Re: Q: Biquad Antennas


Jeff Liebermann wrote:
arthur-temp-3@xxxxxxx (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 jeffl@xxxxxxxxxxxxxxxxxxxxxx
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

.

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