Date: Wed, 6 Oct 2010 17:57:50 -0400
Subject: Re: [TowerTalk] Min spacing to 80m yagi ?
Placing antennas at right angles often worsens interaction problems. There
should be little interaction between 40m and 80m if they're pointing in the
same direction, but who knows what interaction the boom will cause if one is
rotated. The only way to tell is to take the time to model the system -
modeling software is very inexpensive. Your proposed spacing is very
close and your mast has a lot of antenna load, hopefully it's not a 2" mast.
## I had a full size 2-el 80m with the 5-el 20m at various distances
below the 80m,
modeled....and the full size 80m really screws up the 20m badly ! The 20m
up, even when it's a full 30' below the 80m yagi. It's actually the worst,
when it's 10'
below the 80m..and slightly 'better' when it's 7' below the 80m. F/B, gain,
F/S all degrade badly.
3db BW is wider too ! But this is not a full size 80m yagi we are
proposing, so it's apples and oranges.
If the proposed 80 and 20m booms are at right angles, the 36' boom from the
80m yagi will screw up
the 20m yagi. The 35' boom from the 40m yagi will also screw up the 20m yagi,
if the 2 are at right angles.
## With everything pointed the same direction,the 40m won't affect the
20m..... nor will the 40m affect the
80m. The 80 won't affect the 40m. One possible way out of any 80/20 m
interaction is to open off both els of
the 80m, each with a spst-NC ceramic vac relay..when operating on 20m. The
plan was to use vac relays on both
80m els anyway...and run a coax + balun to the 80m REF. This way the tuning
of the 80m REF can be checked, and
ditto with the 80m DE. The proposal was to use Tornado drives on both 80m
els, and 2 x mating controller box's,
the box's have 10 x pre-sets each. They also have a 4 x digit, digital turns
counter. The Tornado drive is just
a pair of motor driven compressible coils, driven by a threaded delrin rod
lead screw. The stock F-12, 3/8" AL tubing coils
are not used The coils are at the feedpoint of each 80m ele. The controller
has a 'park mode',
where it fully compress's both coils, and re-sets the counter to zero. They
use plastic coated 1/4" AL refrig tubing for
the coils, so even when bunched up, the turns won't short out. On paper, the
2-el 80m yagi should cover 400 khz of
the 80m band..with high F/B, F/S and low swr. It would be a lot of tedious
tweaking, initially, but at least it can all
be done from the shack, and with the 80m ant at full height, VS tower fully
nested, and hrs on end with a man lift/
crane / bucket truck.
## per the ARRL mast software, the 2" OD x 3/8" thick, 107 ksi chromolly
mast is good for 101 mph...and 108 mph using
the older EIA-RS-222-C method. [eles of all 3 x yagi's broadside to the
wind]. With all 3 x booms broadside to the wind,
it's 117mph / 125 mph. Highest gusts here in the last 50 yrs was 72 mph...and
that was 10 yrs ago. Rotor is a small
K7NV prop pitch, with his far out super mast clamp. I replaced the F-12
'easy on' mounts with my own version, using
3/8" thick 6061-T6 plates...and also 3 x DX-eng.. 'super duty saddle clamps'
, so nothing will slip on the mast. I also
used the same dx eng super clamps on the boom as well. A torque compensator
was installed at the REF end of the
20m boom. On paper, all 3 x ants should be torque balanced. They will also
alternate sides of the mast.
## One other option I have is to mount the 20m yagi on a 1032 tic ring..at the
72' level. That would provide 28' spacing
to the 80m yagi... but is a pita. I have 2 x spare 1032 rings.
Subject: [TowerTalk] Min spacing to 80m yagi ?
From: "Jim Thomson" <email@example.com>
Date: Wed, 6 Oct 2010 05:34:21 -0700
How close can you get a 20m yagi, below a 2-el 80m yagi ?
I have an option to use a F-12 Sigma 280-S shortened 80m yagi
at the top of the mast, with the F-12 340N at the bottom of the mast.
Spacing between the two of them would be 10.5' If possible, I
would like to sandwich my F-12 520 [20m yagi] between the
80 and 40m yagi. Apparently, you can mount any other F-12 HF
yagi within 36" of the 3-el 340N yagi. If I put the 20m yagi
4' above the 40m yagi... that would leave 6.5' between the 20m yagi
and the 80m yagi. Will this work ?? If it doesn't, one option is
to remove the 20m yagi. Another option is to place the 80 and 40m
yagi's at right angles to each other [ no 20m yagi at all] . I don't think
is required, but would minimize the total wind load.
Plan B would be to replace the 2-el 80m yagi with a
80m rotary dipole...with dipole inline with 20m/40m booms below it. IE:
is then at right angles to both yagi's below it. Due to day time height
the above options are strictly an after dark scenario. I don't have any
software on hand
to model any of the above.
Thanks.... Jim VE7RF
TowerTalk mailing list