I finally got around setting up 10m 4 square (WPX SSB coming :-) and run into
some problems.
First I checked the ACB - 10 unit on the bench, loaded the 4 antenna ports
with 50 ohm resistors, attached 50 ohm dummy and using MFJ 259B checked the
impedances.
I obtained the following
F is min SWR frequency, following done without resistor at dumping port
(looked better).
sw. pos. F R X
1 28.6 58 5
2 NE 27.6 53 0
3 27.8 57 0
4 29.3 56 6
If I connect 50 ohm dummy load to the dumping port then I get
sw. pos. F R X
1 28.2 70 20
2 28.2 67 16
3 28.2 70 19
4 28.2 66 22
When I checked my 160 and 80 m units, loaded with 50 ohm resistors at the
antenna ports and 50 ohm dummy at the dumping port I get dips within the
bands and R around 46 ohms and X = 2 to 3.
When I connected Comtek unit to 4 square of vertical half wave straight
dipoles, bottom tip about 2 ft above ground (modeled individual impedances
around 110 ohm, individual dipole tuned to 26.3 MHz) SWR is way out,
impedances jump all over within the band. On receive I can see some
directivity, but would not dare to load full power into the system.
It appears that Comtek ACB-10 unit is not working properly (judging by
behavior of 160 and 80m units.) On 10m length of wires/traces (abt 5")
between the hybrids, relays and connectors within the unit becomes
significant and might contribute to imbalance. To be picky, the length of
traces should be made known so the feedlines can be properly trimmed. It
appears that there might be something wrong with toroid hybrids.
Nowhere in the instructions does it say to what impedances radiators should
be adjusted to. Only note saying that you are supposed to use 75 ohm foam
quarter wave coax between ACB and radiators and 50 ohm if Inv.L are used
(present lower impedance). Examples are given for 4 square of V shaped
dipoles and mention that you can use the unit with 4 square made of dipoles,
verticals, shortened verticals.
I assume that that 75 ohm quarter waves are used to double the radiator
impedances and connected in parallel when fed 0 deg. phase for middle
radiators.
Question of using current balun. When I add W2DU balun, that changes the
resonant frequency of the quarter wave line from 28.3 to 21 MHz. Obviously
balun adds some coax length. If I shorten the coax to compensate for balun
length, looks like I will be short of coax, plus balun is 50 ohms.
Question on the balun, if I take the feedlines exactly perpendicular to
dipoles, do I need to use balun? What about impedance? (Checking impedance of
W2DU balun at 28 MHz, I get around 50 ohm and X = 20.) Looks like balun is
complicating things. Would it be better to use bazooka balun?
I designed beautiful 4 square on EZNEC and optimized on AO, on paper works
fine, I get some 30 dB F/B, F/S but when erected it in the backyard, thing is
goofy. I wouldn't put full contest gallon to it.
I also tried triangular array using WX0B stackmatch, not too happy with it
either, not much pattern. Looks like back to my trusty Quads and Razors.
Has anyone fooled around with 4 squares that high in frequency, any practical
experiences? At VE1ZZ using 80m unit, there were no problems, but this "VHF"
situation seems to be finicky.
Yuri, K3BU
trying to be NT1E in WPX SSB on 10m
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