comparing antennas

Yesterday I had to quickly choose which of three antennas to put on a
router being used as a client at a distant site (400 meters) that has
marginal reception.

I had asked the people to pick up some 7 dbi omnis before they left
the country (we are in the boonies in Mexico). But they didn't.
"Maybe next time, but could you please get our laptop online now ?"

So, while setting up the router (WRT54G with DD-WRT) at my house,
which is much closer, I brought up the setup pages for DD-WRT for the
client router and for the base station router (Buffalo HP on dd-wrt)

As I watched the signal level on both routers (seperated by about 150
meters), I swapped antennas to see which yeilded the high readings in
DD-WRT. Speed was fixed at 12 Mbps.

Results given in Signal strength, but sig/noise ratio corresponded.
Results wavered by 2 %.

1) Linksys WRT54G stock antenna: 50% on client end, 58% at AP
2) Buffalo WHR-HP-54G stock antenna (supposedly high gain) with sma to
tnc adapter: 48% on client end 58% at AP

3) Belkin "9dbi" panel antenna with 4' of LMR 100 and sma to tnc
adapter: 48% on client end and 58% at AP

4) Linksys stock antenna with "EZ-10" reflector: 58% on client end and
65% at the AP.

I was surprised. I thought that the 9dbi panel would outperform the
others. Of course I did pick it up for $5 as a discontinued item.
Maybe that's why.
Next I thought that the Buffalo antenna would show better than the
linksys.

The reflector was no surprise. They really do help. At the site, the
reflector added about 4 dbi to the S/N ratio but that got it up to 20,
as shown in DD-WRT. I used it with the original linksys antenna since
it seemed as good as the other two I had.


Question: I understand that doing a walk test at fixed connection
speed is best, but still, is this approach totally worthless or is it
reasonable for me to make comparisons in this manner?

Steve

On 21 Feb 2007 16:20:44 -0800, "seaweedsteve" <seaweedsteve@gmail.com>
wrote:

>Question: I understand that doing a walk test at fixed connection
>speed is best, but still, is this approach totally worthless or is it
>reasonable for me to make comparisons in this manner?

Not totally worthless, but close. The problem is reflections and
multipath. It's very difficult to generate real antenna patterns and
gain measurements without either an RF anechoic test chamber, or a
proper test range. The last time I did it for 2.4Ghz, I had to put
both ends of the link on top of a 15ft high fiberglass pole and 12ft
ladder to keep the ground bounce out of the picture. This particular
test was done in a parking lot. We soon found that moving cars around
would create huge variations in apparent gain. You could see people
moving around.

For example, here's a good example of how to do it all wrong:
<http://martybugs.net/wireless/antennacomp.cgi>
The problem is with the 200 meter test range. The antennas at both
ends were well within the fresnel zone. Ground bounce was also
evident. There were also some obvious obstructions (trees, utility
box) in the path. Judging by the photos, it was done with at least
one end at ground level.

Here's a somewhat better way:
<http://www.olotwireless.net/catala/discono.htm>
See the sections "Measuring Radiation Pattern". Note that the other
end of the link uses a high gain yagi antenna to reduce the amount of
RF that hits the ground and bounces. They would have done better if
they had used something higher than a camera tripod, but good enough.

Try this simple experiment. Take a fiberglass pole and put any
antenna on top. Doesn't matter what type as long as the gain isn't
too high (<12dBi). Connect to some remote access point with line of
sight. Now, move the antenna up and down the pole slowly taking
readings at various altitudes. What you'll see are peaks and nulls
caused by a combination of reflections and knife edge refraction. The
variations will probably be substantial. As you get higher and
higher, the effects will be less and less.

Another clue is to notice the wide variations in signal strength
produced by Netstumbler graphs, even when tested from a fixed
location.

Also, please don't use the xx% signal strength numbers when you have
dB (decibels) available in Netstumbler.

--
# Jeff Liebermann 150 Felker St #D Santa Cruz CA 95060
# 831-336-2558 jeffl@comix.santa-cruz.ca.us
# http://802.11junk.com jeffl@cruzio.com
# http://www.LearnByDestroying.com AE6KS

> Try this simple experiment. Take a fiberglass pole and put any
> antenna on top. Doesn't matter what type as long as the gain isn't
> too high (<12dBi). Connect to some remote access point with line of
> sight. Now, move the antenna up and down the pole slowly taking
> readings at various altitudes. What you'll see are peaks and nulls
> caused by a combination of reflections and knife edge refraction. The
> variations will probably be substantial. As you get higher and
> higher, the effects will be less and less.
>

Thanks.

If fact, my test was from an elevated base antenna (12dbi yagi) but
the client end was on my tabletop and certainly included all the
variables you mentioned.

I guess the implication is that even though various antennas are
compared in the same spot, unless they are up high, then just by
having different patterns, one is inviting an often severe variation
in reflections and refractions.


> Also, please don't use the xx% signal strength numbers when you have
> dB (decibels) available in Netstumbler.
>

I know, sorry. My bad. I didn't see a way to use netstumbler on a
client router (or would it work?) but DD-WRT does give dB readings.
My eye got seduced by the bar graph and I didn't realize I'd be
"publishing" numbers.

Next time I'll at least put it up on a pole and use dB readings.

Steve

On 22 Feb 2007 08:45:15 -0800, "seaweedsteve" <seaweedsteve@gmail.com>
wrote:

>If fact, my test was from an elevated base antenna (12dbi yagi) but
>the client end was on my tabletop and certainly included all the
>variables you mentioned.

Well, that's better than usual. Here's a fun test. Setup a link
between any two machines and continuously record the signal strength
over an extended period of time. Use SNMP in DD-WRT and MRTG to make
it easy. Instructions on request. Don't change or move anything on
either end. What you'll find are radical variations over time. It
can be weather related, reflections from moving objects, or just water
condensing on the antenna. Lots of sources of errors. Typical is at
least 4dB variations in signal strength. If you have bad luck and end
up with multiple reflections, probably 10dB overall variations. Kinda
hard to use such numbers as some kind of reference to measure antenna
gain.

>I guess the implication is that even though various antennas are
>compared in the same spot, unless they are up high, then just by
>having different patterns, one is inviting an often severe variation
>in reflections and refractions.

Exactly. That's what the anechoic chamber offers (besides a great
place to have lunch without the cell phone ringing). By eliminating
reflections, you eliminate most of the major sources of errors and
variations. Incidentally, when measuring antenna gain, it's
considered good form to use a reference antenna with known gain. You
make the initial measurement with the reference antenna, switch
antennas, and do it again. The difference in measurments is the
relative gain.

>> Also, please don't use the xx% signal strength numbers when you have
>> dB (decibels) available in Netstumbler.

>I know, sorry. My bad. I didn't see a way to use netstumbler on a
>client router (or would it work?) but DD-WRT does give dB readings.
>My eye got seduced by the bar graph and I didn't realize I'd be
>"publishing" numbers.

Netstumbler gives BOTH the client and the access point's indicated
signal strength and signal to noise ratio (SNR). Scroll horizontally
with the numerical tables to see them. The column headings are not
very clearly labelled so you'll need to do some digging to figure out
what is indicated, peak reading, and in what direction. I tried to
find a Netstumbler reference page that defined the column headings,
but couldn't. You really need all 4 numbers to do a decent job of
determining what's happening. For example, an overpowered transmitter
will yield a much higher signal strength in one direction than in the
other. Any interference might show up only at one end.

>Next time I'll at least put it up on a pole and use dB readings.

--
# Jeff Liebermann 150 Felker St #D Santa Cruz CA 95060
# 831-336-2558 jeffl@comix.santa-cruz.ca.us
# http://802.11junk.com jeffl@cruzio.com
# http://www.LearnByDestroying.com AE6KS

Jeff Liebermann wrote:

> Netstumbler gives BOTH the client and the access point's indicated
> signal strength and signal to noise ratio (SNR). Scroll horizontally
> with the numerical tables to see them. The column headings are not
> very clearly labelled so you'll need to do some digging to figure out
> what is indicated, peak reading, and in what direction. I tried to
> find a Netstumbler reference page that defined the column headings,
> but couldn't.

Netstumbler-->Help-->Help Topics-->user interface-->List View

> You really need all 4 numbers to do a decent job of
> determining what's happening. For example, an overpowered transmitter
> will yield a much higher signal strength in one direction than in the
> other. Any interference might show up only at one end.
>
>> Next time I'll at least put it up on a pole and use dB readings.
>

On Sat, 24 Feb 2007 11:14:54 +0000, Kev <invalid@invalid.invalid>
wrote:

>Jeff Liebermann wrote:
>
>> Netstumbler gives BOTH the client and the access point's indicated
>> signal strength and signal to noise ratio (SNR). Scroll horizontally
>> with the numerical tables to see them. The column headings are not
>> very clearly labelled so you'll need to do some digging to figure out
>> what is indicated, peak reading, and in what direction. I tried to
>> find a Netstumbler reference page that defined the column headings,
>> but couldn't.
>
>Netstumbler-->Help-->Help Topics-->user interface-->List View

Thanks. I was looking all over the web and didn't bother to check the
help. (argh).

>> You really need all 4 numbers to do a decent job of
>> determining what's happening. For example, an overpowered transmitter
>> will yield a much higher signal strength in one direction than in the
>> other. Any interference might show up only at one end.

Oops. Looking at the above help page, Netstumbler does NOT show the
signal and noise for both ends of the link. It apparently shows only
what the local card shows for signal and noise. Well, one of the
numerous sniffers that I've used does show everything, but I seem to
have muddled them somewhat. I'll dig. It's either Kismet, Wi-Fi
Hopper, or one of the network sniffers that can decode 802.11 packets
(WireShark). I'll check when I have time.

--
# Jeff Liebermann 150 Felker St #D Santa Cruz CA 95060
# 831-336-2558 jeffl@comix.santa-cruz.ca.us
# http://802.11junk.com jeffl@cruzio.com
# http://www.LearnByDestroying.com AE6KS

> Incidentally, when measuring antenna gain, it's
>considered good form to use a reference antenna with known gain. You
>make the initial measurement with the reference antenna, switch
>antennas, and do it again. The difference in measurements is the
>relative gain.

I sorta considered the Linksys stock WRT-54G antenna to be a reference
of sort. I assume (I know, I know) it is a classic 2dBi omni rubber
duck. I guess you are saying that the terms and statements should be
made as "gain over reference antenna."

> >> You really need all 4 numbers to do a decent job of
> >> determining what's happening. For example, an overpowered transmitter
> >> will yield a much higher signal strength in one direction than in the
> >> other. Any interference might show up only at one end.

Gotcha.

>What you'll find are radical variations over time. It
>can be weather related, reflections from moving objects, or just water
>condensing on the antenna. Lots of sources of errors.

I now understand about the reflection, refraction issues, but apart
from that is the time issue you mentioned. It seems like if one is
not seeing much variation (1-2 db) over the short-term with the
control /reference antenna, then swapping the antennas quickly and
measuring right away should give a decent comparison? That is if
there aren't any moving objects on the scene....??


My biggest question now is about using Netstumbler or Kismet etc. vs
using DD-WRT to measure. Are you saying that DD-WRT is NOT an
acceptable instrument for signal measurement ?

More specifically; "When running DD-WRT-managed devices on both ends
of a wireless link in a controlled environment and then comparing
numbers (dB and SN ratio) reported on the "wireless status" page on
each end's interface as I switch out antennas (on one end only) would
this be an acceptable way to compare antenna gain?"

If not, why? Is DD-WRT's signal level or SN reporting unreliable or
poorly derived? Is Netstumbler's or Kismet's much better?

Also, I still don't see how Netstumbler or anything else running on pc
would work when using a router in client mode. I'm not able to test
it right now, but it seems that Netstumbler would not have any way of
seeing the signal strength if the client router is connecting the
computer via ethernet cable...



This is particularly useful thread, I think. So many people are
buying and building antennas. We all want to be able to tell which
antennas deliver measurable gain. Most folks probably depend on
marketing hype and anecdotal evidence (I put on one of those machV
doodads and it's really humming now, lemme tell 'ya).

And of course we also forget about that four feet of LMR100 and the
TNC adapter that we're using.

And we have no way of judging the accuracy of what tests are
reported. It would be great to have some understanding and agreement
so that various people could be reporting reliable tests with the
antennas they have.


I wish I could do that little test again, but the Linksys is in use
and cannot be pulled. Has anybody else compared the Buffalo HP's
included "hi-gain" rubber duck to the Linksys's included rubber
duck? Is it likely that "hi-gain" to Buffalo means 2dBi? Or that
the RPA to TNC adapter could eat up any difference?

Anyone else done any testing?

Cheers,
Steve

On 26 Feb 2007 10:51:04 -0800, "seaweedsteve" <seaweedsteve@gmail.com>
wrote:

>I sorta considered the Linksys stock WRT-54G antenna to be a reference
>of sort.

Nope. It's not a repeatable or well controlled antenna pattern or
gain. Inside the rubber radome is just a 1/4 wave piece of coax cable
with the braid peeled back over the jacket. There's no way to build
two of them that are reasonably identical or any way to prevent other
parts of the system (box, coax, connector, table top, etc) from
becoming part of the antenna pattern.

>I assume (I know, I know) it is a classic 2dBi omni rubber
>duck. I guess you are saying that the terms and statements should be
>made as "gain over reference antenna."

Yes. Something like that. Ideally, the reference antenna should be
tested in exactly the same position as the antenna under test.

>I now understand about the reflection, refraction issues, but apart
>from that is the time issue you mentioned. It seems like if one is
>not seeing much variation (1-2 db) over the short-term with the
>control /reference antenna, then swapping the antennas quickly and
>measuring right away should give a decent comparison? That is if
>there aren't any moving objects on the scene....??

Nope. Cancellation and reinforcement as it goes through nulls is
difficult to avoid. A person walking at 1 meter/second will go
through about 8 wavelengths of 16 nulls and 16 peaks. It doesn't take
much to change the signal levels. I find myself using blue masking
tape to old my test setup onto the workbench to keep it from moving
and changing the levels. It's much easier to take large number of
readings over a fairly long period of time, throw out the very high
and very low readings, and average what's left.

Incidentally, I get a kick out of data sheets that claim 3 decimal
place accuracy for antenna gain, when they're lucky if they can
measure the gain to one decimal place accuracy.

>My biggest question now is about using Netstumbler or Kismet etc. vs
>using DD-WRT to measure. Are you saying that DD-WRT is NOT an
>acceptable instrument for signal measurement ?

DD-WRT is fine (including Kismet Drone). All that I was concerned
with was that signal level and SNR measuring should not be measured
just at one end of the link as tx power levels are not guaranteed to
be identical.

>More specifically; "When running DD-WRT-managed devices on both ends
>of a wireless link in a controlled environment and then comparing
>numbers (dB and SN ratio) reported on the "wireless status" page on
>each end's interface as I switch out antennas (on one end only) would
>this be an acceptable way to compare antenna gain?"

That's as good as it gets. There's no guarantee that the actual
numbers are accurate or even repeatable, but that will have to do
under the circumstances.

>If not, why? Is DD-WRT's signal level or SN reporting unreliable or
>poorly derived? Is Netstumbler's or Kismet's much better?

They're all derived from the RSSI level, which is only 8 bits (0-255)
range. It's also not very linear. It's good enough for saying one
antenna is xx dB better than another, but not very good for making
absolute signal level measurements. SNR should be fairly accurate.

>Also, I still don't see how Netstumbler or anything else running on pc
>would work when using a router in client mode.

Netstumbler will NOT see a client radio. Netstumbler works by sending
a "probe request" to any access point and listening for the responses
which contain the SSID. That's called an active probe as the sniffing
computer must transmit something in order to get a response.

Kismet is a passive sniffer, which will extract packets out of thin
air and does NOT need to transmit anything in order to get a response.

>I'm not able to test
>it right now, but it seems that Netstumbler would not have any way of
>seeing the signal strength if the client router is connecting the
>computer via ethernet cable...

It can't. However, it is possible to sniff the traffic betwenn this
client and an access point and extract the signal strength and SNR.
Management packets, going in either direction, all contain the signal
strength and SNR.

>This is particularly useful thread, I think. So many people are
>buying and building antennas. We all want to be able to tell which
>antennas deliver measurable gain.

The easiest way to do that is to compare the antenna with a known
antenna (as previously described).

>Most folks probably depend on
>marketing hype and anecdotal evidence (I put on one of those machV
>doodads and it's really humming now, lemme tell 'ya).

That's why I like RF. It's all magic, hype, and anecdotal rubbish.
Building and designing antennas is very easy compared to testing them.
I've had more than a few antennas that look great on paper (or
printout) and don't work anywhere near what I intended.

>And of course we also forget about that four feet of LMR100 and the
>TNC adapter that we're using.

Some manufacturers also forget about those. They specify the gain of
the antenna, without the slightest consideraion of the coax and
connector losses.

>And we have no way of judging the accuracy of what tests are
>reported. It would be great to have some understanding and agreement
>so that various people could be reporting reliable tests with the
>antennas they have.

True. The most common way to compare antennas is to NOT use the test
results, but rather to compare the antenna models. There are serious
problems with doing it this way, but it does tend to be fairly useful.
There's also a big difference between being able to connect at long
ranges, and being able to stay connected reliably while doing useful
work. Sorry, but I don't have an easy answer. I tend to compare
computer models instead of test results. That opens a whole new can
of worms, but seems to be adequate for purposes of comparisons.

>I wish I could do that little test again, but the Linksys is in use
>and cannot be pulled. Has anybody else compared the Buffalo HP's
>included "hi-gain" rubber duck to the Linksys's included rubber
>duck? Is it likely that "hi-gain" to Buffalo means 2dBi? Or that
>the RPA to TNC adapter could eat up any difference?

Dunno about the antenna comparisons. The RP-SMA to RP-TNC adapters
have very little loss. Might was well ignore them.
<http://802.11junk.com/jeffl/antennas/connector-loss/index.html>
At 2.4GHz, the loss through all those connectors was 2dB or about
0.15dB loss per connector pair. That's essentially zilch.

>Anyone else done any testing?

--
# Jeff Liebermann 150 Felker St #D Santa Cruz CA 95060
# 831-336-2558 jeffl@comix.santa-cruz.ca.us
# http://802.11junk.com jeffl@cruzio.com
# http://www.LearnByDestroying.com AE6KS

Excellent! Muchos thanks for the clarification.

I do have another question:

> >I sorta considered the Linksys stock WRT-54G antenna to be a reference
> >of sort.
>
> Nope. It's not a repeatable or well controlled antenna pattern or
> gain. Inside the rubber radome is just a 1/4 wave piece of coax cable
> with the braid peeled back over the jacket. There's no way to build
> two of them that are reasonably identical or any way to prevent other
> parts of the system (box, coax, connector, table top, etc) from
> becoming part of the antenna pattern.
>

What would you recommend using as a reference? An isotropic (as if I
knew what that is!) radiator? Is there such a thing?


> The most common way to compare antennas is to NOT use the test
> results, but rather to compare the antenna models. ... I tend to compare
> computer models instead of test results.

Yep, that's certainly how I shopped for our base antenna. But many
antennas don't have any published model or even offer and idea of it's
gain (i.e. the Buffalo and Linksys rubber ducks).

And then, sometimes there are numbers (gain) but no model. I wonder
if the published or stated numbers are even close to reality. I
guess that if one has a fixed location for the antenna already then
just swapping out antennas at that location will tell you which to use
there.


>Here's a fun test. Setup a link
>between any two machines and continuously record the signal strength
>over an extended period of time. Use SNMP in DD-WRT and MRTG to make
>it easy. Instructions on request.

This sounds interesting, but I don't have the time right now. I can
see that I really need to sort out SNMP though...might ask for those
instructions later.

Thanks,
Steve

seaweedsteve wrote:
> What would you recommend using as a reference? An isotropic (as if I
> knew what that is!) radiator? Is there such a thing?

Only in yer dreams...Literally, its just a theoretical antenna. A simple
dipole or ground plane antenna would work.

Jeff Liebermann <jeffl@comix.santa-cruz.ca.us> wrote in
news:ta07u2hhtq598rq0fokjedoul85fj32t1u@4ax.com:


> Incidentally, I get a kick out of data sheets that claim 3 decimal
> place accuracy for antenna gain, when they're lucky if they can
> measure the gain to one decimal place accuracy.

Yeah, that's funny. One of the tech's where I work for always recorded
levels of everything 2 decimal places.

S/N Ratio of 38.27 dB
12 dB SINAD @ -112.23

I could see an automated piece of test equipment filling in those number
after averaging readings over a time period, but a human......

On 28 Feb 2007 13:45:34 -0800, "seaweedsteve" <seaweedsteve@gmail.com>
wrote:

> I do have another question:

Groan...

>What would you recommend using as a reference? An isotropic (as if I
>knew what that is!) radiator? Is there such a thing?

I was hopeing you wouldn't ask that question. There are plenty of
reference antennas for the usual FCC type certification tests. They
start at HF and work their way into microwave land. However, the
purpose here is to have a known gain across a wide frequency range.
That's not the case if you're only interested in 2400 thru 2483.5MHz
which is a much narrower frequency range. So, the ideal antenna for
this would have:
1. A constant or known gain from 2400 to 2483.5MHz.
2. Very good VSWR characteristics to keep the coax cable from
radiating.
3. Simple and reproduceable construction.

The first two requirements immediately eliminate any high gain
antennas. In general, above perhaps 19dBi, the bandwidth and VSWR of
the antenna at the band edges is fairly horrible. If you want a wide
bandwidth, the antenna has to be a fairly low gain antenna.

For antenna range tests, it's also desireable to have the reference
antenna be a close approximation of the antenna being tested in order
to reduce or eliminate having each antenna affected differently by the
local environment. For example, if the test antenna is highly
directional, while the reference antenna is not so directional,
differences in ground bounce and reflections from nearby objects will
have an effect.

The only antenna that I know for sure will do the trick is a full wave
(12.5mm) loop antenna with a reflector. It gets about 8dBi of gain,
low VSWR, and is fairly broadband (not sure exactly how wide). It's
fairly close to this Maxrad antenna (which is what I've been using).
<http://802.11junk.com/jeffl/antennas/Maxrad%209dBi/index.html>
The PCB antenna can be replaced by a full wave look and some matching
stubs. A biquad is a similar antenna.

The problem is that no matter what you decide to use, if you want
accuracy, it has to be properly tested on the bench and at an antenna
range. However, if the manufactory has done a tolerable job of
characterizing the antenna, then those numbers can be used instead.
>Yep, that's certainly how I shopped for our base antenna. But many
>antennas don't have any published model or even offer and idea of it's
>gain (i.e. the Buffalo and Linksys rubber ducks).

Yep. That's how I seperate the hype from the technology. Those
products without test results (and published testing proceedures)
probably have something to hide.

See:
<http://www.maxrad.com/product_category_detail.cgi?id_num=9&styleid=8>
for examples of what proper specs and test patterns should look like.
Note that these are test patterns from an antenna range, not from a
computah simulation.

>And then, sometimes there are numbers (gain) but no model. I wonder
>if the published or stated numbers are even close to reality.

They're not. Everybody lies, but that's ok because nobody listens.

>I
>guess that if one has a fixed location for the antenna already then
>just swapping out antennas at that location will tell you which to use
>there.

That might work in one dimension but not two. For example, if you
compare two omnidirectional antennas with the same gain, but different
downtilt patterns, you might get the same gain, but at different
vertical elevations. Lots of other ways to make comparisons come out
weird.



--
# Jeff Liebermann 150 Felker St #D Santa Cruz CA 95060
# 831-336-2558 jeffl@comix.santa-cruz.ca.us
# http://802.11junk.com jeffl@cruzio.com
# http://www.LearnByDestroying.com AE6KS

On Wed, 28 Feb 2007 23:37:26 GMT, DTC <no_spam@move_along_folks.foob>
wrote:

>seaweedsteve wrote:
>> What would you recommend using as a reference? An isotropic (as if I
>> knew what that is!) radiator? Is there such a thing?

>Only in yer dreams...Literally, its just a theoretical antenna. A simple
>dipole or ground plane antenna would work.

Actually, I've seen simulations that come close. Two pairs of crossed
dipoles feed in quadrature works fairly well if the coax cable is
decoupled and fairly small. That's easy at HF and VHF, but the coax
gets in the way at UHF and up.

--
# Jeff Liebermann 150 Felker St #D Santa Cruz CA 95060
# 831-336-2558 jeffl@comix.santa-cruz.ca.us
# http://802.11junk.com jeffl@cruzio.com
# http://www.LearnByDestroying.com AE6KS

Jeff Liebermann wrote:
> On Wed, 28 Feb 2007 23:37:26 GMT, DTC <no_spam@move_along_folks.foob>
> wrote:
>
>> seaweedsteve wrote:
>>> What would you recommend using as a reference? An isotropic (as if I
>>> knew what that is!) radiator? Is there such a thing?
>
>> Only in yer dreams...Literally, its just a theoretical antenna. A simple
>> dipole or ground plane antenna would work.
>
> Actually, I've seen simulations that come close. Two pairs of crossed
> dipoles feed in quadrature works fairly well if the coax cable is
> decoupled and fairly small. That's easy at HF and VHF, but the coax
> gets in the way at UHF and up.

Kind of like an antenna who's name escapes me at the moment, but it starts
with a L. Its a four element cross polarized antenna used for satellite work.

Jeff Liebermann wrote:
> Everybody lies, but that's ok because nobody listens.

Divorce attorneys excluded. They just have a different perception of the truth.

On Thu, 01 Mar 2007 13:01:49 GMT, DTC <no_spam@move_along_folks.foob>
wrote:

>Jeff Liebermann wrote:
>> On Wed, 28 Feb 2007 23:37:26 GMT, DTC <no_spam@move_along_folks.foob>
>> wrote:
>>
>>> seaweedsteve wrote:
>>>> What would you recommend using as a reference? An isotropic (as if I
>>>> knew what that is!) radiator? Is there such a thing?
>>
>>> Only in yer dreams...Literally, its just a theoretical antenna. A simple
>>> dipole or ground plane antenna would work.
>>
>> Actually, I've seen simulations that come close. Two pairs of crossed
>> dipoles feed in quadrature works fairly well if the coax cable is
>> decoupled and fairly small. That's easy at HF and VHF, but the coax
>> gets in the way at UHF and up.

>Kind of like an antenna who's name escapes me at the moment, but it starts
>with a L. Its a four element cross polarized antenna used for satellite work.

Turnstile antenna. It's circularly polarized and has a very wide
hemispherical radiation pattern suitable for covering the entire sky.
It's usually used with a ground plane reflector, which ruins the
isotropic approximation, but adds some gain. I would not want to
build the phasing section at 2.4GHz.

http://www.cebik.com/qex/turns.pdf
http://www.west.net/~jay/turnstile.html
http://www.arrl.org/tis/info/pdf/0210036.pdf
--
# Jeff Liebermann 150 Felker St #D Santa Cruz CA 95060
# 831-336-2558 jeffl@comix.santa-cruz.ca.us
# http://802.11junk.com jeffl@cruzio.com
# http://www.LearnByDestroying.com AE6KS

Jeff Liebermann <jeffl@comix.santa-cruz.ca.us> hath wroth:

>>Kind of like an antenna who's name escapes me at the moment, but it starts
>>with a L. Its a four element cross polarized antenna used for satellite work.
>
>Turnstile antenna. It's circularly polarized and has a very wide
>hemispherical radiation pattern suitable for covering the entire sky.
>It's usually used with a ground plane reflector, which ruins the
>isotropic approximation, but adds some gain. I would not want to
>build the phasing section at 2.4GHz.

I found an isotropic approximation among the samples supplied with
4NEC2 antenna modeling software.
<http://802.11junk.com/jeffl/antennas/isotropic/>
It needs a complex 4 port feed system, is nowhere near 50 ohms, but
looks very close to a perfect unity gain sphere. The 4 feed lines are
not shown in the model and will probably make a mess of the pattern.

--
Jeff Liebermann jeffl@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