[TowerTalk] Problems on a 2el vertical Array

[Jim Lux]

At 12:11 PM 1/9/2004 -0600, Mark Beckwith wrote:
> > 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
>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.

The model actually assumes equal element currents, and the Christman 
phasing scheme deals with the oddball mutual impedances, etc. I will say, 
having tried, that calculating the necessary lengths of coax analytically 
(by solving simultaneous equations, which involve hyperbolic functions) is 
extremely tedious. Much better to let a simple optimizer (like that in 
Excel) find the lengths by iteration.  Or, you can use excel to calculate 
out the currents and voltages along the feedlines and look for a place 
where they are equal. (this is basically the technique described by ON4UN 
in his book).

If you are a glutton for punishment, I have spreadsheets for 2,3, and 4 
elements to do the Christman technique.  You'll need calculated or measured 
mutual Zs.




>Another factor which doesn't help is when DX signals arrive from
>non-straight-line directions or multiple directions.

This is probably the dominant effect.



>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.

You don't want a 50/50 power split, but, a fairly effective way to do the 
job is to put a tuner at each element and a tuner at the power divider.

I wish that LDG or MFJ would provide a version of their tuners that allows 
you to command the L and C over a serial line.  Essentially a computer 
controlled tuner (but not an auto tuner). I modified a stack of LDG QRP 
tuners to do this (by feeding new software in at boot time), but it has 
some serious complexity problems (too many wires, to much fiddling around)

I also wish someone would make an inexpensive current probe that measures 
phase and amplitude (relative to some reference) and returns it over a 
serial line.



>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.


Considering that the broadcast industry uses the tuner approach, it's 
probably a winner.  The real problem is the user interface and the 
"variable angle of arrival" problem.




>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.


You think that will drive you to drink, you should see the scheme I'm 
working on.. individual receivers and transmitters at each element with a 
wireless LAN for interconnect.  No more coax for me, thank you! My feedline 
will be 110V extension cords.