Q: Biquad Antennas
Arthur Shapiro
annotated web sites that describe the modest construction details.
Unless I'm missing something, they differ in one possibly critical area: on
some of them the N jack seems to be electrically connected via screw or
solder to the metal plate (is that a ground plane?) In others, the N jack is
clearly isolated from the metal of the plate.
Is anyone here well versed in antenna theory to assert which of these two
possibilities is optimal?
Art
mi...@sushi.com
Do you have an URL for a floating reflector biquad?
While both are at ground potential, I consider the surface to be a
ground plane when the spacing from the element to the plane doesn't
matter, while a reflector has a critical spacing between the element
and the grounded surface.
A ground plane would be under a vertical antenna.
I'm of the school of grounding the reflector, but there are cases where
reflectors float, such as in yagis.
Jeff Liebermann
You rang? I've been wanting to rant on the topic of biquad
construction. There are far too many erronious biquad articles.
The antenna will work with either a grounded or floating reflector. It
doesn't really matter. I've done it both ways with no obvious
difference in performance.
Note that the 2 quad elements are literally floating above the ground
plane in these models.
http://802.11junk.com/jeffl/antennas/Biquad/index.html
http://www.lecad.uni-lj.si/~leon/other/wlan/biquad/index.html
You might be looking at this construction article
http://www.lincomatic.com/wireless/homebrewant.html
which mounts the N connector directly on the reflector without a coax
extension section to the quad elements. Scroll down to the biquad
section. This is the WRONG way to do it and will not work at all.
Even the author claims horrible performance in the article. Little
wonder why.
This is more of the same, with the antenna wrongly constructed:
| http://www.engadget.com/2005/11/15/how-to-build-a-wifi-biquad-dish-antenna/
The pair of wires shown, between the connector and the biquad elements
should be a 50 ohm coax section, not an open wire pair.
Even more biquads built totally wrong:
http://flakey.info/antenna/biquad/
http://www.sorgonet.com/network/biquad/
with the same problem. Amazing.
This one attempts to solve the problem by moving the connector ground
to the antenna elements:
http://www.lecad.uni-lj.si/~leon/other/wlan/biquad/
However, the ground and exposed center ping lead length are still
excessively long. Also, don't try to drill circuit board material
with a large wood drill bit while holding onto the board. Great way
to rip up your hands.
So much for the wrong way to build a biquad. The following are some
proper biquad constructions articles. Note that the coax cable
extenders *THROUGH* the reflector in all examples. The center
conductor is only exposed when it hits the quad elements and then as
little as possible.
http://homepages.ihug.co.nz/~redwood4/
http://martybugs.net/wireless/biquad/
http://www.vallstedt-networks.de/?Fotogalerien/quad2
http://www.brest-wireless.net/wiki/materiel:biquad
http://www.weijand.nl/wifi/ (dead?)
http://www.trevormarshall.com/biquad.htm
http://pe2er.nl/biquad/
http://pe2er.nl/biloop/
http://pe2er.nl/biquadusb/ (For USB radio)
A really marginal construction idea (mine):
http://802.11junk.com/jeffl/antennas/biquad2/
The mistake I made was to support the ends with nylon spacers. The
nylon barely works at 2.4Ghz and connects to the antenna at the worst
possible high impedance point. Removing the nylon spacers increased
the gain about 1dB. However, I've only measured 8dBi of gain but it
should be 2dB higher, so something else is still screwed up.
Note that there is really only one critical dimension, the cut length
of each quad element. Whether it makes a perfect square is totally
irrelevant. The length of the "loop" should be 1 wavelength or
12.5cm. The problem is from where to where does one measure. The end
that hits the center conductor is easy enough, but where the ground
end hits the coax shield is a problem. If the two quad elements don't
hit the shield in the same exact place, half the length BETWEEN the
elements need to be included in the loop length. It's tricky and
varies with the method of construction. In all cases, the loop length
is never longer than one wavelength (12.5cm) and always slightly less.
It's probably safe to cut the loops a bit short.
However, the cut length is seriously critical. At 2.4GHz, the
difference in length per MHz is:
3*10^10cm/sec / 2400.0 MHz = 12.5 cm
3*10^10cm/sec / 2483.5 MHz = 12.1 cm
(12.5cm - 12.1cm) / 83.5Mhz = 0.005cm per Mhz
With 5MHz channels, you have to be with 0.025cm to hit any channel.
The only reason that it works at all is that the biquad antenna is
fairly broadband and will work reasonably well even if miscut and
mistuned. The VSWR and gain curves versus frequency are fairly flat
over a wide range of frequencies.
The height of the antenna only affects the VSWR (how close to 50 ohms
the antenna appears) and does not affect the center tuning frequency
much.
Without a directional coupler and signal source, or network analyzer,
your chances of getting it perfect are minimal. So, build one:
http://www.qsl.net/n9zia/24swr/
http://www.qsl.net/n9zia/wireless/pwr-mtr-pics.html
http://pe2er.nl/wifiswr/
http://yves.maguer.free.fr/WiFi/page_wifi_yves.html#Mon_mes_antennes_sont-elles_bien_accordees
There was a site showing how to make a directional coupler out of two
pieces of semi-rigid coax cable, but I can't find it.
You can use Netstumbler to test antenna gains:
http://pe2er.nl/antennetesten/
http://pe2er.nl/list_1.htm
http://www.seattlewireless.net/AntennaHowTo
However, everyone seems to make the same mistake. They mount the
antenna on a camera tripod and try to connect to a distant access
point that's also at approximate ground level. The problem is that
the Fresnel Zone extends to the ground an makes a mess of the numbers.
Assuming a typical camera tripod 1.5 meters off the ground at both
ends, the maximum range that can be used for testing is 130 meters
(for 0.8 times Fresnel zone radius).
http://www.terabeam.com/support/calculations/fresnel-zone.php
Even without the Fresnel Zone issues, reflections from the hard street
will cause problems. Maybe do the test over an RF absorbent grass
lawn or park will give better results. It's easy to tell if you've
got reflection or Fresnel Zone problems. Just raise or lower the
antenna. If the gain moves, you've got reflections.
--
Jeff Liebermann je...@comix.santa-cruz.ca.us
150 Felker St #D http://www.LearnByDestroying.com
Santa Cruz CA 95060 http://802.11junk.com
Skype: JeffLiebermann AE6KS 831-336-2558
Arthur Shapiro
>Do you have an URL for a floating reflector biquad?
Here's one; hope it is clickable as the URL is split weirdly as I construct
the message:
http://features.engadget.
com/2005/11/15/how-to-build-a-wifi-biquad-dish-antenna/
Art
Arthur Shapiro
>You rang? I've been wanting to rant on the topic of biquad
>construction. There are far too many erronious biquad articles.
>
So after digesting your pleasantly voluminous message (thank you!), let's see
if I understand. I was really hoping to mount the N jack on the ground plane,
and it sounds like that is OK, even if not universally done in your
references.
But you assert that the critical factor, whether or not the jack is
electrically connected to the ground plane, is that short 15-18mm
gap between the jack and the actual antenna. It HAS to be a piece of coax -
the alternative of two soldered pieces of copper wire or a notched cylinder
of half inch copper pipe will severely detract from performance. Correct? I
assume that's an SWR issue.
And doubtlessly the scraps of cable TV coax I have about the house are the
wrong impedence.
Art
mi...@sushi.com
I pieced together the URL. Pretty funny seeing the dish on the car
roof.
I can't really tell if the ground wire connects to the reflector or
not. Since they didn't use PCB but rather copper sheeting, it is clear
they didn't make a connection from the back.
I saw Jeff's reply and if floating is good enough with him, I would
float it only because soldering to a ground plane is work. Generally
you want to tin the area where the connector will be soldered, then try
to solder the connector without melting the inside. I would much rather
just epoxy the connector to the reflector, which means the only wire
you have to solder is the one that brings ground to the biquad element.
You will undoubtably ruin one or two until you get the hang of
soldering to them, so have a few on hand. If you don't ruin one, then
build another antenna! If you have a ham flea market in your area,
these connectors are a quarter. Buy a handfull.
mi...@sushi.com
If it is really necessary to use a coax fed, then I would solder the
copper pipe to the ground plane. If the RG-8 insulator isn't inserted,
then you can blast the pipe and copper with a torch without fear of
melting. So scratch the epoxy I mentioned in the other post.
I built mine just using copper wire to reach the biquad. However, I
don't have any gear to measure SWR at wifi frequencies, so my
construction may not be optimal.
Jeff Liebermann
>I was really hoping to mount the N jack on the ground plane,
>and it sounds like that is OK, even if not universally done in your
>references.
Ummm.... you missed my point. The coax cable needs to extend all the
way to the antenna. If you mount the coax connector on the reflector,
you will still need to attach a short piece of coax cable or simulated
coax cable made out of tubing, from the connector to the antenna.
That's not easy. It's MUCH easier to run the coax through the
reflector and deal with only one messy soldering job, than to put the
connector on the reflector and end up with two messy soldering jobs.
>But you assert that the critical factor, whether or not the jack is
>electrically connected to the ground plane, is that short 15-18mm
>gap between the jack and the actual antenna. It HAS to be a piece of coax -
No, I said the critical factor was the length of the pieces of wire
used to make the quad antenna elements. The height of the quads above
the reflector is not particularly critical.
What I said about that gap was that it MUST be coax, not two pieces of
wire.
>the alternative of two soldered pieces of copper wire or a notched cylinder
>of half inch copper pipe will severely detract from performance.
Severely is an understatement. As several of those that built it like
that mentioned, it doesn't work.
>Correct? I
>assume that's an SWR issue.
No. It's NOT a VSWR issue. The exposed extra wires in place of the
coax cable become part of the radiating elements and totally mangle
the radiation pattern. There will also be a significant change in
VSWR, but it's not as damaging as having the pattern and gain all
screwed up.
>And doubtlessly the scraps of cable TV coax I have about the house are the
>wrong impedence.
Actually, I use RG-6/u and RG-11/u (75ohm) coax for LOTS of antenna
projects. (RG-59/u is garbage). If I'm trying to squeeze every bit
of gain out of the antenna or match the tests to the model, I use
LMR-240 or LMR-400. However, if I'm trying to throw something
together that's not particularly critical, I'll use 75 ohm coax. The
VSWR and resultant loss is negligible. See:
http://www.qsl.net/n9zia/wireless/75_ohm_hardline.html
Maximum VSWR is only 1.5:1 which is about 0.18dB loss. Not much. The
75 ohm coax actually has less loss than the equivalent length of 50
ohm coax, so the loss is even less.
The big problem is connectors. I use CATV F connectors and quad
shielded RG-6/u. Going from F to RP-TNC or RP-SMA is a bit of a
problem. I make my own adapters or use 2 adapters, but I suspect I'm
losing something in the connection.
Anyway, follow the examples I listed that are known to work. If you
wanna be creative, at least build one that works so you can compare
performance.
Jeff Liebermann
>Here's one; hope it is clickable as the URL is split weirdly as I construct
>the message:
>http://features.engadget.com/2005/11/15/how-to-build-a-wifi-biquad-dish-antenna/
That's one of the examples of how *NOT* to build a biquad. Same
problem I mentioned in my rant. The coaxial cable needs to extend all
the way from the connector to the quad elements. I don't see any easy
way to salvage the design and recommend you consider one of the others
that do it correctly.
Incidentally, you might want to look into a sector antenna.
http://www.qsl.net/yu1aw/vhf_ant.htm (very slow loading)
http://www.brest-wireless.net/wiki/materiel:amos
http://pe2er.nl/wifisector/
More gain, wider coverage area, and rather easy to build.
mi...@sushi.com
http://www.qsl.net/yu1aw/biqaudfeeding.gif
shows the coax feed.
They have a variant I haven't seen before, which using a director with
the biquad:
http://www.qsl.net/yu1aw/2elbiqf5.JPG
Refering page:
http://www.qsl.net/yu1aw/vhf_ant.htm
Arthur Shapiro
<je...@comix.santa-cruz.ca.us> wrote:
>Ummm.... you missed my point. The coax cable needs to extend all the
>way to the antenna. If you mount the coax connector on the reflector,
>you will still need to attach a short piece of coax cable or simulated
>coax cable made out of tubing, from the connector to the antenna.
>That's not easy.
Ahah...I think you're starting to beat some sense into me- thanks!
In my case, I've already purchased the appropriate connector to N pigtail for
my particular card, so I'd really like to use the bulkhead-mount N jack that I
purchased (25 cents? Hell, it was five bucks, but at least is silverplated.)
So mounting on the reflector is logical, and then apparently I'll use a little
stub of some coax soldered to that jack to support the biquad appropriately
off the plane of the reflector. If I've still missed the boat, hit me again.
And to just dwell on one of your points: half-inch copper pipe with a wire
suspended inside is NOT a coaxial equivalent - correct? That's a shame, as it
fits nicely around the business end of the N jack.
I don't seem to have any copper-clad pc boards in the scrap box right now, so
it's time to find either that or some sort of craft store that might carry
copper sheeting.
Art
Jeff Liebermann
>Ahah...I think you're starting to beat some sense into me- thanks!
We try harder.
>In my case, I've already purchased the appropriate connector to N pigtail for
>my particular card, so I'd really like to use the bulkhead-mount N jack that I
>purchased (25 cents? Hell, it was five bucks, but at least is silverplated.)
Sigh. Well, Mr Miso(?) found a good construction article that will
make good use of your N bulkhead connector:
http://www.qsl.net/yu1aw/biqaudfeeding.gif
Please don't duplicate the sloppy screw mounting of the N connector
flanges. I think that's just to illustrate how it goes together.
Otherwise, it's an excellent construction method.
>So mounting on the reflector is logical, and then apparently I'll use a little
>stub of some coax soldered to that jack to support the biquad appropriately
>off the plane of the reflector.
If you must do it with a piece of coax, then make sure that the center
conductor can slide up and down. Extend the center conductor from the
shield. Solder the coax center conductor to the connector center
conductor as close to the insulation on the coax as possible. Then
slide the shield over the N connector and solder it to the connector
or reflector all the way around the connector. Unless you are VERY
good at soldering, you're going to have problems doing this. (At
least I did when I tried it and I'm really good at slobbering, err...
soldering).
Personally, methinks assembling the coax section out of copper tubing
and wire makes more sense. See the URL above.
>If I've still missed the boat, hit me again.
Naw, you're close but are trying to make things difficult. Methinks
the problem is that you don't seem to realize how critical cable
connections can be at 2.4GHz. Even the smallest amount of exposed
center conductor will cause an impedance mismatch. Fortunately, it's
not too horribly critical in the connector extension (as evident from
my use of 75 ohm coax) as long as it doesn't radiate. That's where I
think you missed my previous remarks. Using two wires in place of the
coax feed are a problem because the wires will radiate. If I have
time, I'll do an NEC2 model of this wrong way to build a biquad and
demonstrate the problem.
>And to just dwell on one of your points: half-inch copper pipe with a wire
>suspended inside is NOT a coaxial equivalent - correct? That's a shame, as it
>fits nicely around the business end of the N jack.
It most certainly is a coax cable. If you look at the biquad URL's I
listed as being properly constructed, some use coax cable simulation
out of copper pipe.
The trick is to get the impedance close to 50 ohms. For air
dielectric.
Z = 138 log b/a
Z = impedance in ohms
b = inside diamter of outer tube
a = outside diameter of inner conductor
For 50 ohms, b/a is about 2.3.
Trick. If you need support for the center conductor, I've used
styrofoam peanuts from packing material held in place with hot melt
glue.
>I don't seem to have any copper-clad pc boards in the scrap box right now, so
>it's time to find either that or some sort of craft store that might carry
>copper sheeting.
Try copper roofing flashing from the local hardware store. The stuff
on rolls is fairly cheap.
Sheet aluminum also works, but you might need to do some glue and goo
instead of soldering to the reflector. A sloppy way is to take a
large copper or brass washer and drill 4 holes that align with the 4
holes in the N connector. Stuff the coax braid between the reflector
and the washer and tighten the screws. Instant compression joint.
mi...@sushi.com
I'd going to make another biquad as Jeff really believes the coax is a
must. However, I would go for the copper pipe as it will make a better
base to held to the biquad driven element. I use a pipe cutter, but to
make the tilt on the end, I suppose a hacksaw will be required.
I don't know if this is a good idea, but where you solder the N
connector to the copper, you might drill some hole to act as thermal
relief, i.e. make the copper a less efficient heatsink. I would make
them small as all you are trying to do is break the thermal path.
Again, tin the copper first.
I also want to protect the design from getting banged around. I'm
assuming a fiberglass radome won't be a problem.
Jeff Liebermann
>Even the smallest amount of exposed
>center conductor will cause an impedance mismatch. Fortunately, it's
>not too horribly critical in the connector extension (as evident from
>my use of 75 ohm coax) as long as it doesn't radiate. That's where I
>think you missed my previous remarks. Using two wires in place of the
>coax feed are a problem because the wires will radiate. If I have
>time, I'll do an NEC2 model of this wrong way to build a biquad and
>demonstrate the problem.
Sigh. Well, the above is wrong. I hate it when that happens.
I built a model of the screwed up biquad antenna construction method
with 4NEC2. I expected to find lousy gain, a weird pattern, and sky
high VSWR. What I got instead is normal gain of about 10.5dBi, a
normal antenna pattern, but a sky high VWSR (6.7:1). See:
http://802.11junk.com/jeffl/antennas/biquad-junk/index.html
If you want to play with the NEC2 model, see:
http://802.11junk.com/jeffl/antennas/biquad-junk/biquad-annotated-02.nec
It's still a lousy way too build a biquad due to the very high VSWR,
but it's not as lousy as I originally predicted.
Grumble, grumble...
mi...@sushi.com
No ham would put up with more than maybe 3 on VSWR, so I'm going to go
with the copper pipe. It is also better mechanically.Those wifi cards
get pretty toasty as it is, so the lower the VSWR the better.
Is the director element worth the effort? Those guys are artists when
it comes to bending and solder wire. My results are not nearly as
clean. I can't see how they soldered the director so cleanly without
any mechanical support to hold the wire in place. The usual practice
when you solder is to twist the wires first but clearly that won't do
in this kind of antenna construction.
Jeff Liebermann
>No ham would put up with more than maybe 3 on VSWR,
Thank you for not suggesting an antenna tuner. As for tolerating
VSWR, well my junk HF antenna is really awful should be replaced. I
tolerate high VSWR, but just barely. Unfortunately, my ancient HF rig
doesn't.
If you could make the two wires to be 50 ohms instead of whatever they
turn out with 0.200 air spacing, the antenna could be build without a
coax section. It would be tempting to try a piece of G10/FR4 circuit
board, with a 0.1" wide trace for 0.062 board, with everything else
grounded and copper tape over the board edge. It's stiff enough to
support the biquad. Hmmmm....
>so I'm going to go
>with the copper pipe. It is also better mechanically.Those wifi cards
>get pretty toasty as it is, so the lower the VSWR the better.
The radios made to tolerate infinite VSWR at the antenna if they have
an external RF connector. I'm not so sure about PCMCIA or USB cards
that have had a connector or pigtail added. 50mw isn't going to blow
up or overheat anything. However, the higher power boards are another
story. Dunno. I've never blown up a board and seen no reported radio
failures resulting from antenna experiments. My guess is that some
home made antennas have fairly awful VWSR. There would a flood of "it
blew up my wireless router" postings if there was any possibility of
failure.
>Is the director element worth the effort?
I don't know. I'll have to model it. I don't think it will work with
a wire support. I think it should be insulated. However, after my
last bad guess, methinks it best to wait for me to run a model. the
author YU1AW obviously ran NEC2 models through 4NEC2 to get the plots
on his web pages, but apparently didn't do the same for this antenna.
Methinks it might be an experiment that needs work. Dunno.
>Those guys are artists when
>it comes to bending and solder wire. My results are not nearly as
>clean. I can't see how they soldered the director so cleanly without
>any mechanical support to hold the wire in place. The usual practice
>when you solder is to twist the wires first but clearly that won't do
>in this kind of antenna construction.
Did you see my calculations as to how critical the cut lenths are?
At 2.4GHz, the difference in length per MHz is:
3*10^10cm/sec / 2400.0 MHz = 12.5 cm
3*10^10cm/sec / 2483.5 MHz = 12.1 cm
(12.5cm - 12.1cm) / 83.5Mhz = 0.05mm per Mhz
If you're 0.05mm off in your cut lengths, you're 1MHz off frequency.
Miss by +/- 2mm and you're outside the 2.4Ghz band. Either gets some
decent test equipment so you can do cut-n-try, or be VERY precise in
your cutting and soldering. However, that's only if you want
perfection. The gain and vswr curves for a properly built biquad are
very flat, which means you can be fairly sloppy and still have
something that works.
From carpentry:
Measure twice. Cut once.
Incidentally, I'm borrowing an old Wiltron network analyzer, which
should allow me to post pretty pictures and accurate construction
measurement. One catch. It's broken and I have to fix it first.
mi...@sushi.com
Two things come to mind regarding the measurement accuracy. One, since
it is the length of the loop that matters, maybe we shouldn't be so
concerned about getting the wire to fit the pattern as much as just
measuring the right length of wire, then just bending it.
The other thing that comes to mind is maybe a helix is the way to go.
At least it is very forgiving in accuracy, i.e. it is a wide bandwidth
design. The gain isn't all that high on a volumetric basis, but if you
can't build the biquad on target, you don't achieve the theoretical
gain anyway.
I've noticed the satcom guys use a 4 turn helix and a dish. Now of
course in satcom, you need circular polarization.
Jeff Liebermann
>Two things come to mind regarding the measurement accuracy. One, since
>it is the length of the loop that matters, maybe we shouldn't be so
>concerned about getting the wire to fit the pattern as much as just
>measuring the right length of wire, then just bending it.
Exactly. It's the end to end length that's important, not how well
you simulate a square (quad) element. I sometime make diquads using a
circular loop. Works fine.
>The other thing that comes to mind is maybe a helix is the way to go.
>At least it is very forgiving in accuracy, i.e. it is a wide bandwidth
>design. The gain isn't all that high on a volumetric basis, but if you
>can't build the biquad on target, you don't achieve the theoretical
>gain anyway.
Well, you lose -3dB from the mismatch between circular and linear
polarization. That's a small price to pay for insensitivity to
polarization differences. For a point to point link, two helicals are
insensitive to single bounce reflections. They tend to be very
broadband (if properly matched).
http://www.wireless.org.au/~jhecker/helix/
However, all is not wonderful in helixland. They're difficult to
build. They're difficult to match to 50 ohms. They tend to become
huge for high gains. (You should see the monster pair that I have).
If wound on cheap PVC, the PVC intruduces some loss.
>I've noticed the satcom guys use a 4 turn helix and a dish. Now of
>course in satcom, you need circular polarization.
Some of the SETI and ham satellite nerds also use circular polarized
patch antennas and dual mode cans:
http://www.qsl.net/va3rr/dualmode/dual_mode.htm
What they're trying to accomplish with a helical dish feed is match
feeds pattern to the f/D ratio of the dish. See:
http://www.qsl.net/n1bwt/contents.htm
especially the section on dish feeds in section 6. Sorry, there's no
sub-section for helical feeds yet. The basic idea is to have the -3dB
beamwidth of the antenna hit exactly the edges of the dish. Half the
power stays in the dish, the other half goes off the edge. (That's
part of the 50% typical dish efficiency). The helical is nice because
it's really easy to adjust the beamwidth by simply adding or
subtracting turns.
--
# Jeff Liebermann 150 Felker St #D Santa Cruz CA 95060
# 831-336-2558 je...@comix.santa-cruz.ca.us
# http://802.11junk.com je...@cruzio.com
# http://www.LearnByDestroying.com AE6KS
mi...@sushi.com
Jeff Liebermann wrote:
> On 17 Jul 2006 12:08:48 -0700, mi...@sushi.com wrote:
>
> >Two things come to mind regarding the measurement accuracy. One, since
> >it is the length of the loop that matters, maybe we shouldn't be so
> >concerned about getting the wire to fit the pattern as much as just
> >measuring the right length of wire, then just bending it.
>
> Exactly. It's the end to end length that's important, not how well
> you simulate a square (quad) element. I sometime make diquads using a
> circular loop. Works fine.
I may give that a shot. BTW, the biquad doesn't seem to be in the ARRL
antenna book, or at least in my 16th edition.
>
> >The other thing that comes to mind is maybe a helix is the way to go.
> >At least it is very forgiving in accuracy, i.e. it is a wide bandwidth
> >design. The gain isn't all that high on a volumetric basis, but if you
> >can't build the biquad on target, you don't achieve the theoretical
> >gain anyway.
>
> Well, you lose -3dB from the mismatch between circular and linear
> polarization. That's a small price to pay for insensitivity to
> polarization differences. For a point to point link, two helicals are
> insensitive to single bounce reflections. They tend to be very
> broadband (if properly matched).
> http://www.wireless.org.au/~jhecker/helix/
>
> However, all is not wonderful in helixland. They're difficult to
> build. They're difficult to match to 50 ohms. They tend to become
> huge for high gains. (You should see the monster pair that I have).
> If wound on cheap PVC, the PVC intruduces some loss.
I've been making fiberglass tubes for lightweight containers if
fiberglass is any better than pvc. It is certainly lighter. Not cheap
though. If anyone is curious, I can write it up as there are quite a
few tricks to construct composites in your garage.
>
> >I've noticed the satcom guys use a 4 turn helix and a dish. Now of
> >course in satcom, you need circular polarization.
>
> Some of the SETI and ham satellite nerds also use circular polarized
> patch antennas and dual mode cans:
> http://www.qsl.net/va3rr/dualmode/dual_mode.htm
>
> What they're trying to accomplish with a helical dish feed is match
> feeds pattern to the f/D ratio of the dish. See:
> http://www.qsl.net/n1bwt/contents.htm
> especially the section on dish feeds in section 6. Sorry, there's no
> sub-section for helical feeds yet. The basic idea is to have the -3dB
> beamwidth of the antenna hit exactly the edges of the dish. Half the
> power stays in the dish, the other half goes off the edge. (That's
> part of the 50% typical dish efficiency). The helical is nice because
> it's really easy to adjust the beamwidth by simply adding or
> subtracting turns.
Does the dish reverse the polarization?
Jeff Liebermann
>I may give that a shot. BTW, the biquad doesn't seem to be in the ARRL
>antenna book, or at least in my 16th edition.
There are some that would consider that to be a compliment.
The way a biquad works is quite easy. You start with a single full
wave loop antenna. End to end impendance is 100 to 120 ohms. Put two
loops in parallel, and you get about 50 ohms. The quads are fed
symmetrically (in phase) with a half wave seperation, so there's no
cancellation. Add a reflector, and you get some gain.
>I've been making fiberglass tubes for lightweight containers if
>fiberglass is any better than pvc. It is certainly lighter. Not cheap
>though. If anyone is curious, I can write it up as there are quite a
>few tricks to construct composites in your garage.
I used to buy the fiberglass scrap from the local sailboard and
sailing shops. Fiberglass is way better than PVC. The acid test is
to put the material in the microwave oven and see if it gets hot (or
melts). There are many different types of PVC with different
characteristics. The white stuff stays fairly cool. The grey stuff
gets too warm. The black stuff melts.
Incidentally, an easy test is to take your access point and slide a
piece of PVC pipe over the antennas. If the material is truely RF
transparent, there should be no change.
>Does the dish reverse the polarization?
Yes. Incidentally, the correct term is that it reverses "sense" as in
RHCP and LHCP. Each surface reflection changes the polarization
sense.
stephen
news:g42qb2lefa0eo9oi6...@4ax.com...
> mi...@sushi.com hath wroth:
>
> >I may give that a shot. BTW, the biquad doesn't seem to be in the ARRL
> >antenna book, or at least in my 16th edition.
>
> There are some that would consider that to be a compliment.
> The way a biquad works is quite easy. You start with a single full
> wave loop antenna. End to end impendance is 100 to 120 ohms. Put two
> loops in parallel, and you get about 50 ohms. The quads are fed
> symmetrically (in phase) with a half wave seperation, so there's no
> cancellation. Add a reflector, and you get some gain.
>
> >I've been making fiberglass tubes for lightweight containers if
> >fiberglass is any better than pvc. It is certainly lighter. Not cheap
> >though. If anyone is curious, I can write it up as there are quite a
> >few tricks to construct composites in your garage.
>
> I used to buy the fiberglass scrap from the local sailboard and
> sailing shops. Fiberglass is way better than PVC. The acid test is
> to put the material in the microwave oven and see if it gets hot (or
> melts). There are many different types of PVC with different
> characteristics. The white stuff stays fairly cool. The grey stuff
> gets too warm. The black stuff melts.
graphite content added to make it less UV degradeable?
>
> Incidentally, an easy test is to take your access point and slide a
> piece of PVC pipe over the antennas. If the material is truely RF
> transparent, there should be no change.
>
> >Does the dish reverse the polarization?
>
> Yes. Incidentally, the correct term is that it reverses "sense" as in
> RHCP and LHCP. Each surface reflection changes the polarization
> sense.
>
> --
> Jeff Liebermann je...@comix.santa-cruz.ca.us
> 150 Felker St #D http://www.LearnByDestroying.com
> Santa Cruz CA 95060 http://802.11junk.com
> Skype: JeffLiebermann AE6KS 831-336-2558
--
Regards
stephe...@xyzworld.com - replace xyz with ntl
Jeff Liebermann
>> The acid test is
>> to put the material in the microwave oven and see if it gets hot (or
>> melts). There are many different types of PVC with different
>> characteristics. The white stuff stays fairly cool. The grey stuff
>> gets too warm. The black stuff melts.
>graphite content added to make it less UV degradeable?
Nope. Lots of assorted additives and stabilizers for PVC and vinyl.
Mostly heavy metal heat stabilizers in PVC. The white PVC pipe
contains a little copper. The problem is that the exact formulation
varies somewhat by manufacturer so testing is required.
| http://www.solvinpvc.com/solvinservices/library/generalinformation/pvcadditives/0,998,3349-2-0,00.htm
| http://www.innovations-report.com/html/reports/materials_science/report-32741.html
| http://www.polymeradditives.cn/products.htm
You can also use vinyl (a form of "soft" PVC) for radomes but they
have the same additives as PVC pipe. I've had good luck with white
vinyl rain gutters and surface conduit.
Luddite
>>I may give that a shot. BTW, the biquad doesn't seem to be in the ARRL
>>antenna book, or at least in my 16th edition.
> There are some that would consider that to be a compliment.
> The way a biquad works is quite easy. You start with a single full
> wave loop antenna. End to end impendance is 100 to 120 ohms. Put two
> loops in parallel, and you get about 50 ohms. The quads are fed
> symmetrically (in phase) with a half wave seperation, so there's no
> cancellation. Add a reflector, and you get some gain.
>>I've been making fiberglass tubes for lightweight containers if
>>fiberglass is any better than pvc. It is certainly lighter. Not cheap
>>though. If anyone is curious, I can write it up as there are quite a
>>few tricks to construct composites in your garage.
> I used to buy the fiberglass scrap from the local sailboard and
> sailing shops. Fiberglass is way better than PVC. The acid test is
> to put the material in the microwave oven and see if it gets hot (or
> melts). There are many different types of PVC with different
> characteristics. The white stuff stays fairly cool. The grey stuff
> gets too warm. The black stuff melts.
> Incidentally, an easy test is to take your access point and slide a
> piece of PVC pipe over the antennas. If the material is truely RF
> transparent, there should be no change.
dissapation aside won't the PVC itself detune the antenna?
Jeff Liebermann
wrote:
>dissapation aside won't the PVC itself detune the antenna?
Oh yes. Shoving an antenna into a PVC pipe or tube[1] will cause the
resonant frequency of any tuned wire lengths to drop in frequency
somewhat. The good news is that it's a function of the square of the
distance between the wire and the PVC. Farther away means a smaller
shift. If the PVC pipe is fairly large compared to the antenna, the
effect will be minimal. If I can ever get my borrowed Wiltron network
analyzer to play, I should be able to both demonstrate and measure the
effect. (Yet another project).
[1] Pipe is measured by the I.D. (inside diameter). Tubing is
measured by the O.D. (outside diameter).
mi...@sushi.com
Jeff Liebermann wrote:
> mi...@sushi.com hath wroth:
>
> >I may give that a shot. BTW, the biquad doesn't seem to be in the ARRL
> >antenna book, or at least in my 16th edition.
>
> There are some that would consider that to be a compliment.
> The way a biquad works is quite easy. You start with a single full
> wave loop antenna. End to end impendance is 100 to 120 ohms. Put two
> loops in parallel, and you get about 50 ohms. The quads are fed
> symmetrically (in phase) with a half wave seperation, so there's no
> cancellation. Add a reflector, and you get some gain.
>
> >I've been making fiberglass tubes for lightweight containers if
> >fiberglass is any better than pvc. It is certainly lighter. Not cheap
> >though. If anyone is curious, I can write it up as there are quite a
> >few tricks to construct composites in your garage.
>
> I used to buy the fiberglass scrap from the local sailboard and
> sailing shops. Fiberglass is way better than PVC. The acid test is
> to put the material in the microwave oven and see if it gets hot (or
> melts). There are many different types of PVC with different
> characteristics. The white stuff stays fairly cool. The grey stuff
> gets too warm. The black stuff melts.
The fiberglass I build is basically like what is used on sailboats, but
I cure mine sandwiched in mylar. This restricts oxygen, which for some
reason makes the resin get rock hard. It's a technique used by amateur
rocket makers. Obviously, I'd skip the carbon fiber.
One advantage to building your own tubes is you can adjust the
diameter.
mi...@sushi.com
Jeff Liebermann wrote:
> mi...@sushi.com hath wroth:
>
> >I may give that a shot. BTW, the biquad doesn't seem to be in the ARRL
> >antenna book, or at least in my 16th edition.
>
> There are some that would consider that to be a compliment.
> The way a biquad works is quite easy. You start with a single full
> wave loop antenna. End to end impendance is 100 to 120 ohms. Put two
> loops in parallel, and you get about 50 ohms. The quads are fed
> symmetrically (in phase) with a half wave seperation, so there's no
> cancellation. Add a reflector, and you get some gain.
Shouldn't the full wave loop have a balanced feed?
Jeff Liebermann
>Shouldn't the full wave loop have a balanced feed?
I think you're really asking why there is no balun. The loop is a
balanced antenna, but the coax is not. It's just like a dipole. If
you don't want the coax to radiate, it needs a (sleeve) balun.
http://www.vhfman.freeuk.com/radio/sleeve.html
mi...@sushi.com
Driving a balanced antenna from an unbalanced feed changes the
directional pattern of the antenna. Does your software model this?
Jeff Liebermann
>Driving a balanced antenna from an unbalanced feed changes the
>directional pattern of the antenna. Does your software model this?
No. If you look at the biquad model, the coax cable feed is not
shown. The generator is right at the connection point between the two
quad elements.
http://802.11junk.com/jeffl/antennas/Biquad/index.html
The idea is the balun keeps the coax cable from radiating. I don't
know how much an effect a balun will have on the pattern. Most of the
time, it just shifts the main lobe to one side causing a "boresight"
error. Considering that the biquad beamwidth is fairly wide (60
degrees), methinks the effect is probably small. I'll add it to the
model (later) and see what happens.
Edmund
> On 19 Jul 2006 13:08:00 -0700, mi...@sushi.com wrote:
>
> >Shouldn't the full wave loop have a balanced feed?
>
> I think you're really asking why there is no balun. The loop is a
> balanced antenna, but the coax is not. It's just like a dipole. If
> you don't want the coax to radiate, it needs a (sleeve) balun.
> http://www.vhfman.freeuk.com/radio/sleeve.html
Hi, I was lurking here for some time and after reading all this
in addition to what I already found about biquads, I decided to
make one myself. I really don't know what I am doing :-) and I
wanted to make the biquad for mounting in a parabolic disk, so
I made an 40 mm round mounting pipe on the back.
Which fit exactly in a normal LNB holder.
Yesterday I bought a cable ( 50 Ohm ) and an SMD connector which
fit in my PCI wireless card. The other end I soldered directly
on the antenna. I decided to test without a parabol and guess
what? I see about 15 networks and after some pointing a can
connect and have a good and rather fast internet connection.
I have no idea where this router is and no idea about any gain
I have with my antenna but the standard antenna sees 0 networks.
Bud of course this is indoors and the biquad is outside, with
about 3 meter cable.
I am not sure if it is alright what I have done bud maybe it
is of any use for someone else. I took a 8 mm bold and drilled
a hole through it of 4.5 mm. I used this to support my biquad
and the stripped cable connects OK in the 4.5 mm hole.
I soldered one side of the biquad on top op this bold, and after
the cable was pushed trough a only soldered to center of the cable
to the other end of the biquad. Really easy.
For the moment I use some 40 mm PVC fittings to support and
aim my biquad, which is easy too since I have a 40 mm PVC pipe
fitted on the back of the biquad already :-)
I am a little surprised It went all so smooth.
Jeff Liebermann
>Hi, I was lurking here for some time and after reading all this
>in addition to what I already found about biquads, I decided to
>make one myself. I really don't know what I am doing :-) and I
>wanted to make the biquad for mounting in a parabolic disk, so
>I made an 40 mm round mounting pipe on the back.
Congratulations. Which plans did you use?
This one works nicely:
| http://www.weijand.nl/wifi/
This one has the problem with the lack of a coaxial feed that I was
ranting about:
| http://www.engadget.com/2005/11/15/how-to-build-a-wifi-biquad-dish-antenna/
>I see about 15 networks and after some pointing a can
>connect and have a good and rather fast internet connection.
Well, using Netstumbler or Kismet to measure the signal strength
difference will tell you the gain. The typical biquad has between
8-10dBi. It should be about 6-8dB more signal than with the stock
rubber ducky or whatever antenna.
>I have no idea where this router is and no idea about any gain
>I have with my antenna but the standard antenna sees 0 networks.
Ok, that qualifies as an improvement.
>Bud of course this is indoors and the biquad is outside, with
>about 3 meter cable.
Hmmm... so much for the improvement. Have you tried your radio
outside with the stock antenna for comparison?
>I am a little surprised It went all so smooth.
Oh-oh. Nothing is that easy. Photos?
Edmund
> Edmund <nom...@hotmail.com> hath wroth:
>
> >Hi, I was lurking here for some time and after reading all this
> >in addition to what I already found about biquads, I decided to
> >make one myself. I really don't know what I am doing :-) and I
> >wanted to make the biquad for mounting in a parabolic disk, so
> >I made an 40 mm round mounting pipe on the back.
>
> Congratulations. Which plans did you use?
Well I didn't exactly copy any plan, I just picked what I
needed and came up with some idea's of my own.
I did look good to travormarshall.com and my reflector also
has 30mm lips and is 110 X 123 mm.
>
> This one works nicely:
> | http://www.weijand.nl/wifi/
>
> This one has the problem with the lack of a coaxial feed that I was
> ranting about:
> |
> http://www.engadget.com/2005/11/15/how-to-build-a-wifi-biquad-dish-antenna/
>
> >I see about 15 networks and after some pointing a can
> >connect and have a good and rather fast internet connection.
>
> Well, using Netstumbler or Kismet to measure the signal strength
> difference will tell you the gain. The typical biquad has between
> 8-10dBi. It should be about 6-8dB more signal than with the stock
> rubber ducky or whatever antenna.
>
> >I have no idea where this router is and no idea about any gain
> >I have with my antenna but the standard antenna sees 0 networks.
>
> Ok, that qualifies as an improvement.
>
> >Bud of course this is indoors and the biquad is outside, with
> >about 3 meter cable.
>
> Hmmm... so much for the improvement. Have you tried your radio
> outside with the stock antenna for comparison?
Not possible, it is a bigtower there is no space behind
the computer for the biquad either. In fact, that was the
first reason I needed an external antenna.
What I can do is connecting the biquad inside and see what
network it sees inside, I don't expect much however.
>
> >I am a little surprised It went all so smooth.
>
> Oh-oh. Nothing is that easy. Photos?
I just took a few photo's, they show some nice detail,
maybe you can give some comment about it.
I don't have a home page for this stuff, shall I mail
it to you? Feel free to put it on line if it of use for
anybody.
John Navas
<e9onl1$dd1$1...@azure.qinip.net>:
>I just took a few photo's, they show some nice detail,
>maybe you can give some comment about it.
>I don't have a home page for this stuff, shall I mail
>it to you? Feel free to put it on line if it of use for
>anybody.
Use <http://tinypic.com/>
--
Best regards, FAQ for Wireless Internet: <http://Wireless.wikia.com>
John Navas FAQ for Wi-Fi: <http://wireless.wikia.com/wiki/Wi-Fi>
Wi-Fi How To: <http://wireless.wikia.com/wiki/Wi-Fi_How_To>
Fixes to Wi-Fi Problems: <http://wireless.wikia.com/wiki/Wi-Fi_Fixes>
mi...@sushi.com
Maybe one of those "coax" baluns would do the trick. The most harm it
could do is the loss in the cable.
It's funny that the gain direction of the short loop is opposite (more
accurately perpendicular) to the full wave loop. I've done HF DXing
with a short loop, and the null is amazing. I have need for DFing some
UHF signals, so I'm going to try a full wave loop.
mi...@sushi.com
One of the things to keep in mind is most of these home made antennas
have larger aperture that stock antennas. So even if you don't achieve
the ideal gain, just that fact the "sponge" is larger should deliver a
stronger signal to the wifi card.
Jeff Liebermann
>It's funny that the gain direction of the short loop is opposite (more
>accurately perpendicular) to the full wave loop. I've done HF DXing
>with a short loop, and the null is amazing. I have need for DFing some
>UHF signals, so I'm going to try a full wave loop.
My favorite VHF/UHF DF antenna is any pair of omni antennas, seperated
by 1/4 wavelength, and connected to gether with a 1/2 wave electrical
coax cable and a T connector. The phase of the direct signal to the
antenna with the T connector is exactly 180 degrees phase shift from
the other antenna. That results in cancellation and a really deep
null. One catch is that the notch depth is really frequency
sensitive.
Jeff Liebermann
>> Congratulations. Which plans did you use?
>
>Well I didn't exactly copy any plan, I just picked what I
>needed and came up with some idea's of my own.
>I did look good to travormarshall.com and my reflector also
>has 30mm lips and is 110 X 123 mm.
The original Trevor Marshall photos do not show much detail on how the
feed is built. Look at some of the others posted to get an idea of
what's necesary.
>I just took a few photo's, they show some nice detail,
>maybe you can give some comment about it.
>I don't have a home page for this stuff, shall I mail
>it to you? Feel free to put it on line if it of use for
>anybody.
Sure. Email to je...@cruzio.com. I can post it on my web pile if it
looks interesting.
Jeff Liebermann
>One of the things to keep in mind is most of these home made antennas
>have larger aperture that stock antennas. So even if you don't achieve
>the ideal gain, just that fact the "sponge" is larger should deliver a
>stronger signal to the wifi card.
Yep, but not quite a sponge. The reason that many home made antennas
appear to work is that almost any type of larger antenna is an
improvement over the stock rubber ducky antennas. Even the most
disgustingly mis-cut antenna will have some gain that is greater than
the stock antennas 2dBi. Even when built correctly, the typical
coffee can or biquad are sufficiently broadband that errors in cut
lengths will not have a huge effect on the gain.
What I do is compare the antenna with a known gain reference antenna.
I can't measure the actual gain directly, but I can measure the gain
relative to my standard.
mi...@sushi.com
I agree the gain is higher, but I don't think you can ignore the
aperture either. If the field is uV/meter and your antenna is larger
(i.e. sits in more of the field), you will "soak" in more signal. Hence
my reference to the sponge.
The aperture effect is more evident as you compare the same antenna
design (i.e. same gain) over different frequencies. If the antenna
designs are the same, the lower frequency antenna has a stronger signal
since it has more aperture. [Obviously, these tests are done at the
appropriate frequencies.]
Edmund
> On Thu, 20 Jul 2006 22:04:07 +0000, Edmund <nom...@hotmail.com> wrote:
>
> >> Congratulations. Which plans did you use?
> >
> >Well I didn't exactly copy any plan, I just picked what I
> >needed and came up with some idea's of my own.
> >I did look good to travormarshall.com and my reflector also
> >has 30mm lips and is 110 X 123 mm.
>
> The original Trevor Marshall photos do not show much detail on how the
> feed is built. Look at some of the others posted to get an idea of
> what's necesary.
I did look around and used about the same method as here:
http://www.qsl.net/yu1aw/biqaudfeeding.gif
(the right hand side) with the difference I used a steel wire end
in which I drilled a 4.5 mm hole, instead of the copper pipe and
used bolds on both sides of the reflector. On the back side of
the reflector I fixed an PVC endcap ( Dont' know if that is the
enlish word for it ) with the same wire end.
I clued a small piece of PVC tube in the end cap which make a perfect
fit for the normal LNB holder of a parabolic dish.
As you will see on the photo I shortened this end cap to have
a little more slack for positioning the bequad in the dish
mount.
>
> >I just took a few photo's, they show some nice detail,
> >maybe you can give some comment about it.
> >I don't have a home page for this stuff, shall I mail
> >it to you? Feel free to put it on line if it of use for
> >anybody.
>
> Sure. Email to je...@cruzio.com. I can post it on my web pile if it
> looks interesting.
Maybe you can show it anyway, as how it is not to be done :-)
do...@xrexxqxxbi.usenet.us.com
> My favorite VHF/UHF DF antenna is any pair of omni antennas, seperated
> by 1/4 wavelength, and connected to gether with a 1/2 wave electrical
> coax cable and a T connector.
Is that a pair of "trucker" CB antennas, mounted on the rearview mirrors of
the cab? As I recall, that's pretty directional, right down the highway.
I caught and passed a guy who had the antennas properly spaced (he said)
but in line, at the front and rear of the roof of his van.
He had great range to the sides of the highway, notsomuch down the road.
The proper length of the interconnect is... 3/4 wave, which is 1/2 wave of
physical cable? Or did I say that backwards ;-)
--
---
Clarence A Dold - Hidden Valley Lake, CA, USA GPS: 38.8,-122.5
Jeff Liebermann
do...@XReXXQXXBi.usenet.us.com wrote:
>Jeff Liebermann <je...@comix.santa-cruz.ca.us> wrote:
>> My favorite VHF/UHF DF antenna is any pair of omni antennas, seperated
>> by 1/4 wavelength, and connected to gether with a 1/2 wave electrical
>> coax cable and a T connector.
>Is that a pair of "trucker" CB antennas, mounted on the rearview mirrors of
>the cab? As I recall, that's pretty directional, right down the highway.
Heh-heh. That's very similar but not the same. What you're thinking
of are co-phased" CB antennas. There's a very good reason for having
two antennas on a big semi-truck trailer or tractor. One can't mount
the antenna high enough to not hit the bridges. But if the antenna is
shortened (with a loading coil) and is mounted on one side, the truck
blocks the signal on the other side. So, installing two antennas, one
on each side with a "co-phaseing" harness was a good solution. The
two antenna system works better than one antenna, but not by much.
The harness is suppose to be an odd multiple of 1/4 wavelength 75 ohm
lines, which roughly form a power splitter. In theory, a station
directly ahead should have the signal arrive at both antennas at the
same time where the signals add together forming a slight peak. To
the sides, it also works nicely because only one antenna will hear the
signal and therefore there can be no peaks and nulls.
That works great for big tractors and trailers, but really falls apart
on passenger cars. I've seen an amazing array of "co-phasing"
harnesses all claiming to have more gain than others. Depending on
the length of the cables, it is possible to put a null either dead
ahead or to the sides. The right lengths are about 70" on each side
of a T connector that goes to the radio. That's a bit long for most
passenger car trunks, so the creativity begins.
>I caught and passed a guy who had the antennas properly spaced (he said)
>but in line, at the front and rear of the roof of his van.
>He had great range to the sides of the highway, notsomuch down the road.
Yuck. Actually, the spacing isn't terribly critical, but the coax
cable lengths make a big difference. If he had a null in front and
back, then he really goofed somewhere.
>The proper length of the interconnect is... 3/4 wave, which is 1/2 wave of
>physical cable? Or did I say that backwards ;-)
http://www.signalengineering.com/ultimate/co_phasing.html
See figure 8.
So much for CB antennas.
The scheme I use looks like this:
| |
Ant #1 | | Ant #2
| |
| | Antennas are spaced
| | 1/4 wavelength apart
| | and are identical
radio =======|==========|
^ ^ ^
| | |
any length | |
coax here | 3/4 wave electrical
| coax length
T Connector
The way this works is that it creates a cardoid pattern with a big
deep null toward Ant #1. A signal coming from the left appears at the
T connector. The same signal arrives at Ant #2 1/4 wavelength later.
This signal is delayed through a 3/4 wavelength coax cable resulting
in a 1/2 wave phase shift (180 degrees). When this hits the T
connector, it cancels with the original signal from Ant #1 and creates
a deep null. Both antennas have to be identical for this to work.
It's also a very narrow band system as any change in frequency
requires a change in antenna spacing and coax cable length.