It depends on the ham's reason for wanting a low SWR.
Perhaps they do not want to use an external tuner.
A truly balanced antenna would be extremely rare. I've never seen one
on 75 including center fed, half wave dipoles. Open wire hooked to an
unbalanced antenna radiates as well as hearing noise. A tuner in the
rig, only lets the rig see the proper load over a limited range.
Without a line tuner and an isolation choke at the antenna my modern rig
would be very limited as to frequency range on 75.
How close does the tuner come to letting the rig see R52, J0? Most low
power antenna tuners are quite limited as to range on 160 and 75.
OTOH the feed line, open wire or coax can pick up noise and radiate
signal. It can be surprising how much noise can sometimes be eliminated
with a good choke in the feedline at the antenna that was thought to be
balanced.
For rigs with tune and load controls it's usually not a problem, but
many of today's modern rigs don't like to see much reactance .
Mine? I use an AT5K HP tuner switched between the rig and an AIM. I
tune the feed line / antenna to R=50, J=0 using the AIM with less than 1
W, I tune the amp into a dummy load and then switch the antenna to the
rig. I depend on R and J being correct and a choke at the antenna
making the feed line, more or less balanced. IOW a legal limit signal
with less than a 1W tune up. On 75, with the legal limit, the voltages
can get quite high with much of an excursion in frequency.
73
Roger (K8RI)
On 11/6/2015 11:06 AM, Donald Chester wrote:
Hello,
I have an inverted vee up about 100 feet. It resonates at 3615. From past
experience, how much should I add to each leg to get it down to 3515?
Thanks,
Carol
Hams are WAY too obsessed with SWR. If it's a full size half wave, the
difference between an antenna resonant at 3615 and 3515 wouldn't make a bit of
difference in performance.
VSWR at 2:1 or less is perfectly OK. Even 3:1 will have negligible effect on
the signal if it doesn't trigger a fold-back feature in the radio.
I use open wire tuned feeders on mine, and get equally excellent results all
the way from 3500-4000, but I use a remotely tuned matching network at the base
of the tower. If I wanted to use coax, I would purchase some hard-line (ever
cable TV stuff works fine), cut the antenna to be resonant at 3750, and just
use it and not worry about it.
Don k4kyv
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73
Roger (K8RI)
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