Q: Biquad Antennas

I've decided to build one of these guys, and have been looking at the several
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

Arthur Shapiro wrote:
> I've decided to build one of these guys, and have been looking at the several
> 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

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.

arthur-temp-(E-Mail Removed) (Arthur Shapiro) hath wroth:

>I've decided to build one of these guys, and have been looking at the several
>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?

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/oth...uad/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/h...-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...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/calc...esnel-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 (E-Mail Removed)
150 Felker St #D http://www.LearnByDestroying.com
Santa Cruz CA 95060 http://802.11junk.com
Skype: JeffLiebermann AE6KS 831-336-2558

In article <(E-Mail Removed) .com>, (E-Mail Removed) wrote:

>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

In article <(E-Mail Removed)>, Jeff Liebermann <(E-Mail Removed)> wrote:

>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

Arthur Shapiro wrote:
> In article <(E-Mail Removed) .com>, (E-Mail Removed) wrote:
>
> >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

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.

Arthur Shapiro wrote:
> In article <(E-Mail Removed)>, Jeff Liebermann <(E-Mail Removed)> wrote:
>
> >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

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.

arthur-temp-(E-Mail Removed) (Arthur Shapiro) hath wroth:

>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 (E-Mail Removed)
150 Felker St #D http://www.LearnByDestroying.com
Santa Cruz CA 95060 http://802.11junk.com
Skype: JeffLiebermann AE6KS 831-336-2558

arthur-temp-(E-Mail Removed) (Arthur Shapiro) hath wroth:

>Here's one; hope it is clickable as the URL is split weirdly as I construct
>the message:
>http://features.engadget.com/2005/11...-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.


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

Jeff Liebermann wrote:
> arthur-temp-(E-Mail Removed) (Arthur Shapiro) hath wroth:
>
> >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 (E-Mail Removed)
> 150 Felker St #D http://www.LearnByDestroying.com
> Santa Cruz CA 95060 http://802.11junk.com
> Skype: JeffLiebermann AE6KS 831-336-2558

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