While we all agree all antennas have losses and bandwidth
limits, there isn't much useful in all this subjective
hyperbole about traps having "astonishing losses" and
"optimization on 14.015MHz will make the pattern junk at
14.250MHz".
When it comes antennas we're all victims of marketing and
rumors laced with very little fact. Some of the claims from
the most vocal are laughable, like six dB gain when going
from 50 to 300 radials and special Quagis with 20 dB over
monobander performance. And yes, these are real numbers
offered by real people! Just as bad are the "way better than
a 4 element monobander", "patterns are junk" and
"astonishing loss" claims.
One antenna manufacturer who loves linear loading claimed
traps have 1dB loss per trap, another number obviously
pulled from deep in that hole in our backsides.
I have a line isolator from a company that claims or claimed
50k ohms isolation across HF, and it fact it never reaches
anywhere near that value on any frequency and is only a few
hundred ohms on some bands! Something like this is beyond a
simple mistake, it requires almost zero understanding of
what we are talking about!
A person very well known and well respected for antenna
modeling (and one of the best engineers I ever met) and I
had a good conversation at Dayton about Ham antennas and
gain. He was hired on two different occasions to model
amateur antennas, and in both cases the widely accepted
claims of two different manufacturers were obvious junk
science. (No, he didn't mention who or what they were.)
My 40 meter Yagi at 140 feet, and it is tougher to get a 40M
antenna to be broad than a ten meter antenna, has about 6.2
dBd measured gain at 7000kHz. It has 6.6dB at 7250kHz. This
is compared to a real dipole at the same spot by measuring
500 feet away on a 300 ft tower in the main lobe of the
antenna. It is set for optimum F/B at 7100kHz, but it has at
least a 40dB null in the middle of the rear null on either
7000 or 7200kHz.
What really falls apart in pattern? The ANGLE of the rear
null moves around. I can get a boom truck and fiddle with
the elements until I almost can't measure the rearward
signal at a certain angle, but if an element flexes just an
inch the null moves and the null is 30dB less deep at the
same angle. The overall average F/B and gain doesn't change
much at all, but if I was all focused about one certain
angle on one frequency I could make it sound like a
bandwidth disaster.
If a thread is going to take up 250kB of hard drive space we
should at least try to stay somewhat around the real world
expectations. This stuff is starting to sound like CB
antenna specmanship or a discussions about religion.
73 Tom
_______________________________________________
_______________________________________________
TowerTalk mailing list
TowerTalk@contesting.com
http://lists.contesting.com/mailman/listinfo/towertalk
|