output goes to a 75 ohm load and a 12V battery (gel cell) through an
RF choke.

--
Jeff Liebermann
150 Felker St #D http://www.LearnByDestroying.com
Santa Cruz CA 95060 http://802.11junk.com
Skype: JeffLiebermann AE6KS 831-336-2558

John Smith wrote:

Jeff Liebermann wrote:

...
I also noticed that he cut his elements to a precision of 0.1mm. That
would be correct if the boom were an insulator or the elements were
inserted in insulating sleeves. However, since the 10mm square boom
is metal and effectively "shorts" part of each element, the actual
element length includes part of the diameter of the boom, thus adding
approximately 5mm to each element length (not including the reflector
and driven element). A VSWR sweep test would have shown the problem.


I have always used #8 wire as a boom on ghz yagis' (constructed a 7
element once.) Since the boom is at a current antinode (voltage node),
I ignored the boom.

The boom you mention is rather LARGE, give the freqs in question.

One of my next projects is a GOOD VSWR meter for ghz ... unfortuantly,
on the list of priorities here, it is low on the list ... :-(

What about surplus directional couplers and a bolometer set (various
military versions used to be quite affordable)? Used to show up at
hamfests regularly too, covered 1 to 20 GHz or so in the kit; sorry
I forget the AN number at the moment, but it came with half a dozen
directional couplers for the various bands and lots of adapters.

Michael

On Mon, 08 Dec 2008 00:23:16 -0800, John Smith
wrote:

One of my next projects is a GOOD VSWR meter for ghz ... unfortuantly,
on the list of priorities here, it is low on the list ... :-(

http://pe2er.nl/wifiswr/

I've built several. They work nicely and are a very important tool.
My main inspiration was blowing up my Wiltron Model something SWR
bridge and discovering the exhorbitant cost of a repair or
replacement. The designs are sufficiently similar that I can
substitute. Of course, it's not flat over many decades of bandwidth,
but for fairly narrow bands, it's quite adequate.

I have just tended to take the % signal readings on the chans as good
enough, and trimmed my antennas accordingly. I know, this does NOT take
into account the functioning/power-fluctuations/antenna(s)/etc. of the
AP over these ranges ...

I have a handy distant reference signal for 2.4GHz. It's line of
sight and creates no Fresnel Zone issues, so it's useful for antenna
testing. I start with my "standard" gain panel antenna and take a
measurement. I then take the antenna under test, and compare. I
rarely make an absolute level measurement. It's always relative to
some other antenna of known characteristics.

The only problem is that I have to climb onto my roof to do the
measuring. I'm currently at the tail end of radiation oncology (no
big deal), which makes me kinda tired. I find myself somewhat tipsy
on the roof, so no antenna measurements for a few more weeks. One
might also suggest that it affects my math and memory, but they're
currently just as bad as always.

But, excellent point, glad you mentioned it ... but you are right, my
elements end up being filed down a bit from the figures the calculator
spits out ...

Suggestion: I always build a 3 element yagi first. Just the
director, driven element, and reflector. With the antenna on the
bench (after clearing off the chain saw and bicycle repair debris), I
run a fairly wide sweep using a reflection coefficient bridge or VSWR
bridge to an oscilloscope. I then trim the various elements for
resonance. My sweeper has a marker generator, but an external
generator (or wi-fi trash generator) can also be used. Adding
additional elements does change the tuning a bit, but the really big
frequency determining elements are the first 3 elements.
http://802.11junk.com/jeffl/pics/home/slides/lab.html
Incidentally, don't forget the mounting bracket. I just hate it when
I prototype an antenna, and then find that the bracket, radome,
potting foam, serial number sticker, or other irrelevent hardware
detune the antenna.

Another trick that I use with wire rod type of antennas is to thread
the end of the rods. Attached to these are threaded standoffs. These
can be rotated to adjust the length of the elements. Also, don't
forget to round off or chamfer the ends of each element to improve the
bandwidth.

1AM. Time to give up before I start babbling nonsense.

Warm regards,
JS

Bah-Humbug (T'is the season).

--
Jeff Liebermann
150 Felker St #D http://www.LearnByDestroying.com
Santa Cruz CA 95060 http://802.11junk.com
Skype: JeffLiebermann AE6KS 831-336-2558