Re: Q: Biquad Antennas

arthur-temp-3@xxxxxxx (Arthur Shapiro) hath wroth:

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.

*** 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.

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