[TenTec] RE: Antenna tuner

[Dr. Gerald N. Johnson, electrical engineer]

Sherrill WATKINS wrote:
> 
> Gentlemen:  Please be advised that on h.f., it is a complete waste of time and money to install an antenna tuner at the base or center of the antenna when it can be installed easily at the rig.  <snip>
 - Corn - k4own.
> 

This not absolutely and completely true. Not at all. Consider a couple
relative common situations:

Situation 1: End fed wire. If fed with a remote tuner, the wire need not
radiate immediately at the ham gear and put RF into the microphone and
the operator. Thus a remote tuner can minimize RFI and operator exposure
to high RF fields. At the same time the wire can be more remote from
domestic RF noise makers allowing for quieter receiving. Triple
benefits.

Situation 2: Center fed wire using coax, operated as a full wave wire.
This presents a high (~ 1K) feed impedance which causes a very high SWR
on the coax. Worse yet, if the coax is an odd multiple of 1/4 wave long
the impedance at the shack end of the coax will be very low (~2.5 ohms)
which can be outside the tuning range of most tuners. FAR outside the
range of most commercially made tuners. The currents required at 2.5
ohms Z may be far beyond the capabilities of convenient coax conductor
sizes if there's any power involved; will induce more than trivial line
loss; and lead to far more loss in the tuner since the tuner has to
operate at a very high Q (lots of circulating current) to achieve the
large transformation ratios require to match such a low impedance if the
tuner can match such a low impedance. An open wire feeder is a far
better compromise, but it doesn't take well to running through metal
walls or metal conduit to reach the hamshack (other than the
unobtainable Belden 8290 shielded twin lead for moderate powers). Hence
a coaxial fed remote tuner may be a handier solution.

Beyond these situations (which readily arise since most of our HF ham
bands are harmonically related and that 80 meter dipole is just right
for a full wave center fed on 40, 20, and 10 meters), its probably
expedient to install the tuner where the knobs are in reach of the
operator. On the other hand, the automatic tuner in these situations is
definitely more versatile when located remotely and since there are NO
knobs for the operator to adjust, there is NOTHING significantly wrong
with placing the works of the tuner at the antenna and running a flat
feed line that will radiate less and hear less local noise to the radio
in the more compact shack. Also the impedances the tuner will have to
handle are more reasonable when the tuner is at the antenna than when
the tuner is working through a run of coax, and some popular tuners have
more restricted load impedance (and reactance) range than the impedances
that can be seen at the feed end of a feed line. So the remote tuner may
be able to operated with a lower transformation ratio, and hence lower
loss which may not drastically affect the signal detected at any
distance, BUT will affect the lifetime of the tuner components
dissipating that wasted energy.

Seems to me from these arguments that there is NOTHING fundamentally
wrong with a remote tuner at the antenna. Automation makes it a
worthwhile consideration compared to the automatic tuner in the shack.
There is no doubt that my link coupled series/parallel tuned tuner (a
design that dates from the 30s if not the 20s) is more versatile than a
T or Pi tuner or the L tuner typical of the automated tuner but it can
be a royal pain to change frequency. Sure, the weather proof automated
tuner is not cheap, but the automated tuner in the shack can't tune as
many antennas and is not cheap either.

73, Jerry, K0CQ

Copyright January 18, 2001 by Dr. Gerald N. Johnson

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