It was with great interest that I went through the various
responses to this original question. In those responses, I
saw accuracy, I saw correct responses based somewhat on myth
rather than science .. etc. But no really "bad" responses!
I suggest that everyone take it upon themselves to regularly
read Kurt Sterba's column in WorldRadio .. the one person
that quite simply debunks the myth with science.
Bottom line Alan, is that, like one respondent stated, when
presented with a load impedance other than its
characteristic impedance, coax (and any other feedline)
becomes a transformer (this characteristic is actually
depended upon in some matching strategies .. ).
You changed the length, and hence the transformation
characteristics of the coax between the ATU and the antenna
... very simple situation. Now, the input Z of the "system"
at the antenna coax input is outside the range of your ATU
on the bands you lost. Add/subtract a few feet to your coax
between the ATU and antenna and all will be well again.
This is a very common situation when using balanced line to
feed a multi-band antenna .. one often has to do some
"trimming" of the feed line to find a length (hence input Z)
that the ATU can handle on all bands.
Don't worry about the VSWR between the ATU and antenna
unless you are running high power or the length is very
long.
And finally .. no .. the ATU does NOT FOOL YOUR RIG into
"thinking" that its working into a 50 ohm load .. no way.
If that were true, then we'd have to state that the 110VAC
to 12VAC transformer in your power supply is FOOLING YOUR
RIG INTO THINKING IT'S RUNNING ON 13.8 V!!! Last I checked,
it WAS running on 13.8 VDC, no fooling, my Fluke DVM said
so. Likewise, measure the Z at the input to your ATU when
it presents a flat load to the rig, and you will measure 50
ohm with 0j ..
NO FOOLING.
By the way, you "could" calculate a length of coax to make
your system work on all bands again. The formulas are
simple, and you'd only have to buy (e.g.) a MFJ-259A or some
such to take initial measurements. BUT .. it's probably
quicker to cut and try!
And to stir up a few more "myths"......
Remember, not all good, and not all effecient antennas are
"resonant" ... I've purposely designed antennas for DX
purposes that are NOT resonant due to other characteristics
that were more favorable to the task at hand.
Furthermore, a resonant antenna generally WILL NOT provide
you with 50 ohm input .. there is no law in physics that
states that! E.g., if your 1/4 wavelength vertical on h.f.
(without any matching system) provides you with a 1:1 VSWR
when fed with 50 ohm line, it is not very effecient .. as
the input Z of an effecient 1/4 wave vertical is closer to
35 ohms than 50 ohms (that other 15 ohm of resistance that
lets you see 1:1 VSWR is really LOSS, as in HEAT. Your 1/2
wave dipole is closer to 70 ohms than 50 ohms, etc etc etc.
And finally, as I recede off my soapbox .... remember that
ANY ANTENNA THAT LETS YOU COMMUNICATE WITH THE DESIRED
PARTY, COUNTRY, ETC., IS A GOOD ANTENNA .. even if its made
with wet string. So remember the facts (not myths), but
don't get carried away with perfection. (And, as I tell
some newer hams, if you rely on VSWR to measure antenna
effectiveness .. replace the antenna with a good dummy load
and save the hassle ... it'll be flat all the time, on all
bands. Conversely, this shows the pertinence of VSWR .. its
a tool, not an end .... ).
73 de Chuck
wa0roi
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