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70cm Deep Dish Feed 70cm Deep Dish Feed
Luis Cupido - CT1DMK
Deep dishes are gaining a bad reputation on 70cm for being
difficult to feed efficiently. On higher bands the scalar feeders
(à la VE4MA) provide a good feeding method, but at 70cm
their size make them too obstructive and very inconvenient to
mount at the focal point. Also the well known EIA dipoles will
under-illuminate deep dishes rendering a 6m dish with 0.3f/d
nearly useless.
In this short article I present a 1wl loop feeder with reduced
distance to the back-plane that gets a diagram wide enough for
a 0.3f/d dish with a minimum antenna blockage.
1wavelegnth loop feeder.
The 1wavelength loop antenna is well known for its broad and quite similar E and H
patterns. Its most common configuration has a 1wl closed loop at about 1/8 to 1/10wl
distance from a reflecting plane which is usually bigger than 1wl in diameter. With this
typical design a 0.4f/d dish could be well illuminated but a 0.3f/d dish would require a
broader feeder.
The distance form the loop to the back-plane is one of the key parameters to optimize
the beam width, a smaller distance will widen the pattern while a greater distance will
narrow the pattern (within the 1/8 to 1/20wl range).
However an impedance close to 50Ohm is only found at about 1/10wl from the back
plane.
Another interesting parameter that modify also the pattern is the size of the reflector if
we make it considerably small. Diameters of 1wl or less will begin to produce wider
patterns.
With the intention of making a deep dish feeder as simpler and as less obstructive as
possible the two effects were combined together in order to widen the pattern enough
for a 0.3 f/d dish without lowering too much the impedance at feed point. A good
compromise was found requiring only a small impedance matching (see diagram and
picture for clarification).
The impedance matching consist of an inductor from the feed connector to the loop,
and a small adjustable wire at the opposite position. The inductor is a 1.5 turns of 3mm
thick wire with 20mm diameter and 20mm length. The end capacitor is a 50mm length
3mm thick wire in which the last 15mm are bent for optimizing the impedance match.
The simultaneous Vertical and Horizontal polarization were obtained by making the
same connections and adjust points 90 degrees apart which should be a voltage null
point. However the switching between Vertical and Horizontal need to be done with some care as in practice the connection at 90degrees will distort the pattern a bit and
also changes the input impedance of the other polarity. This problem can be solved by
using 1/2wl lines from the coaxial switch to the antenna feed point, this technique will
place a virtual open circuit at the coaxial connection for the arm that is not selected by
the relay (when making this cables, dont forget to count also the length inside antenna
and relay connectors, length must be measured from the end of the antenna connector
to the end of the contact inside the relay).
Thanks to Peter (EA6ADW) to bring my attention to this kind o feed, and to Jan(DL9KR) for
reporting in dBs over a lot of test QSOs.
.
1/2 wl
V
H
Preamp.
1/2 wl
V / H
RX
relay
T / R
relay
TX
Feed points
Loadind caps
N conn. V
Feed coil V
Load cap H
V
H
reflector 1wl diameter
loop 1wl perimeter
loop high 0.08wl above reflector