[TowerTalk] FW: Receive 4-Square

[Matt]

Correction...

Regarding item (3):  I checked back on my notes and I did not remember it
correctly.  Both the 0 deg and 90 deg port of the hybrid should be operating
at or near the input impedance.  (It was the impedance of the capacitors in
the bridge that is selected for twice the port impedance that I was thinking
of).     

The port impedance needs to be matched to the antenna pairs though, because
each antenna pair is operating in parallel (albeit one is an inverted pair
via the un-un).   A 1/4 section of 75 ohm cable should transform 50 ohms at
the antenna to ~112 ohms.  When these are fed in parallel the resulting
transformation is 56 ohms, which will match very well with a port
characteristic impedance of 50 ohms (this is why these types use the 75 ohm
1/4 wave feed lines - if your antenna impedance is something other than
this, you would need a different transformation - ideally need to end up
with something around 100 ohms for each antenna at the port end of the feed
lines).

Matt
KM5VI



-----Original Message-----
From: Matt [mailto:maflukey at gmail.com] 
Sent: Wednesday, October 01, 2014 8:29 PM
To: towertalk at contesting.com
Subject: FW: [TowerTalk] Receive 4-Square

Hi Roger,

On cursory review, here are some thoughts that come to mind...

>> I sum the two middle elements with a two way hybrid combiner, and the
middle, front and rear elements with a three way hybrid combiner 

(1) How are you achieving the plus and minus 90 deg phase shifts for the
forward and rear elements respectively with respect to the mid elements?  I
see the inverter un-un but is the three way combining all of the inputs
in-phase?  A hybrid quadrature (bridge) coupler would normally be used to
generate the necessary 90 degree phase relationship, or which the front or
rear would be offset by 180 deg.  Your circuit diagram does not elaborate on
the coupler, but it does not look like a hybrid quadrature because there is
no "dump" port.

(2) You mention phase delay line but I did not see that in the on-line
schematic.  If you are using phase delay line feed method, bear in mind that
there will be mutual interactions between the driven elements.  These will
need to be evaluated (through testing) and included in the delay line
calculations.   W7EL has a good program out there to do this.

(3) If using hybrid quadrature, I seem to recall that the mid antenna "port"
of the bridge operates at half the impedance of the "front" / "rear" ports
(would need to look through my notes to verify).  I seem to recall that when
the two mid antennas are connected in parallel, the resulting combined
impedance (half of one antenna) then matches the port.   If you are using a
combiner, the input impedance may be the same as each output and there could
be an impedance mismatch between the 2-way combiner and the quadrature
bridge.

Hope this info helps you & good luck on your project.

Matt
KM5VI




-----Original Message-----
From: TowerTalk [mailto:towertalk-bounces at contesting.com] On Behalf Of Roger
Parsons via TowerTalk
Sent: Wednesday, October 01, 2014 10:49 AM
To: towertalk at contesting.com
Subject: [TowerTalk] Receive 4-Square

I would appreciate some help with my 160m receive 4-square antenna. It has
horrible directivity which I presume means that I have the phasing or
amplitudes wrong.

The array consists of 4 passive vertical antennas, spaced quarter wave
across the sides, each 25' high and top-loaded. Each has 8 50' ground
radials, is resonant on 1850 kHz and is loaded to 75 Ohms - all exactly as
W8JI. I am confident that this part of the array works properly.

There is a pdf copy of the schematic at:


https://www.dropbox.com/s/2wg3kj9r42woqbi/Vertical%20Array.pdf?dl=0

Ignore the excessively complex control circuitry - that bit also works fine!

I sum the two middle elements with a two way hybrid combiner, and the
middle, front and rear elements with a three way hybrid combiner. These are
commercial units and are claimed to have excellent phase and amplitude
balance. The only real difference (I think) between my circuitry and other
published designs is that the two way combiner allows me to use a single 75
Ohm phasing cable rather than two in parallel.

So what have I done wrong?

73 Roger

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