On Tue, 7 Oct 1997 11:12:35 -0600 Jim Henderson <jhenders@zianet.com>
writes:
>Esteemed reflectees:
>
> I missed some of this when the local server was down. Forgive
>if
>some is a repeat, but I thought my experience along this line might be
>of
>interest.
>
> For 2 160 seasons I used a 1/4 sloper of #12 wire, fed 73 feet
>up
>my tower. The tower has 9 guys and no insulators. The desert ground
>here is
>rotten to worse, depending if any moisture came thru. I replaced the
>wire
>with 450-ohn ladder line, installed as one conductor of broad cross
>section
>(ie the ends soldered together) After pruning for the change in L/D
>vice
>the single wire, I got an increased bandwidth (at 2:1 points) of about
>65
>kHz. With my mighty 100w and no Beverages, this is the best system I
>have
>come up with. The Zo is low, maybe 18 ohms, and I have no figure for
>the
>ground loss.
>
> Anyway, I worked some of the top EU guys with the better ears,
>but
>with difficulty.
>
> Last year I read an old handbook, and in it I saw the
>"twinlead
>Marconi", using 300 ohm line and fed against a counterpoise or good
>screen.
>Realizing a good thing here, I installed one made from the 450 ladder
>line
>(all I had) and put the feed up on the tower where the sloper was; so
>now
>it is an "inverted twinlead Marconi". Anyway, the Zo went up to near
>65
>ohms, and the 2:1 SWR points went to 30 khz wide. BUT... the increase
>in
>performance, as reflected in the better DX results was striking. No
>question it inproved things, IMO.
>
> I was later able to borrow a 400w amp, and was then able to
>(almost) compete in the piles with the buch better-equipped stations,
>and
>worked the 9X, V51, and ZS4TX with ease (big thrill in this, thanks
>gentlemen!) But with 100w there was no question it worked better.
>
> BTW, I know of no one using this antenna, and have heard no
>one
>speak of it. Whether or not the math stands up is academic to me, as
>everyone knows the bumblebee can not fly...the proof of the pudding is
>in
>the eating, as it were. Anyway, I made no other changes in the system
>other
>than the antenna. Same placement, same (lack of) moisture, etc. The
>improved results came with the change.
>
> I tried going back to the old #12 wire sloper, and the results
>went
>back to poor. Tried the 450 line fed as one conductor, got the same BW
>improvement but nothing noticable in the performance. Put the Marconi
>back
>up, and I got the better results immediately.
>
> Since I have never seen this arrangement written up in any
>publication, nor heard of anyone using the idea, I'd like to hear from
>anyone that has. And also from anyone that may try it.
>
>73 de Jim, KF7E
>"May the Flux Be With You."
Jim, I believe that you are looking at the beginning of the cage effect.
Since both wires are in parallel it certainly is not a folded unipole.
I often wonder at the accuracy of some published skin effect data (450
Ohm ladder line is usually #18 Copperweld) but that is way out of my area
of knowledge.
About 13 years ago I built a pair of 6 wire cage verticals for 80M and
hung them from tree branches. I could switch them Cardiod or Broadside
and with a few relays and LC networks ran them as 1/2 waves (switchable)
on 40M . All wires were #12 insulated electrical wire and the diameter
was 12" .
To keep it short, the cage or parallel wires "appeared" to make a big
difference in pileup busting. Back then I was not into computers and just
attributed it to conditions.....now I wonder???
For many years I have heard stories of 75-80M types that went from a
regular dipole to a folded dipole and reported signifcant improvements.
Fact or Fiction ???
BTW I still hold the multiplier record for 80M CW (1986 ?) as a single
band op in ARRL DX using that cage antenna. That old QTH was pretty poor
otherwise.
IMO we all need more info on antennas in real world situations.
73 Carl KM1H
-
---
FAQ on WWW: http://www.contesting.com/topband.html
Submissions: topband@contesting.com
Administrative requests: topband-REQUEST@contesting.com
Problems: owner-topband@contesting.com
|