Hi Jim,
That stacking method has been used for VHF antennas and only
works in the "free space" that VHF and UHF antennas see many
wavelengths above ground. At HF, the ground reflection is important
and the antenna must be parallel to the ground to achieve maximum gain.
Tilting the boom of a log (or yagi) tilts the "image" antenna as well and
reduces the gain without changing the radiation angle. W2PV's book
on Yagi design overs this topic very well and the same theory applies
to LPDAs.
I agree that 33 ft spacing would be good for short boom LPDAs (20 ft or
less in boom length) but as you go for longer boom lengths, as in yagis,
the spacing must go up to achieve the maximum available gain from stacking.
The ability to control vertical take off angle is far more important than
the directivity you would get with a long boom. I would always
take 1/2 the boom length and make two antennas stacked ( to take
advantage of vertical angle control) rather than one long boom. At
HF the vertical incoming and outgoing angles can make 10 to 20
dB difference!!!
73,
Chuck...
K1KW
-----Original Message-----
From: jljarvis [SMTP:jljarvis@abs.adelphia.net]
Sent: Tuesday, March 19, 2002 2:00 PM
To: coneal@ma.ultranet.com
Subject:
Hi Chuck,
I have one Tennadyne T8 at present, in a turning radius constrained
situation. Performance is very close to the T10. One scenario would
be to obtain a second, to obtain vertical angle control from the
stack, when I move it to the roomier location.
It would seem to me that for horizontal lpda's, 33' spacing would
give you one wavelength at 10m, and 1/2 wave at 20m, and in-between.
Probably would be the optimal comprise.
On the other hand, if the antennas were NOT horizontal, but tilted
toward each other on the front end, you MIGHT be able to have something
approaching 1/2 wave spacing on EACH band. Haven't done the geometry,
but someone suggested this, off the reflector this morning.
Crazy thought, eh?
Jim
Subject: RE: [Towertalk] LPDA stacking distance?
To: <towertalk@contesting.com>
Date: Tue, 19 Mar 2002 10:42:38 -0500
Hi,
I have and the stacking distance depends upon the characteristis of
the LPDA. High taper factor (above .92 or so) and high spacing factor
(above 0.05 or so) require larger spacing due to the higher gain of these
arrays. For my system, Taper = .94 and spacing of 0.085, optimum
spacing models out at about 1.1 wavelength. Interestingly, the stacking
gain drops very slowly either side of this spacing. I used 55 ft since the
top antena is at 110 feet. This provides best angle fill and provides
a very good range of vertical take off angles with a top-bottom-in phase
-out of phase switch. The 55 ft spacing yields less than 0.2 dB
compromise on 20M (too close) and 0.3 db on 10M (too wide). On 10M
the patterns starts to show secondary lobes in the vertical plane but these
lobes are more than 12 dB down still. For shorter boom LPDAs I suspect
that the spacing will be smaller and I know of a pair of Tennadyne T-10's
that have been stacked with 30 foot spacing and perform quite well. With
LPDAs, you do not need to worry about the detuning effects of stacking.
73,
Chuck...K1KW
Jim Jarvis
Keithley Instruments
Essex Vermont
802 872 5830 voice
802 872 5831 fax
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