On Mon, 29 Aug 2011 08:51:12 GMT, Clifford Heath wrote:

On Sun, 28 Aug 2011 22:09:00 +0200, Helmut Wabnig wrote:
All started because I could not get the PVC tubing which the many
internet sites propose.

Hi. I foxhunt regularly with the Melbourne ARDF community,
and we have the design of 2m sniffer antennas down to a fine art.

The best material for the shaft is 12mm square or u-channel aluminium.

The common antenna element length and spacing recommended with the
tape-measure antenna is wrong - very wrong. For a start, the lengths
vary by too much (in the hope of a wider effective bandwidth), and
the reflector is too close to the driven, the director too far away.
This causes large rear side lobes, which makes sniffing incredibly
frustrating.

For effective sniffing, you *don't* want maximum F/B ratio, and you
don't want maximum gain - what you want is a cardioid pattern with
just one lobe - the front one. Anything else will give you a bum
steer.

The best pattern we've found so far is the VK3VT beam:
http://www.vk3vt.net/VTbeam.html. Most of the recent ones use a
gamma match instead of the one shown.

We build this using elements made up from 3mm fiberglass rod and
co-ax braid, covered by heatshrink. The element goes through a
short copper tube before the heatshrink goes on.

These elements are sufficiently stiff to keep their shape, but
whippy enough to run with through a tree.

Clifford Heath.



Do you think the matching affects the cardioid or F/B pattern?
So far I have neglected any influence from that.

With the YAGICAD optimizer I could not come to a positive end.
This program finds 100 different "optimums" therefore
I turned to 4nec2 now.
The patterns which YAGICAD calculates look good to me.
When I compare the F/B patterns, of course the 4nec2 has a more
detailed "resolution" showing minor side lobes where YAGICAD
is just approximating.

I wonder how important matching is for a pure
direction finding antenna, methinks rather it should not.
Only the coax cable currents are not considered
in the simulator diagrams, there might be an influence.
Therefore matching is important :-)

Thank you for the vk3vt link.

w.
http://www.qsl.net/oe8uww/

On Tue, 30 Aug 2011 09:03:33 +0200, Helmut Wabnig [email protected] ---
-.dotat wrote:

I wonder how important matching is for a pure
direction finding antenna, methinks rather it should not.

You are right, not at all.

Only the coax cable currents are not considered
in the simulator diagrams, there might be an influence.
Therefore matching is important :-)

You should have stopped writing at the first thought. CHOKING is
supremely important, and is a benefit found in good matching design.
Choking can be achieved without matching, however. And choking will
be needed at least at the feed point, and again a quarterwave down the
line.

73's
Richard Clark, KB7QHC

On Aug 30, 11:17*am, Richard Clark wrote that :
On Tue, 30 Aug 2011 09:03:33 +0200, Helmut Wabnig [email protected] --- wrote:
I wonder how important matching is for a pure
direction finding antenna, methinks rather it should not.

You are right, not at all. ...
73's Richard Clark, KB7QHC

Matching, perhaps not so much, HOWEVER, the antenna pattern/phase/
characteristics are *THE* most important aspect of a DF antenna/
system. The DF antenna characteristics must be as close to ideal as
possible. I've been involved in DF since my stint in the Navy and they
went to great lengths to get the antenna sites ideal, for that reason.
Also...

But why I re-posted.... Thanks to Richard's post...
I forgot to mention a bead choke that I used with success on one of
these beams.
They also significantly improved the performance of a four dipole
Doppler type DF antenna
that one of the locals had and knew it was not right, but couldn't
decide what to do to fix it..

We used two cores on the coax to get well above the coax 50 ohms.

The following cores will fit inside the PVC pipe and around RG-58 size
coax.

Mouser: 623-2643540002
Fair-Rite (Mfr) # 2643540002
O.D. 0.562"
I.D. 0.25"
Len. 1.125
250 ohms @ 100MHz. [specs. I did not measure them as I recall]
171 ohms @ 25 MHz.

Finally,

Helmut,
You have quite a bit of reading there with that sliding element.
Can't wait to dig into it.

73, Steve, K9DCI Haven't read this group in ages.

On Tue, 30 Aug 2011 09:03:33 +0200, Helmut Wabnig wrote:
I wonder how important matching is for a pure direction finding antenna,

I don't think it affects sensitivity, which is not really at a premium
anyhow.

A gamma match seems to skew forward-max location a bit to one side,
which matters if you're relying on accurate bearings from a distance,
but overall it doesn't matter much - in some ways it's better to
expect to spiral in on the fox a little anyhow, I reckon.

The gamma match also goes wonky if you put your hand on it, but I
guess that's not surprising. many of the top competitors use them
anyhow - we have a design that's easy to disassemble to put into a
suitcase for air travel.

One of the guys built a much-enhanced version of the NEC software for
the Mac, and he's automated it to automatically optimise (millions
of individual runs with different combinations) a 7-element boom
for 70cm, which is yielding very impressive DF results right across
the band.

Clifford Heath.