On Thu, 27 Jun 2013 13:34:20 -0500, seediq
wrote:
I do not doubt your information here. However, it seems to conflict with
my experiences working 75 meters.
Ummm... is there a weather channel on 75 meters? My "cut the antenna
length" test was intended for VHF, which is a very different beast
from 75 meters. The big difference between VHF and HF is that HF
turns the nearby earth, ground, water table, hills, buildings, fences,
and neighbors into part of the antenna system. For VHF, once I get
out of the near field region, most of that stuff can be ignored
(unless it's also in the Fresnel Zone). I can elaborate more on this
if you like, but I'm not an expert or seriously experience with land
based HF antennas, just marine HF, which is yet a different beast.
I'm stuck at home today with a foot problem. So, I get to sit at the
computer instead of the workbench. I'll throw together a web page
showing that cutting the antenna short does NOT reduce it's gain and
efficiency very much (but does mangle the pattern and VSWR). Stay
tuned.
I work 75 each day using a 75 meter
horizontal loop. I hear the same characters on each day. Often a newbie
pops up with a poor signal. He is in the same area as "the gang" and yet
his signal stinks. Almost invariably we ask him about his G5RV. "Gee
guys how did you know I was using a G5RV?" Poor signals shows up every
time. He is using a dipole that is way too short to resonate on 75
meters. I think they are 82 feet long. It seems to me if VSWR made
little difference, then his 82 foot long dipole on 75 meters should work
just fine. Not trying for a fight, just want an opinion about why we are
hearing this effect.
Can I pass on this right now? I have some definite opinions on HF
antennas and the G5RV, which unfortunately I cannot substantiate with
either experience or calculations. Rather than post erroneous
information, I'll keep my foot in my mouth where it belongs.
However, I can't resist giving you a clue as to what's different
between an excessively short dipole and a real antenna. A hint is
that unless the VSWR is outrageously high or the antenna was made from
barbed wire, nearly 100.0% of the RF that is applied to it gets
radiated in some direction. The some direction is the key. With a
decent antenna, it's going in the right direction. With a not so
wonderful antenna, it's going in useless directions, such as into the
ground. I'll stop there before I get myself into trouble.
Of course they are using tuners to make a match to
their transceivers.
I should also point out that one can always make things worse with an
antenna tuner. Try the loss on 160m and 75m with this Java applet:
http://fermi.la.asu.edu/w9cf/tuner/tuner.html
Tweak the values of Q for the caps and inductors for a more realistic
calculation.
--
Jeff Liebermann
150 Felker St #D http://www.LearnByDestroying.com
Santa Cruz CA 95060
http://802.11junk.com
Skype: JeffLiebermann AE6KS 831-336-2558