Topband: Antenna Comarisons

[dennis o'connor <k8do@mailblocks.com>]

W5UN Dave:
A shortened vertical on 160 should be down from a taller vertical...  
Looks like your real world experiment matches theory...  Also, any 
antenna that measures within  +- 2dB of a model is doing pretty good... 
 I suggest that we check Lee Cebik's site, W4RNL for discussion of the 
perils of Pauline (and antenna modeling)...  You may want to re-model 
your antennas ungrounded,  using radials elevated above ground a few 
feet...  Mininec over estimates the gain of ground mounted antennas 
(under estimates near field ground losses)...  Also suggest we all read 
Moxon's book on "HF antennas for all Locations .  .  .  .  . etc.", for 
a crystal clear discussion on how to estimate loss in a shortened 
antenna by using perspective and simple trigonometry (easy stuff, even 
for the math phobic)...

And, only a half wave of separation between resonant antennas is pretty 
tough for making valid measurements of gain... There will be 
significant field distortion for any resonant <or near resonant> 
structures, especially of the same polarization, within a minimum of 2 
wavelengths, and lesser distortion out to five waves at least...  For 
making comparisons you/we need to detune those antennas not in use that 
are within the near field...  I'm in the process of finishing up  three 
sets of 3 element, 80 meter antennas, and in spite of >500 foot 
separation, modeling shows significant interaction and field 
distortion. particularily between the NE and NW arrays, the SW array 
which has the greatest separation and has it's reflector shielding it 
from the other two suffers the least interaction...  I am building a 
dozen relay boxes to detune the 80m antennas not in use at any given 
moment...
Plus the detuned 80m antennas interact with my 40 meter antennas, so 
there has to be a second detuned state for all three when I am on 40 
meters...  And, one of the the two Rohn 25 towers  distorts the field 
on two of the arrays to the point that I am forced to also detune that 
tower, with another relay and a coil... <sigh>

And, judging antenna performance by looking at the S-meter is fraught 
with error... The biggest error is that different antennas have 
different noise figures that affects where the S-meter reads... I can 
show you that my 80 meter inverted vee at 95 feet has a minimum of 2 
S-units (S+N) over the low angle, ground mounted, 80 m antennas (same 
favored direction) yet I can call a DX station on the vee till I'm blue 
in the face and be ignored, then switch to the array and work the DX 
...  So, the S-meter is not the final arbitor of antenna performance...

You might consider reworking the Tee vertical using relays, and tuning 
it to be either a reflector, or director, or take it out of the field 
altogether, and take advantage of some additional gain that way...

Looking forward to hearing more aobut your experiments on 160, Dave...

Cheers  ...  Denny

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