> Hi guys,
>
> over Christmas I set up a 2el vertical Array for 40m.
[snip]
>The pattern shows, that I should have at least 10dB
>on EACH station and 15-25dB f/b on DX stations.
>I heard only a few DX stations where I really had
>25dB f/b... but this was the big minority (maybe 5
>out of 100). Often I didn't notice any f/b at all.
>(especially on stations closer than 2500km).
[snip]
>It would be great to get some feedback from
>the more experienced Array-builders!
Hi Toby. You didn't say how far down the stations off the back were
actually measuring - all you said was it wasn't the 25db you were hoping
for.
When I build these arrays (I've made a few) if I just build them and split
the power with a TEE and have the different lengths of coax for the phasing,
I might get an S-unit to 10db difference when I switch. That's usually
sufficient for me. (I'm from the "hey, you can tell a difference!" school).
Where you may not be realizing your model is: The model probably allocates
power equally (50/50) between each vertical, right?
In real life when you feed it, the SWRs on the two verticals are different
because of the fact that there's a second energized vertical nearby having
some kind of effect on it, and each is being fed in parallel with something
else with a different length of coax, blah blah blah. Gets too complicated
for me, anyway.
This is the point at which my eyes usually glaze over. Better Men than I
will go further: basically the induced SWR means the power doesn't get split
50/50 - it becomes 60/40 or 70/30 or whatever - not 50/50. Anyhow, if you
plugged in some uneven amounts of energy at your sources in your computer
model, you will see a degradation of the pattern - like you are experiencing
in real life.
Another factor which doesn't help is when DX signals arrive from
non-straight-line directions or multiple directions.
The real-life way to get the power to split 50/50 is to put a tuner at the
base of each vertical and a tuner right before the TEE. Once you get all
tuners to tune out all reflected power at all three points, then your power
is being split in half, and you will experience more like the F/B in the
model.
The people reading who think tuners are blasphemous will probably propose
other answers with a lot of strange symbols and words I don't understand.
Anyway, while they're going through all those histrionics, I'll be on the
air working guys.
:)
Congratulations Toby for getting your MFJ to actually make your parts
imitate your computer model. You must be one of those Better Men. I'll
take a parasitic array any time, mostly because phased ones drive me to
drink. Hic.
Mark, N5OT
_______________________________________________
See: http://www.mscomputer.com for "Self Supporting Towers", "Wireless Weather
Stations", and lot's more. Call Toll Free, 1-800-333-9041 with any questions
and ask for Sherman, W2FLA.
_______________________________________________
TowerTalk mailing list
TowerTalk@contesting.com
http://lists.contesting.com/mailman/listinfo/towertalk
|