On 5/3/2014 8:56 AM, Marsh Stewart wrote:
Someone posted the following on the subject of antenna tuners: "They aren't
adjusting the resonance of the antenna, so what are they tuning?"
I believe you will find that "Match Box" is a trade name.
When I adjust the capacitance and inductance of my "antenna tuner" my
antenna does not change length or height, and the length of the feedline
does not change. Could it be because it is not really an "antenna tuner" but
is actually an impedance matching network?
But as far as your rig is concerned it sees the antenna change its
frequency so R=50 (height) and both C and L (length) are equal.
So it not only changes the resonant frequency, but adjusts the impedance
to the proper value as well.
No, it doesn't change anything in the antenna, even if the "tuner" is at
the antenna, but it changes what the rig sees, to what it wants to see.
Older tube rigs could make adjustments in the output circuit with the
tune and load controls. Early solid state rigs were fixed, or constant
impedance. Most of today's SS rigs have a built in matching
network/tuner of limited range, but typically not enough to cover all of
the 160, or 75 meter bands. A fan dipole will help, but an external
tuner will do the job. The tuner at the antenna reduces the loss in the
coax due to the loss created by the additional SWR. On low bands (160,
75, and even 40)with good, low loss coax the additional loss may be
miniscule and not worth the effort of installing a remote tuner and on
the higher bands the antenna bandwidth may be wide enough that a tuner
isn't needed. Remember too, that there is loss in that tuner. On low
bands it may be more than the loss in the coax. OTOH 160 and 75 are
notorious for being narrow band. Maybe 25 or 50 KHz wide to stay within
what the rig can handle. 160 may require "pruning" even with an
external tuner. There is a reason those tuners are derated when
operating into low impedances.
I said tha fan dipole may help, BUT on 75, or 160 (if you have the
height) the antenna can be highly unbalanced with fairly high currents
flowing on the coax, requiring a substantial choke balun to keep RF out
of the shack. I have a fan dipole on 75, It requires two choke baluns
to keep RF out of the shack.
http://www.rogerhalstead.com/ham_files/AntennaFeed2.htm at the antenna
and another where the coax reaches the tower, It would likely have been
better to put the second choke right after the first, but it was easier
ans faster to put it at the tower and it worked,
Unfortunately, most SS rigs do not have very clean signals and only the
more recent and expensive ones have better signals, or intermod. SS rigs
typically have been running around -30db while the old tube rigs ran on
the order of -50db for IM3. The closer they are to the desired antenna
characteristics, the cleaner the signal. Newer SS rigs are getting
better though.
Another problem using the internal tuner is running an amp, or barefoot.
The internal tuner has to readjust every time you change from direct to
the amp. An external tuner after the amp lets the rig see the same
characteristics without having to retune.
73
Roger (K8RI)
Marsh, KA5M
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