> I would not expect a current balun to change the resonant
frequency (0 ohms
> reactance) of a dipole.
Why not Paul?
Anything that changes the common mode impedance of the
system can change the resonant frequency of an antenna
system. As a matter of fact if the feedline is not matched
changing the length of the feedline will change the apparent
resonant frequency of the antenna.
This is why I didn't feel warm and fuzzy about the
"resonance" mode in the MFJ analyzer. Just like mismatch
loss, it is useless except under very specific conditions.
I found the velocity factor on the 75 ohm coax I used on
> my 40m 4 square to be closer to .7 than the .8 the
manufacturer specified
> and those lines wound up being almost taught between the
box and feed
> point(s). The v/f on the lines for my 80m 4 square were
very close to mfg
> specs. It has been my experience that once I measured the
V/F of a roll of
> cable it remained constant throughout the roll, but
different rolls may have
> different V/F's.
If they do have different Vf they also have different
impedance. Unless conductor size or spacing is changed to
compensate of course.
> I'm treading on thin ice here so please don't take the
following as gospel.
> Something else that may be distorting your observations
would be the 1/4
> wave (75 ohm) impedance transformer/phasing lines. With a
feed point Z of
> 35 ohms (ground plane) the Z at the other end of the line
will be 165 ohms.
> Assuming that the phase shifting box was designed to drive
a ground plane
> array this would require the 0 degree port match 165 ohms,
the 90d port
> match 82 ohms (two elements in parallel) and the 180d port
match 165 ohms.
That ignores mutual impedances. The actual design Z0 should
be much different.
> With a dipole array there won't be much, if any, impedance
transformation
> (feed point Z of each dipole may be 75 ohms +/), the same
shifting box
> should be designed to match 75 ohms, 36 ohms and 75 ohms.
Not quite. Aside from earth effects, mutuals can make the
dipole impedance quite different. The front element can
easily be above 75 ohms and transform down, while the null
direction element will be below 75 ohms and transform
upwards.
Lets just assume the mutuals cause the front element to
double, the rear element to halve. With a Marconi element
the 35 becomes 70. It hardly transforms through the 75 ohm
cable. The rear becomes 17.5, and transforms to 321 ohms.
Now let's just say a dipole would be 75 ohms and have the
same mutuals. The front would become 150 ohms and transform
down to 37.5. The rear would become 37.5 and transform up to
150 ohms.
So we have the front changing from ~80 in a Marconi element
array to 37.5 with dipoles.
We have the rear changing from ~321 ohms in the Marconi
array to 150 in the dipole array.
I'm afraid the idea of considering the noncoupled impedance
of the elements as the impedance the phasing unit sees as a
design value is just like peeing while on a tall tower. You
never know exactly where it will land.
You really have to know the design impedance of the phasing
system, and then model the system to match. The ports can't
possibly work as planned if they are designed while ignoring
mutual impedances.
For an example, consider my 160 meter 4 square. It has 1/4
wl Marconi elements that have about 34 ohms J0 impedance.
There are 1/4 wl lines to the phasing system.
When driven, the impedances at the elements become:
Front element : 14.42 j59
Middle elements 25.5 j 0.8
Rear element 1.7 j 0.8
NONE are 34 ohms!
The impedance at the phasing unit is:
Front 11.87 j40
Mid 94.5 j0
Rear 774 j205
My array is not a conventional 0 90 180 shift array, so you
can't use these values. But rest assured if I had dipole
elements and simply changed the line so the "nonmutual
coupled" elements had the same Z0 as the 34 ohm elements
through 1/4 wl of 50 ohm cable = 72 ohms (the cable has
.2dB matched loss, so the transformation is not to 73.5
ohms) and I used 72 ohm elements through 75 ohm cable to
have the same nonmutual Zo, my phasing would be as accurate
as a rubber watch.
The thing that allows us to do poor designs and remain happy
is the fact these antennas work to some extent regardless of
we mess things up. Nobody is likely to notice a gain loss of
23 dB and a F/R loss of 20 dB. Most people are happy with
20 dB F/R and a few dB less gain.
The only thing really important is how hot the dump resistor
gets, what the SWR meter says, and if the array has mediocre
F/B.
73, Tom
_______________________________________________
See: http://www.mscomputer.com for "Self Supporting Towers", "Wireless Weather
Stations", and lot's more. Call Toll Free, 18003339041 with any questions
and ask for Sherman, W2FLA.
_______________________________________________
TowerTalk mailing list
TowerTalk@contesting.com
http://lists.contesting.com/mailman/listinfo/towertalk
