Rick wrote:
On Sun, 29 Oct 2006 08:23:18 +0000, Ian White GM3SEK
wrote:


BTDT, and succeeded,

Ian, What is BTDT please?

"Been there, done that," which is Usenet jargon for "This statement is
based on personal experience."


--
73 from Ian GM3SEK 'In Practice' columnist for RadCom (RSGB)
http://www.ifwtech.co.uk/g3sek

I am considering re-configuring it for 17 meters.
What do you think...given the nadir of the sunspot cycle. Which is the
better band? (I am aiming this one approx NW from Ohio.)


17 m is a great band for ragchewing. Seems to run at a much slower
pace than 20. Here in NJ it is open most days to Europe. I think
you'd be happy with it, but in my case I find I have my beam NE 90 %
of the time.

Rick K2XT

BTDT = "Been There Done That"

-also BTDTGTS = "Been There Done That Got the T-Shirt"

--

73,Charlie-AD5TH
www.ad5th.com


"Rick" wrote in message
...
On Sun, 29 Oct 2006 08:23:18 +0000, Ian White GM3SEK
wrote:


BTDT, and succeeded,

Ian, What is BTDT please?

"Ian White GM3SEK" wrote in message
...
Sal M. Onella wrote:
I am my club's sole source of FD points for UHF/VHF. I have beams for
2m & 70 cm, but maybe rhombics are better. From San Diego, most of my
DX FD contacts are in the same place: Los Angeles. I don't really
need a steerable antenna.

Well, OK... but it takes a LOT of on-site work before a large rhombic
can show any advantage over one or two decent long yagis. You'd need to
put up four masts in precisely surveyed locations, with enough guy
strength to keep four long spans of wire taut, and then terminate and
match the thing.

BTDT, and succeeded, but it takes far more work than you imagine. When
your club is trying to set up several stations all at the same time, you
may find you don't have enough effort to do it. There's a severe risk of
winding up with no VHF/UHF antennas at all. BTDT too...

And after having spent all that effort on a fixed beam, are you really
prepared to risk a sporadic-E opening in a completely unanticipated
direction?

Impedance matching to a 700 ohm antenna seems to be a daunting task.
If I used a 4:1 balun

That is actually the easy bit. You use the 4:1 balun with an
old-fashioned device called a "universal stub": a half-wave section of
parallel line made from a pair of old yagi elements. One end of the stub
is connected to the antenna, and two clips make a moveable tapping point
for the balun. Towards the far end of the stub, two more clips make a
moveable shorting bar.

Detailed dimensions don't matter at all, because you're going to slide
the shorting bar and the balun along the stub until you find the
combination that gives minimum SWR. It takes all of five minutes.

That was the easy bit... but it's probably the ONLY easy bit of the
whole project.

could I connect two or three independently terminated rhombics (one
over the other) in parallel to the balanced side?

Two stacked rhombics are easy to feed - connect the feedpoints by a
length of open-wire, and then connect the universal stub at the
mid-point. The counter-intuitive thing is that a pair of antennas that
could be hundreds of feet long need to be stacked only a few feet above
each other. But attempting to stack two rhombics will multiply your
rigging problems by a factor MUCH larger than 2!

Having experienced a large rhombic for VHF, I'd stay with yagis.


--
73 from Ian GM3SEK 'In Practice' columnist for RadCom (RSGB)
http://www.ifwtech.co.uk/g3sek



Thanks for the detailed, thoughtful reply. I need to learn more about
matching, as the term "universal stub" is a new one to me. (I know of stubs
only for their value as coaxial tuned traps.) However, a matching task that
takes only five minutes has built-in appeal and WILL be pursued!

If I do a rhombic, I think it will be for 70 cm first, since the size is
easier to manage. The FD location includes a baseball backstop which will
serve as a tiepoint for the near end of the rhombic; if I erect and guy a
single pole off in the desired direction, I can run lines from it to the
baseball backstop and, from these lines, suspend the antenna. Since this is
all play and no work, the complexity is of no concern. If I get it built
and tested before field day, it will be no more time-consuming to set up
than a yagi.

Er ... what about polarization? The HF rhombic is horizontally polarized, I
think. If I do that at 70 cm, I'll be cross-polarized with the rest of the
FM voice world, won't I? I should rotate the whole thing 90 degrees, eh?

An aside: When I was a young sailor (USN), I was stationed at a
communications receiving site in the Philippines. We had over a square mile
of antennas, including dozens of HF rhombics. We could hear anything.